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Estimation of the relationship between intramuscular fat, testosterone, and scrotal circumference in yearling Angus bulls
The objective of this study was to estimate the correlation between intramuscular fat (IMF), testosterone levels, and scrotal circumference (SC) of Angus bulls. Angus bulls (n=317) were put on an 84 day growth trial at an average age of 276.1 + 1.15d with an ADG of 1.51 + 0.013 kg. The study was conducted in 5 replications over a 5 year period. Bulls were weighed every 14 days and individual feed intake was recorded daily. During the first and last week of the trial SC was measured and blood samples were taken for the purpose of measuring circulating plasma concentrations of testosterone. At those same time points a certified ultrasound technician captured images which were later evaluated by the National CUP Lab and Technology Center, Ames, IA to predict IMF, 12thrib fat, and rump fat. Partial correlations were calculated using three different statistical models. Each model included the fixed effect of year. Testosterone, IMF, and SC were adjusted using a regression covariate of either 84d body weight, rib fat, or rump fat. Correlations that differed from zero (P < 0.01) were only consistent in traits adjusted for rib fat. Correlations between SC and testosterone, IMF and testosterone, and SC and IMF were 0.46, 0.42, and 0.59, respectively. There were also correlations between testosterone and IMF (0.12; P < 0.05) as well as SC and testosterone (0.23; P < 0.01) when adjusted for rump fat and IMF and testosterone (0.19; P < 0.01) when adjusted for 84d weight. When adjusted to a common rib fat there were positive, favorable correlations among testosterone, IMF, and SC. This supports the existing literature. Selection for increased IMF in Angus bulls would not be expected to negatively impact testosterone or SC which are two important components of male fertility.
Keywords:
Beef cattle, fertility, intramuscular fat