Purebreeding

Purebreeding is the mating of rams and ewes of the same breed. A purebred flock can be managed as a single flock because all ewes and rams are of the same pure breed. The goal of purebred sheep production should be to supply genetics (seedstock) to the commercial sheep industry. Seedstock are marketed as rams and replacement ewes to other seedstock producers or to commercial sheep operations.
Improvements in purebred sheep should be documented through records. The National Sheep Improvement Program collects performance data from purebred producers and provides them with across-flock EPDs. "EPD" is short for "Expected Progeny Difference." An EPD is an estimate of the genetic merit of an animal for a given trait. Specifically, the EPD of an animal is the expected difference between the performance of that animal's progeny (offspring) and the average progeny performance of all the animals in the breed for that trait. For example, if a lamb has a weaning weight EPD of 2.0 lbs., this means that its progeny will be 2.0 lbs. heavier than the average lamb in the breed. Australia's LAMBPLAN is a similar system for determine genetic worth of a sheep. The show ring generally does a poor job of identifying genetically superior sheep, particularly maternal breeds of sheep whose traits cannot be evaluated by visual appraisal.

Within purebreeding, there are several types of mating systems. Outbreeding is the mating of animals of the same breed but which have no closer relationship than at least 4 to 6 generations. Outbreeding is the recommended breeding practice for most purebred sheep breeders.

Inbreeding is a system of breeding in which closely related animals are mated. This includes sire to daughter, son to dam, and brother to sister. Technically, inbreeding is defined as the mating of animals more closely related than the average relationship within the breed or population concerned. The primary genetic consequence of inbreeding is to increase the frequency of pairing of similar genes.

Inbreeding is essential to the development of prepotent animals — animals that uniformly "stamp" their characteristics on their progeny. Inbreeding may also be used to uncover genes that produce abnormalities or death — genes that, in outbred herds, are generally present in low frequencies. Inbreeding is suggested for only highly qualified operators who are making an effort to stabilize important traits in a given set of animals. In general, inbreeding results in an overall lowering in performance: vigor, disease resistance, reproductive efficiency, and survivability. It also increases the frequency of abnormalities. For example, the spread of spider lamb disease in black-faced sheep is believed to be the consequence of inbreeding.

Linebreeding is a system of breeding in which the degree of relationship is less intense than in inbreeding and is usually directed towards keeping the offspring related to some highly prized ancestor. The degree of relationship is not closer than half-brother half-sister matings or cousin matings, etc. Line breeding is a mild form of inbreeding.


Crossbreeding
Crossbreeding is the mating of rams and ewes of different breed compositions. However, it does not denote indiscriminate mixing of breeds, but rather is a systematic utilization of different breed resources to produce crossbred progeny of a specific type. Crossbreeding is used extensively in the commercial sheep industry and the majority of slaughter lambs are crossbred.
Crossbreeding offers two distinct advantages: 1) heterosis; and 2) breed complementarity. Heterosis or hybrid vigor is the superiority of the crossbred offspring. Mathematically, heterosis is the difference in performance between the crossbred and the average performance of the purebred parent. There are effects of heterosis in the crossbred offspring, crossbred dam, and crossbred ram. In general, crossbred individuals tend to be more vigorous, more fertile and grow faster than purebreds. Effects of heterosis tend to be large for traits that are lowly heritable (e.g. reproduction) and small for traits that are highly heritable (e.g. growth, carcass, and wool). The effects of heterosis are cumulative. Heterosis can be maximized by mating crossbred ewes to a ram of another breed to produce crossbred offspring. Composite breeds such as the Katahdin and Polypay capture most of the benefits of heterosis.

The second major advantage of systematic crossbreeding lies in the ability to utilize breed complementarity. All breeds have strengths and weaknesses. No one breed excels in all relevant traits. Thus, production can be optimized when mating systems place breeds in roles that maximize their strengths and minimize their weaknesses.

Mating Polypay ewes to Suffolk rams is an example of matching complementary strengths of breeds to optimize efficiency of a production system. This cross takes advantage of the reproductive efficiency and moderate maintenance costs of Polypay ewes while producing Suffolk-sired lambs to meet market requirements for fast-growing, heavy muscled lambs. The efficiency of this cross would be much greater than the reciprocal mating of Suffolk ewes to Polypay rams. The latter cross would produce genetically equivalent market lambs (half Suffolk and half Polypay), but fewer lambs would be sold and production costs greatly increased due to higher feed requirements of heavy Suffolk ewes compared to Polypay ewes.

Click HERE to read an article on Crossbreeding Sheep by USDA.

Crossbreeding Systems
There are several systematic crossbreeding systems. Terminal crossing makes maximum use of both heterosis and breed complementarity. It may utilize two, three, or four breeds, and can be as simple as crossing two purebreeds. In terminal crossing, all of the crossbred offspring are sold and replacement ewe lambs must be purchased or produced in the flock by mating a proportion of the flock to rams of the same breed. In a three or four breed terminal crossbreeding system, crossbred ewes and crossbred rams can be utilized in the system to maximize heterosis.

Rotational crossing will also maintain high levels of heterosis. Rotational crossing involves alternating the use of rams of two, three, or more breeds. Ewes are mated to rams of the breed which they are least related. It works best when breeds which function acceptably as both ram and ewe breeds, are utilized.

Roto-terminal crossing involves both terminal crossing to produce market lambs and rotational crossing to produce ewe lambs. The best ewes in the flock would comprise the nucleus flock. They would be used to produce replacement ewes. The rest of the ewes in the flock would be bred to a terminal sire to produce market lambs.

Grading up denotes the repeated crossing of ewes and their female progeny to rams of a single breed, with the ultimate objective of creating a flock that is indistinguishable from purebred flocks of the ram breed. It is used when only rams of the breed of interest are available or affordable.

Crossbreeding is also used to form new or "composite" breeds. Once the crossbred base population has been formed, the flock is managed as a purebred flock. This is how many new breeds are created.

Many of the aforementioned crossbreeding systems are difficult to accomplish in a small flock, which may only have the option of one or two breeding groups. The purchase of replacement females would enable the use of a terminal crossing program. Alternating the use of ram and ewe breeds would maintain maternal and growth characteristics in the flock.