L R Ndlovu
Department of Animal Science
University of Zimbabwe
PO Box MP 167, Mount Pleasant, Harare, Zimbabwe
ABSTRACT
Ruminant livestock production in sub-Saharan Africa is based on forages as the major feed resource. Strategies on the utilisation of these feeds should aim at establishing an efficient rumen ecosystem in order to maximise fibre digestion and optimise microbial protein synthesis. An efficient rumen ecosystem requires fermentable nitrogen and energy sufficient to support the rumen microbial population. A pH of 6.5-6.8 is optimum and advantage could be taken of fibre buffering capacities to maintain the pH. Grasses have poor buffering capacities while legume forages have very good buffering capacities. Legumes appear well suited as forage supplements to grasses. Some examples of forage supplementation are reviewed.
RESUME
Complémentarité des fourrages dans la digestion chez les ruminants: considérations théoriques
En Afrique subsaharienne, l'élevage des ruminants est largement tributaire de la disponibilité des fourrages, lesquels constituent les principales ressources alimentaires du bétail Les stratégies de mise en valeur de ces aliments doivent viser à établir dans le rumen des animaux un écosystème capable de faciliter la digestion de la cellulose ainsi que la synthèse des protéines microbiennes. Celui-ci doit contenir suffisamment d'azote disponible et d'énergie pour entretenir la flore microbienne du rumen. On peut se servir de la faculté stabilisatrice de la cellulose pour maintenir le pH, dont la valeur optimum se situe entre 6,5 et 6,8. Contrairement aux graminées, les légumineuses fourragères possèdent d'excellentes capacités stabilisatrices. Elles semblent particulièrement indiquées comme complément de rations de base de graminées. Quelques exemples de complémentation alimentaire de fourrage ont été analysés.
INTRODUCTION
Ruminant livestock production in most of sub-Saharan Africa is based on forages as the major feed resource. The term forage refers to herbaceous material including grasses, legumes, browseable trees and fibrous crop byproducts. In seasonally dry environments, the main limitations to animal production are the lack of green feed for at least half of the year coupled with the low nutritive quality of forages during most of the period of active pasture growth (Jones and Wilson, 1987). The low nutritive quality of the forage during the growth period is mainly due to environmental stresses such as high temperatures (Van Soest, 1988) and infertile soils (Roberts, 1987).
In recent years there have been several attempts to improve the nutritive quality of the forage resource base through propagation of species with high nutritive value (Dzowela, 1988). However, because of limited land, the quantities of such forages produced are not sufficient on their own to support the current livestock population. Judicious combinations of these feeds with the more abundant low quality forages are needed. This paper discusses the theoretical basis for such combinations.
ANIMAL UTILISATION OF FORAGES
The major determinant of livestock productivity is dry-matter intake which; in turn, is influenced by the palatability, chemical composition and physical attributes of the diet, assuming that the livestock are disease-free. The objective of designing diets must therefore be to optimise animal productivity; feed intake and animal response are not good indicators of quality. Thus, while laboratory analyses of the diets are useful, they are not essential to research progress.
Forage-based feeds yield nutrients to the animal mainly through the processes of fermentative digestion in the rumen. Strategies for the utilisation of these feeds should therefore aim to establish an efficient rumen ecosystem that will maximise the digestibility of fibre within the rumen and optimise microbial protein synthesis. Additional attention needs to be paid to supply of balanced nutrients post-ruminally, if high productivity is expected.
ESTABLISHMENT OF AN EFFICIENT RUMEN ECOSYSTEM
Microbial considerations
The fermentative digestion of fibre in the rumen is carried out by a mixture of bacteria (Mackie and White, 1990), protozoa (Demeyer, 1981) and fungi (Akin and Borneman, 1990). The role of protozoa has been the subject of great controversy which is outside the scope of this discussion (Mackie and White, 1990). Rumen microbes require a source of fermentable nitrogen, usually as ammonia although some species require preformed amino acids and peptides (Russell and Baldwin, 1978). The low nitrogen (N) content of most mature grasses points to a need to combine them with a forage with a high N content. The ideal N concentration in the rumen for efficient digestion has been variously estimated at 50-70 mg/litre (Satter and Slyter, 1974) and at 150-200 mg/litre (Krebs and Leng, 1984). These levels are not easy to maintain in stall-fed animals over a 24-hour period, particularly if the feed is mature grass and it is fed in insufficient quantities. Plant and protein degradation depends on the physical nature of proteins, their release from plant cells, the concentration of proteolytic enzymes and the time available for proteolysis. In tropical forages, more than 20% of plant proteins are present in structures such as the vascular bundle sheath which are resistant to microbial attack (Egan, 1985). The presence of tannins in legumes and browse species may also result in binding of proteins and thus inhibit microbial attack. This may not necessarily have a negative impact on the animal if the proteins can be released post-ruminally and thus be available to enzyme digestion (Mueller-Harvey et al, 1988).
Plant considerations
In the presence of adequate rumen N concentrations, microbes will ferment fibre to obtain energy for growth and synthesis of new cells. The byproducts of such fermentation include the volatile fatty acids (VFAs) acetate, proprionate and butyrate which are the main energy nutrients absorbed in the rumen. Degradation of specialised plant material by microbes varies from tissue to tissue, decreasing in the order: mesophyll and phloem > epidermis and parenchyma sheath > sclerenchyma and lignified vascular tissue (Akin, 1982). Tropical grasses have few mesophyll cells between vascular bundles (as a consequence of adaptation to a C-4 photosynthesis pathway) and have a high proportion of lignified vascular tissue. Both factors combine to lower the degradability of these grasses. As plant cells mature, their cell walls thicken and deposition of hemicellulose and lignin increases, further reducing degradability. Thus diets based on tropical grasses should be supplemented with forages high in readily degradable tissues.
In addition to adequate N and energy supplies, rumen microbes require a stable pH environment (6.5-6.8). Production of VFAs tends to lower the rumen pH and thus there is a need to buffer the rumen pH to the optimum level of 6.5-6.8. Forages encourage buffering through increased salivation (Van Soest, 1982) and by the buffering capacity and cation exchange of fibre (McBurney et al, 1986). Tropical grasses and straws have low ion-exchange and buffering capacities while tropical legumes and citrus have high ion-exchange and buffering capacities (Van Soest, 1988). Interestingly, in legumes this buffering capacity is due to the high lignin content of these species. Lignin has been found to have a high capacity for cation exchange (McBurney et al, 1986). This cation exchange is also important in mineral nutrition, a component of forage quality that is usually overlooked.
The chemical composition of the fibre is important in providing indications of fermentation rates. However, interactions between environment and plant physiology and growth are sufficient to render associations between fibre components and nutritive value unreliable (Van Soest, 1988). In general, a high content of neutral detergent fibre and lignin results in lower fibre degradation compared to a low content of both. Although legumes have more lignin than grasses they are degraded more, mainly because of their high N content and the fragility of their cell walls in addition to their good buffering capacities and higher content of readily degraded specialised tissues of mesophyll and phloem.
Tropical legumes and browse species also contain phenolics, other than lignin, which limit the digestibility of cell wall carbohydrates and proteins. Palatability of feeds is also usually affected (Reed et al, 1988). The most important non-lignin phenolic compounds seem to be tannins. Tannins help to inhibit attack on lignified tissues by fungi and bacteria (Barry and Blaney, 1987). Recent research indicates that condensed tannins are more important than hydrolysable tannins in affecting digestion of feeds (Reed, 1986). Use of browseable tree species as supplements should be done keeping in mind the possible negative effects of tannins.
The next section gives a few examples of the application of these principles in designing forage-based diets for ruminants.
APPLICATION OF THE THEORETICAL CONCEPT TO PRACTICE
One of the biggest challenges in feeding low quality forages is to increase their intake in animal diets. Chemical treatments, while successful, present several practical problems for smallholder agriculture. Addition of higher quality feeds to a poor-quality basal diet is more practicable. If the addition does not result in reduced intake of the basal diet, then the added feed is a supplement. If the addition results in reduced intake of the basal diet but an increase in total intake, then a substitution effect exists. Since high quality feeds are available in small quantities, it is preferred to use them as supplements rather than as substitutes.
A classic case of substitution is reported by Njwe and Olubajo (1989) who fed fresh Guatemala grass (Tripsacum laxum) with various combinations of cassava flour (up to 200 g/day) and groundnut cake (up to 150 g/day) to West African Dwarf goats weighing 9-15 kg. Total dry-matter intake increased with increasing additions of both cassava flour and groundnut cake. Intake of cell walls, acid detergent fibre and cellulose (all components of the basal diet) increased. Fresh Guatemala grass had adequate readily degradable carbohydrates and, having poor buffering capacity, was unlikely to prevent pH falling below 6.2 when supplemented with cassava flour (another source of readily degradable carbohydrate). Adding a protein source that was low in fibre did not improve buffering. It is worth noting that animal productivity in terms of liveweight gain was increased by substituting concentrates for the basal diet. The merits or demerits of this substitution then depend on the economic returns to the farmer.
An example of supplementation given by Ayoade (1989) who fed maize bran (up to 200 g/day) to Malawi indigenous goats fed a basal diet of pigeon pea (Cajanus cajan) pods. Dry-matter intake of the pods was not changed by adding maize bran but total dry-matter intake, and digestibility of dry matter and organic matter, increased. Maize bran presumably supplied readily fermentable carbohydrates which provided energy to the rumen microbes and thus improved cellulolytic activity. The buffering capacity of pigeon pea pods ensured that rumen pH was not severely lowered and thus fibre digestion was not negatively affected.
Nuwanyakpa and Butterworth (1987) found that supplementing a diet of molasses and teff straw with varying levels of Trifolium hay increased total feed intake and apparent digestibility of dry matter, neutral detergent fibre and nitrogen. The results support the hypothesis that legume supplements enhance intake and digestion in ruminants. More important, they also indicate the value of forage with readily degradable tissue (Trifolium hay) as a supplement to crop byproducts with limited readily degradable tissues.
The value of using green forage, particularly leguminous forage, has been described by Ndlovu and Buchanan-Smith (1985), Dixon and Egan (1987) and Elliot and McMeniman (1987) based on stall-fed animals. No reports were found on grazing animals. A possible experiment would involve grazing/browsing on green forage for a limited time (say 1-2 hours) plus unlimited access to mature dry hay. Fodder banks could be used for the restricted grazing on green forage. This area requires further research.
CONCLUSIONS
Livestock productivity from forage-based diets can be improved by making use of current knowledge on the rumen ecosystem and on the qualities of different forage species
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