Selection for increased muscling is not detrimental to maternal productivity traits in Angus cows

Abstract

The aim of the present study was to assess the effect of selection for increased muscling on maternal productivity of a temperate beef cow herd. Cows of predominantly Angus breeding were selected using visual muscle score (1-15 scale, where 1 = lightest and 15 = heaviest muscling) into low- and high-muscled animals, and mated to Angus bulls with low or high muscularity. Initially, low-muscled cows were mated to low-muscled bulls to create the Low line, and high-muscled bulls and cows were mated to create the High line. On discovering that some High cattle carried the myostatin (821 del11) gene, a second High line was created to distinguish between cattle with no copies of the myostatin gene (High line) and those with one copy (HighHet line). Data from 12 breeding cycles, consisting of 2003 joining records, and 1713 resulting weaning records were analysed to assess maternal productivity. Cows from the three lines were similar in weight (547, 548 and 550 kg, P = 0.9), but varied in body composition - from Low to High to HighHet cows, muscling traits increased and fatness traits decreased (all P < 0.001). Compared with Low cows, High cows had a 4.4 units higher muscle score, 10% higher eye muscle area and 21% less fat, and HighHet cows had a 7.1 units higher muscle score, 17% higher eye muscle area and 45% less fat. There were no significant effects of selection for increased muscling on live birth or weaning rates, or weaning weight (all P > 0.1). Dystocia levels of Low and HighHet maidens or cows did not differ significantly, but High maidens or cows had less dystocia (P = 0.013). Low, High and HighHet cows weaned 218, 225 and 216 kg of calf/cow joined.year, indicating similar maternal productivity.