LH Concentration after Kisspeptin Injection for Sexual Precocity Evaluation in Heifers

In order to research precocious sexual hypothalamic maturation, luteinizing hormone (LH) concentration in response kisspeptin injection in heifers was evaluated monthly, from weaning until the breeding season. The tested hypotheses were that LH secretion in response to kisspeptin increases with age and that at the same age, heifers heavier at weaning secrete more LH, in response to kisspeptin, than lighter weaned ones. The experiment was carried out in a commercial farm, from 184 weaned heifers the 25 heavier and 25 lighter were segregated as groups, but kept in the same pasture. The heifers were challenged monthly with 10 μg/kg (IM) of recombinant bovine kisspeptin (American Peptides) from weaning (eight months) until 17 months; 20 min after the injection blood sample was collected. At 17 months the heifers were exposed to bulls (25:1) for 60 days, the pregnancy was evaluated 30 days later. Progesterone and LH quantifications were performed by RIA and IGF-I by Elisa. The pregnancy rate of the heavier heifers was 92% (23/25) while in the lighter group it was 62.5% (15/24), gestational time did not differ between groups (88±6 in heavier and 73±8 days in the lighter group). There were no differences on the LH (3.6±0.4; 3.3±0.2ng/mL), IGF-I (460.6±37.6; 450.2±32.2ng/mL) and progesterone (1.4±0.32; 2.3±0.8ng/mL) concentration between lighter and heavier heifers. The same was observed between heifers that became pregnant or not, for LH (3.4±0.1, 3.7±0.2 ng/mL), IGF-I (447.8±32.8, 478.4±42ng/mL) and progesterone (0.9±1.8; 1.04±1.8) concentration. Considering the last weight before the breeding season, pregnant heifers in the heavier group had greater weight than pregnant heifers in the lighter group (319±5; 272±4 kg), even in the lighter group pregnant heifers were heavier than non pregnant (272±4; 254±4 kg). It was concluded that LH concentration after kisspeptin injection was not useful to evaluate hypothalamus sexual maturation and to be used as a screening tool for sexual precocity in Nellore heifers.

Acknowledgements: CAPES, UNESP, Fazenda Santa Encarnação- Bataguassu-MS, Brazil.