GnRH Inducing LH Release, Nutrition and Plasma Cortisol in High Producing Dairy Cows Postpartum

抄録

The primary objective of this study was to determine how the luteinizing hormone (LH) response to gonadotropin releasing hormone (GnRH) treatment after parturition correlates with the intakes of total digestible nutrient (TDN) and crude protein (CP), and the plasma concentration of glucose and cortisol during the postpartum period in high producing dairy cows (HPDC). The second objective was to determine whether GnRH analogue (GnRH-A) treatment induces ovulation and subsequently ovarian cyclicity in the acyclic cows. The day of normal parturition was designated DAY 0. Twenty-four multiparous Holstein cows were assigned randomly to one of three postpartum groups and accordingly received 200 μg of GnRH-A intramuscularly at either day 10 (DAY 10 group; n=9 postpartum acyclic cows), day 30 (acyclic DAY 30 group; n=4 postpartum acyclic cows: cyclic DAY 30 group; n=6 postpartum cows with functional corpus luteum (CL)) or day 60 (DAY 60 group; n=5 postpartum cows with functional CL). For the analysis of LH concentration, blood samples were collected at 15- or 30-min intervals until 8 h after the treatment. Plasma samples collected before the GnRH-A injection were assayed for LH, cortisol (CORTISOL) and glucose (GLUCOSE). In order to analyze TDN and CP intake until 9 weeks after parturition, 6, 2 and 4 cows were randomly selected from the DAY 10, acyclic DAY 30 and cyclic DAY 30 groups respectively. The CORTISOL (2.3 ± 2.3 ng/ml), GLUCOSE (45.5 ± 7.3 mg/dl), and plasma peak LH concentration after GnRH-A injection (27.6 ± 11.1 ng/ml), as well as the area under the LH response curve (AUC; 3250 ± 1505) and TDN intake at the week of GnRH-A injection as a percentage of requirement (67.8 ± 8.2 %) were smaller than other groups in the DAY 10 group (P<0.05). These items in the weeks following parturition tended to increase as the postpartum period progressed. From the results of DAY 10, we found only two significant positive correlation coefficients by rank (P<0.05) between CORTISOL and peak LH (r=0.72) and between CORTISOL and AUC (r=0.65). From the comparison between acyclic DAY 30 and cyclic DAY 30 groups, no clear correlation could be found between nutrition and pituitary LH release response to GnRH-A. In conclusion, the LH release induced by GnRH-A injection, plasma cortisol level and values of nutritional items in the weeks following parturition tended to increase as the postpartum period progressed. However in the same week after parturition, the LH response to GnRH-A treatment is correlated only with plasma cortisol level, and not with nutrition. Additionally, all the treated cows had single or multiple ovulations, and started or continued estrous cycles of normal length lasting for 18 to 22 days.