Estimated Breeding Values (EBVs)

Imagine you are out to buy a new ram, you could select just on how the ram looks and his weight at 120 days (if you were looking for growth). That is a start, but what if you also knew how his weight compared to that of his contemporaries, that will give you a lot better idea of his genetic potential. What will even be better is to also know how well family members grew (offspring, sire and dam, grand sire and grand dam as well siblings and half sibling). It would also be nice to know how well he or his family members performed on other farms that might use different management systems. This level of information is exactly what EBVs provide.

"You buy a ram not for what he looks like but how his progeny will perform" - Sheep Genetics, Australia

We are part of NSIP (National Sheep Improvement Program) through which our EBVs are calculated, animal performance is compared against over 4000 lambs and their sires and dams in close to 40 different Katahdin flocks each year. EBVs are expressed as a number above or below an “average” for all animals that participate in the program. The “average” is a baseline calculated at fixed point in time, this allows for better tracking of genetic progress over multiple years. EBVs also have accuracies that are based on the amount of information utilized to calculate the EBV. The percentile report helps you see how an EBV value compares against other animals (latest lamb crop) in the program for each trait measured, 0 th band (percentile) is the top (largest or smallest depending on the trait) performing while the 100 th band (percentile) is the bottom performing. The July 2018 percentile report for Katahdin sheep can be found here on the NSIP web site.

 

Some EBVs are correlated with other EBVs, this allows estimates to be created (at a lower accuracy level) even before an EBV can be measured such as estimating WWT once BWT is known (because BWT and WWT are correlated). EBVs can also measure maternal traits in rams, this gives an indication of how ewes produced by the ram will perform (maternal genetics come from the ram and the ewe).

EBV Categories

  • Growth  (BWT, WWT, PWWT)

  • Parasite resistance (WFEC,PFEC)

  • Carcass (PFAT, EMD)

  • Maternal (NLB, NLW, MWWT)

  • Hair Index (in some cases just referred to as Index) which is a calculation using multiple EBVs to maximize pounds of  lamb weaned.

Below we will explain the different EBVs and how we use them to select for and against different traits to produce animals that do well on our farm. Ewes contribute the other 50% of the genetics, so in all the examples below the EBV is divided by two, as only 50% of the genetics come from the sires. Remember that the examples below only look at the genetic potential. Management, weather, random events, chance, etc. play a much larger role in determining if an animal will live up to its genetic potential but only genetics are heritable.

Growth EBVs

Birth Weight (BWT), Weaning weight (WWT) and Post Weaning weight (PWWT) is calculated from weights that we submit at birth, 60 Days and 120 days. These three traits are correlated and animals are typically either high or low in all three, so unfortunately higher growth usually comes with higher birth weight. Sheep with higher growth, tend to have bigger frame sizes which means that they are leaner than animals with lower growth if you grow them to the same body weight. Bigger framed animals tend to have bigger adult size. Ewe lambs need to start putting on some fat in order to go through puberty, so high growth could lead to later sexual maturity.

 

We pair high growth rams with lower growth ewes to moderate birth weight. We measure hogget weight on ewe lambs at 1 ½ year old and select for lower hogget weight to keep our adult weights in check. We only keep ewe lambs that can get pregnant at 7-8 months old to make sure that we do not sacrifice early maturity. Selecting against extreme low fat (PFAT) also helps that our lambs are not too lean.

BWT - measured in kilograms (kg). You can see that WHK 1352 has the largest number (0.738) while NWT 6041 has the smallest number (-0.505). This means that WHK 1352 should produce lambs that have the genetic potential to be  0.62 kg (1.36 pounds) [0.738 -(-0.505)=1.243, but ram only contributes 50%, so 1/2 = 0.62] heavier than NWT 6041.

NSIP ranks BWT from low (0th percentile) to high (100th percentile). This is an EBV that can get you in trouble if you go too high or too low as really high birth weights leads to lambing problems while really low birth weights leads to decreased lamb survival. We pair our rams with ewes so that their offspring should have more moderate birth weights.

WWT - measured in kilograms (kg). You can see that GBR 8202 has the largest number (3.619) while NWT 6041 has the smallest number (0.031). GBR 8202 should produce lambs that have the genetic potential to be 1.8 kg (3.9 pounds) heavier than the lambs produced by NWT 6041 at 60 days old.

NSIP ranks WWT from high (0th percentile) to low (100th percentile). If you are selling lambs by weight at weaning, selecting a ram with a higher WWT can increase profits.

PWWT -  measured in kilograms (kg). You can see that WHK 1352 has the largest number (7.41) while NWT 6041 has the smallest number (0.073). WHK 1352 should produce lambs that have the genetic potential to be 3.7 kg (8.1 pounds) heavier than lambs produced by NWT 6041 at 120 days old.

 

NSIP ranks PWWT from high (0th percentile) to low (100th percentile). If you are selling finished lambs by weight or meat , selecting a ram with a higher PWWT can increase profits, but be sure to keep your eye on carcass traits to preserve quality.

Parasite Resistance EBVs

Weaning fecal egg count (WFEC) and post-weaning fecal egg count (PFEC) is calculated from fecal egg counts that we submit around weaning and a second sample 4 weeks later. The average fecal egg count (fec) for the whole group of lambs that we are testing needs to be at least 500 eggs per gram (epg) when we take these samples. This ensures that there is a high enough parasite challenge to be able to identify the good and the bad. If we were to do fec at a time when there is not a high parasite challenge, most of the lambs would have low fec but not because they are parasite resistant. Having a 0 fec when the other lambs have 500+fec, means a lot more than having a 0 fec when the other lambs just have 50 or 100 fec.

You cannot go wrong selecting for lower WFEC and PFEC. These two EBVs are correlated and usually animals are low or high in both these traits. You will notice that BUL 18403 does not have EBVs for these traits, the farm he is from does not measure it, we will find out how parasite resistant he is once we start measuring the lambs. Being parasite resistant will not make your lambs grow faster (unless they have good growth EBVs too) but they will need to be dewormed a lot less frequently (hopefully not at all) and you will lose fewer lambs due to parasites.

NSIP ranks WFEC and PFEC  from low (0th percentile) to high (100th percentile). The more negative the number, the better the parasite resistance.

WFEC - expressed as a percentage above or below average. The lower this number, the better parasite resistance the ram passes on to his lambs at weaning time. A negative number for this EBV is a good thing. MOF 1837 has the lowest number (-98.58) and WHK 1352 has the highest number (-53.32). This means that the lambs produced by MOF 1837 should have 49.3% fewer parasite eggs than the average lamb and WHK 1352 lambs should have 26.7% fewer parasite eggs than the average lamb at weaning time. All the rams, except BUL 18403 (we do not know, he could be good or bad), should produce offspring with above average parasite resistance.

PFEC - expressed as a percentage above or below average. The lower this number, the better parasite resistance the ram passes on to his lambs after weaning (about 120 days old). A negative number for this EBV is a good thing. MOF 1837 has the lowest number (-100) and GBR 8202 has the highest number (-73.22). This means that the lambs produced by MOF 1837 should have 50% fewer parasite eggs than the average lambs while GBR 8202 lambs should have 36.6% fewer parasite eggs than the average lamb after weaning. All the rams, except BUL 18403 (we do not know, he could be good or bad), should produce offspring with above average parasite resistance.

Carcass EBVs

When our lambs are 80 pounds plus, we do ultrasound scanning to measure the loin eye size and fat cover on the tissue between the 12th and 13th rib, and also weigh the lambs on the same date. Loin eye depth is always standardized to a specific weight so that the EBV tells you how much smaller or larger the loin eye will be at a specific body weight. This data is submitted to calculate fat cover (PFAT) and loin eye depth (PEMD). The convention is to maximize PEMD and minimize PFAT for maximal lean meat production.

 

PEMD and PFAT are negatively correlated with WWT and PWWT, so high growth lambs tend to have less muscle and less fat at a specific weight (100 pounds for Katahdins). There is no down side to select for maximal PEMD but it is hard to do while maintaining growth, as there are few high growth lambs with high PEMD.

 

There can be issues with pushing PFAT too low or too high. If you sell lambs at graded auctions, sell meat or get paid on quality of lamb, there is a premium for the right amount of fat cover, you could produce lambs that do not finish well if you maximize/minimize this trait. The other negative is that ewe lambs do need to accumulate some fat cover before they start cycling, you can delay puberty in your ewe lambs if you push this to the extreme low side.

 

PEMD - expressed in mm (millimeter). The higher this number, the deeper the loin eye. NWT 6041 has the highest number (0.68) while WHK 1352 has the lowest number (-1.289). NWT 6041 should produce lambs that on average have loin eyes that are 1.0 mm thicker than lambs produced by WHK 1352, when the lambs weigh 100 pounds.

NSIP ranks PEMD from high (0th percentile) to low (100th percentile). Increasing this EBV should give you more meaty lambs, but make sure that you do not sacrifice so much growth that you are not able to get to 100 pounds before the lambs have put on too much fat.

PFAT - expressed in mm (millimeter). The lower this number, the less fat cover. WHK 1352 has the lowest number (-1.359) and NWT 6041 has the highest number (0.81). WHK 1352 should produce lambs that on average have fat cover that is 1.1mm thinner than lambs produced by NWT 6041 when the lambs weigh 100 pounds.

NSIP ranks PFAT from low (0th percentile) to high (100th percentile). Decreasing this EBV should give you leaner lambs, but make sure that you do maximize this and end up with lambs that do not have enough fat cover for good finish.

Our 2018 ram lambs were measured for PFAT and PEMD, so the three GBR rams have this trait measured, WHK 1352 and NWT 6041 have EBVs because their sons were measured this year. MOF 1837 and BUL 18403 did not have this measured, but we will measure their offspring (when they are big enough) and see how they do.

Maternal EBVs

For each lamb that is born we submit data on birth type (single,twin,triplet, quad) and rear type (single,twin,triplet,quad). If lambs die or are not raised by their mothers due to any reason, the rear type is adjusted and this data is used to calculate Number of lambs born (NLB) and number of lambs weaned (NLW). Maximizing NLB is not something we select for. We do not feed grain at any stage of production and not all ewes are able to gestate and raise triplets or quadruplets without supplementation. Pushing NLB and not increasing nutrition appropriately can lead to pregnancy toxemia in pregnant ewes, bottle lambs or losing lambs because ewes do not produce enough milk to raise 3+ lambs, and ewes losing too much condition during lactation so they can not breed back successfully. Maximizing NLW has no downside, you want your ewes to successfully raise all the lambs that they have.

We typically select for animals with NLW higher than NLB. This sounds counter-intuitive right, how can a ewe raise more lambs than she has? This is possible because NLB and NLW are both expressed as number of lambs more born / weaned per 100 ewes lambing so as long as your ewes raise all the lambs that they have and other ewes in NSIP lose some of theirs, ewes can have a higher NLW than NLB.

Maternal weaning weight (MWWT) is the milk and mothering EBV. This EBV is calculated by comparing the expected 60 day weight (based on WWT EBV) to the actual 60 day weight. The difference is attributed to the ewes milk production and mothering skill and can be better or worse than expected. We do not select to maximize this EBV as too much milk can be a problem. Ewes that produce more milk are more prone to get mastitis at weaning. Pushing MWWT and not increasing nutrition appropriately can lead to ewes losing too much condition during lactation so they can not breed back successfully.

Remember: A ram will pass on maternal traits to his daughters and influence their NLB, NLW and MWWT. He has NO INFLUENCE in these traits in the ewes that he is bred to, so using a ram with higher NLB will not increase lamb numbers in ewes that he is bred to, but his daughters should produce more lambs.

NLB - expressed as the % more lambs born than average (average is a baseline established as about 210 lambs per 100 ewes lambing). You can see that MOF 1837 has the highest number (9.5%) and GBR 8005 has the lowest number (-0.7%). This means that the ewe lambs produced by MOF 1837 will produce about 220 (half of 9.5% more than 210) lambs per 100 ewes lambing compared to 209 (half of 0.7% less than 210) lambs per 100 ewes lambing for ewe lambs sired by GBR 8005 (once the ewe lambs are mature).

NLW - expressed as the % more lambs weaned than average (see NLB for more detail). You can see that BUL 18403 has the highest number (18.3%) and GBR 8005 has the lowest number (3.3%) This means that the ewe lambs produced by BUL 18403 will have 9.2% (half of 18.3) more lambs that survive to weaning than average and ewe lambs sired by GBR 8005 will have 1.7% (half of 3.3) higher than average survival.

 

MWWT - measured in kilograms (kg). This EBV looks at the contribution that the ewe makes to the weaning weight of her lambs. If you have a ewe that produces more milk and is a better mother to her lambs when compared to the average ewe, her lambs should weigh more than the WWT growth genetics. However a ewe that produces less milk or is not as attentive a mother would produce lambs that weigh less than the WWT growth genetics. BUL 18403 has the largest number (1.6) while WHK 1352 has the smallest number (-0.533). This means that the ewe lambs produced by BUL 18403 should wean lambs that are 0.8 kg (1.8 pounds) heavier than their WWT growth genetics while WHK 1352 would produce ewe lambs that wean lambs that are 0.533 kg (1.17 pounds) lighter than their WWT genetics.

Think of it this way, the genetic part of the weaning growth of a lamb comes from half the WWT genetics of the dam combined with half the WWT genetics of the sire plus the MWWT (milk and mothering) of the dam.

Hair Index

This is a calculated value using WWT, MWWT, NLB and NLW EBVs (0.246 x WWT EBV + 2.226 x MWWT EBV + 0.406 x NLW EBV– 0.035 x NLB EBV)

This index is designed to maximize pounds of lamb weaned per ewe lambing. You can see that BUL 18403 has the largest index (111.5) while GBR 8005 has the smallest index (102.2). This means that ewe lambs produced by BUL 18403 should be able to wean more pounds of lambs when compared to the ewe lambs sired by GBR 8005.

We do not select based on this Index as we do not want to maximize MWWT (which carries the most weight in the equation) and we prefer focusing on the ratio of NLW:NLB to work best in our management system.