The Nuts and Bolts of EBVs

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EBVs are tools that producers can use to select breeding animals that will help them meet flock goals for specific traits such as growth, milk, reproduction, parasite resistance and carcass quality. It is important to understand that selecting for extremes in one trait will often put negative selection pressure on other traits of interest. Often, animals with extremely high EBVs in certain traits will require additional nutritional support or better management to perform successfully at that level. In addition, some traits are antagonistic to other traits, meaning that if you select strongly for one trait you may be inadvertently reversing the progress you’ve made in other important traits. Traits that are directly measured (weights, FEC, Carcass) are generally more heritable than other traits such as milk and prolificacy. Below is a condensed description of EBVs, how they are measured and why they are important.

Maternal Weaning Weight EBV (MWWT):

MWWT EBV provides an estimate of the genetic effects of the ewe’s milk and mothering ability on the weaning weight of her lambs. A higher value generally indicates more milk production and/or higher quality of milk. There is no direct measurement for MWWT; instead, it is a calculated value derived by comparing the actual growth of a ewe’s lambs to the predicted growth of her lambs at weaning. The MWWT EBV is reported as lamb growth in kilograms above or below the breed average at weaning. A producer’s management system will determine how much selection pressure is applied to this trait. Forage-based systems often put more selection pressure on MWWT since pasture-raised lambs rely more heavily on mother’s milk compared to lambs with early access to creep. Ewes with high MWWT EBVs may require more maintenance and/or nutritional support to prevent loss of body condition or to reduce the risk of parasitic infections during lactation. Ewes with low MWWT may have difficulty raising their lambs, especially triplets, to weaning. Lambs from these ewes may require additional nutritional support. Unlike traits that are directly measured (growth, FEC and carcass), maternal traits cannot be accurately measured until daughters start producing offspring.

Example: A ram with a MWWT of .8 has the genetic potential to add .8 kg or 1.8 lbs. of additional growth between birth and weaning. Since the ram contributes half his genetics to his lambs, it is predicted that his lambs will inherit approximately .4 kg (.9 lbs.) of additional weight by weaning.

For a detailed description of this trait: Fact001-Weight-EBVs.pdf (

Weight Trait EBVs (BW, WW, PWW):

Weight Trait EBVs are designed to separate the effects of environment from the animal’s genetics for growth from birth to adulthood. Birth Weight EBVs (BWT) estimate the portion of a lamb’s birth weight that is due to genetics, apart from its prenatal environment. The Wean Weight EBV (WWT) estimates the genetic potential for growth between birth and weaning excluding influences of dam’s milk, forage quality or creep feeding. The Post-Weaning Weight EBV combines information on pre-weaning and post-weaning weight to predict genetic potential for growth to 120 days, independent of maternal or environmental influences. Birthweights should be recorded in the first 24 hours after birth. Wean weights are recorded when the contemporary group averages 60 days old (range of 40-89 days of age) regardless if lambs are actually weaned on that date. Post-weaning weights are usually measured at the early post-weaning period when the group averages 120 days of age (range of 90-150 days of age). Weight Trait EBVs are more accurate when there are wider differences in the weights of the lambs. It is important to include weights on both fast-growing and poor-doing lambs. Selection pressure on weight traits is usually geared towards marketing goals. Higher Wean Weight EBVs are expected to increase the weight at weaning and is an important EBV to select for if lambs are marketed at or around weaning. If marketing older/heavier lambs, selection pressure is focused on post-weaning EBVs. Since growth traits are correlated, selecting for extremes in growth can have unintended consequences such has dystocia due to large birth weights and larger framed lambs (and adults) that may require supplemental feed to maintain condition. Weight trait EBVs are measured in kilograms (kg) above or below breed average.

Example: A ram with a Wean Weight EBV of 2.8 has the potential to weigh 2.8 kg (6.2 lbs.) heavier at weaning. Because a lamb inherits 50% of its genetics from its sire, its offspring are expected to weigh 1.4 kg (3 lbs.) heavier at weaning.

For a detailed description of this trait: Fact002-MWWT-EBV.pdf (

Fecal Egg Count EBVs (WFEC, PFEC):

Fecal Egg Count EBVs evaluate the genetic potential for resistance to intestinal parasites, specifically the Barber Pole worm. The Weaning FEC EBV evaluates early, or innate, resistance in lambs at or around 60 days of age. The Post-weaning FEC EBV evaluates resistance after weaning (acquired resistance) around 90-120 days of age. The WFEC and PFEC EBVs are strongly correlated and have moderate heritability. Unlike other EBVs, a negative value is preferred. Animals with excellent parasite resistance may have FEC EBVs approaching -100%. FEC EBVs are measured by collecting and analyzing FECs on all lambs in a contemporary group when lambs have had significant exposure to parasites. Ideally, lambs should be grazing under warm and humid conditions on pasture for 4 to 6 weeks prior to collection. Accuracy for this trait increases when the FEC average of the contemporary group is high and the range of values is wide. To be significant, a group’s average should be at least 500 eggs per gram, although a group FEC average of over 1000 epg is preferred. WFEC and PFEC EBVs are expressed as a percentage above or below the breed average ranging from a -100 (more resistant to parasites) to positive infinity (less resistant). Because of the moderate heritability of this trait, a breeding ram with excellent parasite resistance (a low negative EBV value) can pass on immediate health benefits to his lambs, especially lambs that are predominantly raised on pasture. Since parasitism is a major cause of morbidity and mortality, selecting breeding rams with very low FEC EBVs can significantly reduce worm burdens in his lambs, improve weight gain, decrease the frequency of deworming and reduce pasture contamination. This trait is more important in a pasture raised system and less so if lambs are raised in confinement. Selection pressure on this trait must be sustained if a preferred level of resistance in the flock is desired.

Example: A ram with a FEC EBV of -90% has the potential to reduce the worm burden in his lambs by 45% since his offspring inherit half its genetics from the sire. It is important to remember that not all lambs inherit this trait equally. The accuracy of this trait is increased when more than one FEC is collected on the animal itself and is compounded when FECs have also been collected on its close relatives (offspring, sibs, half sibs, parents, grandparents etc.).

For a detailed description of this trait: Fact003-FEC-EBVs.pdf (

Reproductive Trait EBVs (NLB, NLW):

Reproductive or Maternal Trait EBVs evaluate the genetic potential for prolificacy, or number of lambs born per lambing (NLB,) and the ability to raise those lambs to weaning (NLW) separate from environmental influences. Reproductive traits have low heritability with most variation due to management and/or environment. Reproductive traits are measured by recording the birth type and rearing type of each lamb. Accuracy depends on correct coding (including stillbirths) for birth type and recording accurate weaning information on lambs including those that were orphaned, grafted or died between birth and weaning regardless of the cause: mismothering, accidental death, or predators. Reproductive Traits are expressed as a percentage above or below breed average. In general, selection pressure is focused on NLW EBVs that are the same or higher than NLB EBVs since ewes having twins are expected to raise twins without fail. Although NLB and NLW are calculated independently, the traits are correlated since a ewe giving birth to multiples will have the opportunity to wean multiples. Selecting for higher NLB EBVs is predicted to increase the number of triplet births and decrease the number of single births over time. With appropriate management and nutritional support, ewes with high Reproductive Trait EBVs may lead to increased profitability since more lambs are available to market. A low input system may prefer NLB EBVs closer to the breed average lambing rate of 220%. Breeding rams with high Reproductive Trait EBVs will have no influence on the NLB or NLW of the ewes he is bred to, only on the future productivity of his daughters.

Example: A ram with a NLB of .12 has the genetic potential to increase the number of lambs born by 12%. Because his daughter receives half her genetics from her sire, his daughter is predicted to give birth to 6% more lambs than the breed average over her lifetime.

For a detailed description of this trait: Fact004-Reproductive-EBVs.pdf (

US Hair Index (Katahdin Index, Ewe Productivity Index):

The USA Hair Index, or Ewe Productivity Index, is a combination of growth and maternal traits designed to maximize total pounds of lamb weaned per ewe lambing. The USA Index is often considered an economic trait because an operation’s profits are expected to increase when more pounds of quality lamb are marketed. EBVs included in the calculation of the US Hair Index include the following: wean weight EBV, maternal wean weight EBV, number of lambs born EBV and number of lambs weaned EBV. NLW is the most heavily weighted component of the calculation. The USA Hair Index is the best “all around” measurement of maternal merit and when selection pressure is applied, it allows producers to select for multiple traits simultaneously. Ewes with a high US Hair Index are either very prolific resulting in large total litter weights from weaning triplets or from ewes that consistently wean fast growing twins. The index will favor a ewe that produces twins and weans both over ewes that produce triplets, but only weans two. Selection pressure depends on management since higher indexing ewes may be too prolific and labor intensive for low input pasture-based operations. Ewes with high US Hair Indices may improve out-of-season conception and lambing rates resulting in more lambs to market in the off season. Accuracy of this trait is dependent on the accuracy of EBVs included in the calculation.

For a detailed description of this index: Fact005-USHair-EBV.pdf (

Carcass Trait EBVs (PEMD, PFAT:

Carcass Trait EBVs evaluate the genetic potential for improved muscling and carcass fat covering. The post-weaning backfat thickness (PFAT) evaluates the degree of fat deposition, while the post-weaning eye muscle depth (PEMD) evaluates the development of the ribeye muscle. Carcass trait EBVs are standardized to a common body weight of 121 lbs. (55 kg.). Loin eye area, closely correlated with eye-muscle depth, is considered highly heritable so rapid improvement is possible with consistent selection pressure. Measurements are made via ultrasound scanning at 90-150 days of age. Lambs must be weighed on the same day as scanning. Ultrasound measurements are taken by a certified technician with approved equipment. Detailed information, including a list of certified technicians, can be found on the NSIP website. Carcass Trait EBVs are measured in millimeters above or below the breed average for the traits. Using breeding rams with positive EBVs for both traits can lead to improved carcass quality at slaughter.

Example: A ram with a PEMD EBV of 0.9 mm has the genetic potential for a 0.9 mm thicker ribeye muscle at post-weaning. Since a lamb inherits half of its genetics from its sire, his offspring are expected to be 0.45 mm thicker than the average for the breed.

For a detailed description of this trait: Fact006-Carcass-EBVs.pdf (

As with all EBVs, “bigger is not always better”. Selection pressure needs to be balanced with an operation’s management, resources and marketing goals. Progress can be slow when selecting for multiple traits at one time, while single trait selection often results in more rapid improvement for that trait. However, the tradeoff may be that you lose progress in other important traits. For new NSIP producers, putting selection pressure on one or two traits while maintaining acceptable EBVs in the other traits is often the best course of action. To learn more about EBVs, including a glossary of EBV terms and practical application of NSIP concepts, visit the EAPK website: Education – Eastern Alliance for Production Katahdins (

By: EAPK Communications Committee


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