Introduction
The main focus of genetic improvement programmes for venison production has been improving growth rates. High growth rate animals achieve slaughter live weights at an early age than low growth rate animals. While efficient conversion of grass into live weight gain is important in running an efficient venison operation, the amount of venison that can be turned into meat cuts for sale is also very important. Heavier animals tend to have a greater weight of meat than lighter animals, but that does not necessarily mean that the carcass has yielded well. Animals slaughtered at the same live weight can vary considerably in the weight of meat they produce.
An estimate of the difference in loin weight in a group of animals with a narrow live weight range (from 113 to117 kg) in the Wilkins Farming herd is around 1.7kg between best and worst animals. Loin weight was estimated from ultrasonic eye muscle area multiplied by the length of the animals back and the density of lean meat. At $8/kg carcass weight, this equates to a difference of $13.60 per head from the loin alone, without including any correlated increase in meat yield from the other areas of the carcass. The loin is also the highest value cut so the difference in ultimate value between best and worst yielding carcasses will obviously be much greater. Assuming a current average figure of $35NZ/kg FOB could be achieved then the difference in value between the best and worst animals would equate to $59.50 in the loin muscle alone.
Ultrasound scanning
In order to estimate meat yield in a live animal, we have to be able to ‘look inside’ the body of the animal. Ultrasound scanning is a medical technology that is able to collect images of tissues in the body. It is was developed for use on humans, but has been widely used in the sheep industry to measure the area of the eye muscle to improve meat production.
Ultrasound scanning works well for scanning farmed animals. The scanner itself is small and portable meaning it can be used on-farm. It does not take long to measure an individual animal and animals can be measured in a crush without the need to sedate. It is also relatively inexpensive so a large number of animals can be measured. However, it is not possible with ultrasound to scan the entire carcass meaning that the results for the eye muscle are used to give an approximation of total meat yield. There are also problems with deer in collecting good images when the stags are in their winter coats.
Ultrasound is a very useful tool in genetic improvement programmes to improving meat yield. It gives better estimates of meat yield than live weight alone, but not as good as more expensive technologies like CT scanning. However, the fact that it is inexpensive, easy to collect and measured on farm means that all of the stags can be measured.
CT scanning
CT scanning is another human medical scanner that has been applied for use in livestock. CT scanner allow us to take very accurate measurements on meat and fat in the carcass of an animal. The CT measurements are effectively as accurate as slaughtering the animals and then dissecting out the meat from the fat and bone in the carcass. However, the stag is still alive after CT scanning and can be used in a breeding programme.
While the accuracy of CT is excellent, scanning is very expensive (hundred of dollars per animal), and only a relatively small number of animals can be scanned in a day. The machine is not portable, so the deer have to trucked to the CT facility rather than scanned on farm. The deer also have to be anaesthetized while they are scanned. As such it is suited to taking a set of highly accurate measurements on a small group of animals. This means that CT scanning is suited to selecting the stud sires for use in the stud rather than for animals for sale to clients. The genetic improvements made in the stud then flow on to the next crop of stags for sale.
Benefits of ultrasound and CT scanning
Selecting for growth rate alone will improve the value of an animal’s carcass because it will have a larger carcass at the same date, or it can be slaughtered at an earlier age. If some meat yield measurements (i.e. ultrasound and CT scanning) are included in the genetic improvement programme, it is possible to improve carcass yield over and above the improvement that come from improved growth rate. In a breeding system that ultrasound scans all of the stags and CT scans the top 10% of stags, we would expect to loss a small amount of the improvement in growth rate (340g per round of selection), this would be compensated for by improvements in the yield of meant in the hindleg, loin and shoulder regions of the carcass. The gains have been estimated at an additional 510 grams of meat per round of selection, including 270g additional in the hindleg, 70g additional on the loin and 150 grams additional in the shoulder. These gains are per round of selection and are cumulative so with five years of selection the gains would be 1343, 373 and 747g more meat in the hindleg, loin and shoulder cuts than would have been the case if the stags had only been selected on the basis of growth rates.
These gains are based on random allocation of hinds to the stud stags. Further gains can be made using ‘assortative mating’ where the best stags are mated to the best hinds, although the level of inbreeding has to be monitored using this technology.
A full report can be seen here
-Neville Jopson (AbacusBio)