Casual factors affecting liveweight gain in north Australian beef herds

Summary

This project investigated the reasons for the large within-herd variation in steer growth identified in extensive beef herds in northern Australia, and the scope for targeted improvement of the  performance of the tail of the annual steer or heifer crop, either by better selection or improved management. Eleven study groups of steers on different commercial cattle properties were inducted at weaning and observed in one of three 12-month periods: 2008/09 (n=1); 2009/10 (n=5); and 2010/11 (n=5).  In addition to liveweight, individual animal measurements included hip height, body condition, flight speed, tick score, buffalo fly count, lesion score, HGP (timing and retention), internal parasite status, and disease status.  
Data analysis did not show any of the measured factors to be major drivers of the observed variation in growth rates within a mob, so no factors emerged to explain the poor doers or tail that typically occur in mobs.  Note that this does dismiss the importance of  factors such as tick, flies, internal parasites and disease as being able to cause production losses from time to time at the mob level. Weaning weight and hip height at weaning appeared to be associated with dry season ADG. The largest dry season weight gains were observed in animals that were taller at weaning and animals that were lighter at weaning. It is important to note that while annual growth rate was higher in those animals that were lighter at weaning, the increased growth rate in lighter weaners was not enough to overcome the weight advantage conferred on those animals that were heavier at weaning.   Pen studies found that variability in post-weaning growth rate is not related to the nutrient status of the animals. It is unlikely that the variability in post-weaning growth rate is genetic or a genotype x diet quality interaction given that the animals responded similarly when provided with identical diets under pen feeding conditions.
This project, therefore, has shown little practical scope for improving performance of the tail of steer, or heifer, mobs in extensive production systems.  Additional and more detailed research may reveal some key causes of variation within a mob and, perhaps, some practical solutions.  In the meantime, improvements in productivity will arise from  improving the average performance of the whole mob through genetic selection and improved nutrition.