Nutritive value and manure quality in supplemented maize stover and grass hay diets for ruminants.
Production of ruminants is highly dependent on utilization of roughages. In tropical regions, particularly Sub-Saharan Africa, the roughages are predominantly of poor quality due to high fibre and low protein content. For these reasons supplementation with protein sources and other feeds with high degradability is essential. Maize is an important cereal in the world, and in Africa it produces the highest amount of crop residue. The stovers left after harvesting maize grain are potentially an important feed resource. The overall goal of this study was to identify nutritive attributes of diets comprised of maize stovers or grass hay and their mixtures with different protein supplements for ruminant production, and to determine the influence of the diets on manure quality. There were three different roughages and three different protein supplements used in the study. The roughages were grass hay and maize stovers harvested when the grain was at milk stage or when the grain was dry (approximately 80% DM content).The roughages were characterized by high fibre and low protein content. The protein supplements included lucerne, Sericea lespedeza and sunflower oil cake. Although lucerne has been widely used as a model protein source for ruminant production there are agronomic and climatic limitations which constrain its widespread use, particularly in smallholder systems. Hence there is a need for alternative protein sources such as those included in the present evaluation. There were three sets of investigations carried out: The first set involved laboratory chemical composition and in vitro rumen fermentation of feeds using automated gas production technique (IVGPT). Over forty feed rations were evaluated. Multivariate cluster analysis was performed to create clusters of diets exhibiting homogeneous nutritive characteristics from IVGPT measurements. The diets were grouped into three different clusters. Ten diets were selected, with each cluster represented, and tested in a second set of investigations. These investigations involved conducting feeding trials using Damara rams. Data collected included intake, digestibility, weight gain and manure (faeces) production. The third set of in investigations involved manure quality analyses. This was done by determining the mineral composition and nutrient release, particularly nitrogen mineralization. The relationships among chemical composition, IVGPT, in vivo and manure quality measurements were determined. Models to predict in vivo performance from chemical composition and IVGPT measurements were derived. Maturity stage influenced the nutritive characteristics of the forages whereby both maize stovers and lespedeza depreciated in quality with age, but the effect was small in the stovers with regard to degradability. The stovers harvested at grain milk, stage showed superior nutritive quality over stovers harvested at dry stage by having higher soluble carbohydrate, lower fibre content and shorter lag time (time taken for microbes to colonize and begin fermenting the substrate) under in vitro fermentation. The positive effects of supplementation with lespedeza included enhancing fermentation of fibre fraction of and decreasing lag time. However, increasing the ratio of lespedeza caused a decrease in degradability, and this was attributed to the presence of tannins in lespedeza. In vitro gas production technique (IVGPT) is currently one of the most widely used methods in ruminant feed evaluation. It measures the gas production arising from fermentation of feeds, and the degradability can be measured by weighing the residues left at the end of incubation. Gas production profile from IVGPT can be fitted in models to derive parameters of fermentation kinetics. Although IVGPT can give several nutritive measurements to help in understanding the fate of substrates in the rumen, this study obtained inconsistent and confounding results in some parameters. These included gas volume, microbial yield and degradability:gas production ratio or the partitioning factor-PF (mg of DM Degraded/ml of gas produced). On the other hand, it was found that time taken to produce half of the maximum gas (T (1/2)) was consistent across rations. The degradation efficiency factor (DEF), calculated as the ratio of degraded material : gas produced X T(1/2), was also consistent across rations. These parameters therefore showed potential of being useful to include as indices for evaluating ruminant diets. The study demonstrated that the measurements derived from gas production technique can be used in multivariate cluster analysis to logically separate feeds into distinct homogeneous groups of nutritive characteristics, and this was helpful in identifying diets for the in vivo trial. The directly measurable variables from in vitro gas production technique which significantly contributed to distinctiveness of the clusters included degradability, microbial yield, T(1/2), PF and DEF. Other significant distinguishing variables for the clusters were derived from kinetic of gas production. They included lag time, gas from the Fibre or soluble fraction of the feed and the rate of gas production from the fibre fraction. Diets induced different performances by sheep, with the best (60:40 grass:sunflower cake) having an intake of 43 g DM/kg liveweight and weight gain of 172 g/d. The poorest performance was obtained in 60:40 milk stage maize stover:lespedeza diet where the intake was 25 g DM/kg liveweight and marginal weight loss of 11 g/d was recorded. Evidence in the literature indicates that lespedeza has a great potential due to adaptation to poor climate and low inputs requirements. Though not determined in this study, the major limitation to lespedeza nutritive value is high tannin content. Reports in the literature indicate that presence of tannins can be of nutritional and health benefits, and negative effect of tannins can be moderated through use of chemical additives. Also, there are reports indicating that lespedeza can be invasive, hence its cultivation may require effective control. Further work is recommended to investigate means of optimizing lespedeza utilization in ruminant production systems, particularly where it can have climatic and economic advantages. Ideally, evaluation of diets should be done by conducting in vivo trials. However, in vivo trials are widely considered to be impractical due to many reasons including high costs, animal welfare and inflexibility. In vitro methods are therefore widely used as alternative to mimic and predict what would happen in vivo. This study showed that in vivo performance of sheep was predictable with high precision (R(2) = 0.75-0.85 for intake, 0.70-0.89 for Digestibility and 0.77- 0.82 for weight gain) by use of models combining chemical composition and IVGPT measurements. The important chemical composition measurements included fibre (NDF or ADF) and protein contents, while important IVGPT measurements included degradability (true or apparent), T(1/2) and DEF. There were confounding factors affecting the validity of gas volume, microbial yield and PF measurements as predictors of nutritive value. Consequently, these measurements lacked consistency and generally had poor relationships with in vivo measurements. Drying fresh manure caused 25 to 89 % loss of ammonium (NH4 +), emphasising the need to avoid drying manure in order to maximise the quality. Mineralization results showed that manure from diets with crude protein content above 170 g/kg can be similar to limestone ammonium nitrate (LAN) fertilizer in supplying N. Notwithstanding, the manures would have additional advantage of improving soil humus and supply more nutrients as decomposition continues. In the overall, the results demonstrated that maize stover or grass hay supplemented with sunflower cake or lucerne can support high ruminant (sheep) production while at the same time yield good quality manure. For practical purposes, diets containing 20-40% of DM contributed by these supplements showed high potential in this dual purpose role. The choice between these two supplements would therefore depend on socio-economic considerations. As for lespedeza, high tannin content is stipulated to be responsible for depression of microbial activity, hence nutritive value. However, given that lespedeza may have agronomic and climatic advantages, plus the fact that the impact of tannin can be ameliorated, it is viewed that lespedeza has potential to boost productivity in specific situations. Further evaluation of lespedeza should therefore focus on detailed chemical composition analyses at its different stages of maturity, in vivo trials using different ruminants and measures to moderate tannin effects.