MATURE cow weight is a consistent theme of discussion among seedstock and commercial beef producers.

Several recent Beef Central genetics columns have looked at implications associated with mature size, ranging from concerns associated with increased feed demands and fertility implications through to impacts on profitability. The emphasis from these articles is that larger cows require more feed and infrastructure, and they may not yield proportional gains in productivity.

While these concerns are well recognised in industry commentary, a recently published study in the Journal of Animal Science has presented additional information identifying the genetic and management factors driving these trends.

The research analysed more than 650,000 records of mature cow traits from Angus herds in both the US and Australia. The study team included Ayooluwa O. Ojo, Gabriel S. Campos, Henrique A. Mulim, Allan P. Schinckel, and Hinayah Oliveira from the Department of Animal Sciences at Purdue University, Indiana; Andre Garcia and Kelli Retallick-Riley from Angus Genetics Inc, American Angus Association, Missouri; and Australians, Christian Duff, formerly of Angus Australia and now with ABRI; and Stephen Miller from the Animal Genetics and Breeding Unit.

The study explored multiple factors influencing mature cow size. While rising input costs, particularly feed, were a key motivator, two additional aims were to:

  • Assess the limited use of Body Condition Score (BCS) in genetic evaluation, particularly in the US
  • Evaluate the feasibility of joint genetic evaluations between Australia and the US.

The study utilised extensive datasets from the American Angus Association and Angus Australia, enabling analysis on a significant scale.

Heritabilities

Heritability estimates for mature cow weight (MWT) and mature height (MHT) were found to be moderate to high in both countries, indicating strong genetic influence and potential for genetic progress through selection.

Estimated heritabilities were: MWT – 0.45 (U.S.) and 0.40 (Australia); MHT – 0.57 (U.S.) and 0.63 (Australia). In contrast, body condition score (BCS) showed lower heritability at 0.18 in both populations, suggesting greater environmental influence and limited potential for change through selection alone.

Repeatability for MWT and MHT was high (0.68–0.77), meaning these traits remain consistent across measurements and need fewer records per animal. In contrast, BCS repeatability was lower (0.31–0.35), highlighting greater variability and the need for more frequent, standardised measurements to improve reliability.

The study found MHT and BCS to be only weakly correlated (0.11 in the US) and not significantly correlated in Australia, confirming that body condition is largely independent of height. MWT and BCS had a modest correlation, meaning heavier cows may hold condition slightly better, but not consistently.

It was proven that MWT and MHT were moderately to strongly correlated with early growth traits, reinforcing that selection for growth (eg, Weaning Weight (WW) or Yearling Weight (YW)) will often increase mature size unless directly managed. BCS had low correlations with growth traits and must therefore be treated as a separate selection trait.

In addition, MWT and MHT were negatively correlated with fat traits such as rib fat and intramuscular fat. BCS showed weak positive correlations with these traits. These results indicate that ultrasound fat measures are not reliable proxies for body condition in selection decisions.

In accessing data from both the US and Australia, it was possible to consider the genetic correlations between the same traits as measured in both countries. It found that the correlation for Mature Height was 0.98; Mature Cow Weight was 0.91 and Body Condition Score 0.65. These high values, particularly for weight and height, support the feasibility of joint genetic evaluations. Shared sire lines and comparable trait definitions will further strengthen this potential.

The moderate correlation for Body Condition Score may reflect genotype-by-environment interaction, likely due to differences in feeding systems (eg, pasture vs. supplementation) and scoring scales. However, the correlation is high enough to justify inclusion in a joint framework if scoring methods are standardised.

While some of these findings re-enforce previous studies, it is valuable to have them reinforced, and at a production level include them as part of an approach to selection decisions. Producers can have greater confidence in using mature cow weight EBVs when selecting sires.

Breeders can constrain mature size while maintaining early growth

Given the trait’s heritability, breeders can constrain mature size while maintaining early growth, provided they monitor both traits during selection.

The research also highlights the importance of monitoring early growth trait selection (eg, YW, WW), as these traits are genetically tied to mature size and can inadvertently increase cow weight if not balanced.  It is also recommended, particularly for seedstock breeders to record cow weights and condition at or near weaning. Even one or two records can improve prediction accuracy for weight and height.

Beef producers should treat Body Condition Score (BCS) as a separate trait. Its weak association with growth and fat traits means direct measurement is necessary. As such, producers shouldn’t assume that condition will track with weight or scanning data.

This most recent study provides solid genetic evidence that mature cow size is under control of heritable traits and is closely linked to early growth. While selection has often favoured growth, this approach brings with it an increase in mature size, unless size is actively managed in breeding decisions.

The findings also show that body condition is a distinct and more environmentally sensitive trait, requiring its own focus in genetic programs.

Selection remains a powerful tool to manage mature size, provided producers also monitor early growth traits and avoid unintentional increases in cow weight. Body condition, being more environmentally sensitive, must be measured directly. With feed costs continuing to rise, using existing data to align cow size with environment and inputs is both timely and essential.

 

Alastair Rayner

Alastair Rayner is Principal of RaynerAg and an Extension & Engagement Consultant with the Agricultural Business Research Institute (ABRI). He has over 28 years’ experience advising beef producers and graziers across Australia. Alastair can be contacted here or through his website: www.raynerag.com.au