Hormonal changes during lactation in cows of three different breeds

Changes of somatotropin (bST), insulin-like growth factor I (IGF-I), insulin (INS) and free fatty acids (NEFA), and milk yield were measured ante partum and during an entire lactation period in groups of dual-purpose cows of the German Fleckvieh (FV) and German Brown Swiss (BS) breeds and of German Black and White (BW) dairy type cows. In total, 26 cows were studied located on two farms. Blood was collected weekly (8 weeks ante partum (ap) until 40 weeks post partum (pp)) and more frequently (10 h, 30 min intervals) at 6 and 2 weeks ap and Weeks 2, 4, 8, 12, 20 and 30 pp. Episodic secretion pattern was recorded only for bST. bST, NEFA and milk showed a clear increase after parturition in all three breeds. NEFA started to rise 2 weeks ante partum. In contrast, INS and IGF-I fell immediately post partum to significantly lower concentrations. As lactation progressed, all of the investigated plasma parameters returned to values seen in pregnancy. The three groups studied of FV, BS and BW cows exhibited significantly different values for bST, INS, IGF-I and milk yield during the first 8 weeks of lactation. NEFA concentrations were significantly higher in BW vs. FV and BS cows. The relation between bST and milk yield was positive and significant in BS cows (r = 0.58). The elevation of bST pp is due to an increase of episodic secretion pattern (baseline, peak frequency, peak amplitudes and rise of suprabasal peak secretion).