Estimated breeding values (EBVs) are predictors of an individual animal’s genetic merit for a particular trait. These values are the most accurate selection tool available because they represent an animal’s genetics independent of environmental factors. EBVs combine data about the animal's unique performance and data from all known ancestors, siblings, progeny, and correlated traits. They simplify and improve selection by providing a robust tool that represents more than merely a single data point at one point in time. EBVs can be used to compare the genetics of two individuals in numeric terms. More information about EBVs can be found in AG-991 Small Ruminant Genetic Improvement and on the National Sheep Improvement Program website. This publication discusses the interpretation and utilization of EBVs in a selection program based on a comparison of two rams (Table 1).

While an EBV reflects the total difference in genetic merit between animals, only half of an individual animal's genes are passed to the next generation. Therefore, only half the difference in EBVs will be realized in the progeny, which is referred to as the expected progeny difference (EPD). Simply put, EPDs are half the value of an EBV. In the small ruminant industry, genetic merit is reported as EBVs. While animals with superior EBVs will also have superior EPDs, understanding the technical definition and calculation of EPDs is necessary to quantify potential differences in progeny performance.

Accuracy (acc.) values are influenced by the amount of data in the genetic evaluation program calculating the EBV. Each EBV has an associated accuracy value. The more data available, the more accurately the program can estimate genetic merit and the closer the “estimated” breeding value is to the “true” breeding value. Accuracy values are often shown below or next to the EBV in a genetic report (Table 1). A producer can have more confidence in breeding values with higher accuracy because there will be less expected change in the EBV with additional information.

WWT: weaning weight; WFEC: weaning fecal egg count; NLW: number of lambs weaned; PEMD: post-weaning eye muscle depth; Mat. Hair Index: maternal hair index (combination of EBVs for weaning weight, maternal weaning weight, number of lambs born, and number of lambs weaned).

The weaning weight (WWT) EBV represents potential genetic differences in weight at 60 days of age (reported in kilograms). In Table 1, Ram A has a WWT of 2.7 kilograms, while Ram B has a WWT of 1.8 kilograms, representing a difference of 0.9 kilograms. Because the individual ram passes half of its genetic material to its offspring, expected difference in WWT between lambs sired by Ram A compared to Ram B (if bred to the same set of ewes) would be 0.45 kilograms (half of 0.9 kilograms). If you prefer to evaluate the weight difference in pounds, multiply 0.45 by 2.2 (there are 2.2 pounds in 1 kilogram) for a total weight of 0.99 pounds. If you were to use Ram A instead of Ram B, you would expect each lamb to weigh about 1 pound more at 60 days of age. If you market lambs at weaning and the current market price is $1.80 per pound, each lamb would be worth an extra $1.80.

Potential parasite resistance at weaning is reflected in the weaning fecal egg count (WFEC) EBV. The lower the WFEC, the greater the parasite resistance. Lower WFEC EBVs are more desirable. This trait is expressed as a percent change in WFEC. In Table 1, the difference in WFEC EBV between Ram A and Ram B is 55%, favoring Ram B; therefore, we would expect progeny of Ram B to have a WFEC that is 27.5% lower than that of the progeny of Ram A.

Prolificacy (number of lambs produced) and lamb survival are represented by the number of lambs weaned (NLW) EBV. The NLW trait is expressed as a percentage. Greater NLW EBV indicates a greater number of lambs produced at weaning time. In the example in Table 1, the NLW EBV for Ram A is 10% greater than Ram B. We would expect daughters of these two rams to differ by 5%. Daughters of Ram A would be expected to wean 5% more lambs than daughters of Ram B. Another way to think about this is that daughters of Ram A would produce an additional five lambs per 100 lambing events compared to daughters of Ram B. The NLW trait is often one of the most economically valuable traits in a breeding operation that retains replacement ewes.

Post-weaning eye muscle depth (PEMD) is the standard measurement for muscling. Greater PEMD is predictive of a heavier-muscled animal. The PEMD trait may be economically relevant for some operations but not for others. Generally, emphasis is placed on this trait in terminal sire breeds and terminal sire selection. In Table 1, the difference in PEMD EBV between Ram A and Ram B is 0.5 millimeters. We would expect lambs sired by Ram A to have 0.25 millimeter greater PEMD compared to lambs sired by Ram B.

The maternal hair index is a predictor of weight of lamb weaned per ewe. This selection index combines the weaning weight, number of lambs born, number of lambs weaned, and maternal weaning weight (milk) EBVs. These selection indexes further simplify selection by combining multiple economically relevant traits into one EBV. If you need to improve ewe productivity, the maternal hair index can be used to make improvements in multiple traits. In Table 1, the difference in maternal hair index scores is 1.8 kilograms. The difference in index scores between daughters of these two rams would be 0.9 kilograms, or 2.0 pounds. Therefore, we would expect daughters of Ram A to wean about 2.0 pounds more lamb weight per litter than daughters of Ram B.

The answer depends on the selection goals of your operation and current production metrics. If your operation has satisfactory maternal performance and needs more parasite resistance, Ram B is likely the best choice. If the operation has ewes that are adapted to your environment and perform well on grass, already exhibiting satisfactory parasite resistance, then Ram A may be the best option to improve weight of lamb produced per ewe.