Effect of feeding legume pods or alfalfa in combination with poor quality grass straw on microbial enzyme activity and production of VFA in the rumen of South African Merino sheep

In the first of a two-phased study, changes in rumen pH and concentrations of VFA were evaluated on diets based on pasture straw (negative control) or straw supplemented with alfalfa (Alfalfa diet), pods of Acacia sieberiana (Sieberiana diet), Acacia nilotica (Nilotica diet) or Leucaena leucocephala (Leucaena diet) for South African Merino rams. In the second phase, the effects of the same diets on the degradation of extracted straw in nylon bags and activities of microbial enzymes were investigated. The pH and total concentration of VFAs in rumen fluid ranged from 6.2 to 6.5 and 30–53 mmol l−1, while degradation of DM and NDF varied from 126 to 269 g kg−1 DM and 233–309 g kg−1 NDF, respectively, among diets. Variations in pH were not significant (P > 0.05), but diet influenced (P < 0.01) the concentration of VFAs, degradation of DM and cell wall constituents as well as enzyme activities of proteolytic and fibrolytic microorganisms. The results showed that the differences between the alfalfa and pod diets might have been due mainly to the presence of tannins in the pods. These compounds limit the growth and/or activity of ruminal microorganisms. However, a limitation in the supply of energy imposed by the pod diets may have reduced the efficiency of N capture by microorganisms leading to a depression in microbial activity. The tested legume pods could be used in limited quantities to supplement low-quality forage. The pods may need to be detoxified prior to inclusion at much higher levels in the diet. © 2003 Elsevier Science B.V. All rights reserved