Quantitative analysis of birth, weaning, and yearling weights and calving difficulty in Piedmontese crossbreds segregating an inactive myostatin allele.

The Piedmontese breed has a high frequency of double-muscling. Animals tested in this breed are homozygous for a guanine to adenine transition in exon 3 (C313Y) of the myostatin (MSTN) gene. This transition seems to be responsible for the double-muscling phenotype. The objective of this study was to compare effects of alternative MSTN genotypes on proportion of assisted calving and weights at birth, weaning, and 1 yr of age. Reciprocal backcross and F2 calves out of Piedmontese-Angus (PA) and Piedmontese-Hereford (PH) dams born in 1995 (n = 82), 1996 (n = 75), and 1997 (n = 144) were evaluated for birth (BWT, kg), adjusted weaning (W200, kg), and yearling (W365, kg) weights and calving difficulty expressed as a proportion of assisted calving (CD). The number of copies of C313Y was assessed in each calf. Data were analyzed with a model that included effects of year, sex, subclasses of proportion Piedmontese (.25, .5, .75) by number of C313Y copies (0 = +/+, 1 = mh/+, 2 = mh/mh), and age of dam as covariate. For BWT, heterozygous mh/+ animals were 3.2 +/- .8 kg heavier than +/+ animals. Homozygous mh/mh animals increased .19 +/- .06 in proportion of CD compared with mh/+ animals. Differences between homozygous animals (mh/mh - +/+) were 5.2 +/- 1 kg for BWT and .21 +/- .06 for CD. Heterozygous mh/+ animals were 9.1 +/- 4 kg heavier at W200 than homozygous +/+ animals. Homozygous +/+ and heterozygous animals were 20 +/- 8 and 24.5 +/- 8 kg, respectively, heavier at W365 than mh/mh animals. Differences between mh/+ and the mean of mh/mh and +/+ genotypes for W200 and W365 were 8.8 +/- 3 and 18 +/- 5 kg, respectively, suggesting dominance effects on postnatal growth. Production of heterozygous animals, to take advantage of the positive impact of one copy of C313Y on carcass traits, may be a viable option when the value of increased retail product yield is greater than the increased cost associated with calving difficulty.