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Performance by feedlot steers and heifers: daily gain, mature weight, dry matter intake, and dietary energetics

University of California, Davis 95616

razinn{at}ucdavis.edu

Abstract

Performance, DMI, diet composition, and harvest data from 9,683 pens of steers and 5,009 pens of heifers that were fed high concentrate diets for 90 d or more were obtained from 15 feedlots from the western United States and Canada. The data set included pen means for more than 3.1 million cattle fed between 1998 and 2004. Performance measurements assessed included ADG, DMI, dietary NE (based on NRC, 1996 values), shrunk initial weight (SIW) and shrunk harvest weight (SFW). Mature final weight (MFW) for cattle in each pen was estimated based on regression of harvest weight against SIW and ADG across all pens. Equations were developed to standardize performance projections (ADG, MFW, and break-even values) and analyze feedlot cattle close-outs. Generally, as diet NE concentration increased, DMI was reduced but G:F, dressing percentage, and yield grade all increased. Pens of cattle with greater SIW had greater ADG, DMI, and SFW but a lower G:F and dressing percentage. Dressing percentage and yield grade were correlated positively. NRC (1996) equations relating gain to NE intake explained 85 and 80% of the variation in DMI of steers and heifers, respectively, with mean ratios of predicted to observed DMI (DMIratio) at 1.000 ± 0.0506 and 0.974 ± 0.0490. However, a significant (P < 0.001) bias in the NRC (1996) estimate of DMI was detected (r² = 0.10 and 0.05, for steers and heifers) between the DMIratio and ADG where DMIratio increased as ADG increased. This was due to inherent confounding of ADG and MFW in the original Lofgreen and Garrett (1968) NE equation. Based on iterative optimization to minimize the difference between expected and observed DMI, revised equations for retained energy (RE, Mcal/kg) were developed for steers and for heifers: RE_steer = 0.0606 x (LW x 478/MFW_steer)^0.75ADG^0.905; RE_heifer = 0.0618 x (LW x 478/MFW_heifer)^0.75ADG^0.905, where LW is mean shrunk live weight. The revised equations reduced the SD of the DMIratio by 5.4% (from 0.0496 to 0.0469) and eliminated the bias in DMIratio that was related to ADG (r² = 0.0006). The similarity between the 2 equations derived for steers and for heifers for estimation of RE from ADG supports the concept that scaling by MFW accounts for energy utilization differences between genders.

Key Words: energetics • feedlot • heifers • performance • steers

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