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Genetic fat

The X Factor – Genetic Fat

(Current Literature review)

If there is one aspect of seedstock breeding over the past 10-15 years that has been consistent, it is the conscious aim to breed genetically leaner animals, largely in response to consumer health concerns. However in recent years, the price we have paid for perhaps going too far in this direction is becoming clear as we discover more about the positive correlations that genetic fat has on many areas of sheep performance. We are not referring simply to fat laid down due to additional feeding; genetic fat or “bred on” fat relates to the differences in genetic capacity of an individual to lay down higher levels of fat compared to another animal. While extreme levels of fat have a negative impact on carcase value in relation to carcase yield and additional costs of trimming and lower Lean Meat Yield (LMY), the value of higher levels of genetic fat in breeding programs is being realised and found to have positive consequences across a number of areas.

Recent research has shown that an extra 1mm of fat (YFat) can have significant impacts on management and profitability. The value of higher condition scores in relation to both conception rates and lamb survival is well known but requires feeding to control condition scores and not all animals respond equally. The value of higher genetic fat levels, especially in the Merino ewe could result in significant savings in feed costs and management.

So what are the benefits of higher genetic fat?

At Lambex 2012, Andrew Thompson from Murdoch University stated that positive section for fat was like getting a genetic insurance policy resulting in more robust ewes that were able to better handle limited feed conditions with reduced weight loss under feed limiting conditions. Ewes with higher levels of genetic fat produced lambs with higher levels of survivability with more lambs to weaning. Dr Thompson said “the arguments for fat included improved energy storage, ability to cope with tough seasons, animal welfare, reproduction and taste and flavour.”

Dr Thompson also found that 1mm (YFat) of additional fat resulted in 18% more lambs born with a higher chance of survival to weaning, in addition to the decrease in the risk of ewe mortality due maintenance of a higher condition score. Analysis of the results from sheep CRC has found a relatively strong correlation between fat (YFat) and lamb survival. The higher levels of genetic fat also resulted in slightly higher birth weights and 15 % higher survival of multiple births. These benefits relate to the levels of genetic fat inherent on the ewe side so what about the impact of using sires with higher levels of genetic fat. Simply using sires that are genetically fatter increased lamb survival by around 5%.

Analysis conducted by Ferguson showed that the effect of additional genetic fat produced an additional 14% and 24.5% of lambs per mm of YFat over the 2 years of the study. This translated to a 2% increase in profitability for a wool enterprise per mm YFat up to a 25% increase in profit for lamb enterprise. There was no significant discount on lamb carcases due to only small variation of fat thickness.

Findings reported by SRS Media found a 20% increase in lamb mortality when using sires with low levels of Fat and Muscle. The finding also found lamb birth weights were less affected in the ewes with higher levels of genetic fat when feed become limiting.

Another research project conducted by Filmer and Adams during the period from 1992 -98 found a positive correlation in Merinos between wool staple strength and lamb survival. So how does this relate to genetic fat? We now know that there is a very good positive correlation between genetic fat and staple strength so what they were also measuring was the effect of not only staple strength but also genetic fat levels on lamb survival. They found a difference of 30% in lamb mortality between the different groups based solely on staple strength. While staple strength is positively correlated to genetic fat levels, breeding for increased fleece weight will reduce genetic fat levels.

The benefits for higher levels of genetic fat in the Merino industry is obvious, but the benefits are not restricted to just the Merino. The same benefits apply across all breeds. Higher fat ASBV’s have been linked with higher fertility and linked to ewes getting into lamb more consistently, or conversely leaner sheep are more likely to have a greater percentage of barren individuals, especially when feed conditions are not favourable. The ability of animals to buffer against weight loss when feed becomes limiting is a major advantage across a whole range of areas relating to management and profitability.

Some of the first research conducted on feed efficiency a few years ago (2006/07) found a positive correlation between the ASBV for fat and feed efficiency across a wide range of sire lines. This was contrary to the general thinking at the time which hypothesised that leaner animals should be more efficient but this had not been verified. It has now been shown that the positive correlation between genetic fat and feed efficiency is real indicating a win/win situation for managing sheep body condition and the cost of feeding to maintain that condition.

The new area where genetic fat is found to have advantages is in relation to Meat eating Quality (MEQ). Butchers have always told us that fat is essential for succulent, great tasting meat but the industry decided to breed against it in response to growing consumer demand for leaner meat that was assumed to be healthier. What we have seen subsequently is the breeding toward lower MEQ to the point where we have some sires in the industry with unacceptable MEQ. While muscling plays a role in this trend, genetically fatter sheep generally have higher MEQ characteristics, especially Intra Muscular Fat levels which is the main driver of MEQ.

Obviously the sheep industry (especially the terminal breeds) has to weigh up the consumer demands for healthier lamb associated with lower levels of fat but this is being offset by the fact that continually breeding toward these goals has resulted in lower MEQ. It becomes a balance between the two aims and sourcing individuals that lie outside the general correlations between leanness and MEQ.  Even allowing for this, there is still the significant advantages that are gained by selecting animals that have higher levels of genetic fat which lead to higher levels of lamb survival, more lambs to weaning, less weight losses in ewes, higher feed efficiency, better fertility and fecundity, higher staple strength and lower CV in wool. All these advantages result in higher levels of profitability and easier management and most will be possible when using sires with higher levels of genetic fat.

There is no doubt given the new understanding of the role genetic fat plays in management of sheep and profitability that it could be the X Factor that explains many secrets as to why some sheep perform better under limiting conditions than others.



“Genetic fat key to ‘bulletproofing’ Merino ewe”, 23 Sep, 2012 By Deanna Lush

“The value of genetic fatness in Merino ewes differs with production system and environmentM. B. Ferguson , J. M. Young  , G. A. Kearney  , G. E. Gardner , I. R. D. Robertson, A. N. Thompson Animal Production Science 50(12) 1011-1016, 2010

“Selecting for Fat Improves Lamb Survival in Merino” SRS Media, Dec 2011

“Lamb Survival link to Wool Staple Strength” N Filmer and N Adams, Farming Ahead No. 187, 2007

“Genetic fat – Bullet Proofing the Merino ewe,” A.N. Thompson, M.B. Ferguson, S.E. John, G. Kearney, G. Rose and J. Young, Ovine Observer, WADAF, 60; 2012