42 The economic effects of using heterozygotes for a non-functional myostatin mutation within a commercial beef production system

Alford AR, McKiernan WA, Cafe LM, Greenwood PL & Griffith GR (2009) The economic effects of using heterozygotes for a non-functional myostatin mutation within a commercial beef production system, Economic Research Report No. 42, Industry & Investment NSW.

Executive Summary

Meat yield is a key driver of profitability in beef production. Meat yield per animal can be increased by investing in particular breed types known to have higher meat yields or by selecting sires within breeds that have high Estimated Breeding Values (EBVs) for meat yield traits. Selection for the phenotypic trait known as "double muscling" or "muscular hypertrophy" is one avenue for increasing meat yield per animal. This trait is associated with higher meat yield, a higher proportion of preferred cuts of meat, leaner and more tender meat, higher birth weights and superior pre-weaning growth rates. However on the negative side, production problems have included reduced fertility, dystocia and lower rates of calf survival.

The application of molecular genetics to improve muscling in beef cattle, and hence meat yield in beef carcases, has generated global interest in recent years. Recent research has identified Myostatin as a potent negative regulator of skeletal muscle mass in cattle. Myostatin is a growth factor that limits muscle tissue growth, i.e. higher concentrations of myostatin in the body may cause the individual to have less developed muscles. Muscular hypertrophy in cattle has been attributed to naturally occurring mutations in the bovine myostatin gene that result in "inactive" or "non-functional" myostatin.

It is now possible to genotype individuals for the myostatin mutation and identify whether they are homozygous (+/+ or mh/mh) or heterozygous (mh/+). Thus it is now technically feasible for beef producers to incorporate selection for the myostatin mutation into their production system. Practically, the heterozygous form would be preferred to the homozygous form to avoid potential problems with fertility and calving difficulty.

The objective of the study reported in this bulletin is to take the findings of some recent experimental results relating to selection for the heterozygous myostatin mutation undertaken by the Beef CRC, and to examine the profitability implications of possible commercial application in the Australian beef industry.

A herd-level economic analysis of the heterozygous myostatin mutation was undertaken using the Beef-N-Omics software package. Inputs into the package included herd costs and returns for a representative self-replacing beef herd turning off young cattle of some 15 to 20 months of age, as published by the Industry & Investment NSW. Other inputs included pasture growth data for a representative good quality pasture system in the North-west of New South Wales, and herd production data based on the experimental results.

A base case herd was set up first. The gross margin of the base herd of 200 breeding cows was $108,105 or $540 per breeding cow or $270 per hectare. Then four scenarios were examined based on different combinations of herd structure and premiums available for muscle score. All scenarios showed that there is a potential economic benefit from incorporating the myostatin mutation gene in the heterozygous form in a commercial beef herd.

When the myostatin mutation was incorporated using the self-replacing system (where it is assumed that the genetic screening test for the myostatin mutation is $50 per female), the increase in gross margin over the base herd was 0.5 per cent or $3 per breeding cow ($2 per hectare) when there was a 1 muscle score increase in the average muscle score of heterozygous animals in the production system. If the outcome of the myostatin gene was an v increase of 2 muscle scores in the heterozygous animals, then the improvement in the gross margin is 4.9 per cent or $27 per breeding cow ($13 per hectare) over the base herd.

In the scenarios where a terminal sire system is applied to utilise the myostatin mutation then the economic benefits are potentially greater. In the case of a 1 unit increase in muscle score over the base herd, a 6.1 per cent increase in the gross margin is achieved over the base herd scenario ($27 per breeding cow or $13 per hectare). If a 2 unit increase in muscle score is obtained from the application of the myostatin gene then a potential 17.7 per cent increase in the gross margin over the based herd is obtained ($96 extra gross margin per breeding cow or $48 extra per hectare).

The relative profitability of a self-replacing herd production system compared to a terminal system, independent of the myostatin mutation, was tested by incorporating a terminal sire production system utilising a Limousin bull using published average bull prices. The use of a Limousin bull over British breed cows was assumed to increase muscle score of progeny by 1 unit, consistent with published results. It was found that the terminal sire system independent of the myostatin mutation achieved a 7.4 per cent higher gross margin, or $40 per breeding cow, than the self replacing herd modelled in this analysis. This terminal sire system using a Limousin bull also had a slightly higher gross margin ($580 per breeding cow) than the terminal sire system utilising the myostatin mutation ($574 per breeding cow), assuming a 1 muscle score increase in progeny for both scenarios.

Finally, the assumed premium for muscle score has a large impact on the potential profitability of introducing this trait into a breeding program. More recent data on muscle score premiums needs to be analysed to check the level of benefit the market is willing to pay for this extra yield of beef.