Polled Dairy Genetics: Facts and Fallacies
Dr. Chad Dechow recalls dehorning as one of the least-preferred-yet-regular tasks that routinely had to be done when he was growing up on a dairy farm. Now Dechow, who is Associate Professor of Dairy Genetics at Pennsylvania State University, is studying polled genetics as a potential means of possibly eliminating the task of dehorning forever.
The American Veterinary Medicine Association (AVMA) cites polled genetic selection as an alternative means of addressing the concerns surrounding pain and potential infection caused by dehorning and disbudding. Dehorning also has been raised as a primary concern of consumer groups. While the AVMA suggests sedation, cauterization, anesthesia and analgesia all are measures that can mitigate the drawbacks of dehorning, selecting for polled offspring can negate the need for dehorning altogether.
Still, breeding for polled dairy animals has a long way to go in the United States. Via a recent webinar sponsored by the Dairy Cattle Welfare Council, Dechow explained that polledness is a dominant autosomal trait, meaning that all offspring of homozygous (PP) sires will be polled. For sires heterozygous for the polled trait (Pp), half of the offspring will be polled from dams that have no polled genes (pp), and three-fourths will be polled if the dams are heterozygous (Pp) for polled. So far, the availability of either homozygous or heterozygous sires in the U.S. artificial insemination (AI) system is fairly limited.
Evaluating data for AI sires in the Holstein, Jersey and Brown Swiss breeds from 2008 through 2018, the number of polled bulls has held steady at about 5%. Interest in, and availability of, polled sires peaked slightly for all three breeds around 2013, but has fallen again since.
Dechow said the industry perception regarding polled genetics has generally been that selecting for the polled trait will cause other performance factors to suffer. “Both registered and commercial breeders generally do not seem to be opposed to polled genetics, but they will not breed for them if they perceive they will be chosen at the expense of other traits like reproduction and milk production,” he shared.
But in a statistical comparison between the Net Merit of horned and polled (either homozygous or heterozygous), Dechow found that concern to be only partially valid. “It is true that the Net Merit of polled sires across all three breeds showed about a 20-30% lag compared to horned animals,” he stated. “But the rate of genetic advancement was virtually identical between the two groups. So while there is an initial sacrifice by choosing polled genetics, producers should not be afraid that they will fall further and further behind with each generation by doing so.”
Other factors that may be holding back the prevalence of polled sires are:
The economic incentive for creating polled genetics is fairly small – about $6.00 in the Dairy Wellness Profit Index (DWP$) for heterozygous polled, and $12.00 DWP$ for homozygous polled.
In terms of commercial herd management, having some polled calves and some horned is not all that appealing. Because disbudding often is done before the presence or absence of horns is clearly evident, many managers will simply process them all the same, creating no real management incentive for polled genetics.
There is concern for inbreeding, which Dechow said is valid, but likely would not occur to a higher degree than already is happening in U.S. dairy breeds.
Finally, and perhaps most concerning, there are global biodynamic groups that are promoting the concept of horns being left intact for cattle to live in their most natural state.
Within a single breed, Dechow said a herd could achieve 75-80% polled offspring over the course of 5-6 generations (10-15 years) by using mostly heterozygous polled sires. That rate could be increased rapidly via gene editing, although that technology has not yet been approved for commercial application. Dechow also noted that the same class of consumers concerned about dehorning also would likely be resistant to gene editing.
Another alternative is crossbreeding. The Norwegian Red breed, for example, has a much higher frequency of polled animals than any of the major U.S. breeds. Dechow suggested that a three-way cross made up of HolsteinXBrownSwissXRed breeds (Norwegian Red, Normandy, Swedish Red, Montbeliarde) could rapidly accelerate polled prevalence while concurrently advancing a number of other positive traits, such as fertility.
“If there are enough polled bulls in a single breed from which to choose, the trait would grow rather quickly, with little impact on long-term genetic merit,” said Dechow. “However, there is an initial investment in intentionally focusing on polled genetics.”
And, in the absence of an organized effort to advance polled genetics, it could happen organically if an outstanding sire emerges who just happens to be polled. “One really high-indexing polled bull could change the picture fairly quickly,” said Dechow.
Sat, 02/09/2019 – 11:37
Shown are two of the 15 Holsteins that were born at Penn State’s Dairy Barns in 2017, the result of a research project to reintroduce valuable genetic variance to the breed.
Source: Dairy Herd