## 4 The Use of Myostatin Genetics to Increase Meat Yield in Cattle
### Adverse Effects of MSTN nt821(del11) on Calving Ease  
It was reported that _MSTN_ c.821(del11) significantly increased muscle score and calving difficulty (rated on a five-point scale from 1 = easy to 5 = caesarean section) and decreased fat depth but did not affect weight at 200 and 400 days in South Devon cattle [40]. The authors argued that the effect on calving difficulty was recessive as the c.821(del11) effect was not additive (i.e., there was no significant effect for heterozygosity), and they noted that there was greater difficulty in calving males than females. The farm also had a significant effect on calving ease. It was suggested by another study the effect of _MSTN_ variation on calving difficulty appeared to be recessive in Piedmontese six backcrosses to Piedmontese \(\times\) Hereford and Piedmontese \(\times\) Angus cows [41]. While not studying the _MSTN_ c.821(del11) allele (the cattle had myostatin p.C313Y variation), the authors suggested that the production of heterozygous animals could take advantage of the positive impact of having one copy of the Piedmontese-derived myostatin p.313Y allele on carcass traits, and that it may be a viable option when the value of increased retail product yield is greater than the increased cost associated with calving difficulty. In a follow-up study, they studied crosses of Belgian Blue, Charolais, Hereford, Angus, and composite MARC III cattle [42]. The Belgian Blue cattle were the source of the _MSTN_ c.821(del11) allele variation. The study revealed that two copies of c.821(del11) resulted in calves that were more likely to die before weaning. Animals carrying one copy were heavier at birth and at weaning, and their carcasses were leaner and more muscled. The authors discussed how much of the observed loss was likely to be related to the increased birth weight of calves, albeit they also noted that the calves in the study were born in the late winter, when adverse weather in combination with the relatively low amount of extra-muscular fat might contribute to calf loss. They concluded that the use of _MSTN_ c.821(del11) is an option to increase retail product yield, but consideration of conditions at calving is important to prevent mortality. A linearly decreased pelvic area (\(p<0.05\)) was reported when the myostatin p.C313Y allele was included in Piedmontese cattle (\(p<0.05\)) [43]. The pelvic opening of double-muscled dams was 10 and 6% lower (\(p<0.05\)) than in non-double-muscled Charolais [44] or cross-bred cows [45], respectively, such that the incidence of dystoca and perinatal mortality was higher in double-muscled cows [45]. In a review of the literature at the time, it was suggested that because of the birthing difficulties encountered by mothers that are homozygous for _MSTN_ c.821(del11), having access to a simple typing method for cattle would prove to be a benefit for the breeder, as mothers heterozygous for _MSTN_ c.821(del11) have a low frequency of birthing complications, and are also capable of delivering calves homozygous for the _MSTN_ c.821(del11) allele [26]. The authors concluded that a decrease in birthing difficulties not only reduces cost, but also reduces the risk of loss of mothers or calves. A large study was undertaken to investigate associations between 21 known myostatin gene mutations and calving and carcass traits in twelve cattle breeds [27]. The _MSTN_ genotypes of 32,770 dam-progeny combinations were used in the association analysis of dystoca, with the genotypes of 129,803 animals used in mixed-model association analyses of carcass weight, conformation, and fat score.