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L. A. Kriese-Anderson, L. E. Mahler, T. D. Brandebourg, M. K. Mullenix, 004 Recommended Duration for Evaluating Feed Intake and Validating the Residual Feed Intake Model in Brangus Heifers, Journal of Animal Science, Volume 95, Issue suppl_1, December 2016, Page 02, https://doi.org/10.2527/ssasas2017.004
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Abstract
Dry matter intake (DMI) was measured on 186 Brangus replacement heifers obtained from two purebred southeastern Brangus breeders. Heifers were delivered to the Auburn University Beef Cattle Evaluation Center (AUBCE) during 2014 and 2015. Seven contemporary groups were assigned based on date of trial and farm. Feed intake trials (70 day) were conducted using Calan gates. Heifers were weighed every 14 days. Carcass ultrasound measurements were taken by a certified ultrasound technician when average age was 365 days. Residual feed intake (RFI) was defined as actual feed intake minus expected feed intake based on maintenance and production requirements. The basic regression model for determining RFI was Yi = β0 + β1ADG + β2MMWT + ei, where Yi = expected DMI, β0 = regression intercept; β1 = partial regression coefficient of DMI on ADG, β2 = partial regression coefficient of DMI on metabolic mid-test weight (MMWT) and ei = RFI. ADG was determined by two methods. Individual animal ADG1 was computed by regressing individual weight on day of test. ADG2 was defined as (Final BW – Initial BW)/days on test. Additionally, ultrasound 12th rib fat depth (ubf) was added to the basic RFI model. A maximum of four RFI values were determined for each individual heifer depending on ADG value and whether ubf was included in the model (RFIADG1, RFIADG2, RFIADG1U, RFIADG2U). To estimate linear relationships between models, regression analyses were performed for RFIADG1 on RFIADG1U, RFIADG2 on RFIADG2U, RFIADG1 on RFIADG2, and RFIADG1U on RFIADG2U. Pearson and spearmen correlations were used to examine correlations among the four models. To assess whether a shorter feeding period could be implemented to accurately determine feed intake and ADG, subsets of the 70 d trials were created based on 14, 28, 42, and 56 d of data. Data subsets were regressed on the entire dataset to determine whether a shorter feeding trial could be implemented. Regression coefficient of RFIADG1 on RFIADG2 was 1.00 ± 0.01 (P < 0.0001), which did not differ from 1 (95% confidence limits; 0.98 < β < 1.01), suggesting models were equivalent. Addition of ubf in the models accounted for an additional 2% of variation for DMI, but reranking of individuals for RFI was minimal. Accurately measuring ADG is the limiting variable in reducing test duration (r = 0.89 between day 70 and 56) for Brangus heifers. Results suggest testing length could be reduced to either 56 (r = 0.93) or 42 (r = 0.88) days.
