Katahdin flock
Bluefaced Leicester ewe
Montadale flock
Purebred Charollais ram
Crossbred ewe
Crossbred Dorper ram
Hampshire x Polypay lamb
Suffolk ram
Photo courtesy of Donner
Trail Ranch
Crossbred Southdown lamb
Grading up to Dorper
Polypays: a composite breed
Breeding systems
Animal breeding is a branch of animal science that addresses the
evaluation of the genetic value of domestic livestock. A breed is
a group of domestic animals with a homogeneous appearance, behavior,
and other characteristics that distinguish it from other animals.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 (NSIP) 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.
Outbreeding
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
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
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.
Heterosis in the crossbred lamb
|
Trait |
Percent heterosis |
| Birth weight |
3.2 |
| Weaning weight |
5.0 |
| Pre-weaning ADG |
5.3 |
| Post-weaning ADG |
6.6 |
| Yearling weight |
5.2 |
| Conception rate |
2.6 |
| Prolificacy of dam |
2.8 |
| Lamb survival |
9.8 |
| Carcass traits |
0 |
| Lambs born/ewe exposed |
5.3 |
| Lambs reared/ewe exposed |
15.2 |
| Weight of lamb weaned/ewe |
17.8 |
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.
Heterosis in the crossbred ewe
|
Trait |
Percent heterosis |
| Fertility |
8.7 |
| Prolificacy |
3.2 |
| Pre-weaning ADG |
5.3 |
| Body weight |
5.0 |
| Lamb birth weight |
5.1 |
| Lamb weaning weight |
6.3 |
| Lamb survival |
2.7 |
| Lambs born/ewe exposed |
11.5 |
| Lambs reared/ewe |
14.7 |
| Weight of lamb weaned/ewe |
18.0 |
Breed complementarity
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.
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.
Terminal crossing
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
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
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
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.
Composite breeds
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.
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