Crossbreeding now proven, yet some farmers unwilling to make the leap

01-06-2003 | Gerard Hall

Profit from sheep is directly affected by their genetic make up. For some it is this lack of genetic potential that limits production.

While most sheep farmers aspire for continual improvement of the genetic merit of their flock, many have never considered looking outside their present ewe breed except when they buy a terminal sire to crossbreed.

While good results can be achieved with purebred sheep, completely different gene mixes introduced through use of other breeds are seen by many as providing scope to lift profit to the next level.

The release of several new breeds in recent years has seen interest in crossbreeding lift to an all time high.

While some farmers are keen to exploit the genetic potential the new breeds may offer no matter where they come, others remain uncertain and in some cases sceptical about the role they have to play and the best way to incorporate the desirable traits.

Hybrid vigour is a well-known phenomenon. It leads to crossbred sheep outperforming the average of the purebreds they were developed from. It is usually thought of as positive but it can lead to detrimental changes in performance. Crossbreeding offers another advantage that is not due to hybrid vigour. This is a better different balance of traits due to the parent breeds being “complimentary” to each other.

After years of running purebred flocks crossbreeding for some looks like a leap headlong into the unknown. In part this is because we know less than we need to in order to predict the gains and losses crossbreeding might give.

Questions farmers are seeking answers to include; should they stick with what they have, change breed completely, add a bit of this and that, or follow a crossing strategy then stabilise a new “breed”.

Over a range of farms and farmers the answer to each question is probably yes but for different reasons.

Since each farm is different, and each farmer’s business plans differ, there will never be an ideal genetic mix for all farms. However, those considering a crossbreeding option should have one common objective-a more profitable and sustainable business.

This may be driven by the desire or need for greater profit but may also be driven by personal satisfaction in producing a product of higher quality and value.

To answer some of the often-asked questions Country-Wide spoke to animal geneticists Dorian Garrick and Mark Young.

Dr Garrick is Professor of Animal Breeding and Genetics at Colorado State University and Massey University while Dr Young is senior adviser, Sheep Improvement Limited (SIL). Both were keynote speakers at the recent series of Lincoln University Animal Industry workshops.

Dr. Young says the key thing about hybrid vigour is that it is a function of three populations, the two parent populations and then their cross. If you have information on less than the three you cannot calculate the hybrid vigour effect.

“Hybrid vigour is quite simply the extent which the cross outperforms the average of the parent breed, it doesn’t necessarily mean it will outperform each of the breeds making up the crossbreed (F1) but it does outperforms the average of them,” Dr Young says.

Traits that respond well to heterosis tend to be the complex traits or those that are less well inherited, those that have a low heritability or genetic component. These include traits of high economic value like fecundity, mothering ability and hardiness.

Traits that are relatively easy to increase through selection are usually less responsive; wool and growth rate traits being good examples.

Another rule of thumb is that closely related breeds (e.g. Romneys and Coopworths) will show less hybrid vigour when crossed than will breeds that are more distantly related (e.g. Merino and Border Leicester).

Another important but often forgotten principle is what happens when you interbreed the first cross (F1) progeny. In essence if you breed (F1) sires and dams together, there will be less hybrid vigour in the progeny. Like wise hybrid vigour also drops if you breed back to either of the parent breeds.

In practice however things are more complicated. One is that hybrid vigour can occur in the lamb or calf itself, the direct effect, or in the ewe or cow, the maternal effect.

The effects are one generation out of phase. Since hybrid vigour declines after the first cross, and the direct and maternal hybrid vigour are one generation out of phase, this effect does not stabilise till the third generation after crossing begins.

It will be delayed further due to linked genes, which are not independently inherited. The good news is that the lift in performance due to hybrid vigour is not lost. It is just that it is hard to predict how much is maintained and for how long.

What hybrid vigour you get depends on how the matings are planned. Mating a Texel x Romney ram to Finn ewes results in offspring that exhibit half Romney-Finn hybrid vigour but no Romney-Texel hybrid vigour. These animals are 1/4 Texel, 1/4 Romney and 1/2 Finn. If a composite of the same breed fractions were stabilised, it would exhibit 1/8 Texel-Romney hybrid vigour and 1/4 of each Finn-Romney and Finn-Texel hybrid vigour.

The composite would therefore have lost half of the hybrid vigour that was present in the first-cross but gained 1/8 Texel-Romney hybrid vigour.

This stabilised composite will have different levels of hybrid vigour from a foundation crossbred of the same composition that has been created by crossing differing parental breeds.

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