United Suffolk Sheep Association

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April/May 2004

Useful Tools to Help in a Successful Sheep Breeding Program 

Larry Arehart 

Frequently discussed has been the question of whether heredity or environment is the more important in the expression of economic traits. Such a discussion here would be of little value because it is now recognized that both are of great importance. The best possible inheritance will not result in a superior flock unless the proper environment is also supplied so that the animals can attain the limit set by their inheritance (selected gene pool). Half-starved and neglected purebreds are truly a disappointment to livestock men, in their appearance as well as their performance. Nevertheless, the best possible environment will not develop a superior flock unless the proper inheritance is also present in ewe, ram or wether. An answer to this statement is the question why don’t dairymen use beef cattle for milk production or why don’t racing enthusiasts include draft horses in their racing string? The answer is simple, the best possible feed, training or care cannot make a record-breaking milk-producer out of a beef cow or a Kentucky Derby winner out of a draft horse. The gene selection was just not present to phenotypically express the breeder goal. 

Superiority due to genes is the only thing that is transmitted from the parents to their offspring. Superiority due to environment will not be transmitted by the parents. However, this superior in environment must be provided for the offspring if they are to be the equal or excel their parents phenotypically and express their genetic potential.  

Heritability Estimates 

Heritability estimates refer to that portion of the phenotypic variance in a population that is due to heredity. Remember the formula: 

Genotype + Environment = Phenotype 

Heritability estimates have been derived by animal breeding scientists comparing relatives in different environmental conditions. For the purpose of this discussion, accept the fact that the following formula defines percent heritability:

 % Heredity = variation due to heredity ÷ variation due to both heredity & environment x 100

Table 1 gives heritability and repeatability estimates of several economic traits for sheep. 

Profitable Breeding Value and Expected Breeding Value 

If you are in the SID selection program what the following discussion entails is what SID is based on except in a much more sophisticated formula, which utilizes as many ancestors as data, is available. 

Suppose the breeder is interested in estimating the Probable Breeding Value (PBV) for an individual for a particular trait. That can be determined from the following formula:

                                                PBV = P + hi(Pi – P) 

Using the above formula we will compute the PBV of an individual for the trail postweaning gain. Let’s say the postweaning gain for a selected individual (Pi) is 1.5 pounds while the average for the contemporaries (P), (that is average gain of all lambs in the flock), was 0.90 pounds. From table 1 we find that the heritability estimate (hi) equals 40 percent. Therefore: 

                                                                PBV = 0.90 + .40(1.5 – 0.90)

                                                                PBV = 1.14 lbs. 

Notice in computing the PBV the performance of the flocks the average of all lambs are used, therefore, comparing individuals to all lambs in the flock. A simple estimate of the EPV (expected breeding value) is equal to half of the improvement over the average or below the average which ever the case may be. In this case the EPD (Expected progeny difference) is equal to 0.14/2 or 0.07 pounds. This is the improvement passed on to the next generation by this individual. Now you can understand why progress is slow even when you are breeding the best to the best. When you are breeding the best to the average, it is even slower. 

Don’t underestimate the small value of 0.07 lbs. Suppose we are talking about a ram. He is being bred to 30 ewes and will be used for 6 years. If the average post weaning feeding period is 75 days his economic value would be {if lambs are worth a $1.00 per pound (75 days X 30 ewes X 1.5 lambs per ewe X 6 years X 0.07 EPD $075 per pound)} $1063 more in increased gain compared to the average ram out of this flock. By the same token, a ram with a negative 0.07 EPD could cost the breeder $1063.

Trait Ratio 

Another useful calculation to identify superior and inferior individuals in the flock is the trait ratio; 

Trait ratio = Individual’s record for a trait ÷ Group average for that same trait x 100                                                                 

 Example: we are comparing ten lambs for post weaning gain. The data is: lamb A weight is 1.25 lbs., B = 1.0, C = 0.75, D = 0.65, E = 1.5, F = 1.25, G = 0.95, H = 1.40, I = 0.50, J = 0.75 lbs., group Ave = 1.0 lbs. To calculate the trait ratio for lamb E is equal to (1.5 / 1.0) X 100 = 150. To calculate the trait ratio for lamb I (0.50 / 1.0) X 100 =50. The interpretation of this is simply that lamb E is 50 percent better than average and lamb I is 50 percent worse than the average. If all lambs were calculated one could rank them from top to bottom and identify lambs that were above the average in the flock as well as those individuals that were below the average in the flock. The ones above the average are the replacements. A reminder – one must compare ram lambs to ram lambs as well as ewe lambs to ewe lambs. Sex does make a difference in postweaning daily gain. 

I use the trait ratio all the time to identify the superior individuals in my flock. Remember, you can only use this tool within your flock. One can set up a method to evaluate any trait using the trait ratio. If you want to identify subjective or quality traits, for example, structural correctness, you might get a committee of five knowledgeable sheep people and have them score you lamb crop using a score from 1 to 10. Ten being perfect and 1 being the worst you can think of. Score each lamb for structural correctness, drop the low and high score on each lamb and average the other three scores. All folks scoring lambs need to do it independently. Using the trait ratio formula you can quickly identify your lamb crop for structure correctness and it will be an unbiased evaluation. 

Table 1