|
Back to Main News |  Print This Page E-Mail this Page
|
News |
We can Breed Healthier Cows
by Nate Zwald
Today, keeping your cows healthy is likely more difficult than getting your healthy cows to milk. Some farmers have taken note of this fact and are looking for bulls that sire healthier cows for the long haul instead of extremely high-producing cows for the short run. We now have tools to help them and they’re found in the Net Merit index.
During the past 10 years, four new traits that directly measure health and longevity have been introduced by the geneticists at the United States Department of Agriculture’s Animal Improvement Laboratory (USDA-AIPL) in Beltsville, Md. They include productive life, somatic cell score, daughter pregnancy rate, and maternal calving ease. A fifth, sire calving ease, has been available for some time.
|
All these traits have been incorporated into Net Merit, and they lead to healthier, long-lived cows. However the combined weight of these five traits only accounts for 31 percent of the index when looking at Holsteins. Even if we include the conformation traits (udder composite and feet & legs composite) as an indirect indicator of health, we still only accounted for 45 percent of the index. This leaves over half of the index still determined by production. Yet, we have made progress. Net Merit is trending away from production and more towards functional traits. Back in 1993, the index was 100 percent production. |
If you’re not using Net Merit or selecting for health traits, you may be asking the question: can we really make a difference selecting for these traits that have less accuracy (lower reliability) when the actual difference (phenotype) between cows is less genetic and more management (ie lower heritability)? We can look two places for answers to these questions.
What’s been done abroad…
In Scandinavian countries, clinical mastitis, SCS, and other health disorders have been part of the selection index for nearly 20 years. This selection pressure really shows when looking at average SCC of cows in Scandinavian countries. The average bulk tank SCC in these countries is less than 200 cells per milliliter, while the average in the U.S. is around 320 cells. Although these countries have not made as much genetic progress for milk production compared to their U.S. counterparts, they have maintained a positive genetic trend while boosting the overall health of their herd.
The past 10 years in the U.S. ...
Let’s take a look back to 1993. If we had used the same Net Merit formula that we use today, we would have more genes in the population from Holstein bulls known for their health traits such as Leader, Duster, and Rudolph, and fewer genes from bulls known for negative health traits such as Mascot, Luke, and Michael. The average daughter of these “health-trait” sires lives 160 days longer, gets bred back 16 days earlier, and is a half a point lower for SCS than the second group of “low-health” sires.
So how much weight should health traits receive in your selection program? This question comes with controversy because many of the early release bulls have low reliabilities for these traits. Therefore, much of the information is based on parent averages instead of actual daughter performance. While we certainly get less variation in proofs when reliability increases, we cannot ignore the differences in low-reliability sires. To put it another way – it is a good bet that a group of five bulls that rank high for health traits but have low reliabilities will have healthier daughters than a group of five bulls that rank poorly for health traits.
A common misconception about health traits is that since the heritabilities of these traits are so low, genetic progress will be limited and therefore, these traits are not worth worrying about in a selection program. It is true that a large portion of the variation in health and longevity between cows is due to management and environmental factors, but why ignore the genetic differences?
|
How a farm manages and cares for its cows is updated and adjusted throughout the cow’s life. However, genetic change is permanent. It doesn’t change from the time the sperm hits the egg and is passed on through generations. That begs the question… If a certain management change was going to permanently make your average cow last a couple months longer, and/or breed back a few days sooner and only took a few minutes of your time, wouldn’t you implement it? Of course you would. And it certainly doesn’t take long to select a sire that improves health traits. |
|
Breeding longer-lived cows...
So, which currently available bulls will sire healthier cows in the next generation? The table below provides a synopsis of the best options among the current group of active sires. For this listing, the Net Merit index was divided into “sub-indexes” for production, conformation, and health.
- The “health” index includes productive life (PL), somatic cell score (SCS), daughter pregnancy rate (DPR), sire calving ease (SCE), and maternal calving ease (MCE).
- The “health and conformation” index includes all the health traits plus udder composite (UDC), feet & legs composite (FLC), and body size composite.
The listings give a sire the same amount of credit for each of the traits as does the Net Merit index. For example, the top bull, Event, gets 259 of his NM$ from health traits, and additional 70 from his conformation traits, which gives him 329 for the health and conformation index. Since his NM$ is $512, this means that he gets 183 points from production (512-329).
For reference, the “health-trait” sires (Rudolph, Duster, and Leader) mentioned earlier average 158 points on the health index and 192 points on the health and conformation index, compared to the group of low-ranking, negative health trait sires (Mascot, Michael, and Luke) which average -152 and -165 respectively on the two indexes. This shows there is certainly a difference between sires. And your herd cannot afford to ignore these traits.
Although health traits are gaining greater importance over time, currently the best index to measure overall profitability in a commercial environment is Net Merit. However, the fastest genetic progress for health and fitness traits alone will be made by selecting sires from one of these indexes, and if that is your true selection goal than these sires may be just what your looking for when breeding your herd.
Posted Jun 23th
