Supplementation of graded levels of Desmodium intortum hay to sheep feeding on maize stover harvested at three stages of maturity: 2. Rumen fermentation and nitrogen metabolism

The utilisation of cereal crop residues as animal feed is limited by deficiencies of protein, fermentable energy and other nutrients. In this study, rumen fermentation and nitrogen metabolism of lambs fed basal diets of maize stover harvested at three stages of maturity and supplemented with graded levels (0, 150, 300 and 450 g per head per day) of desmodium (Desmodium intortum cv Greenleaf) hay were investigated. The maize stovers were harvested at 30, 22 and 12% grain moisture content which were designated as stages I, II and III, respectively. Rumen ammonia and volatile fatty acids (VFA), nitrogen (N) balance and microbial N supply, based on urinary excretion of purine derivatives, were determined. The concentration of ammonia-N in rumen fluid was significantly lower (P<0.05) in sheep fed maize stover harvested at stage III than at stages I and II and increased significantly (P<0.001) with increasing level of supplementation. The total VFA concentration and the molar proportions of acetate and butyrate in rumen fluid were not significantly influenced (P>0.05) by the stage of maturity of the stover. The total VFA content increased with increasing level of supplementation and varied from 64.1 to 113.6 mmol/l of rumen fluid. The N intake, N absorbed and N retained decreased with increasing stage of maturity of the stover and showed a significant linear increase (P<0.05) with increasing level of supplementation. However, use of maize stover as a sole diet resulted in negative N balance at all stages of maturity due to the low N content of the stover. Microbial N supply was not affected by stage of maturity of the stover while microbial efficiency (g/kg digestible organic matter fermented in the rumen) was significantly lower (P<0.05) in sheep fed maize stover harvested at stage III than at stages I and II. The microbial N supply increased with increasing level of supplementation, presumably due to improved degradation of the diet and increased supply of N, fermentable carbohydrates, sulphur and probably other essential nutrients.