Effects of the m gene for muscular hypertrophy on birth weight and growth to one year of age in beef cattle.

182 calves, mostly Angus but also of Hereford and Shorthorn breeding, born in the University herd, were studied, including 55, born during Spring 1970, genotypically distributed as 8 mm, 33 m+ and 14 ++. Eighteen of the 24 homozygotes (mm) had the typical double-muscled conformation at birth (mmY); in the remainder, the typical bulging muscles usually manifested themselves within one month after birth. Averaged over sexes, mmY calves were 4.4 kg heavier (P less than 0.05) than the Angus (++) breed average of 26.6 kg. Compared within-sires, mmY calves were 4.0 kg heavier than m+. Heterozygotes (m+) were 0.4 kg heavier than the normal (++) average. Estimating differences between mm, m+ and ++ for post-natal traits was confined to the 1970 calves because experimental conditions were controlled. Double-muscled (mm) calves had a higher (+7%) 180-day weight compared with normal (++) calves. After weaning at approximately 180 days, until yearling age, bulls and heifers received diets with roughage:concentrate ratios of 75:25 and 70:30, respectively. The post-weaning growth rate of mm calves was depressed (-17%) due to a reduced (-11%) feed intake compared with non-mm controls. Homozygotes had a lower (-4%) liveweight at 365 days, compared with ++ animals. Least squares estimates of the difference (m+ - ++) at birth, 180 days and 365 days were -1.0 +/- 1.0 kg, +7.3 +/- 4.5 kg and +7.3 +/- 6.3 kg, respectively.