CHARACTERISTICS OF SOMATOTROPIN AND PROLACTIN PROFILES IN YOUNG DAIRY SIRES BEFORE AND AFTER A 72-HOUR FAST DURING DIFFERENT SEASONS

Serial blood samples were collected from young (xBAR = 275 d of age) AI sires at 20-min intervals for an 8-h period before and after a 72-h fast. Samples were collected from four different groups of young sires in January (n = 23), April (n = 31), July (n = 27), and October (n = 24). Samples were collected for only 6 h for the July group. Plasma somatotropin (ST) and prolactin (PRL) data were analyzed by PULSAR, a computerized peak detection program. The PULSAR parameters, except for number of peaks detected, were not different (P > .05) when calculated using data from the entire 8-h period or just the first 6 h. Because some endocrine characteristics as defined by PULSAR were influenced by season, deviated values were calculated for those characteristics within seasonal groups by subtracting the mean of that group from each individual observation for all PULSAR parameters. Number of ST peaks was correlated (P < .01, r = .26) with USDA pedigree index (PI) for fat yield (PI(F)), sire predicted transmitting ability for fat (PTA(F); P < .01, r = .30), Northeast AI Sire Comparison (NEAISC) PI(F) (p < .05, r = 24), and sire PTA(F) (P < .01, r = .33). No ST pulsatile characteristics after fasting were correlated (P > .05) with PI. Prolactin peak length was negatively related to USDA PI(F) (P < .05, r = -.23) before fasting and positively correlated (P < .05, r = .26) after fasting. Seasonal differences in pulsatile characteristics may reduce usefulness of endocrine parameters in assessing genetic merit for production traits.