## 4 The Use of Myostatin Genetics to Increase Meat Yield in Cattle
### Adverse Effects of MSTN nt821(del11) on Calving Ease
The mutant _MSTN_ genotypes of c.821(del11), p.Q204X, and p.F94L were all associated (\(p<0.01\)) with increased calving problems when present in either the cow or the progeny, but the c.821(del11) allele had the highest association with calving difficulty when the homozygous deletion was present in either the calf or the cow, although the association between the calf's c.821(del11) genotype and calving difficulty differed depending on the c.821(del11) genotype of the dam. The authors suggested that, depending on the dam genotype, a bull with two copies of the c.821(del11) allele can produce progeny with improved carcass merit, while minimizing calving problems. While [PERSON] et al. [27] noted that more than 90% of the 32,770 recorded calvings required no assistance at birth, 7.5% required some assistance, 1.3% required considerable assistance, and just 0.3% (109 calving events in total) required veterinary assistance. In an interesting contrast, the incidence of, and risk factors associated with, calving assistance and dystoca in pasture-based dairy herds (there was no mention of _MSTN_ variation), from their investigation of 152,641 records of full-term calvings from Holstein-Friesian dams served by artificial insemination (AI) sites of seven breeds, appeared to be higher [46]. The overall average incidence of 'no','slight', 'considerable', and'Veterinary' calving assistance was 68.9%, 24.3%, 4.3%, and 2.5%, respectively, with the notable difference being the higher levels of 'considerable' assistance and'veterinary' assistance for the dairy cattle. Additionally, an analysis of sires currently used in New Zealand dairy production broadens our understanding (Table 1), but also illustrates how _MSTN_ nt821(del11) might be better used. As illustrated in the table, the _MSTN_ nt821(del11) Belgian Blues sires have similar calving difficulty and gestation periods compared to the average Friesian sires (born after 1990 with more than 10 recorded calvings). In total, approximately only 38% of the Belgian Blue (_MSTN_ nt821(del11)) sires used in New Zealand have acceptable calving ease and gestation periods for the industry (and below all beef sire averaged for both traits). In comparison, the average of the Angus sires is 0.29 for calving difficulty estimated breeding values (eBVs), which is approximately 1% fewer difficult calvings than Friesian sires (1.30) (Table 2). What are called 'dairy suitable' Angus sires have a very similar calving ease and gestation length to the Friesian \(\times\) Jersey cross dairy sires. It must also be noted that calving difficulty and carcass merit are reported to be antagonistically correlated in cattle [47], with studies suggesting a moderate-to-strong genetic correlation between calving difficulty and carcass weight in Japanese Black Cattle (0.64 to 0.81) [48] and carcass fleshiness grade in Charolais (0.42) and Hereford cattle (0.54) [49]. In summary, the evidence would suggest that myostatin variation can affect calving ease, but the influence is complex and likely affected by other factors, not just variation in _MSTN_. Appropriate sure selection in conjunction with selection for cows with improved calving ease could be indulged to improve performance.