Giving fertility a boost

Written by Byrony Daniels, Beef Extension Officer, Department of Agriculture and Fisheries

Key messages

  • Both grazing and breeder body condition management are crucial for cows to calve every year.
  • Controlled mating, or seasonal mating, has many benefits including but not limited to more control over cow body condition, as cows are lactating when feed is at its best.
  • Genetics can have a large impact on the fertility of heifers and future replacement breeders. Select bulls with care.
  • Diseases, such as vibriosis, leptospirosis, pestivirus and trichomoniasis, need to be managed to achieve a calf per cow, per year.

To maximise breeder performance, we’re aiming for cows to conceive early in the mating period and for each cow to raise a weaner.

We need to be aware of a few biological realities.

There are 365 days in a year and the average Brahman pregnancy is 290 days. The time between calving and a cow’s first cycle is about 42 days. This leaves 33 days or 1.5 cycles to get cow back in calf if she is to calve every 365 days. We are asking a lot and many cows cannot do it: hence we get calving drift.

We need to manage both grazing, and the cows, to give them the best opportunity to get back in calf quickly.

Giving fertility a boost
Are you happy with your herd’s fertility? Could it be better?

Lack of breeder body condition is the most common cause of poor breeder performance. Table 1 shows the impact of breeder body condition on pregnancy rates.


Table 1 . Breeder body condition at the end of the dry season and expected pregnancy rate

Body condition scoreBody conditionExpected pregnancy rate next mating
1Poor 0-40%
4Forward store60-90%

When cows calve at the end of the dry season their energy requirements double, but feed quality is usually low. They face a period of weight loss and require reserves of body condition to carry them until the seasonal break occurs. Breeder management strategies are aimed at ensuring cows have adequate body condition at calving and minimising weight loss during lactation.

Strategies to minimise weight loss

Stocking rates and grazing management are the most critical issue.

If cows cannot consume their potential feed intake, they will always struggle to maintain condition.

Weaning is the most powerful tool after grazing management as it immediately reduces the cow’s energy requirements, providing an opportunity to recover condition.

Phosphorus deficiency reduces feed intake. Consequently, on deficient country phosphorus, supplements increase feed intake and improve body condition. During the dry season, protein supplements can reduce weight loss but will usually not stop it. Weight loss by lactating cows in the dry season can only be stopped by intake and costly energy supplements i.e. molasses, whole cottonseed, grain or protein meals.

Controlled mating

Seasonal/controlled mating has many advantages.

Cows that calve six to eight weeks before the seasonal break is likely to occur, do not face a long period of weight loss. Consequently, less supplements are required.

Management and labour activities are concentrated, and it’s easier to identify less productive cows (failing to conceive or wean a calf). If you can’t control mate, a good option is to foetal age at pregnancy testing and draft breeders into management groups. You’ll save on mustering, won’t be supplementing those who don’t need it and can more easily wean calves to manage body condition.


Bulls have the most influence on the genetic progress of the breeding herd because they sire many more calves in their lifetime than a cow can birth. We can improve fertility through bull selection if we understand the influence of genetics.

Bulls with better scrotal size estimated breeding values (EBVs) produce heifers that reach puberty earlier.

Good heifer conception rates depend on having heifers cycling at the start of mating. Be careful reading sale catalogues and data. The actual scrotal circumference measurements are not related to the fertility of heifer progeny. This data can only be used to ensure the bull has large enough testicle to meet the demands of mating.

A negative day to calving EBV (DTC) indicates a shorter interval from the start of joining to calving. Variation is mostly due to differences in the time taken for females to conceive after joining. Females with shorter DTC return to oestrus sooner and conceive quicker.

Sires have a large influence on post-partum anoestrus (the period after calving when cows do not cycle) in their first-calf daughters. It is most pronounced in first-calf cows. Some sires’ daughters will not cycle until late in the lactation or after weaning. This results in reduced conceptions and late conceptions. Sires with more negatives DTC EBVs produce daughters that cycle and conceive quicker after calving.

Some rules to remember when selecting bulls to drive herd fertility.

  • Use bulls with above average scrotal size (SS) EBV.
  • Use bulls with below average days to calving (DTC) EBV.
  • Use bulls that pass a full BBSE including above 70% normal sperm in a morphology test.
  • Look for bulls that passed a BBSE @ 600 days.
  • Avoid bulls from dams that missed at the first rebreed.
  • Use bulls out of dams that have had at least three calves in a row naturally.
  • Use balance in traits selection.

Disease management

Vibriosis causes embryonic loss and early abortions and is a common cause of poor fertility, with heifers the most affected. The disease is easily prevented with a bull vaccination program. Bulls require two vaccinations six weeks apart before their first joining and an annual booster. Problems are commonly caused by sale bulls not being vaccinated and annual boosters being missed.

Leptospirosis causes late abortions and weak calves. It is easily prevented with cow vaccination, which also protects people handling cattle from leptospirosis.

Pestivirus is also a common fertility problem that can be prevented with vaccination.

Trichomoniasis is a less common disease spread by bulls which has similar effects to vibriosis but no vaccine is available. Older bulls are more prone to carrying trichomoniasis and vibriosis so culling at seven is recommended and joining new bulls to heifers.

Read more about beef cattle fertility…

Seasonal mating → 

Nutritional management of breeders →

Managing cow reproduction →