HERITABILITY is a commonly raised subject among producers seeking to improve their selection decisions. Often the term is used by people without truly understanding what it is describing, or how it is determined.

Without this basic understanding, making decisions that are more effective in improving the performance of cattle in a herd becomes more difficult.

In practice, bull breeders and commercial livestock producers seek to produce livestock that have superior performance over that displayed by their current breeding animals. Determining how great that improvement within those traits is likely to be is key to making the right choice at joining.

Quantitative traits

When discussing heritability, it is useful to understand that heritability is a measure of the relative importance of genetic and environmental influences on the development and expression of a specific, quantitative trait. Quantitative traits are those traits which are important for livestock production. These are the result of hundreds of thousands of genes. Importantly these traits are not expressed as a distinct form, but fit into a range.

Perhaps the best example of a quantitative trait is growth rate, which can vary from very low to very high. This is opposed to a single form, such as horns, which are controlled by major genes. In comparison to those traits, such as horns, quantitative traits are more influenced by environmental factors.

The measurement of heritability aims to describe the variability in a quantitative trait that is a result of genetics. The result is a ratio based on genetic variance to total variance among a population. The estimate is then provided as a measure between 0 and 1. Measurements that have a higher number indicate genetics have had a stronger influence on the expression of that trait.

In making selection decisions, the most rapid change within a herd occurs when decisions are made around major genes. The decision to breed for polledness, or to change coat colour for example can be implemented without having the complication of environment contributing to the physical (phenotype) appearance of the animals.

However, addressing and selecting for quantitative traits is a more complex undertaking due to the influence of the environment.

Many of the production traits that are of interest to producers, such as growth rate, weight at weaning, carcase and mature weights have high heritability. The practical expectation is that selecting bulls that have superior genetics for these traits in comparison to the cow herd, will result in an improvement in the subsequent progeny.

However there are also many other highly desirable quantitative traits that are of low heritability.  Many of these traits contribute to key profit drivers such as fertility in the cow herd.

A key risk with traits of low heritability can be either considering these traits of low priority or even a disinclination by producers to consider the genetics associated with those traits

A key risk with traits of low heritability can be either considering these traits of low priority or even a disinclination by producers to consider the genetics associated with those traits. This disinclination often occurs when producers assume that low heritability offers little scope for improvement, as the environment plays such a major role in the expression of the trait.

However, this approach overlooks one significant factor – that is, quantitative traits occur across a range, and are not expressed as a single form.

Dr Matt Wolcott

In discussions on this subject with Dr Matt Wolcott of AGBU in Armidale, he says it is important that producers remember that heritability is not the basis by which we should evaluate the potential value of an Estimated Breeding Value (or EPD).

“The critical number is the genetic variation present for the trait (ie, spread in EBVs) at the population level and (in particular) for selection candidates,” Dr Wolcott told me.

Making progress, in other words, is about understanding the range that exists within a breed for that trait. This can easily be seen from current EBV Breed Tables, where each trait is ranked by decile from highest to lowest.

Big variation in some traits

The variation in some traits can be significant.

Dr Wolcott highlighted the example of Days to calving within the Brahman breed.

“Days to Calving (DTC) is a lowly heritable trait (h2 ~ 0.05 – 0.15), but it has a range of -34 to +19 days in DTC EBVs in Brahmans.”

“This means that there is vast opportunity to improve the trait by selection regardless of its heritability,” he said.

If this trait was a key profit driver for Brahman breeders, focussing on those animals that were ranked above the breed average would offer the opportunity to start reducing the time taken between the introduction of a bull and the subsequent calving by the females he mates with.

Perhaps the key message for producers who are looking at herd bulls this year, is to remember that heritability of a trait shouldn’t be the deciding factor in whether to include it in a genetic improvement program.

If a producer has clear profit drivers that need to be addressed, then the economic value of even lowly heritable traits requires those traits be included and improved by genetic selection.

The opportunities to consider the range of variation for traits using EBVs allows producers much greater scope and opportunity to make these decisions, and ultimately move a herd towards greater levels of productivity.

Alastair Rayner is the Principal of RaynerAg, an agricultural advisory service based in NSW.  RaynerAg is affiliated with BJA Stock & Station Agents.  He regularly lists and sell cattle for clients as well attending bull sales to support client purchases.  Alastair provides pre-sale selections and classifications for seedstock producers in NSW, Qld, and Victoria.  He can be contacted here or through his website www.raynerag.com.au