Maximising live calves in your breeding herd

Reproductive performance is major driver of profitability in a beef cow/calf enterprise.

The key factors that affect reproductive performance are:

  • the growth path of heifers from weaning to the onset of puberty
  • heifers achieving critical mating weight at joining
  • heifers and mature cows getting back in calf after their first calf to maintain a 365-day calving interval
  • the proportion of breeders conceiving by the end of the second cycle in the joining period.

Onset of puberty

The point in time at which heifers reach reproductive maturity is controlled mainly by nutrition and genetics. Bodyweight play a greater role in determining the onset of puberty than just age alone. A set of scales and a good eye for body condition is important.

Puberty occurs when heifers reach about 52 per cent of mature body weight. For British beef breeds, this is between 265 – 280 kg.

Joining weights

Critical mating weight (CMW) is the weight at which 85 per cent of heifers are expected to conceive within a six-week joining period. Determining the CMW for your replacement heifers and managing nutrition to achieve this weight ensures that the majority of heifers will be cycling and fully fertile when exposed to a bull. CMW is generally 65 per cent of the typical adult weight of the cow herd. Where 85 per cent or better of heifers conceive in the six-week joining period, a more desirable calving pattern will be achieved, ensuring a more even line calves and, most importantly for the calved heifer, a better chance of returning to oestrus to maintain a 365-day calving interval.  For British breeds the CMW is between 280 – 300 kg. For European, Bos Indicus and larger framed cattle, the CMW is higher.

Post-Partum Interval

The post-partum interval (PPI) is the time from calving to conception. This is ideally about 80 – 85 days after calving.

To achieve the same calving period every year (i.e., a 365-day calving interval), breeders must get back into calf within this period. This puts considerable nutritional demand on the animal, particularly the first calf heifer. First calf heifers represent one of the biggest challenges for a beef producer in maintaining reproductive efficiency.

If this 365-day calving interval is not maintained, over time the mean calving date will become later and as a result the overall calving period will change. As more late calves are born, more cows will fail to join on the next joining resulting in empty breeders. Late drop calves have less time to grow out to weaning, and the period of greatest nutritional demand moves further away from when the pastures are most productive.

There is strong evidence that the body condition score of cows at calving is strongly linked to the PPI. Increases in condition score during late pregnancy through the provision of good nutrition, particularly energy, can reduce the interval between calving and first oestrus for all cows except those in good condition where it has no effect. A similar outcome can be achieved through the provision of good nutrition during early lactation.

To maintain a strict 365-day calving interval, cows that fail to deliver a calf in this interval should be culled.

Culling more mature cows will require retaining more heifers as replacements. More young growing stock puts greater pressure on feed resources. The benefits of higher heifer retention rates do, however, far outweigh the perceived downside of having to allocate quality feed to breeders.

Nutrition is key

Adequate nutrition (principally energy) over and above maintenance and growth requirements is required to drive the onset of oestrus, both in young growing heifers and older cows. Older cattle that have no growth requirements can allocate more energy to reproductive processes than rapidly growing heifers, who have high energy and protein requirements for growth.

The energy intake for pregnant heifers must always be adequate to maintain growth for herself, for the developing reproductive system, and the bone structure, particularly the pelvis to avoid dystocia issues. This requirement has been quoted as being around 40 MJ for maintenance and 34 MJ for each kilogram of body weight gained by the heifer in the growing stage to the point of calving. The developing foetus requires an extra 5 MJ/day for the first five months pregnancy, 8 MJ/day at six months pregnancy, 11 MJ/day at seven months, 15 MJ/day at eight months and 20 MJ/day at nine months pregnancy.

The effects of nutrition on reproductive efficiency in older cows is much less apparent. This can then allow re-allocation higher quality feed to younger cattle.

Key considerations for first calf heifers:

  • their PPI on average is higher than mature cows
  • prioritise the higher quality feed resources to first calf heifers. They are the priority over older cows
  • all breeders should be joined on a rising plane of nutrition
  • if possible, heifers should be joined two – three weeks before the main herd, as this allows an additional two – three weeks for them to cycle post calving and to slot into the main herd to maintain a tight calving pattern.

Setting up the replacement heifer to be a high performing member of the breeder herd starts when she is weaned and ensuring that nutrition is adequate to support a growth path to meet minimum liveweight targets for age.

For more information on feed budgeting, feed values and other helpful tools and calculators, visit the Feeding Livestock website. For additional information on managing the beef breeder herd to improve reproductive efficiency and enterprise profitability visit the Meat and Livestock Australia website.


Author: Cathy Mulligan, Livestock Industry Development Officer

Page last updated: 24 Jan 2024