Sheep Extension Program
Montana Farm Flock Sheep Production Handbook Sheep Selection — Section 2 of 7
By Rodney Kott, Extension Sheep Specialist, Animal & Range Sciences Department, Montana State University
Although the correct feeding of the ewe is probably the most important key to profitability, the genetic makeup of the ewe must also be suited to the demands made upon her. Before deciding upon a genetic program, a producer’s production situation relative to climate conditions, feed resources, facilities, available labor, market options and management level should be defined. Production objectives relative to the desired growth and reproductive rates, balance between lamb and wool production etc. must be determined. Traits that most affect income such as lambing rate, weaning weight, fleece weight and grade are often overlooked since they are difficult to measure. It’s a lot simpler to just pick the biggest and the prettiest ones. Visual appraisal of prospective breeding animals is a multi-billion dollar beauty contest. Sizing up sheep, as well as cattle and swine, does not have the aura of beauty pageants or major sporting events, but it is probably the largest "contest" going on in the US.
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 are possible. The management provided determines the degree to which that potential is realized.
Selection goals vary among breeds and individual producers. The producer should decide which traits are economically important to him and place emphasis on them in a selection program. Heritabilities and relationships of individual traits should also be considered when selecting replacements. Examples of traits which are usually considered when selecting sheep include:
|Reproductive Rate (15% heritable)||Although the heritability for multiple birth is low, progress through selection can be made. Twin and triplet replacements, or a replacement from a dam with a high reproductive rate, should be identified and preference given to them at selection time.|
|Weaning Weight (10% heritable if weight is taken at about 60 days of age and 30% heritable if weight is taken at over 100 days of age)||A lamb’s weaning weight is a good indicator of a ewe’s milk producing ability, as well as an early indication of growth potential. Weaning weights taken at about 60 days of age are highly influenced by the dam’s milk producing ability. Use of these weights (60 day weaning weights) in selecting replacements is usually not recommended as they have not yet had a chance to express their true growth potential. By 100 days of age, much of the dam’s effect is removed and thus these weights favor lambs that have the ability to grow on their own. When weaning weights are used as a selection criteria, the weights should be corrected for age, sex, type of birth and rearing and age of dam.|
|Post-Weaning Growth Rate (30% heritable)||Although weaning weights can be used to estimate an animal’s growth potential, a more accurate evaluation of a individual’s growth potential can be made from a post-weaning growth trial. Post-weaning rate of gain reflects directly on the lamb’s actual growth potential.|
|Wool Production (40% heritable)||Individual producers should determine the economic importance of wool in their sheep operation and apply selection pressure accordingly. Pounds of wool, staple length and fiber diameter are wool traits commonly included in selection programs.|
|Inherited Defects||Defects such as jaw abnormalities, cryptorchidism and inverted eyelids should always be avoided.|
Once the traits which are to be used in the selection program are determined, the producer must then evaluate the genetic potential of possible replacement ewes and rams for these traits. The desired genetic potential of sheep can be evaluated by different procedures. They include (arranged in order of increasing intensity and accuracy): pedigree, visual appraisal including show ring winnings, performance testing or measurement or the individual’s own performance or measurement of progeny performance (NSIP). The selection of ewes can often best be accomplished using some form of mass or simplified selection system. On the other hand, selection of rams should involve as accurate evaluation process as possible. Very few commercial cattle enterprises do not use performance data (EPD’s) to select herd sires. However, this is not the case in the sheep industry.
Management for genetic improvement requires a mix of art and science and may involve a varying degree of chance. By properly planning and developing goals for a selection program and then 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.
Keeping a good set of records on ewes and rams in the breeding flock and watching for weakness within the flock can help in assuring steady positive progress within your flock. These records can be extremely useful in identifying the most productive ewes within the flock and thus replacement stock which may have superior genetics. There are many different production record-keeping systems used by individual sheep producers. In many instances they are tailor made to fit individual needs. Production records such as the one shown in figure 4 are relatively simple and easy to keep. However, the basic principle still applies — some kind of record keeping pays off. Once a producer decides which traits are important, they must then figure out how to identify those sheep that are superior. 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, is the bigger lamb a single, 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 trait easily changed by visual appraisal is mature body size. The only consolation is that until recently there was not a better way.
Quicker genetic progress could be made by standardizing environmental conditions as much as possible and objectively measuring differences in production — hence the development of On-farm and Central performance testing. Although performance testing does not eliminate environmental influences, they are controlled to some degree. This changes our ability to identify those animals, and in particular rams, that are superior. Although we have greatly reduced the differences due to environment, chance still plays a significant role in our selection program.
As a result of rapid progress in genetic research and advances in computer technology, tools have become available to access 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), developed by the American Sheep Industry, expected progeny differences (EPD’s) are made available to cooperating 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. They 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 applicable 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 animal would be smaller or finer.
|EPD — A prediction of the performance of future progeny of an individual. It is a measure of only the genetic differences between animals.|
|Sheep A||Sheep B|
|Type of Birth||+.10 lambs||-.10 lambs||Daughters from Sheep A will have a 11% [(+.1) - (-.01) = +.11] higher lamb crop than those from Sheep B.|
|Weaning Weight||+3.12 lbs.||-1.02 lbs.||Daughters from Sheep A will have an average weaning of 4.14 more pounds than those from Sheep B.|
|Fleece Weight||+.04 lbs.||+.14 lbs.||Daughters of Sheep A will shear .10 lbs. of wool per shearing less than those from Sheep B|
|Fleece Grade||-.20 microns||+.10 microns||Daughters from Sheep A will be .30 microns finner than those from Sheep B.|
The ram’s contribution to the profitability of a sheep operation is frequently neglected. The greatest impact of selection on sheep performance can be made through ram selection. Small producers who feel that their sheep flock is not large enough to justify purchasing a quality ram should consider renting one, or purchasing one in partnership with another producer before using a poor quality ram. Remember, it does not take too many pounds of lamb and/or wool to justify using a good quality ram, as opposed to an average one.
Careful selection of rams can benefit the producer in two ways. It contributes to the production efficiency of every lamb and to the genetic improvement of economically important traits in the herd. In Montana most commercial sheep flocks produce their own replacements but purchase their rams from purebred or seedstock producers. Since relatively large numbers of ewe lambs are needed for replacements and often detailed production and genetic records are not available (increasing the role chance plays in the selection process) genetic progress through ewe selection is limited. In most sheep flocks 80 to 90 percent of the genetic progress comes from ram selection and only 10 to 20 percent comes from the selection of ewes.
The amount of genetic improvement made in commercial sheep flocks is primarily 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, only about two generations (6 to 8 years) behind, 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. What happens if the selection goals of the purebred producer and the commercial producer differ? For example, if the purebred producer’s selection is based only on show-ring type traits, commercial producers purchasing rams from that producer are making progress in those same show-ring traits that the purebred producer is. However, little or no progress is made in the traits that are economically important (reproductive rate, total pounds of lamb weaned, etc.) to the commercial producer.
The profit or loss of a sheep enterprise often is determined by the type of rams that are used. It is always difficult to determine how much money one can justify paying for a ram. However, a good ram is worth at least four times the value of a good market lamb. Selection of a ram is a major decision, not something to be done on the spur of the moment. Nearly always the best ram will prove to be by far the cheapest. Likewise, a poor ram will certainly be expensive.
Expected change in a commercial sheep flock when rams are purchased from a purebred flock in which genetic improvement is being made.