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The 8th Revised Edition of the Nutrient Requirements of Beef Cattle: Protein and metabolic modifiers
The 8th Revised Edition includes updates to the proteins and digestive and metabolic modifiers sections of the report reflecting new information since the 7th Revised Edition was published. The MP system was adopted in the 7th Revised Edition. It accounts for rumen degradation of dietary protein and separates requirements into the needs for ruminal microorganisms and the needs of the animal. Rumen degradable protein (RDP) provides ruminal microorganisms with various sources of nitrogen (N). In contrast, ruminally undegraded protein (RUP) is not hydrolyzed in the rumen. The amount of RDP required is based on prediction of synthesis of microbial CP (MCP). Published data from studies using cattle fitted with intestinal cannulas were used to develop and evaluate empirical equations for prediction of MCP based on total digestible nutrient intake (TDNI), fat-free TDNI (FFTDNI), and CP intake as independent variables. Equations based on TDNI and FFTDNI are provided to estimate MCP depending on the ether extract percentage of the diet. The MP supply is absorbed amino acids from protein digested in the intestine, supplied by microbial protein and RUP. Reported values for RUP digestibility are variable but most estimates for forages are less than 60%. The digestibility of RUP for forages was decreased from 80% to 60%. Regression analysis of literature data based on the dual-labeled urea isotopic approach was used to update equations to estimate urea N kinetics. A more complex rumen model is needed to include recycling directly in the beef cattle nutrient requirements model. A number of feed additives and other compounds that improve animal health and the efficiency of nutrient use, increase growth rate, and decrease the environmental impact of beef cattle were reviewed. These include compounds that alter rumen fermentation, additional aspects of gastrointestinal tract function, or post-absorptive metabolism. Review of feed additives with potential to provide an alternative to inclusion of dietary antibiotics such as plant secondary metabolites, direct-fed microbials, and feed enzymes was included. Ionophores change microbial populations in the rumen and improve feed efficiency. Predicted DMI is decreased by 3% when monensin is fed. In addition, dietary ME was increased by 2.3 or 1.5% for monensin or lasalocid, respectively, to account for improvements in ADG and feed efficiency when feeding ionophores.
Keywords:
Beef cattle, ionophores, microbial protein synthesis, urea recycling