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0907 PD: The high value of low heritability

Nate Zwald Published on 31 August 2007

Editor’s note: The following benchmarks have been compiled using data reported by dairies enrolled in Alta Genetic’s AltaAdvantage program, a progeny testing program. More than 182,500 cows in 175 herds participate in the program nationwide.

Brutal fact: Heritability is often misunderstood. Among even some of the A.I. industry’s own people, heritability has been described as the probability that the resulting progeny will inherit a trait. Dairy producers not surprisingly push back on “low heritability traits,” saying things like, “We will make less progress,” or “We won’t make a noticeable difference,” or “It takes so many generations to actually change those traits.” It is worth the time to clear up the confusion.

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The traits most commonly referred to as “low heritability” are the health traits – Productive Life (PL), Daughter Pregnancy Rate (DPR), Somatic Cell Score (SCS). The most common misconception is that although the PTA for PL or DPR is high for particular bulls, the heritability is so low that progress will be slow and minimal in a single generation. It’s a myth that needs busting.

We need to make it clear what heritability means. We also need to explain the economic value of a trait is much more important than the heritability of a trait.

What is heritability?

Heritability is the proportion of the total amount of variation in a trait between groups due to genetics. Think about two cows in two different herds. How much of their milk production difference is due to genetics, and how much is due to management? It turns out that about 30 percent is due to genetics and 70 percent is due to management and environment. Therefore, milk has a heritability of 0.30.

What about the difference between pregnancy rates between herds? Management and environment account for the majority of variation between daughters (96 percent), so the influence of genetics is minor at 4 percent. Thus, DPR has a heritability of 0.04. But it would be wrong to conclude that DPR is insignificant as a result.

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Perspective is important

When we evaluate genetics, we need to remember that we do it within a herd. Within that herd, we also evaluate within a specific lactation group and within a specific time of freshening. What this means is that we reduce the impact of management and environment differences in our comparison. We effectively have more “control” over the evaluation to better isolate the “impact” of genetics. So, even though the overall heritability is low for a trait like DPR, within a given environment and management situation where all cows are managed exactly the same, the true differences between genetic lines is much clearer. The daughters are showing the differences their genetics make.

For proof of this point, refer to the August issue of Progressive Dairyman. In that issue, we benchmarked genetics within a herd to identify the performance of daughters sired by bulls with low or high health and production traits. Notwithstanding the low heritability, the differences were very significant and worth a lot in terms of better performance and higher profit.

The proof sheet & heritability

The genetic value reported in a sire’s proof, or the PTA calculation, already takes the heritability of a trait into account. This means the PTA value, for the low- and high-heritability traits, indicates the amount of gain (or loss) that can be made in a given generation. So, the PTA is the progress the dairyman can expect to make with his selection.

Think of a herd with a PG rate of 20 percent (140 average days open) and a ME305 average milk production of 25,000. If the dairyman selects AltaBLASTOFF (+3.0 DPR, +831 lbs milk) to create 100 AltaBLASTOFF daughters, then we expect those daughters on average to have a 23 percent pregnancy rate (and 128 days open) and 25,831 ME305.

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It isn’t just theory, either. With an extreme bull like AltaBLASTOFF, we expect more milk from cows that get pregnant faster compared to their herdmates. An analysis of DCOMP records at an AltaAdvantage® dairy that milks 1,800 cows proves the point. AltaBLASTOFF daughters get pregnant faster than herdmates. The daughter performance is better than average, with a daughter pregnancy rate 13 percent points better than contemporaries and more than 500 pounds more milk. This is obviously real progress on so-called low heritability traits.

Focus on economic value

The progressive dairy producer should not let confusion about heritability prevent them from using the genetic tools to make improvement in their herd. Health traits are economically important, and making improvement in these areas can have a huge economic impact.

For example, there are many traits that have a very high heritability, but no economic importance. In other words, we can make a lot of progress for these traits very quickly, but it will not make a more profitable cow. A couple of examples are coat color and stature. Coat color has a heritability of 100 percent; it is completely controlled by genetics. Another example is stature, which has heritability near 0.50. Even though we can make cows a lot bigger (or smaller) and “redder” in one generation, what is the economic value?

By comparison, progressive dairymen can see the economic value of more fertile cows, with fewer incidences of mastitis and calving difficulty or death, that last longer. These genetic features make a more profitable production unit.

Management tips for selection

Many progressive dairy producers want to make improvement, for example, in the areas of reproduction, udder health and calving ease. If you are one of those dairy producers, consider the following selection tips:

Define your goals

What are the most significant health problems you are dealing with on your dairy? You can start with identifying the most common reasons for culling in your herd. Is it reproduction, milk production, mastitis? This information gives you the basis to make breeding decisions that will have a positive impact.

Choose your tools

The development of health traits offers dairy producers some powerful tools to help correct for low reproduction, high incidences of dystocia and the like. Identify how important each is to you, and place a proportionate emphasis on these traits when choosing proven sires for your dairy.

Customize the solution

National indexes, like NM$ or TPI, place a default emphasis on health and other traits. NM$, for example, places 6 percent and 9 percent emphasis on calving ability (stillbirth and calving ease) and daughter pregnancy rate, respectively. So don’t assume the national indexes reflect your goals and needs, and work with your A.I. supplier to customize an index that works for you. Your situation may demand more emphasis than this. Many progressive producers have custom indexes created for their dairy, and they are very pleased with the results.

Demand accuracy

As with all sire proof information, the quality of the data going into the evaluation determine the quality of the results. In typical U.S. young sire testing programs there is an estimated 25 percent misidentification rate among sires and daughters. Accurate evaluations also depend on whether the daughters are tested in conditions that reflect those on your dairy, and on the number of herdmates they are compared to. PD

References omitted but are available upon request at

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