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...When one contemplates the benefits of crossbreeding and hybrid vigor
it is important to remember that traits that are least heritable are
the traits that respond best to crossbreeding. Traits of importance
in the sheep industry would be the maternal traits, like lambs born, livability,
out of season breeding ect. which have low heritability (<30%).
Maximum hybrid vigor is accomplished with a 3 breed crossbred ewe mated
to a terminal sire. An example of this might be a Finn Dorset Rambouillet
ewe mated to a Suffolk or Hampshire ram. One loses some of the maternal
hybrid vigor when the crossbred ewes and rams are intersee mated rather
than using purebred ewes. The easiest system to generate the example ewe
above is to breed western Rambouillet ewes to FinnDorset crossbred rams.
Thus it only requires one purebred ewe breed and someone else to produce
the crossbred rams.
Hybrid vigor indicates the amount of superiority of the crossbred as
compared to the average of the two pure breeds. So for growth with a Finn
ewe mated to a Suffolk ram the offspring would be expected to grow about
6-10% faster than the average of the two breeds which might still be less
than Suffolk. This leads to a need to discuss that use of breed complementarity
to maximize the benefits from crossbreeding and hybrid vigor. In the
example of mating the Finn ewe to Suffolk ram we are using complementarity
correctly. The maternal superiority of the Finn ewe and superior growth
from the Suffolk ram. Using a Finn ram as terminal sire would not make
sense. However an awful lot of producers use blackface terminal sires
on the materanl side. They do not intentionally do this. It is the fact
that the speckled face lambs grow faster and get bigger so we keep some.
(Within a few generations) the flock is commercial
black face ewes with not much hybrid vigor.
The next four tables are from a paper that Dr. Kreg Leymaster wrote
for the Iowa Sheep Symposium several years ago.
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- Table 1. Relative production of alternative crossbreeding system.
| System |
Mating scheme (a) |
Percent Replacements (b) |
Relative % Purebreed (c) |
Cost advantage (d) |
| (1)Purebreeding |
A |
29% |
100% |
0 |
| Rotational |
ABr |
24% |
134% |
18% |
| Rotational |
ABCr |
23% |
143% |
21% |
| Composite |
ABc |
25% |
125% |
14% |
| Composite |
ABCc |
24% |
131% |
17% |
| Composite |
ABCDc |
23% |
138% |
19% |
| (2)Terminal |
Term X A |
29% |
122% |
13% |
| (3)Terminal |
Term X AB rotational |
24% |
146% |
22% |
| Terminal |
Term X ABC rotational |
23% |
153% |
24% |
| Terminal |
Term X AB composite |
25% |
141% |
20% |
| Terminal |
Term X ABC composite |
24% |
145% |
22% |
| Terminal |
Term X ABCD composite |
23% |
150% |
23% |
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- Note a: A, B, C, D and Term are distinct breeds.
- Note b: Percent of ewes in the flock that need to be mated to produce
replacement females in a self contained system.
- Note c: Production relative to pounds marketed in a purebred
flock.
- Note d: Based on assumption that 70% of cost
is overwintering ewes...value is under-estimated when a large terminal
breed is used.
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- Rotational assumes the use of purebreds, while the composite system
uses intersee matings of the initial crosses.
Table 2. Average estimates of individual heterosis.
(Offspring from purebred ewe mated to purebred ram of another breed).
Or, if you look at Table 1, it compares (1) to (2)
| Trait |
% Heterosis |
| Conception |
2.6 |
| Number born |
2.8 |
| Postnatal survival |
9.8 |
| Weaning weight |
5.0 |
| Weight weaned/ewe exposed |
17.8 |
Table 3. Average estimates of maternal heterosis. (Crossbred ewe from
purebred ewe mated to purebred ram of another breed)...Note
that basis has changed. Basis in Table 2was purebred lambs. Basis in Table
3is the "winner" from Table 2. Or, looking at Table 1, Table
3compares (2) to (3)
| Trait |
% Heterosis |
| Conception |
8.7 |
| Number born |
3.2 |
| Postnatal survival |
2.7 |
| Weaning weight |
6.3 |
| Weight weaned/ewe exposed |
18.0 |
Table 4. Average estimates of paternal heterosis. (Crossbred ram from
purebred ewe mated to purebred ram of another breed). As
in Table 3, the basis is the "winner" from Table 2.
| Trait |
% Heterosis |
| Conception |
4.7 |
| Number born |
2.4 |
| Postnatal survival |
0.0 |
| Weaning weight |
0.0 |
| Weight weaned/ewe exposed |
6.5 |
The last factor that ones needs to consider when undertaking a crossbreeding
system is flock size and facilities. If one has less than 100 ewes a two
breed rotation might be more than complex enough. In most cases people
should probably purchase replacement ewe lambs rather than raising their
own. Facilities impact the crossbreeding system because of breeding pens
or pasture for keeping different rams and their ewes separate.
The last five systems listed in Table 1 all show
a +20% reduction in cost relative to a purebred system. 20% is a huge
cost advantage when you are marketing a "commodity". That cost
advantage occurs because the winter feed bill is directly proportional
to weight of the flock. The cost of feeding the ram is negligible, so
it makes sense to have the huge ram and medium sized ewes. Most bang.
Least bucks.
There is one more advantage to using a "terminal"
breed system. At Michigan markets, black and speckle-faced lambs commanded
a 10% premium over generic white-face lambs. That kind of money is hard
to walk away from.
The counter-argument to using a cross-bred flock
is the loss of purebred sales. That is money you have to walk away from
if it is historically less than 20% of your gross sheep income. The basic
fact is that everybody is optimistic when it comes to money. They are
a little optimistic about costs but wildly optimistic about income.
Dan Morrical, PhD Sheep and Grazing Specialist
337 Kildee Hall
Iowa State University Ames,IA 50011-3150
ph 515-294-0847
fax 515-294-3795
morrical@iastate.edu
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