Recent industry reports have suggested that over time, the average breeding cow in Northern Australia has increased substantially in size and that this may be contributing to lower branding rates due to a potential negative correlation between mature cow weight and fertility. There is however, a lot more to fertility than just mature cow size and there is a lot more to the economics of mature cow size than fertility alone.
Growth traits often have a degree of unfavourable correlation with fertility in cattle. It is however, possible to make genetic improvement in both areas simultaneously, providing that breeding program your source your bulls from includes the measuring of both.
In self replacing lines of beef cattle where commercial female progeny are to be kept as replacement breeders, it is often favourable to use genetics that grow less after the stage of life when the steer progeny are slaughtered thus producing lower maintenance cows. Again this routine requires measurement of both traits in order to “bend the curve” and to defy antagonisms. It is therefore crucial, that nucleus (stud) breeding programs weigh mature cows at weaning time as well as weighing young stock at weaning and then at yearling to 18 months.
Many stud breeders submit young stock weights for genetic analysis, but very few are submitting mature cow weights or adequate fertility records. This is likely to be contributing to the larger, less fertile cows that are being observed at the grass roots level of industry today.
As well as subjective visual estimates of maturity patterns and frame score, cattle breeders with appropriate records can also use Mature Cow weight figures in combination with birth weight 200, 400 and 600 day growth figures as an objective method of “bending the curve”.
Mature Cow weight values are estimates of the genetic differences between animals in cow weight at 5 years of age (1825 days of age). In most cattle production systems however the target sale age for steer progeny and surplus heifers is much earlier than this (e.g. 200-800 days). So although all weight traits are correlated, it is possible to identify and select curve bending individuals that grow rapidly up to target steer slaughter age (e.g. 700 days), then slow down their growth over the next 1125 days or so.
Genetically lowering mature cow weight, whilst also increasing steer growth rate really starts to pay dividends when you are selling young stock into higher value markets, as the price received per kilogram of steer is higher than that received per kilogram of cow. Examples of this may be seen when selling steers to markets that offer MSA grading premiums or when lot feeding young cattle for export markets. New Indonesian live-export weight limits should also be considered as they have recently changed the game.
Also as your breeder herd becomes more fertile, having curve bending cows of a more moderated mature size becomes more profitable as a more fertile cow’s job description is more about producing a calf each year and less about weighing heavy over the hooks when she doesn’t.
So the first take home message is not to rush out and buy bulls with low 600 day growth figures as an attempt to improve fertility, but instead buy bulls that will add fertility through hybrid vigour and selection for fertility. Secondly select bulls that have the correct genetic growth curve to breed cows that suit your production and marketing system.
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