Use Birth Weight as a Selection Tool

"Would you look at the size of that lamb!" We sometimes hear that statement in the lambing barn, but hopefully, most lambs arrive with reasonable birth weights. However, lambs outside reasonable birth weights may need to be culled from the flock. Small lambs and overly large lambs both often result in added labor at lambing time as well as increased losses. So, how can producers utilize birth weights as a selection tool in order to manage lamb survival?

Birth weight should begin with a discussion on reasonable birth weights. Most lambs should weigh between 8 to 12 pounds at birth, depending on mature size of the sire and dam. The genetic heritability is low, but producers can still manage birth weights through genetic selection. The National Sheep Improvement Program assists producers with calculating estimated breeding values (EBVs) in order to assess the genetic influence on birth weight.

Environmental conditions have a larger effect on birth weight. However, because producers manage ewes as a group, producers can genetically select for average birth weights by culling lambs with low or high birth weights, as well as ewes that produced those lambs. The best option is to develop EBVs for this trait.

We refer to the characteristics we see visually for any trait as an animal's phenotype. Both genetics and the environment in which the animal is raised contribute to that animal's phenotype. In other words, the birth weight combines animal genetics with environmental influences. By making comparisons between genetically linked animals raised under different conditions, NSIP can calculate the genetic influence on a trait.

Lifetime Wool from Australia notes that 70% of lamb mortalities between birth and weaning occur within the first 48 hours after birth. The greatest factor affecting birth weight is ewe nutrition during late gestation, particularly during the last third of gestation. Nutrition not only affects birth weight, but also affects colostrum quality and quantity, as well as the vigor and health of the lambs.

Other environmental conditions that affect lamb birth weight include number of lambs in the litter, sex of the lamb, dam's age, parental breeds, plus ewe size and how many times she has lambed. "Birth weight of lambs: influences and consequences" (Alexander, 1974) explores the consequences of birth weights outside an acceptable range. In general, large lambs often lead to dystocia (lambing difficulty), which can lead to lamb death, and may even lead to ewe death. Small lambs often suffer from hypothermia and die due to chilling. Their smaller size means that they have a greater surface area as compared to their body weight, which results in a more rapid heat exchange with the environment. In addition, smaller lambs often have shorter and sparser coats. Both extremes in birth weights require extra labor on the part of the shepherd to ensure lamb survival.

"NSIP EBVs", written by Dr. David Notter, discusses two EBVS related to birth weight: birth weight and maternal birth weight. Birth weight (BWT) EBV estimates the direct genetic influence on how much a lamb weighs at birth. This helps producers to manage birth weights for better lamb survival within their individual flock. BWT EBV can be particularly important for flocks producing mostly single-born lambs with high birth weights. This EBV is also useful for producers with flocks producing large-sized twins and triplets. In this instance, producers who wish to decrease birth weights would select for negative BWT EBVs. On the other hand, flocks with high lambing percentages may need to select for higher birth weights in order to improve lamb survival if those twins and triplets have low birth weights. Producers who wish to increase birth weights would select for positive BWT EBVs. If a producer is satisfied with lamb birth weights, then that producer would select for BWT EBVs near 0.

Producers should keep in mind that birth weights have a positive correlation with weaning and post-weaning weights. This means that as weaning and post-weaning weights increase, birth weights tend to increase also. EBVs help producers to manage this correlation so that fast-growing lambs with high weaning and post-weaning weights maintain birth weights within an acceptable range. Many years ago, the beef industry experienced many problems with dystocia due to increasing birth weights as a direct result of selection for heavier weaning weights. That industry learned to control that effect by selecting genetics for both low birth weight and fast growth, which leads to heavier weaning weight.

Notter states, "Maternal birth weight (MBWT) EBV estimates genetic effects of the ewe on the birth weight of her lambs. This EBV mainly reflects the quality of the uterine environment provided by the ewe and may also be influenced by ewe effects on gestation length." The usefulness of this EBV relates to the uterine environment. A favorable uterine environment should lead to healthy, vigorous lambs at birth, while a poor uterine environment can result in small, weak lambs at birth.

Appropriate birth weights also benefit buyers. Buyers want average-sized lambs for the same reasons as producers; therefore, producers should take into account the type of flocks into which they sell breeding stock. What is the typical mature ewe size for these flocks? What birth weights would be acceptable for these flocks? Satisfied buyers often return to buy more sheep, and they serve as a valuable marketing tool when they share their positive experiences with other sheep producers.

As with any selection process, select EBV values for several traits to move genetics of a flock toward that flock's ideal. In this manner, producers seek to develop sheep that best match their operation's resources and markets. For more information on understanding EBVs, and how to use them, visit the National Sheep Improvement Program website and select the resources tab.