As the spring bull sale season gathers momentum in northern NSW and Queensland, many bulls will be assessed by commercial producers and graded against a myriad of requirements.
Ideally, selection decisions are driven by profit-oriented breeding objectives and the need to improve the genetic composition of a herd to achieve a production outcome.
The ability to make selection decisions based on access to performance records has helped producers increase their confidence in their overall choices. However, there are some important caveats to consider in the decisions made.
One of the first steps is to objectively review a producer’s performance record of particular prefixes or sires. While many bull breeders will be offering animals with EBVs this year, not all producers will have demonstrated the same level of rigour in recording and submitting data for analysis.
While recording large amounts of data can improve accuracy, producers can also record just one or two traits, or even just submit a DNA sample, and still obtain reference values for their bulls being sold. The result is a set of reference values that can be used in a selection decision. It is important to note, however, that these reference values based on lower levels of recording will tend to have lower accuracy.
In practice, lower accuracy means that there is greater potential for EBV values to change as more data is collected, and so a bull that may meet a producer’s requirements at first glance can easily be eliminated as more is known about him and his genetics.
For producers looking to establish a shortlist of bulls to physically inspect, assessing accuracy can mean the difference between purchasing a bull that is a great fit and one that later turns out to be a marginal choice. An effective way to determine where to start looking for bulls is to look for breeders who are thorough in their registration and have bulls with greater accuracy, particularly for traits that are important to a specific breeding objective.
Many producers make their breeding decisions based on the heritability of traits and their correlation with other traits. Understanding what genetic correlations mean is fundamental to this process.
Perhaps even more important is to think about the importance of these correlations in a practical sense.
Genetic correlations
Genetic correlations describe how two traits, on average, tend to vary (or not) in common. This is expressed as a number between -1 and +1. Traits with a negative correlation describe cases where, when one trait increases, there is a corresponding decrease in the other, while positive correlations describe a common increase (or decrease) for both traits.
Correlations closer to the absolute number of -1 or +1 describe cases where a change in one trait is strongly associated with a change in the other, while lower numbers (closer to 0) essentially mean that for those pairs of traits there will be no pattern to how one changes relative to the other.
Dr. Matt Wolcott from the Armidale Animal Genetics and Breeding Unit provided insight into genetic correlations as part of a contribution to Bush Agribusiness’ recently released ‘Top Studs – 2024’ report.
Dr. Matt Wolcott
“Genetic correlations are estimated as part of the development underlying BreedPlan and are a key component of its multi-trait evaluation,” said Dr. Wolcott.
At the analytical level, genetic correlations help to maximize the information obtained from the recording made by seed breeders.
Dr. Wolcott notes that “for producers trying to make selection decisions in stockyards, trying to account for trait correlation is not only extremely complex, but also largely unnecessary.”
As part of Matt Wolcott’s contribution to “Top Studs,” he summarized average heritabilities and genetic correlations between traits in BreedPlan evaluations for temperate cattle breeds.
The figure below shows that genetic correlations tend to be moderate to weak, with the only strong relationships observed between certain weight traits or the two measures of fat depth (ribs and P8).
Mean (diagonal) heritabilities and genetic correlations between trait pairs in BreedPlan evaluations for temperate cattle breeds – Dr Matt Wolcott AGBU; Source: Bush Agribusiness, Top Studs 2024. Click on the graph for a larger view.
In practice, this means that while carcass weight, for example, is moderately heritable (meaning there is potential for improvement through selection), the genetic correlation with other aspects of carcass composition (rib and P8 fat, eye muscle area, retail beef yield and marbling) is at best moderate and not all in the same direction.
Dr Wolcott explains this by saying: “In this example, there are only moderate to weak genetic correlations between carcass weight and other aspects of carcass composition, and both negative and positive correlations can be observed between carcass characteristics in the BreedPlan evaluation for temperate cattle breeds.
“Therefore, relying on carcass weight EBV values to provide useful information about a bull’s genetics for other aspects of carcass composition will be, at best, ineffective.”
Critically, genetic correlations describe what is observed on average and they contribute most to estimating breeding value when records of correlated traits are rare or missing.
In cases where all traits of the selection objective have been recorded and included in the evaluation, the impact of genetic correlations on estimates of selection value becomes negligible.
Matt Wolcott’s advice to producers is to recognize that “accurate recording of correlated traits will always trump expectations about how traits vary in common, at the genetic level, as described by their genetic correlations.”
This means that a key question in assessing the suitability of a breeding operation as a source of bulls for your herd is: are they recording the traits that are important to my breeding goals?
Alastair Rayner is the General Manager of Extension and Operations at Cibo Labs and Director of RaynerAg. Alastair has over 28 years’ experience advising beef producers and livestock producers in Australia. You can contact him here or via his website www.raynerag.com.au