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Major dairy breeds moving to composites for bull proofs

Use of genetic scores makes new model more responsive to changes in the dairy population

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Lactanet Canada will move to composite indexes for major genetic traits as early as April, 2021 as it aims for a more responsive system for classifying dairy cattle.

The change will affect breeders of Holstein, Ayrshire and Jersey cattle, while other dairy breeds will continue to be scored using the existing formula for combining descriptive and defective traits.

Why it matters: Lactanet leadership believes the change will create bull proofs that are more reflective of the current needs of dairy breeders.

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Lactanet Geneticist Allison Fleming, who presented about the change during the organization’s recent Open Industry Session series of online seminars, says one hold-up could be the planned transition by Holstein Canada – which administers the classification system through which the trait scores are determined – to two separate major genetic traits for feet and legs.

The major traits that will be composites are: mammary system, feet and legs, dairy strength and rump – with feet and legs expected, in keeping with the Holstein Canada direction, to be expanded in the coming months to foot and mobility.

Currently, scoring for each of these traits is based on combining classifier evaluations for 24 descriptive traits – with potentially two new descriptive traits (locomotion and front leg view) to be added under a mobility score.

Defective attributes identified by the classifier are also factored into the animal’s final major trait score and overall composite score.

Early in a bull’s lifetime, the evaluations from his daughters tend to be reflective of the bull’s strengths and weaknesses.

But as the breed evolves and as researchers learn about relationships between traits, adjustments are necessary in the weighting of different traits. And thanks to the multitude of daughter evaluations for older bulls, there can be significant lag time before fully understanding how he fits into the adjusted demands.

Composite indexes for the major traits go a long way toward decreasing that lag time, Fleming says.

Phenotypes continue to be the base

Physical observations by the classifier — the phenotype — will still be used as the basis for the composites. But instead of the current formula using scores assigned by the classifier, it will be genetic values for those traits —developed through generations and generations of research — which will be plugged into the new system.

Fleming notes there are still some things the Lactanet board will need to decide on, but the plan currently is to have this in place for bull proofs beginning April 2021.

Speaking to Farmtario, Fleming described the composites as “a different way to look at the scorecard traits,” and used the new scorecard trait of udder floor as an example.

Since December, 2020, classifiers have been providing udder floor evaluations — in keeping with research showing the shape and teat angle on the bottom of the udder affects the major mammary system overall trait.

Now, with enough of this data, Lactanet is able to calculate udder floor genetic evaluations for the bulls. Older bulls, for which the majority of the classification data was gathered prior to udder floor coming into use, wouldn’t have this reflected as accurately using classification scores under the existing system.

Only newer bulls would have udder floor reflected in their overall scores. But once the genetic values and the resulting composite indexes come into use, all bulls will be on a more equal footing.

For the dairy genetics sector in general, she added, “we’re going to be more able to make some adjustments if we see the need.”

Stature as an example

An example is stature in the Holstein breed. Over the generations, it has become clear that selecting in favour of some of the character traits linked to higher milk production has also led to increasing size — or stature — of Holstein cattle. This can result in problems with cow comfort, agility, and fitting into smaller stalls.

To counteract this, effort has been put into decreasing the effect on stature as a result of breeding for these other parameters. But that process has taken years because the proofs for bulls are based on potentially several years worth of daughter classifications —stretching back to when stature was not a concern.

With composite indexes, this type of adjustment will be achievable much more quickly, Fleming says.

Defective traits, which can negatively affect a cow’s classification and ultimately a bull’s rating, are not accounted for through a composite index. “With the composite, we don’t have a good way to put the defectives into the formula,” Fleming said.

But, depending on the trait, this isn’t necessarily a bad thing; for the most part, Fleming said. The defectives occur at low frequencies and their overall effect isn’t always well-understood. Less-desired parameters aren’t being completely disregarded, however. “On a bull’s detail page, we will still display the frequency of defectives.”

Composite indexes are already in use in the United States, as well as a number of other dairy nations.

About the author

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Stew Slater

Stew Slater operates a small dairy farm on 150 acres near St. Marys, Ont., and has been writing about rural and agricultural issues since 1999.

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