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:
- make big changes in the performance of your flock, or
- 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
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
Crossbreeding can be used effectively to do the following things:
- 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.
- 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.
- 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
- 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.
- 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.
- 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
Figure 1. Chart showing the average number of lambs born for Breed A and Breed C.
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
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
Crossbreeding is a very effective tool and can be used to enhance
the efficiency of your operation as long as you have a specific