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Identifying and rearing suitable replacement heifers to calve at two years

Identifying and rearing suitable replacement heifers to calve at two years

The current national average age at first calving in suckler-bred heifers is 32 months, with only 23% of heifers calving for the first time at 24 months of age.

When compared to 74% of dairy-bred heifers calving for the first time at 24 months of age, there is huge potential for improvement in this important reproductive key performance indicator.

The top 10% of commercial suckler herds and research herds are consistently calving heifers at 24 months of age. Work from Teagasc Grange has highlighted that a heifer calving for the first time at 36 months of age consumes 65% more grass, 96% more silage and 33% more concentrates than an equivalent heifer calving at 24 months of age. Calving at two years of age will increase farm productivity, but careful planning and a high level of technical efficiency is required to ensure consistent success.

Cow type and sire genetics

Selecting suitable replacement heifers should begin with a visual assessment to ensure that the heifer has all of the necessary functional traits to make a suckler cow. The following visual traits should be assessed as a minimum:

  1. Good legs and feet are paramount for longevity;
  2. Docile, so as to maintain farm safety and improve labour efficiency;
  3. Four teats to indicate a normal udder.

Once the heifer is deemed functionally suitable for breeding, it is important to consider has she met her target weight for age. Weight-for-age targets are outlined in Table 1; these targets underpin successful calving at 24 months. In addition to ensuring that a heifer is well enough grown and capable of conception by 15 months of age and calving at 24 months of age, the ability of a heifer to meet these targets will also be transmitted to her progeny. Therefore, failure to achieve weight targets should make a farmer question whether the heifer is suitable as a replacement animal.

No heifer should be bred at less than 60% of mature weight at 14 months of age; failure to achieve this target will normally mean the heifer is less likely to reach puberty. Mature cow weight can be defined as the average weight of cows in their 4th+ parity when the calf is 200 days old. Weight ranges are outlined in Table 1. Once bred, it is important to remember that the heifer must continue to grow and some preferential management should be considered if heifers are underperforming.

The 'Euro-star' Replacement Index is a useful tool for determining the potential of a cow’s daughters as a suckler cow and the potential beef performance of the progeny from those daughters. If the heifer identified as a potential replacement resides in a herd registered with ICBF HerdPlus, this information is easily accessible through the 'Beef EuroStar' report and farmers should assess her traits and, where possible, her parents traits.

Assessing the predicted transmitting ability (PTA) and the reliability of parent traits will give the most accurate assessment of the heifer’s phenotypic performance. For any trait, a higher reliability gives greater confidence that that the predicted phenotype will be displayed. Farmer’s should take their cow herd averages for various maternal traits such as milk and reproduction into account when deciding which heifers to select as replacements. Replacement heifers should always have a better PTA compared to cows and farmers should try to improve traits of interest by strategically selecting heifers that improve on areas where the herd is poorest without negatively influencing other traits.

Rearing potential replacement heifers

The onset of puberty in heifers is dictated by the animal’s genetics and early-life nutrition. Failure of replacement heifers to reach puberty by the beginning of the breeding season negatively impacts breeding policies and herd conception rates. At farm level, if a farmer is rearing his own replacements, they can implement a number of management practices to ensure good growth rates are achieved. Have a breeding policy to ensure cows have plenty of milk, implement excellent grassland management to ensure that cows can milk to their potential and that there is sufficient high-quality grass available to the heifer calf when she begins to graze.

Post-weaning, it is important to ensure sufficient availability of high nutritive value feed. Ensuring that only high-milk producing dams to breed replacements will also give the best chance of increasing pre-weaning growth rate. If average daily gain (ADG) is poor, supplementing concentrates should be considered to maintain high growth rates. If heifers are sourced from the dairy herd, there will normally be more opportunity to intervene with concentrates. Research from Teagasc Grange has shown that Aberdeen Angus (AA) x Friesian heifers fed to grow at 1.2 kg/day from four-to-eight months of age reached puberty 70 days earlier than heifers fed to grow at 0.6 kg/day. In contrast, when Limousin (LM) - and AA-sired suckler-bred heifers were fed a high or low plane of nutrition after eight months of age, to grow at 1.0 kg/day or 0.6 kg/day, the difference in age at puberty was only 13 days. Therefore, high ADG pre-weaning is critical to have heifers cycling at 15 months. In addition to the above, maintaining a high health status in the herd is very important to ensure that a consistently high ADG of the heifer to ensure that she meets her weight for age targets, outlined below.

Table 1: Growth targets for replacement heifers calving at 24 months




Live weight gain


Target live weight 


How is this achived?
Birth 0   45  
Weaning/housing 8 1.1 275-300
  • Good grazing management
  • High milk in cows
Turnout 12 0.6 335-375
  • Good quality silage and meal
Breeding 14 1.0 380-420
  • 60% of mature body weight
  • Early turnout
Housing 2nd winter 20 0.8 540-570
  • Good grazing management
Calving 24   550-590
  • 80% of mature body weight
  • In correct body condition
Birth to first calving ADG (kg)   0.7    

An important outcome from the research above was highlighting that at 15 months of age, 80% of heifers that had grown at 1.2 kg/day were pubertal compared to only 40% being pubertal after growing at 0.6 kg/day between four-and-eight months of age. Although after a 12-week breeding season, conception rates are normally comparable in this situation, the six and eight-week conception rates will be approximately 12 and 13% better for heifers that were already pubertal at the beginning of the breeding season. Research from both the USA and Canada has shown that heifers calving in the first 21 days of their first calving season had increased longevity in the herd (based on pregnancy rates) and weaned one more calf in their lifetime, compared with heifers that calved after 21 days. Therefore, developing heifers to conceive early in the breeding season is critical for heifer longevity in the herd.

Suitable sire selection

Irrespective of breed, the most important trait that a prospective bull should have is low beef heifer calving difficulty (<8%) with an associated high reliability for this trait. Once satisfied that the potential bulls for use are easy-calving, then the farmer can shorten the list of bulls for use based on the traits that fit their system best.

Post-calving care of the replacement heifer

After calving, heifers should be given good ‘quality’ feed to help them meet their energy demands. If housed indoors, they should be given high-quality grass silage (>70% dry matter digestibility), and concentrates should be considered if silage is of poor quality or if BCS is very low. First-calvers should be turned out to grass as early as possible to give them a chance to ‘build’ condition before breeding again. Following the weaning of her first calf, the ‘heifer’ usually has a lower BCS compared to mature cows. If the calf is of adequate weight, early-weaning should be considered so that the heifer is given additional time to recover before her next calving.

Cow mature weight

Beef cattle can take up to four years to reach their mature weight. There is no scientific evidence that calving at 24 months of age will reduce the mature weight of a cow. As with all animals, any setback due to poor health, restricted nutrition or anything else that reduces growth may negatively impact mature weight. The above impacts on mature weight can be prevented by high levels of technical efficiency in a suckler system.

This paper by Colin Byrne, Mark McGee, Martina Harrington, Alan Kelly and David Kenny was first published in the Beef 2022 ‘Supporting Sustainable Beef Farming’ open day book.