Age for beef heifers at first mating

Well grown British/European breed heifers can be mated at 15 months to calve at 24 months.

The two main advantages of mating heifers as yearlings instead of two- year-olds are:

• heifers mated as yearlings produce an extra calf during their life in the herd
• high producing mothers can be identified earlier, which allows for a shorter generation interval and hence, quicker genetic progress

The main cost of mating heifers as yearlings is the need to feed weaner heifers so that they achieve a minimum liveweight at mating. However, this is offset by the cost of maintenance requirements of the heifer for an additional 12months, particularly in southern systems.

Critical mating weight (CMW) is defined as the weight when 85% of heifers fall pregnant after 2 cycles. As a guide it is recommended CMW weight is 55-60% of mature body weight. Therefore, British breed heifers should weigh at least 300-330kg at joining, while the later maturing European breeds should weigh at least 330-400kg depending on their frame score (MLA, 2013).

Fertility of heifers mated as yearlings

Low pregnancy rates and poor calving percentages in mobs of heifers mated as yearlings are usually associated with inadequate liveweight at mating — as liveweight is the most important factor determining the onset of oestrus. Pregnancy rates of more than 85% can be regularly achieved with yearling heifers that have reached critical weight before mating.

Calving difficulty

Calving difficulty (dystocia) is known to be more prevalent in first calving heifers compared to multiparous cows. This is regardless of if the heifer first calves at 2 years old or 3 years old.

Well-grown mated yearlings should have no more difficulty calving than heifers joined first as two-year-olds, despite the more mature frame of three-year-old heifers. This is because calf size increases with the increased frame of the dam, and older heifers tend to be fatter at calving, both of which can lead to calving difficulty.

However, heifers should not be mated prior to reaching critical joining weights for their breed as this will increase risk of dystocia. A small portion of heifers may reach puberty as early as 5 months therefore it is critical to ensure bulls are separated from heifer calves before this time to prevent early mis-mating. If accidental mis-mating does occur a veterinarian should be consulted.

Growth of heifers mated as yearlings

Provided nutrition is managed to ensure consistent good growth rates are achieved from weaning to calving, heifers joined as yearlings will achieve very similar mature weights as those joined later. Target growth rates will vary with breed, genetics and weaning weight however in general ~0.7kg /day from weaning to calving is suitable. In addition, if nutrition and parasite control is carefully managed post calving any differences in mature weight are further reduced.

Fertility after first calving

Heifers, particularly those calving at two years, take considerably longer after their first calving to return to oestrous. In many cases, heifers may become pregnant late or fail to conceive altogether. This can be overcome by ensuring that only well grown heifers are mated, heifers calve in good body condition, (condition score 3), and are well fed after calving to be joined at minimum of condition score 2.5. Parasite management post calving in heifers is also important.

Joining heifers, 2-3weeks earlier than the main herd is a management option that enables the heifers more time after calving to begin cycling before the start of mating.

Having a 6-week joining period will increase the proportion of heifers calving earlier compared to 9-weeks. Fixed time artificial insemination programs can also be used to increase the number of heifers calving earlier. An earlier shorter calving period increases the recovery time for heifers before the next joining. Therefore, increases the number successfully re-bred and reducing early cull rates.

Milk production and calf growth

Young cows calving at two years produce less milk and wean lighter calves than older cows, but can still produce good calves if calved in good body condition and are well fed after calving.

Although the calves of yearling joined heifers are lighter at weaning, the following calves are as heavy as if the heifers had calved at three years. The first calf of yearling joined heifers, although lighter, therefore represent an extra year of production and genetic gain, as opposed to heifers calving at three years of age.

References

More Beef from Pastures. Minimum liveweights of weaner heifers. 2013. Meat and Livestock Australia.