DO fatter Merinos produce less wool than leaner Merinos - many farmers think they do, but is there a strong correlation between body fat and wool growth?
Also, would less wool cut per head from fatter Merinos be acceptable in some enterprises if farmers could run fatter Merinos at higher stocking densities and make up the wool cut difference with more sheep shorn per hectare?
These are just some of many questions a team of Murdoch University researchers, led by Dr Andrew Thompson and assisted by Sarah Blumer, hope to be able to answer.
They are a little over half-way through a four-year project funded by Australian Wool Innovation (AWI) looking at genetic traits in relation to feed conversion, whole body energy efficiency and profitability of sheep enterprises.
The aim is to use selectable traits to try to establish a more accurate estimation of per hectare profitability for some sheep farming systems.
An add-on to the Merino Lifetime Productivity (MLP) program, their Genetic Evaluation: Productivity, Efficiency and Profitability (GEPEP) project has over the past two years involved extensive repeated measurement and condition scoring of 640 three-year-old wethers from 29 MLP sires, in 2016-drop and 2017-drop cohorts.
Using the Department of Primary Industries and Regional Development's (DPIRD's) specialised animal house at the Katanning research facility, 160 wethers at a time spent 35 days in individual pens on estimated maintenance feeding with extensive measurement and condition scoring before and after.
Apart from ultrasound scanning for fat, a total body water technique was used to help establish body composition, involving injecting each wether with deuterium (a stable 'heavy' isotope of hydrogen), Ms Blumer said.
Proxies for each sire team were also DEXA (Dual Energy X-ray Absorptiometry) scanned to further establish body composition in relation to percentages of lean tissue - which is mostly water and stores less energy - and fat tissue which stores more energy for when a sheep needs it, she explained.
Then each sire team was split into two with half of the progeny from each sire team going onto a "more extreme diet" with the expectation they would lose weight, Ms Blumer said, while the other half were on ad libitum (unlimited feeding opportunity) so they could eat as much of the diet as they wanted.
Measurements continued, including periodic wool growth using hair dye bands to mark growth periods and skin biopsies to count follicle density and to try to determine skin thickness.
The end result was a massive amount of data which over the next two years will inform economic modelling to determine the role of energy related traits to evaluating the profitability of different genotypes.
The feed conversion data, coupled with the body composition data, allowed researchers to calculate a whole body energy rating in megajoules for each wether.
Individual energy ratings, averaged to a 61 kilograms liveweight, allowed researchers to directly compare the fat deposition energy performance in sire teams under the different feeding regimes.
"One of the end results, adjusted for liveweight, was a three megajoule or 20 per cent range in intake between sire groups under ad libitum conditions," Ms Blumer said.
"Also, when fed at levels expected to achieve liveweight maintenance, liveweight change for each sire team differed, from actual liveweight maintenance ranging through to losing more than 90 grams per head per day.
"For every kilogram of liveweight gained, the proportions of fat and lean tissue can be quite different.
"Given that the value in that fat tissue when you want to draw down on it, is more efficient (than lean tissue energy storage), those animals with a propensity to convert feed to fat are considerably more efficient."
So far though, the research has not shown a reliable correlation between a more energy efficient fatter wether and more wool growth.
"Based on analysis to date there does not appear to be any relationship between fatness and wool cut and this indicates plenty of scope to select animals that carry fat and produce lots of wool," Ms Blumer said.
"That's part of what we will be doing over the next 12 months, making sure other traits we might be selecting for are not negatively affected and that the correlations are understood so farmers can make a call.
"If depositing energy is more valuable in your system then maybe you might be prepared to take a cut in your wool but it may be possible to have the best of both worlds.
"We are looking at dollars per hectare, rather than dollars per head.
"If farmers were to take a cut on some traits, because you can run more sheep per hectare, then it might be more profitable on a per hectare basis."
Dr Thompson said the research was broadly based on the premise "a fatter animal is a profitable animal".
"That's the concept that we need to lock away and be 100 per cent certain on," he said of the research team's task.
"(For example) there's a perception in the industry that you can't have a fat animal and cut wool," Dr Thompson said.
"But we think you can and we're hoping to be able to identify those animals."
But the research has already shown the potential major economic benefit from fatter sheep - sheep efficient in converting less feed to more fat - will come from being able to run more of them in the same paddock.
"The benefits from a fatter animal will come from running higher stocking rates," Dr Thompson said.
"Experimental modelling shows the benefits are potentially quite big.
"(But) if you chose to just run fatter animals - the same number at the same rate, but fatter - then the benefits appear to be much less.
"Potentially this is a big change in how we look at the value of animals so understandably we have to be pretty confident in the data, hence it will take some time to translate into changes in how some ram breeders and commercial producers might select their rams.
Before the project is completed the Murdoch research team hopes to rerun all of its tests with ewes, sisters of the wethers, under grazing conditions to establish the heritability of whole body energy and feed intake and their correlations with existing traits such as wool cut, carcase, reproduction and disease traits.
