The objective of this study was to examine the influence of negative energy balance in dairy cows post partum on the LH release after a single GnRH application. Also the development of body condition, body weight and milk concentration of betahydroxybutyrate, Acetoacetat and phosphoenol pyruvate was analyzed to evaluate a possible correlation with energy balance. For 30 Holstein-Friesian cows daily dry matter intake, milk yield and weekly body weight was monitored. Based on this data, the negative energy balance per day was calculated for each cow as the difference between energy needed for maintanence plus lactation and energy consumed. The cows were divided in four different groups. Group 1 (n = 7): Cows treated with 500 µg GnRH (Fertagyl®) intramuscularly when the most negative point of energy balance (nadir) was reached. Group 2 (n = 6): Cows treated with the same dose of GnRH when about 30 % of the individual value of nadir was already compensated. Group3 (n = 10): Cows treated with the same dose of GnRH when about 60 % of the individual value of nadir was compensated. Controls (n = 7): Cows received a placebo (0,9% NaCl solution) on different stages of negative energy balance. All cows were either in interestrus or in anestrus, as determined by plasma estradiol-17? levels at the time of treatment. All animals did not experience any postpartum disorders. Blood samples were collected every 20 minutes beginning one hour before and ending four hours after treatment. LH was determined using enzyme linked immuno sorbant assay (ELISA). Body condition and body weight was evaluated weekly. Ultrasonic measurements of the backfat thickness were made over the area between the hook- and the pin bone. Weekly milk samples were taken and analyzed for betahydroxybutyrate, Acetoacetate and phosphoenol pyruvate. The LH release of group 1 cows was significantly higher than in every other group. Group 2 and 3 cows were showing no difference. In group 4 no changes in LH concentration could be detected before and after placebo administration. None of the metabolic parameters tested was significantly correlated with energy balance. In this study no statement was made about the biological quality of the released LH. We conclude, that the sensitivity of the pituitary gland for GnRH is not directly affected by the level of energy balance post partum. It is possible, that the interval from calving to nadir (i.e. the slope of the negative energy balance curve) has a higher influence of the sensitivity of the pituitary gland than the absolute level of negative energy balance.
