The influence of genetic and environmental factors on performance of Brahman heifers grazing pastures in northern Australia

Context Between animal variation in performance is a determinant of productivity in the northern Australian beef industry. Understanding factors that contribute to this variation is critical for selection of superior genotypes and optimising the pasture animal interface. Aim To determine factors that influence performance of cattle grazing tropical pastures through co-ordinated grazing and pen studies. Methods Within a cohort of 56 yearling heifers, 32 were allocated to an 11-month grazing trial, whereas 24 were grazed, then individually fed hay in pens. At the end of the trial, pasture cattle were ranked according to liveweight (LW) gain and the top- and bottom-ranked animals were allocated to high-performance (HP) or low-performance (LP) groups. Residual feed intake (RFI) of heifers in pens was determined and correlations between pasture and pen performance were estimated. Pasture and diets were characterised using the Botanal method (pasture only) and near-infrared reflectance spectroscopy analysis. Activity on pasture was measured using global positioning system (GPS) collars. Pasture data were divided into dry and wet seasons and the grazing trial was analysed as a factorial, with performance group and season as the two factors. Key results There were seasonal differences in most measured variables of animal production, behaviour and rumen fermentation consistent with changes in diet quality. Diet selection, activity on pasture and rumen fermentation characteristics did not to differ between LP and HP groups. Differences in RFI of hay-fed heifers were attributed to variation in feed intake. There was a correlation (r2 = 0.5) between performance of heifers in pens and on pasture in the wet season. Conclusion Seasonal variations in pasture biomass and nutritive value had an overriding effect on LW change in growing beef cattle, but among-animal variation in LW gain was still apparent. This could not be attributed to any of the measured variables on pasture; however, RFI data suggested a genetic influence on intake, which was supported by estimated breeding values. Implications Several possible reasons for differences in performance of grazing cattle were ruled out by this research. Methods for measuring intake and efficiency of grazing cattle are required to understand and manage the grazing animal.