THE EFFECT OF EXOGENOUS GROWTH HORMONE ON THE ENDOCRINE STATUS AND THE OCCURRENCE OF ESTRUS IN GILTS

Abstract

Fifty-six gilts of Yorkshire and Landrace breeding were selected at 86.8 ± 0.8 kg body weight (BW) and given an intramuscular (im) injection of 400 IU PMSG plus 200 IU hCG to stimulate ovulation. From 14 d after gonadotropic stimulation, gilts were exposed to a boar to detect a subsequent spontaneous estrus. At the onset of this first observed estrus, gilts were allocated to receive daily injections (i.m.) of growth hormone (pGH, 90 μg kg−1) from either 14 d (GH14, n = 21) or 17 d (GH17, n = 20) until 22 d after the onset of the first observed estrus. A third group of gilts served as controls (n = 15) and received vehicle buffer. Blood samples were obtained by jugular vein puncture at 3-d intervals from 14 to 29 d, inclusively. Gilts were slaughtered 30–32 d after the first observed estrus at which time their ovaries were recovered for the determination of ovulation rates. All control gilts and all but one GH17 gilt exhibited normal estrous cycles. However, of the 21 gilts assigned to GH14, only 9 (43%) had normal estrous cycles (P < 0.001). In gilts exhibiting a second estrus, there was no effect of pGH treatment on the duration of the estrous cycle (20.4, 20.9 and 20.5 d) or on ovulation rate (14.6, 13.9 and 13.5) for GH14, GH17 and controls, respectively. Serum assays revealed that pGH injections resulted in decreased serum concentrations of thyroxine (P < 0.01) but increased concentrations of triiodothyronine, insulin and glucose (P < 0.001). The present data confirm an adverse effect of pGH on ovarian function. However, the adverse effect is only evident when the pGH injection regime encompasses days 14–16 of the estrous cycle. Key words: Gilts, growth hormone, estrus, ovulation rate