An initial look at the nutrient requirements of heavy finishing steers

Abstract Text:

Although there has been a long-term trend toward finishing cattle at increasingly heavier weights, diets are formulated based on nutrient requirements extrapolated from lighter cattle. A dataset derived from 40 studies containing 216 means for treatments testing beta-agonists during the final 14 to 42 d of finishing was subjected to a meta-analysis to determine if the Nutrient Requirements of Beef Cattle, 2000 (NRC) correctly predicted energy and protein requirements for heavy finishing steers. Reported DMI and NE_g were used to partition DMI into feed required for maintenance and feed available for gain. Net energy of maintenance was determined as 77kcal/kg BW^0.75. Feed available for gain and diet NE_g were used to calculate retained energy and a regression approach was used to generate NE_g predictive equations. Metabolizable protein intake (MPI) requirement was determined by regressing empty body protein gain (g/kg BW^0.75) on MPI (g/kg BW^0.75). Best-fit models were determined using an F-test with a value of P < 0.05. Predictive NE_g ­and MPI equations for cattle not fed a beta-agonist suggest that, at low rates of gain, they are more efficient at nutrient utilization than predicted by the NRC but less efficient than predicted by the NRC at higher rates of gain. Cattle finished at lighter weights transitioned from being more efficient to less efficient a higher rate of gain. In cattle fed a beta-agonist, NE_g requirements decreased across all weight classes and rates of gain in a consistent manner. The MPI requirement in cattle fed beta-agonist followed the same trend as cattle not fed a beta-agonist; cattle were more efficient at MPI utilization than predicted at low rates of gain but less efficient than predicted at higher rates of gain. However, at the same end weight, beta-agonist-fed cattle retained greater efficiency of MPI utilization at greater rates of gain compared to cattle not fed a beta-agonist. This data suggests that expectations of steer performance at a given NE_g should be re-evaluated, which would allow for more precision in targeting specific feedlot out-weights. Also, MPI is undersupplied in high-performing cattle fed or not fed a beta-agonist. 

Keywords: cattle, energy requirement, protein requirement