There is no substitute for selecting directly for reproductive performance in sheep, new research has shown, following industry debate about the impacts of selecting for fat and growth on fertility.
The importance of body reserves and condition score to provide ewes with a buffer against periods of poor nutrition and during pregnancy is widely recognised by breeders. But Merino breeders are being urged to directly measure for reproduction and ewe condition if both factors are considered economically important traits, rather than selecting on the indirect traits of fat and/or muscle levels to improve reproductive efficiency.
A principal scientist at the Animal Genetics Breeding Unit, Dr Daniel Brown, said the impact of fatness and muscling on many aspects of production had resulted in debate throughout the sheep industry as to the relative emphasis that breeding programs should place on fat and eye muscle depth.
“The bottom line for sheep breeders is that selecting on fat is not a substitute for selecting directly for reproductive performance,” Dr Brown said. “Due to the number of complex interactions between traits, the most important advice is for Merino breeders to measure directly the traits that contribute to their farm profitability.
“The best way to account for these interactions and achieve balanced genetic improvement is to use multiple trait selection indexes that consider the value of a range of individually measured traits.”
Hamish Chandler, Manager of Sheep Genetics said, the genetic relationships observed in data collected by MERINOSELECT breeders and the Sheep CRC Information Nucleus flock (INF) indicated animals with genetically higher fat and muscle tend to have more favourable breeding values for reproduction rate and ewe condition score, among other things. “However, they have less favourable breeding values for traits such as fleece weight, fibre diameter, and lean meat yield,” Mr Chandler said.
Australian Sheep Breeding Values (ASBVs) provide estimates of the genetic merit with respect to a wide range of traits including post weaning and yearling fat depths, and eye muscle depth. Analysis based on all MERINOSELECT data, shows that, on average, an increase of 1mm in the yearling fat or yearling eye muscle depth ASBV, ewes will increase their average number of lambs weaned by around five per cent.
“But sheep producers should take caution because the results can be hugely variable, ranging from -20% to +20% in different flocks and seasons. There are also many cases where no significant relationship was observed at all between increased breeding value for fat and reproductive efficiency,” Mr Chandler said. “The mix of favourable and unfavourable correlations with fat means that a selection strategy focused solely on improving fat will have many and varied consequences for different traits across the flock.”
Mr Chandler said it was also likely that the value of fatness varied with environmental conditions. “This issue is currently being examined in more detail using the Sheep CRC Information Nucleus database,” he said. “The outcome of this study will assist in understanding the genetic importance of fatness and reproduction rate of ewes. This ongoing R&D conducted by Sheep Genetics for both MERINOSELECT and LAMBPLAN is important as it provides new information for updating and improving trait ASBVs and Indexes.”
A new Practical Wisdom Note developed by the Sheep CRC is now available to provide breeders with more detailed information about the genetic relationships between fat, eye muscle depth, reproductive performance and other traits of economic importance in Merino breeding programs. The Practical Wisdom Note, ‘Fat and eye muscle depth in Merino breeding programs’, is available at www.sheepcrc.org.au.
Media contact: Michael Thomson on 07 4927 0805 / 0408 819 666