Teagasc National Beef Conference – The myostatin gene in beef breeding: balancing muscling and calving ease

Katie Quiqley and Ross Evans, ICBF.

Content taken from the 2025 Teagasc National Beef Conference publication.

Introduction

The role of the myostatin gene is to regulate muscle growth and development. Some breeds carry naturally occurring DNA variants within the myostatin gene. These variants can render the gene inactive, unable to carry out its function of regulating muscle growth. Consequently, these myostatin variants are associated with double muscling. All animals have two copies of the myostatin gene, one inherited from the dam and one inherited from the sire. Depending on whether the animal inherits a normal copy or a mutated copy of myostatin, the animal may be termed homozygous normal/wild-type, heterozygous, or homozygous mutant (Figure 1).

  • Homozygous wild-type: the animal has inherited the normal (non-mutated) copy from both parents. The animal does not carry any myostatin variant.
  • Heterozygous: the animal has inherited one normal copy and one mutated copy. For example, an animal inherited the normal copy of the gene from the dam, and a Q204X copy from the sire.
  • Homozygous mutant: the animal has inherited a mutated copy of myostatin from both the sire and the dam. For example, an animal inherited the F94L copy of the gene from the dam, and the F94L copy of the gene from the sire.
Figure 1. The terms homozygous and heterozygous explained. The grey coloured copy of the gene denotes the normal or wildtype copy of the myostatin gene. The green coloured copy of the gene denotes the version of the myostatin gene with the F94L variant.

Impact of myostatin variants

Different myostatin variants vary in their effects, as some are more disruptive to gene function than others. Those classed as disruptive include nt821, Q204X, nt419, E226X, C313Y and E291X, while three other variants, F94L, S105C and D182N are not as disruptive in terms of gene function. Alongside this, calving difficulty and carcass merit are antagonistically genetically correlated in cattle, meaning that bulls selected to minimize calving difficulty generate calves with inferior carcass merit (Berry et al., 2019). A study carried out by Ryan et al. (2022) found that myostatin variants F94L, Q204X, and nt821 were associated with heavier, more conformed and leaner carcasses than their non-carrier counterparts, while E226X was associated with a more conformed and heavier carcass only. While both nt821 and Q204X were associated with improved carcass merit, they were also associated with more difficult calvings when present in the dam or the calf, most likely due to a narrower pelvic opening in the dams combined with those dams giving birth to larger calves.

Myostatin frequency in beef cattle breeds

Segregating myostatin variants in Irish beef cattle include F94L, nt821, Q204X, E226X, and C313Y. Table 1 outlines the frequency of segregating myostatin variants in purebred breeds. The C313Y variant is almost specific to the Piedmontese breed. One copy of the E226X variant is carried by around 10% of Shorthorn and 9% of Parthenaise cattle. F94L is most prevalent in the Limousin and Aubrac breeds, with >78% of both breeds
carrying two copies of F94L and >20% of both breeds carrying one copy of F94L. F94L also segregates in the Charolais and Droimeann breeds. The nt821 variant is fixed in the Belgian Blue breed, however, it segregates at lower frequencies in many other breeds, such as Parthenaise, Speckle Park, Shorthorn, Aberdeen Angus, Aubrac and Salers. One copy of Q204X is carried by 26.6% of Charolais, 11.6% of Limousin and 3.8% of Speckle Park purebred cattle.

Table 1. Percentage of animals carrying one (1) or (2) copies of segregating myostatin variants in the most common beef breeds (based on purebreds only).

An analysis of 1,123,336 beef purpose cattle with genotypes available for nine myostatin variants (C313Y, D182N, E226X, F94L, L64P, nt821, nt419, Q204X and S105C) indicated that approximately 45.9% of beef cattle carry at least one copy of the myostatin variant. The majority of this group (31.8%) carry just one copy of a myostatin variant, while the remaining 14.1% carry two copies of a myostatin variant, which can be ubdivided into 4.4% carrying two copies of a different myostatin (for example, F94L and Q204X) variant and 9.6% carrying two copies of the same myostatin variant (for example, nt821 and nt821) (Figure 2).

Figure 2. Percentage of beef purpose cattle with genotypes for nine myostatin variants that carry one copy of a myostatin variant, two copies of a different myostatin variant or two copies of the same myostatin variant.

Mart prices for suckler-bred cattle carrying a myostatin variant

A review of mart prices for suckler cattle carrying none, one or two copies of F94L, nt821 and Q204X for the six years 2020-2025 inclusive showed that, on average, animals carrying one or two copies of a myostatin variant were sold for slightly higher prices when compared to those not carrying any myostatin variant; however, this was dependent on which myostatin variant the animal carried. The smallest price difference was observed in those carrying the F94L variant. On average, those carrying one copy of F94L made €20 more than those not carrying any copy of a myostatin variant, while those carrying two copies of the F94L variant made, on average, €65 more than those carrying one copy of the same variant. In cattle carrying one copy of the nt821 variant, an average of €92 more was made when compared to those not carrying any copy of a myostatin variant, while cattle carrying two copies of the nt821 variant made, on average, €233 more than those carrying one copy of the same variant. Those carrying one copy of the Q204X variant made, on average, €113 more than those not carrying any copy of a myostatin variant. Those carrying two copies of
the Q204X variant made, on average, €133 more than those with one copy of the same variant. Trends in commercial beef value (CBV) for these three myostatin variants indicate that the most profitable cattle also have higher CBV values. While these insights are useful, it is important to remember that market value is influenced by many other factors, including breed, management and overall genetic merit. Data for the four years 2022-2025 inclusive is shown in Table 2.

Table 2. Median mart prices, mart weights, age and commercial beef value (CBV) for cattle carrying 0, 1 or 2 copies of the most common myostatin variants; Q204X, nt821 & F94L (based on suckler animals only).

Variation in calving difficulty and carcass traits

Data in Table 3 shows Predicted Transmitting Ability (PTA) figures for beef cow calving difficulty and carcass weight for a group of AI sires that are myostatin carriers and have ≥90% reliability on the beef cow calving difficulty sub-index. Taking the first group of 14 sires that carry two copies of the F94L myostatin variant, the median calving difficulty is 4.6% but the calving difficulty PTAs range from 2.1% to 9.7%, while carcass weight PTAs range from 14.2 kg to 37.4 kg. Similarly, the calving difficulty PTA for AI sires carrying two copies of the nt821 myostatin variant range from 5.2% to 15.8%, while carcass weight PTAs range from 19.6 kg to 44.8 kg. This data reiterates the importance of viewing an animal’s myostatin status alongside their breeding indexes and sub-indexes. Myostatin status is only one of many factors influencing calving difficulty and carcass merit traits and, therefore, should never be interpreted in isolation.

Table 3. Median, maximum and minimum beef cow calving difficulty and carcass weight PTAs for AI sires
carrying myostatin variants and having ≥90% reliability on the beef cow calving difficulty sub-index.

Myostatin status

Advances in genomics has meant that the myostatin status can be inferred from a routine genotype taken for genomic evaluation purposes. The genotype status for nine myostatin variants (C313Y, D182N, E226X, F94L, L64P, nt821, nt419, Q204X, and S105C) are provided by ICBF as part of an add-on service and is available for high quality myostatin genotypes only. These results can be viewed by ICBF members through their genomic
profiles on ICBF.com or via the ICBF HerdPlus app and are also available to breed societies on pedigree certificates and sales catalogues

Summary:

  • The myostatin gene regulates muscle growth and development.
  • DNA variations within the myostatin gene (for example F94L, Q204X, nt821) are associated with carcass merit and calving difficulty.
  • These myostatin variants are present in many beef cattle breeds, such as Aubrac, Belgian Blue, Charolais and Limousin, among others.
  • Myostatin status is additional information that may be used when making breeding decisions, but it should be interpreted alongside breeding indexes.