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, 100 th band (percentile) is the top (largest or smallest depending on the trait) performing while the 0 th band (percentile) is the bottom performing. The February 2020 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).
Growth (BWT, WWT, PWWT, HWT)
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.
Birth Weight (BWT), Weaning weight (WWT) ,Post Weaning weight (PWWT) and Hogget weight (HWT) is calculated from weights that we submit at birth, 60 days,120 days and 18 months. These four traits are correlated and animals are typically either high or low in all four, so unfortunately higher growth usually comes with higher birth weight and adult 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(HWT) 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). RAM A 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, difference between BWT of Ram A and BWT of Ram B] heavier than RAM B.
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). RAM A should produce lambs that have the genetic potential to be 1.78 kg (3.9 pounds) heavier than the lambs produced by RAM B at 60 days old.
If you are selling lambs by weight at weaning, selecting a ram with a higher WWT can increase profits.
PWWT - measured in kilograms (kg). RAM A should produce lambs that have the genetic potential to be 3.7 kg (8.1 pounds) heavier than lambs produced by RAM B at 120 days old.
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.
HWT - meadured in kilograms (kg). RAM A should produce lambs that have the genetic potential to be 2.375kg (5.3 pounds) heavier than lambs produced by RAM B at 18 months old.
Unfortunately there is not an EBV for adult weight so this is the best measurement we have to select AGAINST adult ewes that are too big by trying to minimize this EBV while maintaining higher growth at younger ages. We all want lambs that grow to butcher weight fast but then stop growing so our maintenance needs for adult ewes are not too high.
NSIP ranks all the growth EBVs from high (100th percentile) to low (0th percentile).
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. 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 (100th percentile) to high (0th 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. RAM B has a lower number (-88.58) than RAM A (-53.32). This means that the lambs produced by RAM B should have 44.29% fewer parasite eggs than the average lamb and RAM A lambs should have 26.7% fewer parasite eggs than the average lamb at weaning time.
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. This means that the lambs produced by RAM B should have 49.7% fewer parasite eggs than the average lambs while RAM A lambs should have 38% fewer parasite eggs than the average lamb after weaning.
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. RAM B (0.68) has a bigger loin eye EBV than RAM A (-1.289). RAM B should produce lambs that on average have loin eyes that are 1.0 mm thicker than lambs produced by RAM A, when the lambs weigh 100 pounds.
NSIP ranks PEMD from high (100th percentile) to low (0th 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. RAM A (-1.359) has much smaller far EBV than RAM B(0.81). RAM A should produce lambs that on average have fat cover that is 1.1mm thinner than lambs produced by RAM B when the lambs weigh 100 pounds.
NSIP ranks PFAT from low (100th percentile) to high (0th percentile). Decreasing this EBV should give you leaner lambs, but make sure that you do not maximize this and end up with lambs that do not have enough fat cover for good finish.
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 how many more lambs born per 100 lambings as compared to the average (baseline). Ram A has 4.8 and Ram B has 9.3. Daughters produced by Ram B will have 2.25 more lambs per 100 lambings than Ram A if their daughters we born from a mating to the same ewe. That is 9.3/2 - 4.8/2 = 2.25. Profligacy is lowly heritable, the majority of the variation is because of management / environment / chance. On our farm with our management a NLB of around 1.7 produces twins.
NLW - expressed as how many more lambs weaned per 100 lambings as compared to the average (baseline). Ram A has 13.8 and Ram B has 11.4. Daughters produced by Ram A will wean 1.2 more lambs per 100 lambings than Ram A if their daughters we born from a mating to the same ewe. That is 13.8/2 - 11.4/2 = 1.2. Weaning ability is lowly heritable and very dependent on how many are born, the majority of the variation is because of management / environment / chance. On our farm with our management a NLW of around 3.2 weans twins.
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. RAM B (0.128) has a larger number than RAM A( -0.533). This means that the ewe lambs produced by RAM A should wean lambs that are 0.064 kg (0.14 pounds) heavier than their WWT growth genetics while RAM A 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.
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.
This means that ewe lambs produced by RAM A should be able to wean more pounds of lambs when compared to the ewe lambs sired by RAM B.
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.