Final report details
Additional measurements on muscle line cattle
April 14, 2010
February 1, 2012
This project studied the effect of selection for divergent muscling in an Angus cow herd on the performance of the steer progeny and the maternal productivity of the cow herd. Twenty years of selection have resulted in the herd comprising a Low muscle line (D muscle score); a High muscle line (C muscle score); and a Myostatin line (B muscle score) with one copy of a mutation in the myostatin gene causing muscle hypertrophy. High steers had the same growth rates, but improved feed efficiency and retail meat yield than Low steers, with no difference in meat quality. High muscled cows had similar calving rates and maternal performance to Low cows on good and poor nutrition. Myostatin steers had similar growth rates, and improved feed efficiency and meat yield to High steers, with no significant differences in meat quality. Myostatin cows had the same calving rates and maternal performance as High cows on moderate to good nutrition but their performance appeared to be reduced after 18 months on poor nutrition. Industry can be confident that moderate increases in muscling in cows will not reduce maternal productivity, but caution should be taken if using a myostatin gene to further increase muscling.
Final report summary:
There is growing industry concern that market signals driving enhanced carcase yield of progeny (decreased fatness and/or increased muscling) may have adverse impacts on maternal productivity traits in the cow herd. The value of higher yielding carcases is recognised in the marketplace, with premiums of 14c/kg liveweight for an increase in muscle score class being paid across all market categories (McKiernan 2002). Excessively fat carcases are also discounted between 5-30c/kg carcase weight and in some cases up to 80c/kg. However, industry has shown reluctance to increase cow herd muscularity, and has also become concerned about the impacts of continued selection for increased growth and reduced fatness on maternal productivity traits.
Increased carcase yield offers producers the opportunity to increase the total amount of product from the same number of animals. Increasing the muscularity of the female component of a beef herd enables producers to achieve a rapid rate of improvement in the muscling and yield of resultant progeny. While such an improvement will provide economic returns from the sale of progeny with more meat, it is important to establish the impact that such a change will have on other traits such as feed efficiency, meat quality and maternal productivity.
This project was designed to address these issues using the NSW DPI Angus muscling selection line herd. Twenty years of selection for muscling have resulted in the cow herd comprising a Low muscle line (D muscle score average); a High muscle line (C muscle score average); and a Myostatin line (B muscle score average) with one copy of a mutation in the myostatin gene causing muscle hypertrophy. The results confirm the improvement in meat yield with selection for muscling in steers, and address the question of the effect of selection for muscling on feedlot feed efficiency and meat quality in steers. The effect of selection for muscling on maternal productivity is directly evaluated using results for heifer and cow fertility and performance under divergent nutritional conditions.
Major findings include:
• No difference in feedlot growth and finishing weight of steers from the three muscle lines
• Feed intake decreased with increased muscling, leading to an improvement in feed efficiency (Net Feed Intake)
• Dressing %, meat yield, and muscle to bone ratio increased with increased muscling
• Myostatin steers had reduced fatness, but there was little difference between Low and High muscle line steers
• No significant differences in meat quality between the lines
• Myostatin steers had lower muscle pH and lighter meat colour
• No differences in calving rates or female productivity in heifers or cows from the three lines on reasonable or better nutrition
• Calving ease worse in first calf Myostatin heifers than Low or High heifers, but no difference in calving ease between the lines for subsequent calving
• No significant differences in calving rates or weaning weights after 12 months on Low nutrition, but indications that the Myostatin cows on Low nutrition weaned slightly smaller calves
• Significant drop in calving rate in Myostatin cows after 18 months on Low nutrition, but no change for Low or High muscle cows, nor for Myostatin cows on Medium/High nutrition
• Sale value of cull cows and heifers increased with increased muscling
These results provide no evidence of any difference in performance between cows from the High (C muscle average) and Low muscle (D muscle average) lines, nor in the growth rates of their progeny under good, moderate or short term low nutritional conditions. This should put to rest the long held perceptions of poor performance due to higher muscling, at least for moderate increases in muscling. The advantages of the High muscle cows include: improved feed efficiency and increased meat yield in the progeny with no detriment in meat quality resulting in higher value output; the cows’ increased value when cast for age; and no accompanying detriment to maternal productivity. This should encourage producers to select for increased muscling in their replacement females.
The High muscle bulls used in this herd were the highest muscled bulls without the myostatin deletion which could be sourced from Angus industry herds at the time, and had an average muscle score of B+ (range B to A-). Hence, within the Angus breed at least, producers can safely use the highest muscled bulls currently available to breed replacement heifers without fear of reducing the performance of the breeding herd.
The incorporation of the myostatin deletion into the cow herd, on the other hand, should to be treated with care. Whilst there is an advantage in using the gene to further increase the carcase value of the progeny, the heterozygous Myostatin cows (B muscle score average) have shown signs of reduced productivity after 18 months on a low plane of nutrition. Although further data is needed to confirm this result, it may be that the myostatin gene is best utilised through terminal sires. If heterozygous myostatin females are to be kept for breeding they should be run under more favourable nutritional conditions to maximise their advantage and limit their exposure to nutritional stress.
With the wide range of both muscling and fatness being driven by genotypic and environmental factors, the capture of the changes in cow body composition, resulting fertility and the calf performance on divergent nutrition, the data from this project will form a valuable component in the further development of the Beef CRC Maternal Productivity Model.
New South Wales Department of Primary Industries