Crossbreeding of Sheep

Crossbreeding occurs when two distinctly different breeds of animals are mated to each other. Crossbreeding is an effective tool, which can be used to:

1. make big changes in the performance of your flock, or
2. it can be used to create huge management problems. It can particularly become a problem when you are trying to increase the size of your flock significantly.
   

Producers often start crossbreeding because they want to try different breeds. It is very satisfying to use a new breed of ram on your ewes and see a distinctly different result in the lambs. There are so many different sheep breeds that we are continually hearing about a new or different breed that has 'so many good traits'. It can be very tempting to try several new breeds over time. Without a plan on how the crossbreeding will be managed, the flock soon becomes a mix of a bunch of different breeds that are difficult to manage successfully.

Crossbreeding can be used effectively to do the following things:

1. Improve the performance of the whole production system by crossing complementary breeds.
   Most often this is done by using maternal type breeds for ewes and terminal type breeds for rams. This is integrated into the whole production system. For example, Dorset ewes may be used for the ewe flock because they have good maternal traits, breed out of season and are easy keeping sheep. Suffolk rams may be used to produce heavy market lambs. The Suffolk ram will increase the growth rate and size of the market lambs. This way the whole production system is more efficient than if just the Dorset or the Suffolk breeds were used.
2. To produce animals of intermediate performance from extreme parent breeds.
   To create individual animals of intermediate performance rather than to match different breeds to different roles in the production system. For example, a Border Cheviot may be crossed with a Rideau for a once a year lambing system. The resulting cross will have more lambs than a Cheviot but not be quite as hardy.
3. To upgrade to a different pure breed.
   Many producers use this method to change breeds. For example, someone with a Suffolk ewe flock may decide that they want to breed on an accelerated program and to do that effectively with their system they should have a Dorset flock. The easiest way to change particularly from a cost point of view is to start buying Dorset rams. After a few generations, the flock will essentially be Dorset.
4. As a step in creating a new synthetic or composite breed.
   New breeds are generally made up of some combination of existing breeds. Crossbreeding is the first step in creating the new breed. Crossing continues until the foundation animals have the planned mix of breeds and then the foundation animals are mated amongst themselves. At this point the original breeds are no longer used.
5. To introduce a single gene into an existing breed.
   An example of this is the Booroola Dorset. The Booroola gene was found in the Merino breed of sheep. To move the Booroola gene into the Dorset breed, Dorsets had to be crossed with Merinos to get the gene into the Dorset breed. Then the Dorset cross Merino sheep were bred Dorset for several generations keeping the Dorsets which passed on the Booroola gene.
6. To take advantage of heterosis.
   When two animals are crossed together it is expected that the performance of the progeny will be the average of the performance of the parents. Heterosis or Hybrid Vigour is the name for the increased performance above the average of the parents that you get when crossing two different breeds. Heterosis is the opposite of inbreeding. Crossing two very different breeds together creates animals that have more heterozygote gene pairs and fewer homozygous pairs than the purebreds. This results in animals that have higher performance particularly in reproduction, survival and fitness traits. In the figure below, the average number of lambs born is shown for Breed A and Breed C. The average performance if you crossed these two breeds would be expected to be 1.5 lambs born, but in actual data the average number born turned out to be 1.7 lambs born. The extra 0.2 lambs born is due to heterosis between the two breeds. If you then take the AC progeny and cross them back to Breed A or C there will be less heterosis than in the original cross.

Figure 1.

There are a couple of important aspects to using crossbreeding for heterosis. The most noticeable effect of heterosis is seen in the reproduction, survival and fitness traits. For reproduction traits you have to remember that there will not be more lambs born from the Dorset ewe that is crossed to the Suffolk ram. There will be more lambs born from the Dorset cross Suffolk ewe lamb when she becomes a parent. The other important aspect of heterosis to remember is, depending on the breeds being crossed and the trait that is being measured, heterosis may have no value. For example, the objective of one crossbreeding plan is to get as many lambs born as possible. If you look at the figure below for Breed A and Breed C, it is obvious that crossing these two breeds will not give you the most lambs even though there is a lot of heterosis. You would have the most lambs by only using Breed C.

Many producers crossbreed to enhance the effectiveness of the production system and try and take advantage of heterosis by using maternal animals for the ewe flock and terminal animals to produce market animals. The most important part of this system is to plan how you will produce your replacement ewe lambs. All lambs sired by a terminal sire should be sent to market rather than kept as replacement females for your operation. This is because maternal traits like milking ability and number born tend to be negatively correlated to terminal traits like muscling and lean yield. This method should also show heterosis for lamb survival if you are using prolific ewes in the maternal line.
If you are increasing the size of your flock it is also important to keep your ewe flock uniform with animals of similar size and performance. This makes management much easier because the nutritional needs of the animals will be similar. Keeping ewes of different breeds and crosses can be very complicated if you are lambing on an accelerated program using some prolific sheep. For example, you will have open ewes, pregnant ewes at different stages of pregnancy and lactating ewes with singles, twin and triplets that may all have different nutritional requirements. Different breeds will have different nutritional requirements in all of those stages and could almost double the number of groups of animals that have different feed requirements.

Crossbreeding is a very effective tool and can be used to enhance the efficiency of your operation as long as you have a specific plan.