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The use of genetic markers to aid in selection decisions to improve carcass and growth characteristics is of great interest to the beef industry. However, it is important to examine potential antagonistic interactions with fertility in cows before widespread application of marker-assisted selection. The objective of the current experiment was to examine the influence of 2 commercially available markers currently in use for improving carcass traits, the myostatin (MSTN) F94L and μ-calpain (CAPN1) 316 and 4751 polymorphisms, on heifer development and reproductive performance. In Exp. 1, beef heifers (n = 146) were evaluated for growth and reproductive traits over a 3-yr period to determine if these polymorphisms influenced reproductive performance. In Exp. 2, heifers representing the 2 homozygous genotypes for the MSTN F94L polymorphism were slaughtered on d 4 of the estrous cycle and reproductive tracts were collected for morphological examination. In Exp. 1, there was a tendency (P = 0.06) for birth BW to be affected by MSTN with the Leu allele increasing birth BW in an additive fashion. Additionally, MSTN significantly affected the proportion of pubertal heifers by the start of the breeding season (P &lt; 0.05) with the Leu allele additively decreasing the proportion pubertal; however, this did not result in a delay in conception or a decrease in pregnancy rates during the first breeding season (P &gt; 0.15). The GT haplotype of CAPN1, which was previously associated with decreased meat tenderness, was associated with an additive decrease in birth BW of the first calf born to these heifers (P &lt; 0.05). In Exp. 2, there were no differences between the MSTN genotypes for gross or histological morphology of the anterior pituitary, uterus, or ovaries (P &gt; 0.05). From these results, we concluded that the MSTN F94L and CAPN1 polymorphisms can be used to improve carcass traits without compromising fertility in beef heifers. The influence of these markers on cow performance and herd life remains to be determined. While the delay in puberty associated with the MSTN F94L polymorphism did not negatively impact reproductive performance in heifers, caution should be used when combining this marker with other markers for growth or carcass traits until the potential interactions are more clearly understood.

A polymorphism in myostatin influences puberty but not fertility in beef heifers, whereas µ-calpain affects first calf birth weight1