by Rodney Kott, Extension Sheep Specialist, Montana State University


"Accurately identifying rams and ewes that excel
is the key to a successful selection program."


Commercial sheep producers sell their grass and labor in the form of lamb and wool. The value of saleable product produced on a given land area is a function of the quantity and quality of lamb and wool. Production efficiency and ewe profitability can be maximized by correctly matching the biological type of sheep produced with the available feed resources, labor, weather and other environmental factors. Critical factors in matching sheep and the environment are reproductive performance, milk production potential and mature size. Accurately identifying rams and ewes that excel is the key to a successful selection program. In any sheep operation the genetic selection of individual animals and breeds and how we develop mating systems will determine the potential level of lamb and wool production. This sets the parameters of the production that is possible. The management provided determines the degree to which that potential is realized.

Management for genetic improvement requires a mix of art and science and may involve a varying degree of chance. By utilizing the most accurate tools economically appropriate to evaluate the genetic worth of replacement animals, the role that chance plays in the genetic progress of a sheep enterprise can be minimized.

Ram selection is responsible for approximately 90% of the genetic change in a sheep flock. The amount of genetic improvement made in commercial sheep flocks is primary dependent on the genetic progress being made by the purebred or seedstock flock from which the rams are being purchased. As a rule of thumb the genetic merit of a commercial sheep flock increases at the same rate as the flock from which rams are being purchased. In short, whatever genetic progress or lack of progress that is being made by the purebred or seedstock producer is transferred to the commercial producer through purchased rams.

Identifying those sheep that are truly superior is a difficult task. Remember, what a person sees is not usually what they are getting. Less than half of what can be seen visually is due to genetic differences. The rest (over half) is due to what geneticists refer to as environmental differences -- did one eat more feed, etc. The only portion of a sheep’s superiority that can be passed on to its offspring is the portion that is due to genetic differences. In many cases those differences are masked by the environmental differences. Knowing this, we must conclude that we are probably not doing a very good job of picking those sheep that might change things such as lambing rate, weaning weight, etc., by visual appraisal. The only consolation is, that until recently there was not a better way.

As a result of rapid progress in genetic research and advances in computer technology, tools have become available to assess the differences in animals due to genetic differences. When this knowledge is properly applied, rapid changes in levels of performance can be achieved. Through the National Sheep Improvement Program (NSIP), expected progeny differences (EPD's) are made available to cooperating Targhee breeders.

Through the use of the performance records of genetically related animals, an animal’s own performance and a big computer, the actual genetic producing ability of an animal can be separated from that component which is due to environment. EPD’s are developed from a complex set of calculations which combine potentially large amounts of information on individuals and close relatives. While it is not important we know how EPD's are calculated, it is important that we understand that EPD's provide an accurate comparison of animals genetic ability.

An expected progeny difference (EPD) is a prediction of the difference between the future progeny of an individual and the performance of a theoretical reference animal with a zero EPD. EPD values are expressed as plus or minus deviations from a zero base point in units appropriate for each trait. EPD's below zero usually reflect low relative merit for a particular trait. However, for fleece grade a negative EPD is usually desired since that sheep would be finer.

As the name "Expected Progeny Difference" implies, EPD’s allow us to compare the relative expected progeny performance of individuals within a breed. For example, if two rams having EPD’s for weaning weight of +2.0 and –1.0 are bred to random ewes in the same herd , we would expect their lambs to differ in average weaning weight by 3.0 pounds (2-(-1)).

View Text-only Version Text-only Updated: 11/23/2005