The Effects of Supplemental GnRH on Reproductive Performance in Lactating Holstein Dairy Cows During the Summer and Winter.

Abstract

During the summer trial lactating dairy cows were randomly assigned to four treatment groups. These groups included a control group that received no hormonal supplement, and three groups that received supplemental gonadotropin releasing hormone (GnRH) on either day 5, day 11, or both days 5 and 11 post insemination. The cows during the winter trial were randomly assigned to three treatment groups, a control, and cows receiving hormonal supplements on day 5, or days 5 and 11 post insemination. Blood samples and rectal temperatures were taken 9 and 2 days prior to insemination and every other day there after beginning on day 5 post insemination and ending around day 30. Pregnancy was checked on day 30 and then again between days 45 and 60 by ultrasonography. Daily maximum and minimum temperatures and humidity values were recorded in order to calculate the temperature-humidity index and determine the level of heat stress the cows were experiencing. Serum progesterone (P4) and cortisol concentrations from all samples were analyzed using RIA. Environmental information for the summer indicated that the cows experienced an overall mild heat stress by an average THI of 76, where as the winter environmental data showed an average THI of 43, within the thermo-neutral range. However, on individual days during the summer the THI reached values between 79 and 85, indicating that cows experienced medium heat stress. Serum P4 concentrations were data was analyzed between days 11 through 17 post insemination, the expected luteal phase, during the summer. Treatment groups were compared and indicated that the serum P4 concentration was greater in GnRH-D11 treated cows than control cows (P < 0.05) and than GnRH-D5 treated cows (P < 0.05). Concentrations of P4 between groups during the winter were also analyzed between days 11 and 22 post insemination, however the results were different. GnRH-D5 and GnRH-D5+11 were both greater than control concentrations, 6.87 &#177; 0.69 (P < 0.09) and 7.10 &#177; 0.56 (P < 0.05), respectively. Winter progesterone concentrations were greater than those of summer between treatment groups (P < 0.05), indicating that there was an effect of environment on hormonal levels. Analysis of cortisol concentrations during the summer indicated no significant difference between treatment groups, as was the same for winter groups, and there was no difference between seasons. Administration of GnRH appeared to have a beneficial effect on pregnancy rates during the summer between days 45 and 60. This was determined when all groups receiving the hormone supplement were combined (P < 0.05). In conclusion, the administration of GnRH to heat stressed cows increased P4 concentrations and appeared to have a positive effect on pregnancy rates.