Since life began, animals best adapted to their environment have survived and produced the largest number of offspring. For example, most breeds of sheep that originated in the British Isles survived only if they were born in the spring when the temperature was mild and feed was available. That is natural selection.
Selection should be a part of all breeding sheep production enterprises. It is effective for almost all the important economic traits in sheep. No selection program, however, can improve all these economically important traits at once. Generally, the more traits involved in selection, the less improvement will result for a single trait. The first step in any selection program is to identify the traits of greatest economic importance. They may be growth rate, carcass merit, fleece traits, or reproductive efficiency.
The improvement that can be made depends on:
- Accurate measurement of the trait.
- Complete records on the flock.
- The amount of selection pressure applied.
- The amount of variation of the trait or different traits among individuals within the flock. If the sheep do not vary genetically, then no improvement can be made. If they vary greatly, then improvement will be rapid when producers select only the individuals that excel in the expression of important traits.
- The heritability of the trait. Variation in any economic trait is caused by genetic differences and environmental differences. Variation that results from differences in heredity is broadly defined as heritability.
HAF Researchers have estimated the heritability (the ability to “pass on” traits to offspring) of the economically important traits (table 1). Generally, if the heritability estimate is less than 20 percent, progress is slow. A heritability estimate of 20 to 40 percent is considered medium. A heritability estimate greater than 40 percent is high.
Purebred breeders should be committed to improving the economically important traits of their breed. Their breed serves as a source of genetic material for crossbreeding and for improving the industry.
On the other hand, commercial sheep producers might find it more profitable to crossbreed. Some economically important traits that can be improved only slowly within a breed can be improved more rapidly with effective crossbreeding. An example is rate of reproduction. By most estimates, the heritability of reproductive traits is low. However, hybrid vigor (expression of a trait above the average of the dam and sire for that trait) exists for rate of reproduction. Generally, crossbred ewes exhibit a higher reproduction rate, produce more milk, and their lambs are stronger at birth.
| Heritability of traits in sheep | |
| Trait | Percent |
| Birth weight | 0.15 |
| Weaning weight (60 days of age) | 0.20 |
| Weaning weight (120 days of age) | 0.25 |
| Mature body weight | 0.40 |
| Rate of gain (post-weaning) | 0.40 |
| Face cover | 0.35-0.55 |
| Skin folds | 0.20-0.50 |
| Grease fleece weight | 0.25-0.60 |
| Clean fleece weight | 0.25-0.60 |
| Clean yield | 0.30-0.40 |
| Staple length | 0.30-0.65 |
| Fleece grade | 0.20-0.60 |
| Multiple birth | 0.10 |
| Milk production | 0.10 |
| Ewe productivity | 0.20 |
| Loin-eye area | 0.35 |
| Fat thickness over loin eye | 0.30 |
| Carcass weight | 0.35 |
| Retail cut weight | 0.45 |
| Dressing percentage | 0.10 |