Season of testing and its effect on feed intake and efficiency in growing beef cattle.

This study sought to assess whether residual feed intake (RFI) calculated by regressing feed intake (DMI) on growth rate (ADG) and metabolic mid-BW in 3 different ways led to similar estimates of genetic parameters and variance components for young growing cattle tested for feed intake in fall and winter seasons. A total of 378 beef steers in 5 cohorts were fed a typical high energy feedlot diet and had free-choice access to feed and water. Feed intake data were collected in fall or winter seasons. Climate data were obtained from the University of Alberta Kinsella meteorological station and Vikings AGCM station. Individual animal RFI was obtained by either fitting a regression model to each test group separately (RFIC), fitting a regression model to pooled data consisting of all cohorts but including test group as a fixed effect (RFIO), or fitting a regression to pooled data with test group as a fixed effect but within seasonal (fall-winter or winterspring) groups (RFIS). Two animal models (M1 and M2) that differed by the inclusion of fixed effects of test group or season, respectively, were used to evaluate RFI measurements. Feed intake was correlated with air temperature, relative humidity, solar radiation, and wind speed (-0.26, 0.23, 0.30, -0.14 for fall-winter and 0.31, -0.04, 0.14, 0.16 for winter-spring, respectively), but the nature and magnitude of the correlations were different for the 2 seasons. Single trait direct heritability, model likelihood, direct genetic variance, and EBV accuracy estimates were greatest for RFIC and least for RFIO for both M1 and M2 models. A significant genetic correlation was also observed between RFIO and ADG, but not for RFIC and RFIS. Including a season effect (M2) in the genetic evaluation of RFIO resulted in the smallest heritability, model LogL, EBV accuracy, and largest residual variance estimates. These results, though not conclusive, suggest a possible effect; of seasonality on feed intake and thus feed efficiency. [ABSTRACT FROM AUTHOR]