Critical mating weight
Critical mating weight is the target weight for maiden heifers when they’re first joined to bulls to achieve an 84% pregnancy rate in a six-week joining period, or two complete reproductive cycles. This is based on the expectation that 60% should conceive within the first cycle of three weeks and that 60% of the remaining 40% will conceive in the second cycle of three weeks (60% cycle 1 + (24% cycle 2) = 84%).
Critical mating weight varies between breeds and within breeds but can be calculated if heifers are weighed just prior to joining and accurate foetal ageing is performed at pregnancy testing.
Both age and weight at puberty are highly heritable traits but age at puberty is affected by growth rate – faster growing animals reach their critical mating weight at a lighter weight and a younger age.
Weight at puberty is a very practical and easy-to-measure trait that commercial beef producers can establish for their own breed of cattle and implement in their heifer management programs (see table).
The critical mating weight has not been established for all breeds of cattle in northern Australia. It varies within breeds and is heavier than age at puberty because the first few reproductive cycles after reaching puberty are not always fertile.
Why aim for such a short joining period?
A common scenario in northern Australia is to select replacement heifers on size and looks without knowing if they attained their critical mating weight.
Bulls are put in with heifers just prior to the start of the wet season and joining occurs continuously into the dry season as the heifers grow and reach puberty. Conception rates when pregnancy tested in the middle of the year are usually good but no consideration has been given to the spread of the pregnancies and when they will calve down.
Consequently, a heifer that conceives late will fail to get back in calf the following season.