Factors associated with divergent post-weaning live weight gain in northern Australian beef cattle.

Summary

A large variation in liveweight gain (LWG) exists in cattle grazing crude protein (CP) deficient pastures after weaning across northern Australia. This occurs even when N supplements are provided. It is anticipated that increasing the LWG of the tail 20% of weaners across northern Australia could have a significant impact on production and income of these enterprises. To address this issue the mechanisms responsible for this large variation need to be defined. This project examined a range of factors that may be associated with the divergence in liveweight gain evident in weaner cattle in northern Australian beef herds. It did this by measuring liveweight change of animals under commercial and experimental conditions and measuring the change in concentration of a range of circulating factors at key stages during the experiment.
The main findings of this project were:There was variation in liveweight change within a mob of Bos indicus steers after weaning. There was a 15 kg (10%) difference in liveweight between the heaviest 20% of the mob and lightest 20% of the mob, 91 days after weaning.
There was no difference in LWG of the highest and lowest LWG steers post-weaning, when fed moderate (cavalcade; 0.47 kg/d) and low (Mekong grass; 0.28 kg/d) protein diets, within pens.
Steers that had the highest LWG post-weaning, also had a small but significantly higher LWG (0.58 kg/d) than steers that had the lowest LWG post-weaning (0.55 kg/d) when grazing over the subsequent wet season.
After approximately 12 months, steers that had the highest LWG post-weaning were 12 kg heavier than steers that had the lowest LWG post-weaning. This difference in liveweight over a 12 month period is unlikely to be of significance under commercial situations.
There were no differences in circulating albumin, creatinine, glucose, insulin-like growth factor-1 (IGF-1) or urea concentration at weaning or 91 days post-weaning for steers of different post-weaning LWG.
Circulating IGF-1 concentration decreased from weaning to post-weaning.
There were no differences between steers of different post-weaning LWG in circulating albumin, creatinine, glucose, IGF-1 or urea concentration, when fed low or higher protein diets.
Circulating glucose, IGF-1 and urea concentration were higher, and circulating creatinine concentration was lower, in steers fed a moderate protein compared with a low protein diet.
Circulating IGF-1 at weaning and 91 days post-weaning was positively correlated with liveweight at weaning, 91 days post-weaning and at the end of the subsequent wet season, although it was not correlated with average daily liveweight change at any stage of the experiment.

The results suggest that the variation in post-weaning LWG of weaner steers is probably related to variation in how animals respond to the stress of weaning and marking, supplement intake or grazing behaviour, rather than any inherent genetic difference.