Factors influencing the variability in performance of cattle grazing tropical pasture

Context. Whereas it is known that there is variation in liveweight gain (described as performance in this paper) within a group of grazing cattle, the causes of this variation are not well understood. Aim A 2-year grazing study in northern Queensland was conducted to understand the relative influence of diet selection, grazing behaviour and rumen fermentation on variation in performance of growing cattle. Methods. Eighty-nine Bos taurus x B. indicus (similar to 6 months old) growing steers were grazed as a group on mixed tropical pasture for 23 months. Pasture was characterised for biomass, species and nutrient composition. Cattle were ranked according to overall liveweight (LW) gain and the top (high performance, HP) 20 and bottom (low performance, LP) 20 head were compared for diet composition, rumen fermentation and grazing behaviours. Results. Pasture biomass, and nutritive value of pasture and diet varied between seasons and years of study. The HP cattle achieved overall LW gains 20% greater than those of the LP cattle mainly due to higher rates of gain in the wet season. However, the nutritive value and rumen fermentation characteristics were similar for both LP and HP cattle, although there was evidence that crude protein and digestibility were modestly higher in diets of HP cattle, especially in the first wet season. Activity, measured as distance travelled, declined as pasture biomass declined, and cattle appeared to favour pasture with higher legume content. Conclusion. It is concluded that divergence in performance of cattle on tropical pasture could not be fully explained by measurements taken in this study. Diet selection for plant components with improved nutritive value probably played a small role. Differences in initial LW between the LP and HP groups may have been a contributory factor. Implications Despite comprehensive measurements of factors that influence efficiency and performance on pasture, we were unable to identify definitive causes. Genetic variation in feed intake or efficiency of digestion needs to be investigated.