The energetic efficiency of rumen microbial protein synthesis in cattle given silage-based diets

Abstract

Two experiments are described, in each of which cattle were given diets consisting of wilted grass silage, either as the sole feed or with protein and/or cereal supplements. The apparent efficiency of rumen microbial protein synthesis in the reticulo-rumen, expressed as g nitrogen (N) per kg organic matter (OM) apparently digested in the rumen (OMADR), was measured for each diet by using 35S as a marker of microbial protein entering the small intestine. Low values for the apparent efficiency of microbial protein synthesis were observed when silage was given alone. In Experiment 1, supplementation of silage by soya-bean meal increased the mean efficiency of microbial protein synthesis from 25.2 to 31.0 g N kg −1 OMADR. In Experiment 2, supplementation of silage with barley and soya-bean meal increased the efficiency of microbial protein synthesis from 24.3 to 39.0 g N kg −1 OMADR; supplementation with barley alone or barley plus formaldehyde-treated soya-bean meal had an intermediate effect on microbial protein synthesis (30.1 and 31.4 g N kg −1 OMADR, respectively). Supplementation of diets with untreated soya-beam meal was observed to maintain rumen ammonia-N concentrations above 50 mg N 1 −1 throughout a complete sampling period.