Selection indexes work

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Selection indexes can remove some of the complexity out of using expected progeny differences (EPDs) for bull selection by boiling an animal’s genetic potential down to a single number, typically related to economics. And beyond their convenience, use of indexes can facilitate significant genetic progress on several traits at the same time within a herd or a breed, says University of Guelph animal scientist Steve Miller, PhD.

Miller spoke to a group of ranchers at a recent seminar preceding the Colorado State University and Leachman Cattle of Colorado bull sales. Most major breed associations and some individual seedstock producers have developed and offer selection indexes, but they are a relatively new tool for the beef industry. Breeders of other livestock however, have used them for some time.

Miller showed the audience a familiar photo of broiler chicken carcasses, comparing the type of chickens raised in the 1950s with those in production today. Modern chickens are dramatically larger and meatier at every age compared with 1950s chickens. In fact, they go to market in one-third the time and require one-third the feed to reach market weights. Selection indexes, Miller says, allowed that progress.

He also provided examples from the dairy industry, noting that past selection in dairy cattle focused on milk production to the detriment of some other traits. Breeders then began using selection indexes that include EPDs for longevity, health, fertility and other traits along with milk production. Use of a multi-trait index has resulted in improved cow longevity, lower somatic-cell counts and better fertility while simultaneously improving milk production.

CSU animal scientist Milt Thomas, PhD, also spoke about indexes at the conference. He stressed that indexes facilitate multi-trait selection, allowing positive progress in several traits, even when some of those traits typically conflict with each other. Birth weight for example, tends to relate to yearling weight, making it difficult to find bulls with EPDs for low birth weight and heavy yearling weight. An index however, can help balance the positives and negatives and identify bulls that are at least acceptable for both traits.

In recent years researchers and breeders have measured growing numbers of cattle for individual feed intake and developed EPDs for feed-efficiency traits such as residual feed intake, average daily gain and feed per gain. By incorporating those EPDs into an index, a breeder can identify animals that excel in the usual growth and carcass traits while consuming less feed than comparable animals.

CSU operates a feed-intake unit that uses electronic identification and scale-equipped feeders to precisely measure feed intake on individual cattle. This type of system, using actual feed intake and weight gains to determine feed efficiency is the best way to measure the trait. Eventually Thomas says, genomic predictions will play a bigger role as scientists correlate the DNA markers with actual feed-efficiency data. CSU’s facility, he notes, can test a maximum of about 600 bulls each year, while genomic testing could potentially rate any bull.

A major research project, the National Program for Genetic Improvement in Feed Efficiency in Beef Cattle involves several universities and multiple breeds and aims to help producers incorporate selection for feed intake into their multi-trait selection strategies.

Indexes provide effective and efficient selection tools, but experts advise producers to study any index and understand its goals before using it to guide their selection. Some indexes, for example, focus almost exclusively on post-weaning gain and carcass traits, making them good for selecting bulls for terminal crosses, but not for bulls intended to sire replacement heifers.


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anonymous    
April, 01, 2013 at 11:18 AM

Two caveats on using the indexes: 1) Individuals with the same index can vary tremendously on individual traits, especially important ones that are not part of the index. 2) Almost all of the indexes apply to producers retaining ownership at least through finishing. Those marketing at or shortly after weaning should ignore most of the indexes, unless they can effectively market any genetic advantage for post weaning traits.

John Maday    
Colorado  |  April, 01, 2013 at 11:51 AM

Good points. Bull buyers need to understand what an index includes, and focus on the traits that pay based on their management and marketing plans. Our upcoming April magazine has a feature article that explores the use of selection indexes in more depth.


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