During my PhD I did a very large experiment to try and further understand the physiological consequences of differences in muscling, fatness and growth breeding values in Merino sheep.
It was the last and largest of the five experiments that I completed within my PhD. Upon reflection there was enough data (and work!) in that experiment for it to be a PhD on its own but hindsight is a powerful thing.
The experiment used sheep purposefully selected for a large range in muscle and fat breeding values. Across the experiment I found a lot of differences between the genotypes, not all of which I completely understand yet. One of the components of this experiment involved injecting small doses of adrenaline into the jugular vein of sheep and collecting blood samples at VERY frequent intervals to assess the physiological response to stress and whether it differed between genotypes.
To measure the mobilisation of fat tissue under this small stress, I analysed the resultant blood plasma for None sterified fatty acids (NEFA), NEFA are the building blocks of fat and are mobilised from fat storages when required. Some muscle types can use the NEFA directly as a source of energy and NEFA are also transported to the liver where they can be converted into glucose and provide energy for the rest of the animals physiological functions.
When I initially analysed these results (after several months of character building time in the laboratory). I was quite disappointed that there was no correlation between fat mobilisation (as measured through NEFA plasma concentrations after adrenaline) in high and low fat genotypes and high and low muscle genotypes. It is a while ago so I can't remember why I even ran the analysis, maybe in desperation to find a result, but I analysed the data for a correlation with the breeding value for coefficient of variation in fibre diameter (CV). To my surprise they were correlated.
This data showed that sheep with genetically low CV had a lower response to adrenaline and mobilised less NEFA. I've got a few theories on why this may be but I won't bore you with that detail - any more than I already have!
The important thing is that sheep with a low CV when cruising around doing their thing, are less likely to mobilise fat tissue when they get spooked by a magpie flying over or a dog barking in the distance for example. This helps to explain the favourable correlation between CV and fatness. Low CV sheep also tend to be genetically fatter as well.
Fast forward twelve years and I find myself sitting in the shearing shed at Triggervale on Friday listening to Associate Professor Andrew Thompson (AKA Thommo) talk about the work underway at Murdoch University understanding whole body energy that we've discussed previously. Thommo had some preliminary results of the whole body energy (mainly driven by fatness) and it showed that the breeding value for CV was more highly correlated with whole body energy than even fat itself (only just).
The correlation between CV and whole body energy was 0.62. The slope of the line is about 2% in whole body energy per 1% reduction in CV breeding value. You may remember that a 10% difference in whole body energy is predicted to improve farm profitability by 40%. So this is a pretty big deal.
Breeding for lower CV has been a focus for some breeders for some time. This is for two reasons, both of which are associated with wool processing and therefore wool price. Lower CV is associated with higher staple strength and these two traits have a high genetic correlation. Lower CV also results in less fibres over 30 micron - or higher comfort factor as it is often reported which may also be rewarded by wool buyers - although it is a lot less important than staple strength in determining wool price.
It seems to me the additional benefits of CV to the animal will add significant value over and above that of wool quality and price. When you look back at all of the genetic correlations of CV with other traits of importance it becomes a really important trait.
Sheep with lower CV are genetically likely to have;
- Higher growth rates
- Higher staple strength
- Better muscling
- More subcutaneous fat
- Improved worm resistance
- A greater store of whole body energy
- A lower response to stress
This work has been completed on fine-wool sheep and I have no idea how this would play out in stronger wooled sheep. I don't fully understand what is going on here but it seems obvious that finding rams with genetically lower CV makes a lot of sense (that is a very negative number for the YDCV breeding value). Obviously CV is heavily influenced by the environment so it is important that we are looking at a breeding value for CV rather than the raw number itself.