POSTWEANING PLASMA AMINO ACID PATTERNS AND NITROGEN CONSTITUENTS IN YEARLING BULLS AND HEIFERS WITH DIFFERENT RATES OF GROWTH

Abstract

Jugular blood samples were collected from 63 yearling Hereford bulls and 76 yearling Hereford heifers of similar genetic background at the end of a 160-day postweaning test. Plasma protein, amino nitrogen, and urea were determined on samples from all animals. Plasma free amino acids were determined on the 12 highest and 12 lowest gaining bulls and heifers, selected on the basis of 365-day weight. Plasma protein averaged 8.41 g/100 ml, amino nitrogen 56.38 μg/ml, and urea nitrogen 17.19 mg/100 ml for the 63 bulls. In the 76 heifers, plasma protein averaged 7.52 g/100 ml, amino nitrogen 52.90 μg/ml, and urea nitrogen 7.65 mg/100 ml. Plasma protein, amino nitrogen, and urea nitrogen were not significantly different (P > 0.05) between the high- and low-gaining groups within sexes. The concentrations (μmoles/100 ml) of the individual plasma amino acids did not differ significantly (P > 0.05) between the high- and low-gaining groups of bulls or heifers. When calculated on a proportional basis (molar %), the individual plasma amino acids also did not differ significantly (P > 0.05) between the two groups within sexes. Proline and tyrosine were negatively correlated (P < 0.01) to postweaning gain in the bulls, whereas isoleucine and aspartic acid were negatively correlated (P < 0.05) to postweaning average daily gain in the heifers.