Hybrid vigour can be a powerful force to increase or even decrease the reproductive performance of sheep, but just how effective it is to increase the mature live weight remains sceptical.
Doug Fowler of Sheep Pregnancy Screening said any infusion of a new bloodline to an existing bloodline needs to be evaluated carefully.
"When Merino ewes are mated to rams of different bloodlines, there may be hybrid vigour in the first cross progeny," Dr Fowler said.
"The amount of hybrid vigour depends on a variety of factors, however the performance of the first cross of the two bloodlines will be halfway between the performance of the two pure bloodlines if there is no hybrid vigour and greater than this where there is hybrid vigour."
In regards to the mature live weight of a sheep, Dr Fowler said based on genetic theory he would not expect a big change in live weight by switching to a different bloodline.
"The thing is, if that switch is made to another bloodline in hope of increasing the size of the animal, then a change might happen to other traits," he said.
"And in fact, if there is a change in size due to a change in the bloodline, it may have nothing to do with hybrid vigour.
"There are so many things that could impact on mature live weight and it just could be that the particular ram they chose to increase the hybrid vigour just might have a set of genes that are very positive for increasing mature live weight.
"It has nothing to do with hybrid vigour, it has to do with that particular ram having genes for increased mature live weight."
He said where hybrid vigour has its biggest impact is survival.
According to NSW Agriculture in Merinos the crossing of bloodlines can lead to substantial lifts in production rates.
Research carried out at Trangie Agricultural Research Centre and Orange Agricultural Institute showed simple crosses of purebred rams of one bloodline mated to purebred ewes of another bloodline were four per cent more fertile and had a seven per cent increase in lambs weaned per ewe mated.
Dr Fowler said in regards to lamb survival though, all different kinds of aspects matter.
"For example, if you were to have a group of ewes lambing down and they were all twin bearing ewes (they were all in the same category) and there were two different bloodlines in those and they were all lambing under the same circumstances - such as weather, mob sizes, paddock topography, and predators - then you would expect on the genetic theory there would be higher survival rates among those with higher hybrid vigour," he said.
Dr Fowler said the genetic comparisons that were conducted were based on the research at Trangie involved thousands of genetic evaluations, not on the odd one or two.
"If you go back to the Trangie single selection flocks, where a lot of this work came from, those single selection ram flocks ran for over 20 years," he said.
"They conducted genetic correlations and heritability coefficient based on thousands of individuals."
