Calving at two years of age: identifying and rearing suitable replacement suckler heifers

Chloe Higgins Mark McGee Martina Harrington David Kenny Alan Kelly

The current national average age at first-calving in suckler-bred heifers is 32 months with only 19% of heifers calving for the first time at 22-26 (~24) months of age. There is huge potential for improvement to 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. Delaying age at first-calving from 24 to 36 months of age decreased net margin per hectare by 12%, which is mainly attributed to additional feed requirements. 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

As part of the heifer selection process, farmers should clearly define their farming system, understand their target market, the type of animals that are required for the market and the cow traits that are required to produce the correct type of animal. Farmers should look at their current cow herd and decide if the herd is of adequate genetic merit for what they are trying to achieve, i.e. good enough Replacement Index or Terminal Index. Evidence from the ‘Maternal herd’ at Grange has highlighted that there are differences in dry matter intake and milk yield between cows sourced from the suckler herd compared to beef × dairy cows sourced from the dairy herd. These differences in milk yield can influence calf pre-weaning growth. While weaning efficiency, defined as calf weight as a proportion of cow weight when the calf is 200 days old, may be better in ‘first-cross’ dairy cows, the additional ~0.75 units of carcass conformation benefits in the progeny from the suckler-bred cow and aforementioned differences in feed intake are very important where a farmer is operating a calf-to-beef system.

Identifying suitable heifers for breeding

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

• Good legs and feet are paramount for longevity,
• Docile to maintain farm safety and improve labour efficiency,
• Four teats to indicate a normal udder,
• Without other visual issues that may impair her function over her lifetime,

Once the heifer is deemed functionally suitable for breeding, it is important to consider whether she has met her target weight-for-age, which underpins successful calving at 24 months (Table 1). In addition to ensuring that a heifer is sufficiently developed 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 passed on to her progeny. No heifer should be bred at less than 60% of her estimated mature weight at 14 months of age; failure to achieve this target will mean the heifer is less likely to have reached puberty. 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 these daughters. If the heifer 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 (a higher reliability gives greater confidence that the predicted phenotype will be displayed) of parent traits will give the most accurate assessment of the heifer’s phenotypic performance. Cow-herd averages for various traits should be considered when deciding which heifers to select as replacements. Replacement heifers should always have a better PTA compared to cows. Traits of interest should be improved by strategically selecting heifers that are superior in areas where the herd is poorest without negatively influencing other traits.

Rearing potential replacement heifers

The onset of puberty in heifers is dictated by genetics and early-life nutrition. The failure of replacement heifers to reach puberty by the beginning of the breeding season negatively impacts breeding policies and herd conception rates. Where replacements are reared on the farm (rather than purchased) several management practices to ensure good growth rates are achieved can be implemented including having a breeding policy to ensure cows have plenty of milk and implementing excellent grassland management to ensure cows can milk to their potential and that there is a 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. If pre-weaning average daily gain is poor, supplementing concentrates should be considered to maintain high growth rates. Research from Teagasc Grange has shown that Aberdeen Angus × 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 and Aberdeen Angus 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, respectively, the difference in age at puberty was only 13 days. Therefore, high live weight gain pre-weaning is critical to have heifers cycling at 15 months. In addition to the above, maintaining a high herd health status is very important to ensure consistently high live weight gain of the heifer to ensure that she meets her target weights (Table 1).

¹Early-maturing breeds and first-cross dairy heifers will be at the lower end of weight range, late-maturing breeds will be at the upper end of the weight range.

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. While after a 12-week breeding season the 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 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 in selecting an AI sire or stock bull for use on beef heifers is low calving difficulty (<8%), combined with a high reliability value (>80%) for this trait. Attention should also be given to the myostatin status of the bull. Bulls carrying disruptive myostatin mutations, such as NT or Q variants, should generally be avoided for use on first-calving heifers due to the increased risk of calving difficulties. In many herds, replacement heifers represent the most genetically advanced females available. Consequently, these animals can be strategically targeted for maternal breeding programmes aimed at producing future replacement females, either with conventional or sexed semen. Once a team of easy-calving bulls has been identified, farmers can then further refine their selection based on additional traits that best suit their production system.

Pre- and post-calving care of the replacement heifer

During their second winter, replacement heifers should be monitored closely and dosed and vaccinated as required to maintain good health status. Heifers should have a minimum body condition score (BCS) of 2.75 (scale 0-5) to ensure that heifers are ‘fit’ but not ‘fat’ pre-calving. If BCS is much lower than this, the heifer will be weaker at calving, have a poorer colostrum quantity and quality and will be slower to return to breeding. If BCS is too high, heifers are at a greater risk of calving difficulty, and subsequent rebreeding could be delayed. Ideally, first-calving heifers should be penned separately from mature cows to reduce bullying. Ensuring that heifers have enough feeding and lying space is also important. Provision of a suitable pre-calving mineral is essential to reduce the risk of dystocia, retained placenta, post-calving infections and metabolic disorders.

Following calving, first-calving heifers require high-quality feed to meet their energy demands. If housed indoors, they should receive high quality grass silage (>70% dry matter digestibility) supplemented with approximately 1.5–2.0 kg concentrates per day until turnout. Additional concentrates should be offered where silage quality is poor or where body condition is below target.  First-calvers should be turned out to grass as early as possible, ideally 4–6 weeks before the commencement of the breeding season. This will enable them regain body condition before breeding again to ensure high submission and conception rates early in the breeding season, and reduced calving-to-conception intervals.

Following weaning of her calf, the ‘first-calver’ usually has a lower BCS compared to mature cows. If the calf is of adequate weight, early-weaning should be considered to allow the heifer additional time to recover body condition 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 impact mature weight. The above impacts on mature weight can be prevented by high levels of technical efficiency in a suckler system.

Compiled and edited by Mark McGee and Paul Crosson, Teagasc, Grange Animal & Grassland Research and Innovation Centre, and first published in BEEF2026 – Driving Sustainable Performance, additional reading from BEEF2026 is available here.