Effect of rumen-degradable protein balance deficit on voluntary intake, microbial protein synthesis, and nitrogen metabolism in growing double-muscled Belgian Blue bulls fed corn silage based diet

¹ Faculté universitaire des Sciences agronomiques, Passage des Déportés, 2, B-5030 Gembloux, Belgium

^* To whom correspondence should be addressed. E-mail: beckers.y{at}fsagx.ac.be.

	Abstract

Two trials were conducted to evaluate the effect of rumen-degradable protein balance (OEB) deficit on voluntary intake (trial 1), microbial protein synthesis, and N metabolism (trial 2) in growing double-muscled Belgian Blue bulls. In trial 1, 6 bulls (339 ± 26 kg initial BW) were used in replicated 3 x 3 Latin square and received for ad libitum intake a diet of 60% corn silage and 40% concentrate (DM basis). Three different concentrates were formulated by adding urea at the expense of barley to give similar dietary contents of intestinal digestible proteins, NE for fattening, and fermentable OM, but with different levels of OEB. Thus, 2 levels of OEB deficit (-23.7 and -9.2 g OEB/kg of DM) were compared to a diet providing a slight OEB surplus (5.3 g OEB/kg of DM). Voluntary DMI decreased linearly (P = 0.02) with decreasing rumen-degradable protein balance. This decrease in intake could explain the linear decrease in ADG observed when negative OEB diets were fed. In trial 2, 6 bulls (304 ± 12 kg initial BW) with cannulas in the rumen and proximal duodenum were used in a replicated 3 x 3 Latin square and fed diets similar to those used in trial 1 at an intake level of 85 g of DM/BW^0.75. Diurnal variations of ruminal NH₃-N and plasma urea-N concentrations were greatly influenced by the level of OEB in the diet. No differences in NDF and starch degradation in the rumen, microbial-N flow at the duodenum, or efficiency of microbial protein synthesis in the rumen were noted among the different levels of OEB in diets. The reductions of the OEB value from 5.3 g/kg DM to -9.2 g/kg DM and -23.7 g/kg DM were associated with reductions of 26.5% and 48.8% in urinary N output. Absolute amounts of N retained by the bulls increased significantly with the level of OEB in diets. Indeed, 51.4% of the incremental supply of N was excreted between -23.7 and -9.2 g OEB/kg DM diets, and 74.6% of the incremental supply of N was excreted between -9.2 and 5.3 g OEB/kg DM diets. Feeding diets characterized by an adequate intestinal digestible protein supply and a rumen-degradable protein balance close to -10 g OEB/kg of DM could be a feeding strategy to reduce N losses from the farm with little impact on the animal performance and voluntary intake. Reduced rumen degradable protein balance may reduce N excretion in the environment, but may also result in decreased N retention.

Key Words: Bulls, Nitrogen Utilization, Rumen-Degradable Protein Balance, Rumen Fermentation, Voluntary Intake