What is the most common breeding method for sheep?
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 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.
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.
Crossbreeding is not an immediate cure-all. The results will be disappointing with poor quality parent stock or with a breed or breeds that are not adapted to the environment. For a successful crossbreeding program, use breeds that contribute the highest combination to the economically important traits. For example, if a producer wants to market lambs, then ewes from breeds that excel in the following characteristics should be selected:
• Reproductive efficiency.
• Desired time and length of breeding season.
• Conception and lambing rate.
• Milk production.
• Maternal instincts.
Crossbred ewes with these characteristics could then be mated to rams of another breed to produce market lambs. Traits to look for in the ram breeds are: