Use of plasma hormone and metabolite levels to predict breeding value of young bulls for butterfat production

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Abstract
The purpose of the experiment was to investigate the possibility of using concentrations of hormones and metabolites in plasma of potential breeding bulls early in life as indirect selection criteria for butterfat production. Sixteen Red Danish bull calves were included in the experiment, 10 were offspring of parents selected for high butterfat production for two or three generations, and six were from parents selected for low butterfat yield. The average estimated relative breeding values of the calves in the two groups were 111 and 91.At 3·5 and 7 months of age blood samples were collected every 6 h during 2 days of ad libitum feeding, 5 days of fasting and 2 days of refeeding. The samples were analysed for glucose, urea, free fatty acids, insulin and thyroxine. At 3·5 months plasma concentrations of free fatty acids were significantly correlated with breeding value under all feeding conditions examined. The increase in plasma glucose during refeeding at both ages was positively related to breeding value. Plasma insulin during the 1st day of fasting was positively correlated with genetic merit at 3·5 months of age and negatively correlated at 7 months. Plasma thyroxine was negatively correlated with breeding value at both ages, but the correlation was only significant at 7 months. Multiple regression analyses showed that a large part of the variation in breeding value for butterfat production could be described by plasma levels of hormones and metabolites when more than one variable was included in the model. With six variables in the model 0·67 and 0·73 of the variation could be accounted for at 3·5 and 7 months of age respectively.The results of the experiment have to be considered with caution because only one sire was used per group, the parents were preselected, and the number of animals was small. The results, however, do indicate that plasma levels of hormones and metabolites in the early life of potential breeding bulls, measured after suitable physiological challenges, may be useful as selection criteria for butterfat production of their offspring.

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