Understanding “Number of Lambs Weaned” (NLW) Estimated Breeding Values

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We’ve been members of NSIP since 2006 and I must confess that I’ve never really understood what all went into the calculation of “Number of Lambs Weaned”, let alone Genomically-enhanced Estimated Breeding Values (GEBV) for NLW. Seems most of my ewes had NLW GEBV that were about the same as their NLB GEBV, with a few exceptions. Some were a little lower and some were a little higher, and since they are correlated, I didn’t pay a whole lot of attention to these slight differences until I bought a ram last year with a NLB of 0.04 and a NLW of 0.13.

How can a ram have a NLW GEBV three times higher than his NLB GEBV?? Did he come from a dam that was used as a foster ewe where rejected lambs were grafted on to her because she was a super mom with fantastic milk? Nope, it turns out that rationalization was completely wrong. Any increase in NLW from grafting would be small. However, if a dam fails to raise her own lambs, her NLW GEBV would be negatively impacted. To understand the NLW GEBV better, I asked the experts: Dr. Ron Lewis, Lynn Fahrmeier and Dr Jim Morgan to explain to me how an animal can have such wide differences in NLB and NLW GEBV. This is a summary of their explanations as I understood them, albeit significantly simplified.

First, it was explained to me that the vast majority of data on a particular animal doesn’t just come from the animal itself but from all of its close relatives, especially the performance data of its daughters and granddaughters. It also includes data from its half-sibs (same sire or same dam) and their daughters and granddaughters. So that’s a lot of animals and a lot of variability, especially when these relatives are sold into different flocks and lamb under different management systems.

It’s important to remember that maternal traits are lowly heritable, meaning the vast majority of differences in performance are due to the environment: management, feed, climate, physical condition of the ewe and ram at breeding, etc. The most useful data comes from an animal’s own performance and that of her close relatives when they are part of large contemporary groups. For instance, retaining a large group of ewes that are the same age from the same sire that have lambed together around the same time year after year, will result in ewes with more accurate maternal GEBVs since they are being compared to a good number of contemporaries with fewer variables to control. By retaining daughters out of the same ram and submitting birth and weaning data on their offspring over time, it becomes easier to predict both the ewe and her sire’s genetic merit for maternal traits.

Simply put, the calculations for NLB and NLW predict how well an animal can pass along beneficial genes for these particular traits. If I have a group of ewe lambs with a NLB GEBV of 0 and compare that to a group of ewe lambs with a NLB GEBV of 0.10, I can predict that the second group will have 10 percent more lambs born over her lifetime than the first group. Similarly, if those ewe lambs have a NLW GEBV of 0.15, I can predict that the group will also wean 15% more lambs. As I decide which ewe lambs to save in my flock or which new ram to buy, I can use GEBVs to help me decide. If I want to increase the number of lambs born and I have already addressed management issues that might affect this, I want to purchase a ram that ranks higher in NLB GEBV. By the same token, if I am happy with the number of twins and triplets, then I need to search for a ram whose NLB GEBV is about the same as the current average of my mature ewe flock. And, if my ewes are having trouble keeping their lambs alive to weaning, I first need to look at management changes and then select strongly for NLW.

The NLW GEBV is often considered a “survival trait.” While “Number Born” drives the “Number Weaned” GEBV (a ewe having triplets has the opportunity to wean triplets), the NLW GEBV is also genetically impacted by GEBVs for growth and parasite resistance, and vice versa. In other words, a ewe that consistently singles will usually have no trouble raising a healthy, fast-growing single who is less likely to die from starvation or parasites compared to a smaller triplet born lamb having to compete with its siblings for nutrition, thus making it more susceptible to parasites and illness in general. So, the next time you’re buying a NSIP ram, look closely at the NLW GEBV, ask about his dam’s lambing and weaning history, and also consider his GEBV for growth and parasite resistance. If you’re looking for a ram with solid genetics for twinning and the ability to raise them through weaning, you may not need a high NLB GEBV, but you will appreciate a higher NLW GEBV.

By: Roxanne Newton, EAPK Communications Committee

Reviewed by: Dr. Ron Lewis, Dr. Jim Morgan and Lynn Fahrmeier


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