Enhancing Production, Nutritional Qualities and Utilization of Fibrous Crop Residues in Smallholder Crop-Livestock Systems: Potential Intervention Options and Progress Toward Sustainable Livestock Production

Abstract

Large quantities of cereals and grain legume crop residues (CRs) such as straw, stover and haulms are produced every year. They are used as a major and cheap source of livestock feed in developing countries especially during the dry season. However, the overall contribution of CRs as feed for ruminants is much less than the potential estimates because of several factors. In crop-livestock systems, most estimate of cereal CRs production and quality traits available in literature are based on the whole residue and do not represent farmer’s practices/context related to the various uses of residue fractions. In addition, there has been a strong focus on post-harvest interventions, but little adoption has been observed, with less emphasis on pre-harvest interventions and residue handling, storage and utilization. This book chapter aims to review the determinant factors and the reasons for low adoption and explores potential intervention options for improving whole-farm productivity and sustainability as a ‘win-win’ solution. Overall, understanding the local/on-farm socio-economic factors and practices/methods is crucial to estimate the production and quality of CRs/CR fractions actually available as feed for ruminants, and to select and promote the possible intervention options that are more practical for wide adoption by the smallholder farmers.

1. Introduction

Due to the growing population and the associated changes in urbanization, income increase and change in dietary preferences by the growing middle class, the total demand for meat, milk and eggs is projected to almost double worldwide by 2050, particularly in the developing world [1, 2]. These necessitate an increase in feed production and efficient utilization for improving livestock productivity to meet the ever-increasing demands for animal-source food (ASF) consumption. However, the competition for food and feed on a fixed land base present significant challenges towards sustainable feed and livestock production system [1, 3, 4]. Poor or inadequate livestock feed, seasonal variability of feed gaps and imbalanced feeding to livestock have been identified as the major constraint to animal production in most developing countries [2, 5]. Recent estimates indicated that 61% of all herds faced a metabolizable energy (ME) feed gap, and 55% a crude protein (CP) gap between actually supplied feed and calculated requirements at attainable milk production levels in East African smallholder farms [2]. Shortages of arable land and water in the smallholder system constrained options for improving the feed resource base through increasing the cultivated area under improved forages or intensive pastures management [6]. Moreover, concentrate ingredients or compound feeds are generally either not accessible or in short supply and most often expensive for wider use as supplement in low-quality forage-based diet of ruminants. Feed losses are most often high (up to 30–50%) in cut-and-carry systems [2]. Feed can contribute up to 70% of total production costs of ruminants [5, 7]. This triggers efficient utilization of available feed resources for ruminant feeding particularly those not competing with human food like crop residues (CRs) including reduction in wastage and widening the feed resource base through exploring non-conventional potential feed resources [5, 6, 8]. Globally, CRs production has increased from 3.8 billion metric tons in 2004–2005 [9] to about 5.3 billion metric tons in 2020–2021 [10], with both cereals and legumes contributing 82% while the rest are contributed by sugar crops 10%, tubers 5%, and oil crops 3% [9]. Crop residues contribute about 70% of feed resources in India on DM basis [11]. With increasing land allocation for crop production and decreasing grazing lands, dependence on fibrous CRs mainly from cereal and legume crops such as straw, stover and haulms for livestock feeding is expected to increase including crop thinning during crop growing period [12, 13, 14, 15]. Case studies in Ethiopia also revealed this scenario [16, 17]. Crop residues may be the only source of feed in late dry seasons or drought periods particularly in semi-arid areas with low and erratic rainfall [18]. The large quantities of CRs are contributed by cereals which usually accounting over 50% of the crop biomass dry matter (DM) [18, 19]. Generally, the potential of CRs as ruminant feed has not been fully exploited in smallholder crop-livestock systems, particularly given the ever-increasing land allocation to crop cultivation at the expense of grazing lands. This is partly due to the fact that: i) CRs particularly from cereals are bulky and generally have low nutritive value that limits voluntary intake (VI) [20, 21, 22], ii) Improper management of the bulky and fibrous CRs contributing to DM and nutrient losses and low productivity of ruminant livestock in the tropical regions [5], iii) the bulky nature of CRs that requires significant investment and labor for collecting, storing, handling, transporting, and processing also limits the use of CRs [10, 22, 23], and iv) competing demands for CRs in smallholder systems, most importantly for soil fertility, and for household energy and construction, and other uses [14, 15, 24, 25, 26].

All of these factors led to inadequate supply CRs for ruminants particularly during the dry season in sub-Saharan Africa (SSA). This necessitates improved CR production in the same unit of land and efficient utilization of CRs for livestock feeding [5, 15, 27, 28, 29, 30, 31, 32] in smallholder farming systems. First, improving the production of CRs in the same unit of land does not require additional water or land or other farm inputs to produce [15, 29]. Second, ruminant livestock have a huge potential to utilize low-quality fibrous CRs and convert them into high-quality human food [3]. A new analysis of the feed/food debate also revealed that to produce the equivalent amount of ASF (meat, milk, or eggs) much less human-edible feed is needed in ruminant systems than in monogastric systems (6 vs. 16 kg of human-edible feed dry matter (DM) per kilogram of protein products) [33]. However, less information is available on feed interventions related to CRs (less than one in ten articles dealt with CRs) compared to cultivated forages and fodder trees, despite the huge contribution and potential of CRs as a ruminant feedstuff in small-scale ruminant systems across tropical regions [34]. There have been few studies focused on either pre-harvest [12, 15, 31, 35] or post-harvest [5, 32, 36, 37] intervention options for enhancing the production, nutritive value and utilization of CRs as feed for ruminants. Many attempts have been made to upgrade post-harvest CRs using physical, chemical and biological treatments but little adoption has been reported. This stimulated targeted improvement of CRs pre-harvest through breeding and selection at source [15, 28, 29, 30, 38].

The increasing global demand for biomass as food, feed, industrial raw material and a source of energy results in an increasing challenge on sustainable agriculture particularly in SSA [39]. Crop residues are a key element of the interaction between crops (soil mulching) and livestock in many smallholder crop-livestock systems in developing countries, and residue management is a major issue affecting the sustainability of crop-livestock systems. On the other hand, the most estimate of CRs production and quality traits available in literature either is based on the whole residue especially from tick-stemmed cereal crops and do not represent farmer’s practices/context related to the various uses of residue fractions. This calls for revisiting this traditional methodological problems and optimizing allocation of crop residues for improving whole-farm productivity. Hence, in addition to increasing CRs production and nutritive value, and reducing losses during harvest and post-harvest management of CRs, balancing the use of residues for other competing uses such as for soil amendment and forage has also been suggested [26, 40, 41] as an important strategy for improving whole-farm productivity and agricultural sustainability in smallholder crop-livestock systems [3] including the likely adoption of the technology. Through this book chapter, the authors sought to bridge the gap between the conventional research approach/methodology and practical application, providing a comprehensive assessment of fibrous CRs in system wide perspective, recommend possible approach while estimating the amount and quality of CRs that is actually available for feeding livestock and other uses according to farmer’s practices/context and also contribute to the global effort toward more sustainable use of CRs as a ‘win-win’ solution for improving whole-farm productivity.

This chapter contributes to the existing literature on CRs by i) synthesizing the quality attributes and feeding values of cereal and legume CRs for ruminants, ii) summarizing determinant factors for the availability, nutritive value and utilization of CRs as feed for ruminants, iii) describing methodological problems and recommending possible solutions for characterizing the quality attributes of residue fractions and/or whole residue, thereby estimating the amount of CRs that is actually available for feeding livestock and optimizing allocation of crop residues for improving whole-farm productivity and iv) exploring potential intervention options for improving the production, quality and utilization of CRs as feedstuff for ruminants in smallholder crop-livestock system as a ‘win-win’ solution for improving whole-farm productivity and sustainability. As a pre-harvest intervention, we focused on cultivar/varietal choice and those crop agronomy aspects that affect the yield and quality of CRs and could likely be easily modified in smallholder crop-livestock, mainly on cereals and grain legumes or pulses CRs since they are commonly used as livestock feedstuffs in smallholder systems.

Therefore, the aim of this chapter is to present the potential intervention options suitable for smallholder farmers that could likely be easily applied and adopted in smallholder crop-livestock systems

• to improve the production and quality of CRs as ruminant feed

• that reduce feed wastage during harvest, storage and feeding, and improve the feed utilization efficiency by ruminants

A further goal is to provide the scientific community on key gaps and recommend possible approach while estimating the amount and quality of CRs that is actually available for feeding livestock and other uses according to farmer’s practices/context and also to contribute to the global effort toward more sustainable use of CRs as a ‘win-win’ solution for improving whole-farm productivity.

2. Quality attributes and feeding values of cereal and legume crop residues for ruminants

Generally, senescent cereal CRs are characterized by a high fiber and low N (<1.12% DM) contents and low digestibility [13, 21, 42, 43, 44, 45], an unbalanced mineral composition [45, 46] and low VI [20, 22, 47]. For instance, the voluntary DM intakes of ruminants fed on cereal CRs alone ranged from 1.4–2.0% of live weight (LW)/day for maize stover, < 2.0% of LW for rice straw, 1.6–2.3% of LW for tef [Eragrostis tef (Zucc.) Trotter] straw and 1.1–2.1% of LW for wheat straw [13, 43, 44, 45, 48, 49]. As a result, the feeding value of low-quality forages like cereal CRs is also generally far below the maintenance requirements of adult ruminants; when fed exclusively CRs diets ruminants most often lose weight with low VI [43, 45, 50]. For instance, sheep-fed maize stover alone for about 63 days [45] and 92 days [13] in Ethiopia resulted in the live weight and carcass weight loss by about 42 and 26 g/head/day, respectively, and this was likely because of the negative nitrogen balance as observed in [13]. Similarly, feeding tef straw alone did not support the maintenance requirements of cross-bred calves in Ethiopia. There was a LW loss of about 75.8 g/head/day during the feeding period of 111 days [43]. In contrast, haulms of grain legumes such as cowpea, groundnut, haricot bean and faba bean can have better nutritive value compared to cereal CRs and supported daily LW gain in sheep [51, 52, 53, 54] and Arsi bulls (47 g/head/day) [55] when fed as sole diet.

Nonetheless, cereal CRs have the potential to be used as a coping strategy for a certain period of time during critical feed shortage/drought with an acceptable body weight losses but without or with minimum losses of live animal through death. For instance, Singh et al. [51] reported that feeding exclusively chopped coarse-stemmed cereal CRs diets such as millet, sorghum and maize as expected resulted in the weight loss of the Yankassa rams by 45.4, 59.7 and 65.9 g/head/day, respectively, during 70 days of the dry season in Nigeria, but there was no single mortality or sickness observed. This shows that low-quality forages like cereal CRs can be used to keep the animals alive for some time during critical periods.

3. Determinants of crop residues availability, quality and utilization as feed for ruminants

3.1 Harvest and post-harvest management and utilization practices

Crop residues which are the major roughage source during the dry season are either stored opportunistically or grazed in situ in smallholder crop-livestock systems in SSA [35]. In areas where there are relatively high demands for CRs as feed for ruminants, the harvest and post-harvest management and utilization practice of CRs in smallholder are often associated with considerable levels of dry matter (DM) and nutrient losses due to delayed harvesting of the residue after grain maturity [20, 56, 57], during post-harvest residue management practices such as condition/method and duration of CRs storage [32, 56, 58, 59] and feeding practices such as during stubble grazing or during harvesting, transporting, processing and stall feeding practices like with or without feeding troughs [32, 60, 61, 62]. Several authors reported the substantial losses of forage DM and nutrients during harvest, storage and feeding in smallholder system mainly due to lack of appropriate feed harvesting, processing and conserving technologies and exacerbated feed shortage in SSA [36, 58, 59, 63, 64].

Crop residue utilization practices through direct stubble grazing result in wastage through trampling and spoilage, but allow animals to select the most nutritious plant parts and the return of nutrients to the soil [60, 62]. Different studies in Eastern and Southern Africa [58, 61, 63, 65, 66, 67] have shown that collection and storage of CRs is a common practice to mitigate dry season feed shortage. But the extent of practicing collection of CRs and storage for later use varied among farmers depending up on several factors such as the relative importance of the residue as feed, the bulkiness and difficulties for transportation, and other competing uses [63, 66], and agro-ecology, production system, labor availability, intensification and market access [20]. In smallholder systems traditionally CRs are stored throughout the dry season mostly in loose form without cover. Moreover, baling of CRs and use of feeding troughs and storage sheds were uncommon in most reports [58, 61, 68]. Crop residues stored in open without having any cover that exposed in rain and temperature extremes accelerate a sharp decline in their DM and feeding value probably due to rotting, exposure to rainfall or destruction by termites [68]. The extent of such losses limits the amount of CRs that is actually available for feeding livestock. However, studies in East Africa [32, 69] indicated that using storage shades and feeding troughs has a huge potential to reduce feed loss during storage and utilization.

3.2 Competing demands for crop residues

In smallholder mixed crop-livestock system, CRs (straws and stovers) are increasingly important as feed for ruminants in developing countries [18] and have many uses other than as ruminant feed [18, 20, 24, 25] such as to enhance soil fertility through soil mulching, to provide material for construction, to provide household energy and other uses [14]. Among the competing demands for CRs, the demand for CRs for soil mulching is one of the interventions under conservation agriculture (CA), and sustainable intensification practices has been increasing and exacerbated the situation in most crop–livestock systems. Retaining part of the CRs produced in crop fields is being advocated as one of the bases of CA to sustain agricultural intensification [14, 18, 24, 70, 71]. However, the relative contribution of CRs as feed for ruminants showed substantial variation depending on the combined population and livestock density gradients in South Asia and SSA. For instance, the contribution of CRs in livestock feeds in 2011 increased from low-density to high-density areas and can range from 50 to 78% in South Asia and from 10–60% in SSA [18]. Most farmers in Burkina Faso and Western Kenya retain around 80% or more cereal residues on their fields [18, 25]. As a result, many studies reported crop residue trade-offs in smallholder crop–livestock systems [18, 24, 25, 72]. On top of these uses of CRs, CRs markets are emerging in SSA and India [22, 73, 74, 75] triggered by feed shortage. On the other hand, burning of CRs is also common practice in many regions in Africa and Asia [5, 23, 76] mainly associated with high cost, labor shortages, lack of suitable technology for efficient utilization of CRs without delay of the subsequent cropping season and lack of technical knowledge and market access of the CRs [23], which also has an adverse effect on the soil, environmental and human health [5, 76]. As a result of the recent government regulations against open field burning of rice straw and the increasing use of straw for various purposes such as fodder for ruminants, for mushroom production, for fuel (heating and biogas) source, for board or paper production and also for organic fertilizer production, mechanized collection of CRs such as rice straw in baled form is becoming popular in Asia [77]. The extent of such competing uses and burning of CRs limits the amount of CRs that is actually available for feeding livestock.

In summary, though large quantities of CRs are potentially available for feeding to ruminants, but the overall contribution of CRs as feed for ruminants is much less than the potential estimates indicated above, because of: a) the other competing uses, b) the losses associated with the harvest and post-harvest CRs management practices through direct stubble grazing and/or in harvesting, processing, condition and duration of storage and feeding practices and c) the burning of CRs. All these factors need to be considered in national and regional assessments of crop residue inventory and feed balance and promoting increased use of this resource as ruminant feed. Promoting suitable technology that may have a strong likelihood of adoption and improving farmers’ technical knowledge for efficient residue management and utilization, balancing the use of residue fractions for various uses for improving whole-farm productivity and linking farmers to CRs markets are potential intervention areas. Furthermore, the traditional methods need to be revisited and discussed below.

4. Revisiting the traditional methodological problems

As discussed above, CRs have various competing uses in smallholder farmers and unlikely that whole residue exclusively used as feed for ruminants. For instance, cutting height at harvest of whole plant cereal species varies within smallholder crop-livestock systems. Farmers either cut high or low at ground level depending up on method of harvest, the livestock density, labor availability, access to fodder market, and the type of crop species. For instance, many combine-harvested crops such as wheat and barley are cut high. However, crops on small-scale farms where there is no combine harvester and straw are scarce and may be cut at ground level manually using sickle [78]. Among thin-stemmed cereals in Ethiopia, wheat is harvested at higher cutting height (about 20–30 cm or more from the ground) than tef which is harvested close to the ground so as to maximize the volume of straw harvested. Similarly thick-stemmed cereals like maize and sorghum, stover are usually harvested either to ground level or at high stubble height; the upper part of the stover usually removed from the field or stored in situ for later use as feed during the dry season while the lower part of the stover most often left in the field as mulch, or for stubble grazing, or other uses [14, 26]. In addition, the maize ears are also removed from the stalk right in the field leaving the rest for in situ grazing in North-western Ethiopia (Figure 1).

Figure 1.

Maize cut at high stubble height; the lower stover left in the field for stubble grazing, while the upper part of the stover is stored in situ for later use during the dry season in North-western Ethiopia (left) (Source and Photo credit: 136), maize ear removed from the stalk in the field leaving the rest for in situ grazing in Western Ethiopia (middle) (Source and Photo credit: 67), and sorghum grain removed from the stalk in the field and the stover cut at ground level and conserved for dry season feeding in Chiro district, Eastern Ethiopia (right).

Generally, the extent of CRs removal from fields usually increased with increased labor availability, livestock density and market access [14, 79]. For instance, [25] reported that much straw or stover is used as mulch in an agro-pastoral system of Burkina Faso, because of low quality as feed for ruminants, absence of livestock or shortage of labor. This has implications for how CRs are allocated to various purposes, conserved and fed and the measurement of the nutritive value of those parts of the stover actually used as a ruminant feedstuff. However, in contrast to this practice, in most reports found in literature regarding estimates of the cereal CRs production and nutritive value available as feed for ruminants refer to the whole residue harvested at ground level especially from tick-stemmed cereal crops and do not represent farmer’s practices/context related to the various uses of residue fractions. Such measurements and literature values may be poor indicators of nutritive value and the contributions of the stover used for ruminant feeding and mislead feed budgeting and prediction of animal performances. Hence, the traditional methods need to be revisited and should represent farmer’s practices/context related to the various uses of residue fractions while estimating the production and the proportion of the residue fractions allocated for various uses and characterizing the quality of CRs fractions actually available as feed for ruminants.

5. Optimizing allocation of crop residues for improving whole-farm productivity

In addition to increasing CRs production and nutritive value and reducing losses during harvest and post-harvest management of CRs, balancing the use of residues from thick-stemmed cereals such as maize stover for other competing uses particularly for soil amendment and forage has been reported as an important strategy for enhancing both crop and livestock productivity and sustainability [26, 41]. Optimal allocation of CRs like maize/sorghum stover for feed and soil mulching can be attained through manipulating the cutting height at harvest according to the circumstances and demands for CRs. Increasing the cutting height of whole plant cereal species can increase the quality of CRs removed from crop fields as feed for ruminants while leaving some amount of the lower part of the CRs in the field either to be grazed or used as soil mulching and for other uses [26, 41, 80, 81]. In line with this, manipulating cutting height at two internodes below the lowest ear of maize stover at grain maturity provided nearly about two-third of the stover as upper part for feed (6.38 t/ha) and the rest one-third of the stover as lower part (3.82 t/ha) could be left in the field available for mulch, stubble grazing or other purposes [26]. Similarly, available literature in China suggested that the upper fraction of the maize stover could be removed from the field for feed while the lower stover fraction is of comparable value for soil amendment and found as a feasible option to enhance both crop and livestock productivity through best use of maize stover fractions [41, 81]. This simple harvest management strategy of the residues from thick-stemmed cereals coupled with choice and use of suitable dual-purpose food-feed varieties provides the most appropriate proportions of upper and lower stover fractions according to circumstances and need for optimal allocation and improves whole-farm productivity.

6. Enhancing the production, nutritive value and utilization of crop residues for ruminants: potential intervention options

Improving voluntary intake and digestibility of low-quality CRs through upgrading and/or supplementation has been the major emphasis [13, 45, 82]. However, little adoption of upgrading post-harvest CRs technologies has been reported [83]. Much less effort has been put into pre-harvest intervention strategies mainly crop management factors, multidimensional crop improvement and post-harvest residue management, utilization and feeding practices [83, 84]. The importance of focusing more on pre-harvest strategies also highlighted [34] for improving crop residue yields and quality in the same unit of land without affecting the primary product which may have a strong likelihood of adoption.

6.1 Pre-harvest intervention options

6.1.1 Enhancing whole plant value: progress towards developing dual-purpose crops through multidimensional crop improvement approaches

Plant breeding and selection criteria using multi-trait and whole-plant (i.e. food and fodder) model against the single (grain)-trait model has been the focus of research in public and private crop-improvement programs towards whole-plant improvement/optimization [11, 15, 85]. Using dual-purpose/food–feed crops improve both grain/pod yield and biomass yield and residue quality without requiring additional land and water [86] and enhance animal productivity [52, 53, 87]. The following subsections discuss the two separate approaches towards developing dual-purpose crops through multidimensional crop improvement [15].

6.1.1.1 Exploiting cultivar-dependent variation for improving ruminant productivity

This approach is through exploiting the variations among the existing cereal and grain legume cultivars in both grain/pod and residue yields and residue quality traits such as nutrient composition, digestibility, voluntary intake and animal productivity, which is relatively quick and requires lower investment than the second approach, that is, targeted genetic enhancement [88]. Substantial variation in CRs quality among genotypes within cereal crop species without sacrificing grain yield has been reported. Chemical composition and in vitro digestibility of CRs vary among genotypes within species of maize [30, 89, 90], sorghum and pearl millet [15, 29, 86, 87, 91], wheat [92, 93, 94], barley [50, 95] and rice [11, 96]. For instance, the stover CP content and in-vitro organic matter digestibility (IVOMD) of sorghum cultivars varied by about 5.5 and 10 percentage units, respectively [91] while rice cultivars varied in straw IVOMD by about 5 to 10% units [88]. This has a great potential to improve ruminant productivity. For instance, calculations from feeding standards [97, 98] indicate that an increase of 5 percentage units in dry matter digestibility (DMD) would be expected to increase ME intake of ruminants by about 20% and could, for example, increase growth rate of a 300 kg animal from 0.15 to 0.30 kg/day. In addition, an increase in forage NDF digestibility by a one percentage unit could increase the dry matter intake (DMI) and milk production by about 0.17 and 0.25 kg/day, respectively [99].

In an experiment using a sorghum stover of the lower and higher quality varieties (IVOMD 47 and 52%, respectively) resulted in milk improvement from (10 to 15 kg/day) in dairy buffalo fed in densified total mixed ration having about 50% of sorghum stover inclusion rate, due to higher IVOMD and hence higher feed intake [15]. Large variation (P < 0.05) was observed among barley genotypes in daily live weight (LW) gain of sheep ranged from −143–18 g/head/day as a result of the higher straw digestible organic matter intake (DOMI) (as g/kg LW0.75/day) for the superior barley cultivar than for the inferior one when fed as sole diets [100].

There also appears to be large potential for selecting dual-purpose genotypes of the CRs of grain legumes such as cowpea and groundnut [52, 73, 101, 102, 103], faba bean [54, 104], chickpea [105, 106], lentil [107, 108] and common bean [109] without sacrificing pod/seed yield. In addition, the variation in leaf loss observed among common beans genotypes while approaching seed maturity [110] suggesting the need to include this trait as a selection criteria [101]. For instance, sheep fed exclusively groundnut haulms diet from 10 different cultivars in India, nitrogen retention was positive and 70% higher (from 6.7 to 11.4 g/head/day) while weight gains in sheep were more than two-fold (from 65 to 137 g/day) for the superior groundnut genotype than for the poor-quality genotype [52].

Generally, the variation observed among cultivars in most crops in terms of grain and residue yield and residue quality traits [15, 28, 30, 90] was found to be promising for simultaneous selection of superior cultivars both for grain and straw traits. However, the recent advances in dual purpose rice and wheat research [88] suggest that this approach is not promising for all crops. For instance, it looks promising for rice but not for wheat that may need targeted genetic enhancement for simultaneous selection of superior wheat cultivars both for grain and straw traits [88]. Furthermore, the differences observed in residue quality traits among cultivars in most studies were relatively smaller than the differences in the grain yield and residue yield. However, the overall increased in nutrient yields (e.g., both digestible dry matter and ME productivity of the residue) per hectare through use of dual-purpose cultivars has a great potential to improve ruminant productivity/ha/year.

6.1.1.2 Targeting genetic improvement towards specific traits: recent advances toward developing dual-purpose crop cultivars

The second approach towards developing food-feed (dual-purpose) crop cultivars is through targeted genetic enhancement, using conventional [42, 111, 112] and molecular breeding approaches [113, 114], and has greater potential for impact. The recent results [115] demonstrated the feasibility of incorporating genomic prediction as a tool to improve maize stover quality traits such as IVOMD and ME through genome-wide association study. This will allow maize breeders to select for stover quality traits more quickly and cost-effectively while avoiding the need for field or lab-based phenotyping and significantly reducing the need for additional testing resources and to develop new dual purpose maize varieties without sacrificing grain yield. However, these approaches require more investment and time compared to simply exploitation of already existing variations [15].

6.1.2 Crop management options

Exploring the potential crop management options that can contribute to the improvement of yield and quality of CR has been reviewed by [12]. As simple crop management options to improve the production and quality of CRs as ruminant feed that could likely be easily applied and adopted in smallholder crop–livestock systems such as i) manipulation of plant density, ii) improved crop protection practices and use of disease-tolerant food-feed crops and iii) fertilization are discussed below.

6.1.2.1 Modification of planting density

In most maize growing areas of Africa, farmers use lower plant densities at harvest than the recommended rates (e.g., in Nigeria 5.3 plants/m² irrespective of varietal difference, environment or management practice, and in Ethiopia it varied from 4.4 plants/m2 to 5.3 plants/m2, depending on cultivar maturity), imply that maize yield gap could potentially be due to lower plant density [116, 117, 118]. On the other hand, maize plant densities up to 8.5 plants/m² are recommended under intensive production in North America [119], because the newer maize hybrids were more tolerant to high plant population than the older hybrids [118]. As a simple management option, increasing maize plant density of medium maturing genotypes from the recommended 5 to 7 plants/m² in Ethiopia increased dry matter yields of both grains, by 9% (from 6.5 to 7.1 t/ha) and of stover by about 21% (from 11.7 to 14.1 t/ha), while the stover digestible dry matter yield/ha was increased by about 20% (all P < 0.05) [120]. This is in accord with the previous findings in North America [121] who reported that increasing maize grain and whole plant yield through increasing plant density from 4 to 10 plants/m². This might be attributed to the higher crowding stress tolerance of modern hybrids [122] and has higher optimum plant densities for grain yield than older hybrids even under sub-optimal nutrient conditions [123] and suggests the potential of improving maize residue yield through higher plant density with modern hybrids without compromising grain yield and residue quality. However, several factors could influence the optimum maize plant population density; most importantly moisture availability, soil fertility, hybrid maturity group and row spacing [124] need to be considered.

Recent study using sorghum varieties released for grain in Ethiopia [125] indicated that increasing the plant density of sorghum from the recommended 12.5 to 100 kg/ha resulted in similar total biomass yield but increased the stover IVOMD and decreased concentrations of fiber fractions and stalk thickness. This will have a great potential for improving animal production associated with higher intake as a result of higher quality and thin-stemmed stover. In another study, the possibility of increasing sorghum grain and stover yield through increasing the plant density from the recommended 26,600 plants/ha to 53,300 plants/ha has been demonstrated in Mali, albeit it varied depending on plant density, N fertilization, and variety [126]. Generally, the potential constraints to the adoption of technologies are the more nutrients and water needed with higher plant densities.

6.1.2.2 Improved crop protection practices and use of disease-tolerant food-feed crops

There are also substantial amounts of forage dry matter and nutrient losses due to poor management crop diseases, pests and weed. There also appears to be a huge potential for enhancing the productivity and feeding value of cereal and legume CRs and thereby improving animal productivity and profitability in smallholder crop–livestock systems in SSA through improved management of crop diseases, pests and weeds [35, 127]. Moreover, high adoption rate has also been reported through development and promotion of disease-tolerant, food–feed crops such as groundnut and sorghum in India [35]. This is due to the substantial benefits obtained by farmers in terms of not only in improved fodder availability by >50% and > 60% in groundnut and sorghum, respectively, but also in improved fodder nutritive value (dry matter digestibility and dry matter intake by 10–15% and by 10–32%, respectively). As a result, milk yield and net returns to farmers increased by about 0.44 kg/day and 25–29%, respectively. Similarly, Lukuyu et al. and Lenné and Thomas [31] and [35] also reported that improved fodder availability (green maize fodder through thinning’s and dry residue/stover after grain maturity) by about 40–166% and 118–409 kg/season, respectively, through the use of improved management practices such as resistant/tolerant dual-purpose maize varieties for streak virus disease and/or stem-borer along with the higher planting population density than the recommended and the push-pull strategy for stem-borer and weed management.

6.1.2.3 Fertilization

Improving total plant biomass both crop residue and grain yields including crop residue quality particularly the N concentration of the crop residue through application of fertilizers (particularly N and P) at sowing and/or days after crop emergence has been well documented [12, 56, 71, 128, 129]. Improving yields of pearl millet grain and stover, the yields of both digestible dry matter and ME of the stover and stover N content have been reported in India through increasing N fertilizer rate [128]. This could potentially have large effects on the amount and feedstuff quality of the CRs available for livestock, thereby reducing the feed deficit gaps [129]. According to [129] increasing fertilizer level over farmers practice in the semiarid region of Karnataka, southern India resulted more increase in residue yield than residue quality attributes, but the overall increase in residue nutrient yields/ha (e.g., ME productivity/ha) potentially has large effects in ruminant productivity (up to 40% higher milk productivity/ha/year). The appropriate use of fertilizer to enhance CRs availability for both CA and fodder for ruminants in smallholder systems in Africa has also been discussed [71]. However, cost of fertilizer, poor market access and the recurrent drought are some of the constraints for adoption in smallholder farmers [70, 84, 130].

6.1.3 Timely harvesting and rapid removal of the CRs soon after grain maturity

Farmers usually harvest grain crops either soon after grain maturity or sometime after grain maturity. In SSA, cereal CRs are often harvested when they are fully matured, dried, and senesced on the field [37]. In the latter case, farmers intentionally left to dry in the field for about 1 to 4 weeks after grain maturity of most cereal crops depending up on labor availability, season (main vs. short rainy season), and cropping system (double/relay cropping) [19, 20, 37, 57]. However, the reduction in nutritive value of CRs as a result of delayed harvesting after grain physiological maturity associated to the greater loss of the most digestible plant parts such as of leaves due to sun-drying and lignifications [20, 37, 57, 128]. For instance [57] reported that maize harvested at a grain DM content ranging from about 70 to 80% gave higher leaf-to-stem ratio, higher CP and lower NDF contents in stover without significant change in grain and stover yield than harvested at grain DM content about 88–90%. Similarly, delayed harvest of finger millet straws for about 10 days after grain maturity of the crop in India resulted in decrease (P < 0.05) in IVOMD and in vitro cell wall digestibility by about 4 (from 61–57%) and 5 (from 48–43%) percentage units, respectively [56]. Leaf loss and losses through senescence could be greatly minimized through removal of the CRs from the field soon after grain maturity [20, 57, 128]. These highlight the need for training farmers and awareness creation toward the benefits of timely harvesting and rapid removal of the CRs soon after grain maturity.

6.2 Post-harvest intervention options

Straw/stover is underutilized in some African countries mainly due to lack of appropriate feed harvesting/collection, transportation, processing, conserving/storage and feeding technologies for efficient utilization [32, 36, 58, 59, 63, 64, 69, 131, 132]. Labor shortage during harvest, lack of transportation, cost effectiveness and lack of awareness are also other factors contributing for the inefficient utilization of CRs by smallholder farmers [61]. Previous studies [32, 69] showed that proper handling of bulky CRs during harvesting and transportation and use of appropriate storage and feeding facilities that minimizes losses may improve feed availability and utilization that would help in coping with dry season feed shortages. In this regard recently [37] reviewed straw and stover collection, preservation and storage practices and technologies to improve crop residue quality using physical (chopping, pelleting and densification), chemical with acids and alkalis and biological treatments with selected micro-organisms or their products in SSA. However, the following sections focus on the potential intervention options suitable for smallholder farmers that reduce feed wastage during storage and feeding including recent technology like spin-off technologies from second-generation biofuel to improve the quality of CRs suitable for small and medium business enterprises. In addition, options for improved utilization of low-quality CRs on farms such as through supplementation, residue management while feeding and including animal management options also discussed.

6.2.1 Box baling of crop residues

Box baling technology, used to make bales made by trampling CRs into wooden frames placed on the ground and manually tied with locally available rope inserted in the frames, before the CRs, is simple and cheaper method for conserving roughages that do not require mechanization [36]. The potential and limitations of this technology summarized (see [68]). Box baling of CRs reduces transport cost, suitable for storage, and helps in feed budgeting, but it requires initial capital and labor. However, box baling or baling of CRs has not been promoted widely in East Africa and was uncommon in most reports in smallholder crop–livestock systems most importantly due to less awareness by smallholder farmers [36]. Farmers commonly store CRs in loose form resulted in dry matter and nutrients losses. However, there are few exceptions. For instance, 71% of the surveyed respondents in Kebbi state, Nigeria, employed baling of the CRs, and 22% of them store and preserve the CRs in silos, while few (7%) of the respondents use metallic drum to store and preserve the CRs [133].

6.2.2 Using storage sheds and feeding troughs for reducing losses

Poor conservation/storage of straws and stover has been reported in smallholder crop-livestock systems in SSA [58, 59, 64, 131, 134] and southeast Asia [44]. About 90% of farmers in East Shoa, Ethiopia, store CRs in the open uncovered [65]. However, there are few exceptions. For instance, in South Gondar zone, Ethiopia where cereal CRs stacked after threshing under a roof shade made of grass and wood, around homesteads and covered with plastic sheets, and inside their house and on trees [135], and in peri-urban dairy producers in Ethiopia where CRs most often stored under roof in loose form [136]. In southeast Asia, livestock farmers collect and stock-pile rice straw in a simple shed usually made from locally available materials or stored in piles outdoors [44]. Improper storage and feeding methods can lead to substantial amounts of forage waste. Maize and sorghum stover are collected and stacked mostly in the field uncovered for later use [131, 134]. These practices reduce the quality and efficiency of utilization compared to the crop residue conserved around homesteads and under shade conditions. For instance, dry matter loss of cereal CRs can reach up to 20–25% in Alamata district, northern Ethiopia [131] and about 10% in Southern Ethiopia [132], and that of pulse residues up to 7%. Part of this loss is attributed to the traditional uncovered storage practices of CRs that is exposed to temperature fluctuations, termites and untimely precipitation [131]. As rain leaches through uncovered stacks, it also washes away soluble carbohydrates and reduces the quality of the residue and animal performance.

Storage conditions and duration are also the major factors that influence the DM and nutrient losses mainly during bad weather condition [35, 137]. In line with this [58] reported that the CP concentrations and IVOMD of both tef and wheat straws showed consistently decreasing trends with the increased storage durations from 0 to 6 months in central highlands of Ethiopia but the nutrient losses were higher in straws stored in open air than those stored under shelter. The results of another study in Ghana using grain legume residues such as cowpea, groundnut and soybean haulms stored under various conditions over a duration of 120 day [59] also showed that DM quantity reduced by 14% and 35% when haulms stored in the best (using polythene bags and under roof/rooms) and worst (on rooftops or tree-forks) conditions, respectively. Storing CRs in an open field uncovered is also associated with the formation of mycotoxins. Storage of CRs under cover not only reduces the DM and nutrient losses but also reduces the absorption of moisture especially during bad weather conditions and hence reduces the formation of mycotoxins compared with the storage of CRs without cover [137]. These losses can be further minimized using improved feeding practices such as using improved wooden feed troughs along with feed storage sheds. For instance, it has been reported that using feed storage sheds and wooden feed troughs reduced feed wastage during storage and feeding by ∼30–50% in Ethiopia [32] while in Tanzania 20–30% feed wastage reduced through the use of feed troughs during feeding [69]. The technologies have been successfully demonstrated in Ethiopia and received acceptance by farmers in intensifying systems since they can also save labor time by about 10–20% and economically feasible [32].

6.2.3 Pre-treatments of low-quality CRs for improving fiber utilization

Technologies to overcome the poor digestibility, low animal intake, and very low protein content of CRs have been developed for pretreatment of CRs before feeding to animals. Pretreatments of low-quality fibrous CRs by chemical, physical and biological treatments with fungi or their combinations to improve their nutritive value (digestibility and VI) and fiber utilization are well documented [37, 120, 138, 139, 140]. Chemical treatments of low-quality fibrous forages with acid or alkali break down fiber and improve their digestibility and VI [141]. Urea treatment may also increase the nitrogen content of the forage, which is the critical nutrient in low-quality fibrous CRs for ruminants, but its efficacy depends on sufficient urease and moisture [140]. The most practical chemical treatment is urea treatment since urea is widely available and easy to handle compared with NaOH, H2O2, acids or KOH. However, with the exception of some Asian countries like India and China, urea-ammonia treatment of CRs, has limited adoption and impact in smallholder farmers in SSA [83], due to its limitations for large-scale application, seasonality, costs involve and hazard issues, such as toxicity and environmental pollution, variable efficacy that depend on the presence of urease and quality of the residue to be treated and the type of residue like fine vs. course-stemmed cereal residues, are among others. Treatment of straw with lime solution [CaO/Ca(OH)2] improves fiber degradability and provides complementary effects in combination with urea in increasing degradability and incrementing both the calcium and nitrogen contents of the treated straw, but it has longer solubility in water compared to NaOH or urea [44].

Physical treatments are applied to reduce the particle size of CRs and improve VI as a result of the reduced ruminal fermentation time [83, 139], thus providing easy entries or access of the rumen microorganisms for degradation. These physical treatments include soaking, chaffing, chopping, shredding, pulverizing, pelleting, steaming pressure and gamma irradiation. Among which, soaking straw/stover overnight in water which brings softness between the lignin and cellulose component of the residue to enhance intake and digestibility is a common and economical treatment of CRs [44]. Chopping also has relevance for field application and most often practiced by smallholder farmers, but the lack of low cost and suitable chopper has been the main determinant for its adoption. Grinding, chopping or pelleting had beneficial effects in breaking down the cell wall contents of cereal residues [44]. Crop residue densification through compacting, briquetting or pelletizing may also enhance the use of bulky forage like CRs as livestock feeds [5, 77, 142] through enhancing handling, storage and transportation and reducing the associated costs [77, 143], and also allow formulating total mixed diet or mixing with other feed additives. This needs initial investment to purchase pelletizing or compacting machine which has been beyond the reach of smallholder farmers, and the benefits derived may be not attractive, but it could be suitable for cooperatives/unions and private sector to commercialize crop residue-based densified products. Reduced particle size or processing strategies such as pelleting increase the rumen outflow rate and enhance VI [144], with possible negative effects on feed utilization [145]. This causes less time for rumination and less exposure to microbial degradation, and consequently reducing degradation and digestibility of the straw. Hence, the balance between the particle size and the retention time/passage rate of the ingested treated straw should be properly considered while using these techniques [44]. Recently spin-off technologies from second-generation biofuel such as ammonia fiber expansion that uses steam-pressure-thermal treatment has been reported [146] and found to be promising to degrade lignocellulose and improve digestibility, intake, productivity and profitability. For instance, steam explosion improved VI by 4% of live weight in male sheep and total live weight gain of 3.92 kg. But this does not use any chemicals, does not take much treatment time and is generally simple. A treatment cost-benefit ratio is of at least 1:2 [15], but it appears not feasible on smallholder condition and should target small and medium business enterprises [146].

Most studies on biological treatment techniques have inconsistent results. The major drawback is the strain of the fungi to be used and its capacity to degrade lignocelluloses [44]. Among the biological technologies, white-rot fungi are well known for their potential for degradation of lignocelluloses, but several strains degrade easily digestible carbohydrates and often resulted in dry matter and nutrient losses [140]. Although efforts have been made to reduce DM loss while treating straw using solid state fermentation with ligninolytic fungi [83], generally many biologically treated CRs are not economical, and the process has not yet optimized under field conditions [147]. Incubation period is another limitation for its practical application in treating CRs [44]. Recently, [148] also reported that potential adoption of biological treatments of CRs has limited due to suitable strains used to avoid or minimize carbohydrate degradation (loss), the inconsistencies in animal response, and the high technical skills needed. In addition, the limitations on availability of resources to produce and handle large quantities of fungi or their enzymes for practical and field application, and the concerns and problems to be addressed and overcome (e.g., some fungi produce toxins so proper care should be considered) while using biological treatment of straw [44]. It has been recommended that more research is needed to improve the aforementioned constraints for enhancing crop residue quality and improving fiber utilization through biological treatments [37, 140]. Enzyme treatment of CRs to increase degradability and animal performance is not common under smallholder production systems because of the additional input costs involved as well as the limitation of skills for using enzyme products [44].

Generally, treated low-quality fibrous CRs usually support modest level of animal productivity and require additional supplements to provide sufficient nutrients for high-producing animals.

6.2.4 Supplementation of cereal crop residue forage diets for improved utilization and animal performance

Low-quality fibrous CRs, particularly those from cereals, are usually high fiber content, poor digestibility, very low VI and deficient in nitrogen, sulphur, phosphorous and other micro and macro minerals as feed for ruminants. The critical ones are nitrogen followed by sulphur as they are the main determinants primarily to inhibit fermentation process for fiber digestion in the rumen and thus reduce VI [138]. Forage legumes and legume straw (haulm) generally have high nutritive value (digestible energy and CP), which can decrease the use of purchased concentrate ingredient or compound feed supplements and of associated costs [102, 149, 150]. For instance, supplementation with locally available feed resources like legume straw (haulm) may provide N for N-deficient cereal straw/stover to stimulate fiber digestion in the rumen and thus increase ME intake of cereal CRs [13, 102, 149]. Utilization of low-quality fibrous CRs would be improved, and modest level of animal productivity could be obtained through supplementation 0. For instance, supplementation of small amounts of legume haulms ranging from 150 to 450 g DM/head/d improved dry matter and nutrient intake, N balance and enhanced animal performance (carcass weight and body weight gain) in sheep in East and West Africa [13, 45, 102]. Moreover, supplementation of non-protein nitrogen (NPN) (e.g. urea) is also another option for improved utilization of low-quality forages [138]. However, this needs simple and practical management options to provide NPN supplements with low risk in smallholder systems. Among the categories of various safe ways of supplementing urea as NPN supplements like in the form of urea–molasses multi-nutrient block (UMB), slow-release forms of urea, and using a sticky urea–molasses solution that are common in Australia, Europe and North America [120], providing the urea in UMB supplements has been widely promoted in SSA and Asia, so ruminants can only consume small amounts through the day [83]. In addition to NPN supplementation, including some sulphur in the form of ammonium sulphate or elemental sulphur is also needed as substrates of rumen microorganisms to optimize the rumen microbial fermentation and fiber utilization [120]. These improve feed efficiency and potentially would reduce the intensity of enteric methane emission per animal product produced [144, 151].

6.2.5 Residue management: increasing the amount on offer

One simple physical method to increase voluntary intake and productivity is increasing the amount on offer without changing the diet composition for improved intake and animal productivity [144]. Depending up on the nature of the feed, the optimum level of excess feed on offer ranges from 15 to over 40%, so that ruminants have a chance to preferentially select and consume the more nutritious parts of the residue such as leaves than the thick stems, maximizing intake and thereby improving productivity [139, 152, 153, 154, 155]. On the other hand, increasing the feed intake of ruminants decreases the retention time of feed in the rumen due to higher passage rates, decreases the proportion of ingested and absorbed nutrients and energy associated with animal maintenance, leading to a decline in CH4 losses per unit of DMI [144]. In contrast to stubble grazing approach, in the hand-feeding CRs approach obviously there are more refusals available that should be integrated with improved refusal management so that refusals can be effectively returned to cropland as organic matter or used for other purpose as a ‘win-win’ intervention so as to improve whole-farm productivity with little additional labor associated with handling, that is, for example, see [40]. Moreover, the latter approach should also have the greatest potential in the areas where supply exceeds the demands for CRs as feed for ruminants [120].

6.3 Animal management options: improved utilization efficiency of low-quality fibrous CRs

The two approaches that have been suggested to enhance the efficient utilization of low-quality fibrous feedstuffs under animal management option are 1) selectively breeding for better suited animals for improved feed efficiency [156, 157] and smaller metabolic body weight [158], and 2) the application of rumen biotechnology such as altering the rumen microbial composition or using genetically engineered rumen microbes [159, 160, 161, 162], or a combination of both approaches [144]. However, genotyping an animal for feed efficiency is costly, particularly high capital requirement and multiplication of superior animals have been cited as the major determinant factors for improved adoption of ruminants selected for efficient utilization of low-quality fibrous forages [20, 144]. According to the recent report [144], genetic selection for feed efficiency is not yet a breeding objective in most ruminant systems due to the lack of genomic tools designed to predict feed efficiency. On the other hand, several technical difficulties must be solved before the latter approach will be possible [161]. Some of the constraints of genetic manipulation as listed by [162] are categorized mainly related with the incompatibility of modified microbes in the rumen environment and the existing regulatory concerns of the public.

7. Conclusions

From the scientific findings discussed in this chapter, the following important conclusions can be drawn:

1. Given the diverse characteristics of smallholder farmers, understanding the local/on-farm socio-economic factors and practices/methods on CRs management and use is crucial to select and promote the possible intervention options that are more practical for wide adoption by the stallholder farmers.

2. As a pre-harvest intervention options that may have a strong likelihood of adoption for enhancing CRs yields and quality in the same unit of land without affecting the primary product:

   1. Development and use of suitable dual-purpose crop genotypes and crop management practices would increase the availability (digestible dry matter and ME yields of CRs) fodder for ruminants.

   2. Timely harvesting and rapid removal of the CRs soon after grain maturity would also reduce the loss of the most palatable and digestible plant parts such as leaves and leaf sheaths and lignified.

3. Promoting post-harvest intervention options such as

   1. Simple, low cost and practical forage conservation technologies (e.g., box baling) along with proper storage practices (e.g., storing under shed/cover), and the use of feed troughs while feeding would help in reducing feed losses and coping with dry season feed shortages.

   2. Applying suitable crop residue treatment options (e.g., chopping) and locally available low-input supplementation for livestock and residue management while feeding (increasing the amount on offer) would increase the residue quality, intake and ruminant productivity.

   3. Optimal allocation of CRs of residue fractions mainly from thick-stemmed cereals as feedstuff and mulch including the use of feed refusals/leftovers and animal manure as fertilizer enhance both livestock and crop production and sustainability that also needs the close integration of animal nutritionists and plant/soil scientists.

4. Furthermore,

   1. The traditional methods need to be revisited and should represent farmer’s context related to the various uses of residue fractions while estimating the production and quality of CRs/CR fractions actually available as feed for ruminants and other uses.

   2. Linking farmers to CRs markets and improving farmers’ knowledge through continuous promotion and training on the improved technologies/practices are also crucial for improved availability and adoption.

Conflict of interest

The authors declare no conflict of interest.

Author contributions

All authors have made significant contribution.