University of Kentucky College of Agriculture, ASC-224 Food and Environment Cooperative Extension Service Crossbreeding Considerations in Sheep Debra K. Aaron, Animal and Food Sciences rossbreeding is the mating of individu- Cals from different breeds. To a certain extent, it is a simple concept, but em- barking upon a crossbreeding program, in sheep or any other livestock species, involves long-term decisions. The primary benefits of a crossbreed- ing program are heterosis and breed complementarity. Heterosis Heterosis, or hybrid vigor, refers to the increased vitality or “doing ability” of the crossbreds as compared to the average of the parental breeds. The only way to get heterosis is through crossbreeding. Heterosis is expressed by the cross- bred lamb (individual) and by the cross- bred ewe (maternal). Crossbred lambs, on the average, are more vigorous and have a higher survivability than purebred lambs. They may also have a faster preweaning Table 1. Potential heterosis effects in sheep growth rate. The 1F (first cross) crossbred Crossbred Lamb (Individual) Crossbred Ewe (Maternal) ewe exceeds the straightbred ewe in Trait Heterosis (%) Trait Heterosis (%) many traits, especially those related to Birth weight 3.2 Fertility 8.7 reproductive fitness and longevity. Weaning weight 5.0 Prolificacy 3.2 Do all traits respond to crossbreeding? Preweaning ADG 5.3 Body weight 5.0 Postweaning ADG 6.6 Fleece weight 5.0 Traits that are lowly heritable (reproduc- Yearling weight 5.2 Lamb birth weight 5.1 tive traits, traits influencing resistance Survival to weaning 9.8 Lamb weaning weight 6.3 to stress, survivability and longevity) Carcass traits ≈ 0 Lamb survival to weaning 2.7 are the ones that respond the most to Lambs born/ewe exposed 11.5 crossbreeding (that is, have the greatest Lambs reared/ewe exposed 14.7 Weight of lamb weaned/ewe 18.0 amounts of heterosis). These are traits exposed that are difficult or slow to improve Source: Sheep Production Handbook, American Sheep Industry Assoc., Inc., 2002 ed., vol 7. through selection. In general, as herita- bility increases, the amount of heterosis decreases. Carcass traits, for example, are highly heritable. Selection results in fairly However, when total productivity (for Breed Complementarity example, lamb survivability and growth quick genetic improvement for carcass No one breed of sheep is best for all traits, but crossbreeding has little to no rate) is considered, heterotic effects ac- cumulate to provide a rather substantial traits. So, the second primary benefit of effect. Crossbreeding is often used to a crossbreeding program is the advanta- improve performance of low heritability improvement over straightbred sheep. Also, mating F ewes to a ram of a third geous use of breed complementarity. traits. Average heterosis effects for vari- 1 Complementarity refers to the combin- ous traits are shown in Table 1. breed to produce crossbred offspring can maximize heterosis. ing of desirable traits from two or more The effect of heterosis may seem -mi breeds into one animal. The idea is that nor when only one trait is considered. the strengths of one breed will make Cooperative Extension Service | Agriculture and Natural Resources | Family and Consumer Sciences | 4-H Youth Development | Community and Economic Development up for the weaknesses of another breed. duction, longevity Table 2. Classification of some common sheep breeds For example, the Hampshire is known and good mother- Specialized Sire Specialized Dam General Purpose as a heavy mature-weight breed that ing ability. Most (Ram) Breeds (Ewe) Breeds Breeds produces fast-growing lambs with desir- ewe breeds are Cheviot Border Leicester Cheviot able carcass characteristics. However, whitefaced. Some Dorper/White Dorper Columbia Columbia Hampshire Corriedale Corriedale Hampshires are not especially noted for breeds are clas- Oxford Finn Dorset prolificacy or mothering ability. Because sified as general Shropshire Katahdin Montadale of this lack, the Hampshire is referred to purpose because Southdown Merino Polypay as a specialized “sire” or “ram” breed. The they possess traits Suffolk Polypay Texel Polypay is known primarily as a special- that make them Texel Rambouillet Romanov ized “dam” or “ewe” breed because of its equally suitable as Romney prolificacy, milk production and overall either a ram breed Targhee mothering ability. They are not as well or a ewe breed. known for rapid growth rate, muscling General purpose or carcass quality. So, if a Hampshire ram breeds provide a balance between meat of market lambs. Rotational systems use is mated to Polypay ewes, the offspring and wool, are adaptable to a range of en- rams of two or three breeds in alternating benefit from the maternal environment vironmental conditions and are the best generations. Heterosis increases with the provided by the ewe breed and can be choice for small flocks where crossbreed- number of ram breeds used, but manage- expected to grow faster and have more ing programs are not feasible. ment is more complicated because ewes desirable carcasses as a result of the To maximize the benefits of breed must be separated into different flocks ram breed’s contribution. Because the complementarity, breeds must be chosen during the breeding season. The primary Polypay is a more prolific breed than wisely. A rule of thumb noted by various advantage of rotational systems is that, the Hampshire, the producer can also authors about other livestock species is, unlike terminal systems, they generate expect to wean more lambs than would “If you aren’t happy with the performance replacement ewes. be weaned from straightbred Hampshire of the straightbreds then you probably sheep. won’t like the crossbreds either.” Almost Two-Breed Terminal Cross The efficiency of mating Hampshire all sheep producers will choose to include In the two-breed terminal cross rams to Polypay ewes would be much the breed of ewes currently in their flock, system (Figure 1), rams of one breed greater than the reciprocal mating simply by default. The key is to find the are mated to ewes of a second breed, of Polypay rams to Hampshire ewes. breeds that complement each other in a resulting in crossbred lambs. This system Although the resulting offspring are production system. allows flexibility in establishing selec- genetically the same (half Hampshire tion goals, takes maximum advantage and half Polypay), fewer lambs would be Crossbreeding Systems of breed complementarity and yields expected and production costs would What crossbreeding system works 100 percent of the potential heterosis in likely be increased due to higher feed best? Answering this question involves the crossbred offspring. The method is requirements of the heavier Hampshire balancing complexity and flock size with relatively simple because it involves one ewes as compared with Polypay ewes. the ability to maintain heterosis. There breed of ewes and one breed of rams. Instead of Hampshire rams, South- are two types of crossbreeding systems Only one breeding flock is necessary. down or Suffolk rams could be bred to that are feasible for most commercial However, the two-breed terminal cross the Polypay ewes. Like the Hampshire, sheep producers: terminal (two- and is not a self-perpetuating system. All these breeds are considered to be ram three-breed) and rotational (criss-cross crossbred lambs are marketed (hence, the breeds. Or, Hampshire rams might be and three-breed). Terminal cross systems name, “terminal cross”). Therefore, it re- used on White Dorper ewes. On the are widely used to maximize production quires input of straightbred ewes into the ram side, this takes advantage of the traits described previously. Work at the University of Kentucky has shown that Figure 1. Crossbreeding the White Dorper, although not generally systems involving two or Two-Breed Terminal Cross thought of as a ewe breed, has enough three breeds in terminal All lambs sold and rotational systems maternal ability to be used on the dam Ram Breed A Ewe Breed B side and has the advantage of not hav- ing to be sheared. Classification of some common breeds is shown in Table 2. No replacement females Only takes advantage generated; must be purchased ½ A x ½ B of heterosis expressed lambs In general, specialized ram breeds are or produced in another ock. by crossbred lamb. characterized by rapid growth, muscular- ity and carcass quality. Most ram breeds Three-Breed Terminal Cross are blackfaced. Maternal (ewe) breeds All lambs sold tend to have high fertility, high milk pro- Breed A Breed B 2 Breed C ½ A x ½ B + Cull ewe lambs replacement and surplus male lambs ewe lambs Takes advantage of heterosis expressed ½ C x ¼ A x by crossbred lamb and crossbred ewe. ¼ B lambs Criss-Cross (Two-Breed Rotational Cross) Male lambs and cull ewe lambs sold Assumed initially that ewes A-sired replacement ewe lambs within the ock can be divided into those of predominantly Breed A genetics, which will be bred to a Breed B ram, and those of predominantly Breed B, which Breed A Breed B will be bred to a Breed A ram. During each successive gen- eration, replacement ewe lambs are exposed to the breed of ram B-sired replacement ewe lambs to which they are least related. Three-Breed Rotational Cross Male lambs and cull ewe lambs sold C-sired Breed A A-sired replacement replacement ewe lambs ewe lambs Breed C Breed B B-sired replacement ewe lambs system at some point. Replacement ewes Criss-Cross (Two-Breed It requires three breeding flocks. Replace- must either be purchased, or a portion of Rotational Cross) ment rams are the only animals that have the original ewes must be bred to rams to be purchased. The main advantage of of that breed to produce replacement ewe The criss-cross, or two-breed rotation- the three-breed rotational cross over the lambs. al (Figure 1), is the simplest long-range criss-cross is that it maintains a higher crossbreeding system.