Mature cow size impacts on replacement heifers
Does the mature cow weight impact on the size or age the heifers should be at joining?
Mature cow weight impacts the size of heifers at joining, but with careful management, should not impact on the age. Past research has shown that well managed heifers which calve at two years of age (i.e., joined at 15 months) have a higher lifetime productivity than heifers calving at older ages.
Ideal heifer weight at joining
Ideally, heifers should reach 65 per cent of their mature cow weight by the time of joining. This helps to ensure adequate in-calf rates at the end of the joining period.
If the mature cow weight of your herd is 550 kg, heifers should be at least 357 kg at the start of joining. This means the average growth rate of the heifers needs to be 0.71 kg LW/day from birth to joining.
If the mature cow weight of your herd is 750 kg, heifers ideally should be 487 kg and above, with an average growth rate from birth to joining of 0.97 kg LW/day.
Feed requirements vary with heifer size and weight
While this difference in growth rates may not sound like much, it results in the larger framed, heavier weight group of heifers requiring approximately 40 per cent more feed to reach their target joining weights at the same age of those heifers from the smaller framed, lighter weight cows. This needs to be considered when planning your business’ beef breeding targets.
Target market informs decisions about animal size
Ask yourself – What size mature cow do I really need in my system? What market am I looking to target? Where will the progeny of my cows end up?
Larger framed cattle generally produce larger cuts of beef. Do you sell into the butcher or supermarket trade? Butchers and supermarkets do not require large cuts of beef. Consider how much more you will need to feed those larger framed cattle.
If you breed for larger framed, heavier weight cows, you may find you need to reduce the herd size of your cows to better match their feed requirements with the feed you can grow on your property, otherwise you may find you have high bought-in feed costs.
Consider rib fat levels
Results from a Beef Cooperative Research Centre (CRC) Maternal Productivity Project showed while joining weight was the most important factor influencing heifer fertility, rib fat level also had an influence. It was found that heifers with a higher rib fat level had an 8 per cent higher pregnancy rate when they were joined for nine weeks, and a 12 per cent higher pregnancy rate when they were joined for six weeks. Using Estimated Breeding Values (EBVs) and selecting for rib fat could assist in improving heifer fertility in your herd.
However, good management of the heifer’s nutrition is important to allow that rib fat trait to be expressed. The order of energy use in the growing animal is laying down bone, then muscle and lastly fat. You need to be able to provide enough feed, of high enough quality, that it allows the heifer to lay down adequate fat levels, while still growing bone and muscle.
Impact of failure to meet joining weight requirements
If you cannot match the growing heifer’s feed requirements, and they fail to meet the target of 65 per cent of mature cow weight by 15 months, you may find an increased number of the heifers do not get in calf at joining.
Monitor your growing replacement heifers regularly through weighing to see whether they are on target and visually or manually assess their fat cover to see if they are at ideal condition scores. Particularly with the larger framed heifers born from the heavier weight cows, you may find you need to early wean those calves in more challenging seasons and supplement with high quality feed, such as silage or pellets, to ensure the heifer reaches the target joining weight, with adequate fat coverage.
To meet feed and nutritional requirements and for tips on monitoring condition of replacement heifers, use the information in the Beef Cattle Drought Feeding book, and the feed budgeting and calculator tools available on Agriculture Victoria’s Feeding Livestock website.