Department of Animal Science
Obafomi Awolowo University
Les cultures principales qui poussent en zone tropicale humide d'Afrique de l'Ouest de desquelles sont utilisés des résidus pour l'alimentation des chèvres et des moutons ont été identifiées. Il s'agit du maïs, du sorgho, du riz, des arachides, du niébé, du palmier, de l'igname, du manioc, de la banane plantain et du cacao. Les grandes quantités de résidus produits à la récolte ou durant les différentes étapes de transformation des produits à la ferme (pailles, fanes, tiges, feuilles, écorces) ne sont pas utilisés d'une manière optimale à cause d'un certain nombre de contraintes.
Une des contraintes majeures est la faible valeur nutritive de ces résidus de culture. Ils ont une faible teneur en azote, une teneur en fibres élevée et sont faiblement digestibles de telle sorte qu'ils ne couvrent que rarement les besoins d'entretien des chèvres et moutons adultes. Ils ont besoin d'être enrichis par différents traitements ou encore par une supplémentation appropriée afin qu'ils remplissent parfaitement le rôle qu'ils pourraient occuper dans l'alimentation des chèvres et moutons.
On estime que pour des raisons économiques et pratiques, les traitements les plus susceptibles d'être employés dans les systèmes de petit élevage sont: le hachage et l'humidification (traitement physique), l'ammonification par traitement à l'urée, ou aux alcalis en utilisant les cendres caustiques des récolte (traitement chimique), l'ensilage avec ou sans déjections animales ou urée (traitement biologique). Pour les mêmes raisons l'urée, les excréments de volailles, les feuillages et herbes récoltés par ramassage (Gliricidia, Leucaena), les épluchures de tubercules et les bananes impropres à la consommation ont été suggérés comme étant les supplémentations les plus appropriées riches en protéines et énergie.
Sur la base des données et de la littérature disponibles, on a conclu que les résidus de récolte ont un rôle significatif à jouer dans l'alimentation des ovins et caprins des régions tropicales humides de l'Afrique de l'Ouest. On a souligné, cependant, que des programmes de nutrition basés sur les résidus de récolte ont besoin d'être testés au niveau du village ou du petit élevage de telle manière que d'éventuelles contraintes quant à leur utilisation et efficacité puissent être identifiées et éliminées.
The major crops grown in the humid tropics of West Africa, and which provide residues of potential value for goats and sheep were identified. These include maize, sorghum, rice, groundnut, cowpea, oil palm, yams, cassava, banana/plantain and cocoa. The large quantities of residues produced at harvest or during primary on the farm processing of the crops (stovers, straws, haulms, vines, leaves, peels) are not being maximally utilized because of a number of constraints.
One of the major constraints identified is the generally low nutritive value of these crop residues. They have low nitrogen, high fibre contents and are poorly digested such that they barely meet the maintenance requirements of adult goats and sheep. They need to be upgraded by treatment or appropriate supplementation in order to exploit fully the role they could play in goat and sheep nutrition.
It is suggested that because of economic and practical considerations the treatments most likely to be adopted in small scale farming systems are: chopping and wetting (physical), ammoniation via urea treatment, alkali treatment using the caustic ash of crop residues (chemical), ensiling with or without animal manure or urea (biological). For the same reasons, urea, poultry excreta, browse (gliricidia, leucaena) crop foliages, tuber peels and reject bananas were suggested as the more appropriate protein/energy supplements.
It was concluded on the basis of available data in the literature that crop residues have a significant role to play in the nutrition of goats and sheep in the humid tropics of West Africa. It was emphasized, however, that feeding strategies built around crop residues need to be tested under village or small scale farm conditions so that constraints to their adoption and effectiveness can be identified and eliminated.
The humid tropics of West Africa occupy a total land area of 707 000 km2, stretching from the lower third of Nigeria up to Guinea. According to Jahnke (1982) annual rainfall in this zone is well over 1500 mm, and the crop growing season can be as long as 270 days or more. This zone (humid tropics) corresponds ecologically, to the derived savannah and rainforest zones described by Keay (1959).
The goat and sheep population in the humid zone of West Africa is presented in Table 1, together with the cattle population for comparison. It is clear that goats and sheep are much more important, population wise, and represent a major undeveloped food resource.
Table 1. Goat and sheep population in humid West Africa (000)
|Country||Goats||Sheep||Total small ruminants||Cattle|
|Nigeria||5 621||3 476||9 097||857|
|Ghana||1 200||990||2 190||558|
|Côte d'Ivoire||816||874||1 690||65|
|Total||8 010||5 669||13 679||3 365|
Source: Jahnke, 1982.
Three main production systems for goats and sheep are common in humid West Africa: (i) low-input extensive or subsistence system based on free grazing of roadside forages complemented with kitchen wastes, crop residues and by-product supplements when available. (ii) Intensive cut and carry feeding of confined or tethered animals on grasses, browse, crop residues and by-products. (iii) Commercial grazing of sheep flocks that may also benefit from crop residue and by-product feeding.
In other words, crop residues play a significant role in the nutrition of goats and sheep in this area. Nevertheless, for reasons which will be discussed later, the various crop residues and by-products produced have not been maximally utilized for feeding goats and sheep. Experimental and practical evidence showing the important role these feed resources could play in the nutrition of goats and sheep will be reviewed in this presentation.
Crop residues and by-products are often used interchangeably. There is a need to differentiate between the two terms as they may serve slightly different nutritional roles. Crop residues are the materials left in the field after harvesting the target crops (maize stover, cassava tops, maize cob, cassava peels). They are usually fibrous, low in nitrogen and widely spread geographically because they are produced on the farm. They form the basal or principal feed in small-scale farming systems during the dry season.
Agro-industrial by-products, on the other hand, are produced mainly after processing of crops for the production of a main product that may be radically different from the starting crop. They may be rich in nitrogen (oil seed cakes, brewery and flour milling by-products) and may be both low or high in fibre (sugar cane bagasse, palm press fibre). Since they are produced at factory sites, they are less widespread geographically. With a few exceptions, they are used mainly as supplements and not main or basal feeds. A list of major crop residues and by-products produced in the humid zone of West Africa is shown in Table 2.
Table 2. Major crop residues and agro-industrial by-products produced in humid West Africa
Rice mill feed
|Wheat (imported)||-||Bran, offals|
|Barley (imported)||-||Spent grains|
|Oil palm||Empty fruit bunch||Palm pressed fibre Palm kernel cake|
|Rubber||-||Rubber seed meal|
Peels and rejects
Peels and rejects
|Sweet potatoes||Vines and peels||-|
|Pineapple and citrus fruits||-||Pulps|
CROP RESIDUE INVENTORY
Estimates of the available quantities of the major crop residues for the whole of Africa excluding North Africa in 1980 are shown in Table 3. Maize stover and rice straw estimates for humid West Africa in 1984 are also shown in Table 4.
Table 3. Available quantities of major crop residues in Africa
|Sugar cane||Tops||17 070|
Source: FAO, 1980
Table 4. Maize and rice straw quantities available in humid West Africa (000 Tonnes)
|Nigeria||1 600||3 200||1 100||1 100|
|Total||3 220||6 440||2 754||2 754|
1 Estimated yield assumed crop residue to grain of 1:1 for rice, and 1:2 for maize (owen, 1976).
The two tables give an idea of the large quantities of crop residues available for feeding livestock. According to Sansoucy and Emery (1982), if all the available residues were fed to livestock, about 3 kg dry matter would be available to each tropical livestock unit daily. (One tropical livestock unit being one adult bovine weighting 250 kg at maintenance.)
Unfortunately, only a small fraction of the available residues is currently being used for livestock feeding. A number of constraints to the optimum utilization of these crop residues have been identified (Sansoucy and Emery, 1982; Owen, 1985).
These constraints include (i) a lack of knowledge of where the residues are produced, when, where and to which animals they could be fed. (ii) The difficulty and expense of collecting, handling and storing large quantities of bulky crop residues. (iii) Seasonality of production, which should be an advantage, since most crop residues are produced during the dry season when the quantity and quality of forage is low. In actual fact, the seasonal production often constitutes a constraint to optimum utilization, because the large amounts produced cannot be all utilized when available, and have to be stored for future use. (iv) Alternate uses such as for fuel, construction, composting, pharmaceutical use, etc. Such alternate uses may be more profitable than for animal feeding. (v) The apparently poor nutritive value of the residues.
POTENTIAL FEED VALUE OF CROP RESIDUES
Theoretical considerations: From a nutritional standpoint, the plant material is made up of two fractions - the cell contents and the cell wall. Cell contents, which are usually highly digestible, constitute only a small fraction of the dry matter of crop residues, and hence make only a minor contribution to the feed value of crop residues. The cell walls which constitute the major fraction of crop residues may be highly or poorly digestible, depending on the relative proportions of its component partslignin, cellulose, hemicellulose, silica, and how they are complexed with each other.
In other words, the relative proportions of the cell are major factors that will determine the value of a particular crop residue for animal feeding. Another factor which has to do with the animal is that for efficient utilization of any feed material, the ruminant requires a supply of (i) fermentable energy and nitrogen, micronutrients such as sulphur and phosphorus, and roughage for the development of an adequate or efficient rumen ecosystem. (ii) By-pass protein and energy for the complementary needs of the animal as a whole (Preston, 1982; Preston and Leng, 1981). Apparently, a deficiency of any of the above leads to poor feed intake and utilization.
Nutrient contents of crop residues: The potential value of crop residues as nutrient sources for goats and sheep is well demonstrated in Tables 5 and 6. Nutrient concentrations vary from as low as 2 percent crude portein for maize stover to as high as 27 percent for cassava tops. The wide ranges observed within a particular crop residue for specific nutrients are attributable to factors such as variety, age of residue or stage of harvest, physical composition, i.e. the proportion of stems and leaves, length of storage, cultural and harvesting practices. Considerable caution needs to be exercised therefore when using mean values to formulate diets or predict animal performance.
A close scrutiny of Tables 5 and 6 shows that, in general, crop residues have fairly high cell wall contents, low fermentable carbohydrates and except for leguminous residues, low protein content. As indicated earlier, such materials cannot sustain an efficient rumen ecosystem, and are therefore poorly fermented in the rumen. The very high T 1/2 values of most of the residues confirm their poor degradability in the rumen. Such slow rates of degradability mean slow movement of the material out of the rumen, and therefore low intake, and low total digestibilities.
Voluntary intakes and digestibilities of crop residues: A review of some published studies in which crop residues were fed as the sole or major material confirmed the above. Cheva-Isarakul and Cheva-Isarakul (1985) fed adult wethers, weighing about 30 kg, five different varieties of rice straw ad libitum to estimate voluntary intake and digestibility. On the average the sheep consumed about 2.2 percent of their body weight or 52 g/kg metabolic weight of the straw. Dry matter digestibility was 49.8 percent. Suriyajantratong and Senakas (1985) reported higher dry matter intakes for sheep fed groundnut haulms (2.9 percent BN), with a dry matter digestibility of 52 percent. Alhassan et al. (1984) reported lower values for goats. Mean dry matter intakes were 1.1 (sorghum stover), 0.7 (maize stover), 2.0 (sorghum leaves) and 0.8 percent body weight (cowpea vines). Dry matter digestibilities were 52, 53, 57 and 47 respectively. According to Doyle (1982) and Pearce (1984) such low dry matter digestibilities coupled with low intakes may not satisfy maintenance needs. If diets based on crop residues are to be productive, the residues must be upgraded to improve the nutritive value.
Improvement of nutritive value of crop residues
Three alternatives are available for improving the intake and digestibility of fibrous residues. These are (i) treatment of the residue to improve biodegradation, (ii) appropriate supplementation with additional nitrogen, readily available energy and minerals, and (iii) a combination of treatment and supplementation.
Treatment of crop residues: Available treatments are listed in Table 7. Many of them are unsuitable for use in small scale farming systems found in humid West Africa. The various treatment methods are briefly discussed below in terms of their mechanism of action, effectiveness and suitability.
Table 5. Proximate composition of major crop residues
|Crop residue||% Dry matter|
|Crude Protein||Organic Matter||Crude fibre||Ether extract||Ash||NFE|
|Sweet potato tops||90||20–22||82–83||15||3–3.5||17–18||42–46|
|Empty oil palm fruit bunch||56||3–4||95||-||6–8||5||-|
Source: Adapted from several authors.
Table 6. Detergent fibre content and in situ degradability of major crop residues
|Crop residue||% DM||In situ degradability|
|Cell wall (NDF)||Cell content||ADF||Lignin||Cellulose||Hemicellulose||Silica|
|T 1/2 (Hr)||% Dm losses (24 hrs)|
Source: Adapted from several authors.
Table 7. Currently available treatments for improving the nutritive value of crop residues
|Soaking and wetting||Alkali treatment|
|(i) NaoH||Grinding/chemicals||Composting Ensiling|
|Chopping||(ii) Ca(oH)2||Pelleting/chemicals||Fungal growth|
|Grinding and pelleting||(iii) KOH|
|(v) NHYOH NH4OH|
|Ball milling||(i) H2SO4|
|Gamma irradiation||(ii) HCl|
|High pressure steaming||Oxidation|
|(i) Sulphur dioxide|
|(iii) Chlorine and Chlorinated compounds|
Physical treatments: Soaking and wetting crop residues is unlikely to improve their intake or digestibility. It might in fact result in reduced feed value because it causes substantial losses of soluble cell contents with a resultant decrease in digestibility. Thus, according to Dumlao and Perez (1976) dry matter losses of 8–14 percent resulted from soaking rice straw for 3 days. A 5 percent reduction in nylon bag degradability of soaked rice straw was reported by McManus and Choung (1976). Ibrahim and Pearce (1982) also reported a reduction in the soluble cell material and in vitro digestibility of cowpea vines soaked in boiling water for 30–90 mins. Increased intake of crop residues following soaking or wetting have however been reported (Chaturvedi, Singh and Ranjhan, 1973) occasionally. Although wetting and soaking are simple procedures that small scale farmers can easily adopt, the benefits are not clear.
Chopping crop residues before feeding may reduce wastage and facilitate feeding. Since chopping does not alter cell wall structure, it generally does not improve digestibility, although Adu and Lakpini (1983) reported that lambs fed chopped groundnut haulms performed better than those fed long haulms in terms of intake, digestibility and growth. Intake of certain crop residues such as maize and sorghum stovers and rice straw may be improved by chopping, although there are reports that chopped and long rice straws are equally well consumed and digested by sheep (Devendra, 1983). Nevertheless, chopping long crop residues to manageable lengths before feeding is recommended.
Grinding and pelleting are more severe physical treatments which reduce particle size. Reduction in particle size has the dual effect of increasing rate of passage and hence increasing intake, as well as increasing the cellulosic surface area exposed to microbial attack in the rumen, with the resultant increase in digestibility. Excessive reduction in particle size such as that achieved by ball milling or fine grinding may reduce digestibility because of increased rate of passage, although Minson (1982) pointed out that digestible dry matter may still be high under such circumstances. In spite of the reported positive beneficial effects of grinding by the usual methods on intake and digestibility, small scale farmers in the humid zone may not adopt the procedure because of economic considerations.
High pressure steam treatment exerts both physical (separation of cell wall structures) and chemical effects (cleavage of cell wall constituent bonds, degradation of hemicellulose, formation of acids which hydrolize other constituents) on crop residues. These processes may improve the digestibility of the treated materials and the net yield of available digestible dry matter (Doyle, Devendra and Pearce, 1986). These improvements have not always been observed during in vivo evaluations. Negative effects on intake and digestibility have indeed been reported (Garret et al., 1981; Rangnekar et al., 1982). Overtreatment is probably responsible for such negative results, as the optimum conditions of treatment for different residues have not been well defined. Such a costly, energy intensive and marginally effective treatment method cannot be recommended for our target users.
Gamma-irradiation according to Doyle, Devendra and Pearce (1986) may reduce resistance of fibrous residues to physical degradation without the necessity for fine grinding. McManus et al., (1972) indeed noted that irradiated rice straws had a shorter mean retention time in sheep than non-irradiated straw, suggesting that irradiation rendered the straw more susceptible to physical breakdown. Walker (1984) also reported that irradiation solubilizes cellulose, hemicellulose and lignin in the cell wall. In vivo results, however, do not confirm these apparently beneficial effects, as the procedure has in general been shown to depress dry matter digestibility, and to have no effect on voluntary intake (McManus et al., 1972). The process obviously has no practical application for farmers in humid West Africa.
Chemical treatment: The three classes of chemicals currently being used to treat fibrous residues are alkalis, acids and oxidizing agents. All three are capable of weakening cell wall component complexes (lignin-carbohydrates), solubilizing the components (lignin, cellulose etc.), and increasing the swelling capacity of the cell wall, thus facilitating microbial enzyme entry.
Sodium hydroxide is generally regarded as the most effective alkali for improving the digestibility of crop residues. Substantial increases in in vitro as well as in vivo digestibilities and intake of treated crop residues have been reported (Ibrahim and Pearce, 1983; Doyle, Devendra and Pearce, 1986), although the increases observed for in vivo digestibilities are usually lower (Doyle, 1982). In spite of the effectiveness of sodium hydroxide in improving crop residue feeding value, it can hardly be recommended for use on small scale farms in humid West Africa, because of problems of availability, cost and handling. What could be recommended, and which under limited experimentation appeared to be as effective as sodium hydrodixe, is the use of the caustic ash of some crop residues.
Adebowale (1985) reported high levels of potash in the ash of a number of crop residues. Potassium concentration in cocoa-pod ash for example was 44 mg/kg. The amount of hydroxyl ions (OH) present in the ash solution as sodium hydroxide and potassium hydroxide was 20.5 and 28.7 percent respectively. Titrimetric and potentiometric analyses showed that about 4.4 kg of cocoa-pod ash was equivalent to 1 kg of sodium hydroxide. In subsequent feeding trials, the author fed maize straw treated with varying levels of cocoa-pod ash solutions in complete diets to goats, and reported that dry matter and digestible energy intake of goats fed the treated maize straw diets were higher than those of control goats fed untreated maize straw diets. Growth rates were 42 and 20 g/day.
Smith and Osafo (1987) also evaluated cocoa-pod ash solutions as a treatment medium for crop residues. They reported a linear increase in the rumen degradability of the dry matter, acid detergent fibre and neutral detergent fibre of crop residues treated with cocoa-pod ash solutions equivalent in alkalinity to 2, 4, 6 and 8 percent NaOH. Apparently the rumen degradability rates and extent of residues treated with cocoa-pod ash solutions and sodium hydroxide of equivalent alkalinity were similar. The authors suggested that using suitable crop residue ash solutions as sources of alkali for treating fibrous residues may be a better alternative to sodium hydroxide treatment since in the village setting, these ashes are already being used to make soap, and the farmers are therefore used to handling them. The technology is cheap, local and sufficiently simple for use on the farm.
Other chemicals that have been successfully used to improve the nutritive value of crop residues are shown in Table 7. Apart from urea, most of the other treatments may not be suitable for use in humid West Africa mainly because of problems of handling, availability and cost. Urea is used as fertilizer, and is currently used to treat crop residues in developing Asian countries such as Bangladesh, Indonesia, Sri Lanka and Thailand. Well planned studies need to be carried out to determine optimum treatment conditions for crop residues peculiar to the humid tropics.
Physico-chemical treatments: There is evidence that combining physical treatments such as milling, grinding and steaming, which decrease particle size, with chemical treatments, increase the effectiveness of the chemicals (Thiruchittampalam and Jayarisuya, 1978), although the effects may not always be additive (Coombe et al., 1979). In any case, such severe physico- chemical treatments may be out of reach of village farmers in humid West Africa.
Biological treatments: Of all the available biological methods, ensiling appears to be the only feasible and valuable method for our target users.
Composting causes substantial losses of organic matter leading to an increase in ash and lignin content (Doyle, Devendra and Pearce, 1986), with a resultant decrease in nutritive value. Fungal and purified enzyme inoculations have also been used, and have been shown to be useful only under ideal conditions of temperature, pH, aeration and moisture content. Ensilage of crop residues with forages or animal wastes appears to be a simple effective means of improving crop residue quality.
Reports from Asia indicate that silages made from rice straw and (i) water hyacinth (Chhibbar and Singh, 1971), (ii) potato vines (Krishna, 1982), (iii) poultry litter (Neog and Pathak, 1976) supply enough nutrients for maintenance. In Cameroon, Fomunyan and Meffeja (1986) fed goats maize stover ensiled with wet brewers grains, and obtained digestibility values similar to those reported for sodium hydroxide and ammonia treated maize stovers.
It can be concluded from the foregoing that simple processing such as cropping, ensiling with urea or animal manure, or chemical treatment with crop residue ash solutions are the more practical and economical means of improving the nutritive value of crop residues under the small scale farming context.
According to Devendra (1985) the characteristics of a maintenance feed for adult reminants are: a crude protein level of 6–7 percent, a dry matter digestibility of 50–55 percent and a dry matter intake of about 1.7 percent of body weight. Data summarized in Table 8 show that crop residues rarely meet these requirements.
Table 8. Voluntary intake and digestibility of selected crop residues
|Crop residue||%CP||Intake % BW||Dry matter digestibility||Source|
Source: 1. Alhassan, 1984
2. McManus et al., 1972
3. Smith and Adegbola, 1982.
Chemical or other treatments reviewed earlier may improve intake and digestibility, but unless adequate supplementation of deficient nutrients is made, much of the additional energy released will be inefficiently used. Adequate supplementation is therefore required for efficient utilization of crop residues.
Preston and Leng (1981) have suggested that to optimize the utilization of crop residues, nutritional supplements should provide the following: (i) fermentable energy (ii) fermentable nitrogen (iii) micronutrients e.g. S.P. and B vitamins (iv) roughage (v) by-pass protein (vi) by-pass energy. The first four ensure an adequate rumen ecosystem, while the last two complement the needs of the animal as a whole. In other words, adequate supplementation is essential for proper utilization of crop residues. Some of the potentially valuable supplements are listed in Table 9.
Table 9. Sources of nutritional supplements to crop residues
|Fermentable nitrogen||Urea, animal manure|
|Fermentable carbohydrate||Molasses, cane juice, citrus and pineapple pulp, palm-pressed fibre, cassava chips, cassava peels, reject banana/plantain, rice bran, maize bran.|
|Roughage-micronutrients||Forages such as gliricidia, leucaena, water hyacinth, cassava tops, sugar-cane tops, banana leaves and pseudostems…|
|Bypass protein||Oil seed cakes, leucaena, gliricidia, other tannin-rich forages, fish meal|
|Bypass energy||Corn, broken rice, rice polishings.|
Practical examples of adequate supplementation using some of these supplements are briefly considered. Olayiwole, Timberger and Akinola, (1978) fed ensiled sorghum stover to sheep with or without urea-molasses supplementation. Supplementation increased intake from 33 to 41 g/kg metabolic weight, and crude fibre digestibility from 40 to 57 percent. More impressive results were obtained by Sudana and Leng (1985) when they supplemented sheep fed a basal wheat straw diet with urea-molasses (fermentable nitrogen and energy) and cotton seed cake (bypass protein). Their results, summarized in Table 10, show that supplementation improved rumen fermentation (352 mgNH3 - N/litre), and promoted a faster (90 g/day) and more efficient (8:1) growth rate.
Table 10. Performance of lambs fed wheat straw with appropriate supplementation
|Diet||Rumen NH3 level mgNH3-N/ litre||Straw intake gDM/day||Total intake gDM/day||Liveweight change (g/day)||FCR gDM/g|
|Straw plus Urea + molasses (Block)||262||421||498||10||50:1|
|Straw + block + Cotton seed meal||352||480||675||90||8:1|
Source: Sudana and Leng, 1985.
Alhassan and Akorfur (1982) reported a significant increase in the digestibility of rice straw fed to West African Dwarf sheep, when the straw diet was supplemented with oil palm slurry. Winugrolo and Chaniago (1983) reported an increase in the nitrogen content (1.0 to 2.4 percent) and in-vitro dry matter digestibility (35 to 47 percent) of rice straw treated with urea-ammonia. The treated and untreated straws were then fed to goats with or without cassava leaves. The results summarized in Table 11 show the beneficial effect of forage supplementation.
Table 11. Performance of goats fed rice straw supplemented with cassava leaves
|Diet||Dry matter intake|
|75% NH3-treated rice straw plus 25% cassava leaves||1721||84|
|75% untreated rice straw plus 25% cassava leaves||1518||45|
|100% NH3-treated rice straw||989||27|
Source: Winugroho and Chaniago, 1984.
Acceptable dry matter intakes and growth rates were obtained in goats fed diets consisting entirely of crop residues by Soedomo-Reksohadiprodjo (1985). The combination of crop residues was such that the basal feeds (corn stover, sorghum stover and sugarcane tops) were well complemented. The supplements used as shown in Table 12 were groundnut vines, cassava tops and leucaena leaves.
Table 12. Dry matter intake and growth rate of goats fed solely on crop residues
|Performance data||BASAL Diets1|
|Corn stover||Sorghum stover||Sugarcane tops|
|Dry matter intake g/kg metabolic weight/day||78.5||74.8||67.3|
|Growth rate (g/day)||56.3||55.0||50.0|
1 All three diets supplemented with groundnut haulms, cassava tops and leucaena leaves.
Source: Soedomo-Reksohadiprodjo, 1985.
Evidently, supplementation along the lines suggested earlier will improve crop residue utilization by goats and sheep. The choice of supplement must tilt towards the more readily available and less costly alternatives. As availability and cost of the various supplements listed in Table 9 may vary from one area to another, no blanket recommendation can be made. Nevertheless, forages such as leucaena, gliricidia and cassava tops, urea-molasses mixtures in liquid or block form, poultry manure, and peels of tubers may play increasingly significant roles as supplements to crop residue feeding. By-pass protein sources such as oil seed cakes and fish meal may be too expensive for use in the small-scale farming system.
At the level of productivity obtained under unimproved feeding systems in the small-scale farming setting, goats and sheep do not often show symptoms of mineral deficiencies or respond to mineral supplementation. Responses to mineral supplementation only occur after the major nutrient imbalances have been corrected. A feeding strategy based on treated and supplemented crop residues may correct these major nutrient deficiencies and improve productivity to such an extent that mineral requirements increase. Mineral supplementation may then become important, not only to avoid deficiency problems, but also to improve performance further.
According to Little (1985) crop residue based diets are most likely to be deficient in sodium, copper and phosphorus. These are the same minerals found to be marginal or deficient in tropical grasses (Kabaija, 1985). Preston and Leng (1986) reported that most straws are deficient in the same three minerals in addition to sulphur, cobalt and calcium. The high concentrations of oxalates and silicates in some of the straws, such as rice straw, may further reduce the availability of calcium and magnesium, which are lost as silicates and oxalates in the urine and faeces.
Table 13. Mean mineral contents of selected crop residues
|Sweet potato vines||14.4||2.7||6.8||0.2||2.7||10.6||32||0.4||0.31||-||0.9|
|Sugar cane tops||4.4||2.6||4.5||0.8||-||3.2||22||-||-||85||-|
Source: Little, 1985
Very little work has been done in general in the area of ruminant mineral nutrition. It is possible therefore that goats and sheep fed mainly on well supplemented crop residues may become deficient in other minerals. There certainly will be a need for mineral supplementation studies to identify and correct the major mineral deficiencies likely to be associated with feeding strategies based on crop residues. Mineral contents of selected crop residues are shown in Table 13. Meanwhile, as suggested by Little (1985), routine provision of salt supplements to animals fed crop residues is necessary.
The abundant crop residue resource present in the humid tropics of West Africa needs to be harnessed and utilized effectively in the nutrition of goats and sheep. Efficient utilization of the residues is possible through treatments, supplementation and a combination of these, and feeding strategies built around the more common and available residues and supplements in a particular area need to be developed for that area.
These feeding strategies need to be evaluated under farm conditions, as only on-farm trials can accurately assess whether the feeding packages would be acceptable economically and socially to the farmer. Furthermore, constraints that may not be fully appreciated under laboratory conditions, but which are real to the farmer (limited water supply for NaOH treatment or absence of straw chopping equipment) will be identified and hopefully eliminated.
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