Estimated breeding values aid sheep selectionWritten by Natasha Wheeler
“Estimated breeding values (EBV) are a tool that we can use to do a better job of making sheep that will work for our customers,” stated Professor Emeritus Dave Notter with the Department of Animal and Poultry Sciences at Virginia Tech.
EBVs provide producers with comparative data to use for sheep breeding selections, and the National Sheep Improvement Program (NSIP) reports data for a variety of traits.
“Body weight traits have been the key element of genetic evaluation in the sheep industry since the very beginning of genetic evaluation programs,” noted Notter.
The most common of these include birth weight, weaning weight, post-weaning weight and yearling weight. Sheep producers are also interested in hogget weight, which is a term used in Australia and New Zealand to refer to animals that are about 18 months of age.
“If we start recording yearling weights and hogget weights more regularly, it gives us a way to see what the adult weights of the ewes are likely to be,” he explained.
An ideal growth curve, he said, would begin with a lamb born at a moderate birth weight so that the lamb is big enough to get up, nurse and thrive but not too big for the mother to deliver.
“That lamb needs to grow like a house on fire until sale time, either as a feeder or a finished lamb. For the industry, he needs to keep growing until the day he goes to market,” he commented.
He continued, “A ewe needs to be big enough to breed at seven to eight months of age, and she needs to get plenty big enough to raise her first lamb. Then, growth needs to flatten off so adult maintenance costs stay low, condition is maintained and the animal can thrive on pasture and range.”
Unlike cattle, large birth weights are not a common problem in sheep. Notter shared data illustrating that underweight lambs, or those less than about 6.5 pounds, have a reduced chance of survival compared to larger lambs.
“Birth weight EBVs are something we don’t really need to pay a lot of attention to unless we are getting a lot of underweight lambs that are dying or we have some really huge lambs that have trouble getting out,” he explained.
Weaning weight, yearling weight and hogget weight EBVs, he argued, are likely more relevant to selecting for an optimum growth curve.
Lambs born, weaned
“The number of lambs born and number of lambs weaned are traits that are extremely important economically,” Notter added.
EBVs for these two traits are something that NSIP has placed a lot of emphasis on.
“We want to optimize, rather than maximize, those EBVs,” he commented. “If we want to wean a 200 percent lamb crop, which we talk about a lot in the sheep industry, we have to drop at least 2.25 lambs per ewe lambing.”
Data shows that the frequency of triplets born goes up significantly as the number of ewes having twins gets higher than 65 percent.
“We can have triplets. We just have to keep them alive,” he said.
Fecal egg count EBVs are another trait that producers can consider when selecting for the flock.
“The fecal egg count EBV is used to select for parasite resistance and is being used almost exclusively for Katahdin. It is increasingly being used for other breeds as well,” Notter stated.
Data collection is somewhat challenging, as fecal samples must be collected from the rectum of lambs at times when parasites are present in the environment, but for producers who are interested in parasite resistance, he believes that further developing the EBV will provide a useful tool.
“Moving on to some other traits, greasy fleece weights, fiber diameter and staple lengths are standard EBVs in NSIP,” Notter continued.
Ultrasound fat and muscle depths are also standard EBVs, and NSIP uses a number of indexes to optimize trait combinations.
“Wool production, separate from meat production, is not in the cards anymore. Animals have to be multi-purpose producers of lamb and wool,” he commented.
By using indexes, or mathematical equations based on certain trait values, emphasis can be placed on specific EBVs to meet optimum criteria for a given objective.
“Indexes are not static. They are dynamic,” Notter explained, saying that they occasionally need to be reworked to fit the trait changes seen across breeds through newer generations.
“If we use our EBVs in our selection program, genetic change will occur. Some traits deserve emphasis only when there is an opportunity or a problem. Otherwise, we should continue to place emphasis on the traits with more clearly documented economic importance,” he stated.
Examples he provided for opportunistic EBVs included birth weight, ultrasonic fat and ultrasonic muscle depth.
Notter added, “Early growth, post weaning growth, number of lambs born and weaned, maternal ability, fleece weights and fleece diameters are the core economically important traits.”
He emphasized looking at the number of lambs born and the number of lambs weaned when selecting for the flock.
“Good indexes are also increasingly necessary to properly use EBVs. They need to have a sound economic basis, and if they are done right, our customers will thank us for it,” Notter said.
Dave Notter was the featured speaker for a National Sheep Improvement Program webinar that aired on Aug. 25, 2015.