Effects of the F94L Limousin associated myostatin gene marker on metabolic index in growing beef heifers*

Objective: Our objective was to evaluate fasting heat production in cattle with and without the GDF8 mutation. Materials and Methods: The heifers used in this study were genotyped for the myostatin(MSTN) gene mutation to determine their MSTN genotype as either homozygous normal for phenylalanine at amino acid position 94 of MSTN (0 copy; n = 5) or homozygous for F94L variant in MSTN (2 copy; n = 6). Oxygen consumption was measured using portable headbox respiration calorimeters, and heat production was calculated. Results and Discussion: Body weight was not different between treatments (P = 0.33), but it did differ across days (P < 0.01), increasing as heifer age increased. Oxygen consumed in liters per kilogram of metabolic BW was greater for 0 copy heifers than the heifers with 2 copies of the F94L MSTN variant (P = 0.03), on all days measured. Oxygen consumed and heat production decreased as heifer age increased in all heifers irrespective of genotype (P ≤ 0.03). Respiratory quotient had a tendency to be greater for heifers with 2 copies than for heifers with no copy (P = 0.07). Average daily gain measured across the whole study (121 d) was greater for heifers with 2 copies of the F94L MSTN variant. Implications and Applications: Therefore, it is likely that cattle with 2 copies of the F94L MSTN substitution have lower maintenance energy requirements than cattle without the MSTN gene. Thus, genotype, if known, should be considered when assessing energy requirements of beef cattle.