## 5 The Establishment of Integrated Beef and Dairy Systems
#### The Use of Myostatin Genetics to Improve Meat Traits in Beef-Sired Diary-Cross Calves  
Variations in _MSTN_ may have a role to play in producing superior beef from dairy cows. For example, it was revealed that one copy of the myostatin p.F94L leucine allele in beef-on-dairy breeding systems (Limousin/Angus bulls over Jersey/Holstein cows) did not affect gestation length, birth weight, percentage of unassisted births, feedlot average daily gain, live weight at harvest, hot carcass weight, or dressing percentage, but did result in lower marbling scores and increased muscularity, as evidenced through larger, more beef-shaped rib-eyes, lower USDA yield grades, and greater carcass cutout yields (both boxed beef and retail yields) [59]. In this respect, Lim-Flex(r) Angus-cross sires are homozygous for the leucine-encoding allele of myostatin p.F94L. The leucine allele was derived from the Limousin breed in this Angus \(\times\) Limousin cross, and it is claimed that Lim-Flex sires can be used to assist in modifying the phenotype of dairy crossbred calves to resemble more closely that of pure Angus cattle. This results in increased visible muscling in the live animal, causing an alteration in the shape and size of the carcass to meet the specifications required for certification as CAB. A survey was conducted of 1530 calves in a trial involving 125 farms [60]. The authors revealed that whereas purebred dairy calves are destined almost exclusively for veal production, beef \(\times\) dairy crossbred calves are also destined for beef production after fattening on either the dairy farm of birth, or by specialised cattle fatteners. In veal production, compared with Belgian Blue-sired calves (taken as the reference, and presumably _MSTN_ c.821(del11) heterozygous), Limousin-sired calves had a smaller average daily gain and lighter slaughter and carcass weights, while Simmental-sired calves had a similar growth rate, lighter carcass weights, smaller dressing percentage, and smaller muscularity scores. In the case of young bulls and heifers fattened on the dairy farm of birth, Belgian Blue, Piedmontese, and Limousin-sired calves performed similarly; the only exception was that Piedmontese-sired calves had a greater dressing percentage. Belgian Blue and Limousin-sired calves performed similarly when fattened by specialised beef producers. In both veal and beef production, the effects of dam breed were less important than sire breed. It also confirmed that the _MSTN_ c.821(del11) allele in cattle has benefits when crossed with dairy breeds to produce superior progeny [27]. Cattle with two copies were heavier at birth, leaner, and had a higher proportion of muscle mass than animals with zero or one copy. In contrast, heterozygous cattle were heaviest at weaning and had the highest live weight, whereas animals with zero copies had the highest fat content. It is also interesting to note that one study has revealed an effect of _MSTN_ variation on dairy traits, specifically the amount of milk saturated and unsaturated fatty acid; hence, the pleiotropic effects of myostatin variation may need to be considered [61]. This would be of less importance if dairy cows were mated to beef bulls carrying myostatin variations like p.F94L or c.821(del11), but the variation occurred in New Zealand (NZ) Holstein-Friesian \(\times\) Jersey-cross cows, and the _MSTN_ variation described was in the first intron of the gene [61]. In a recent mating of a homozygous _MSTN_ c.821(del11) Angus bull across Holstein-Friesian and Holstein-Friesian \(\times\) Jersey-cross cows in New Zealand, the progeny have a compelling Angus phenotype (i.e., well-muscled, predominantly black colour, and polled; Figure 1). These cattle were all sired by the Angus bull [PERSON] (The Aberdeen-Angus Cattle Society, Perth, Scotland, UK; Identification Number UK170032 100606), who was a double carrier of _MSTN_ c.821(del11).