ENDOCRINOLOGY OF NUTRITIONAL FLUSHING IN GILTS

As part of a nutritional flushing study, the endocrine status of gilts was compared to gain further knowledge of the mechanism(s) that mediate the restoration of ovulation rate. From 45.3 kg BW to puberty, 42 gilts were fed 2.0 kg d-1 (L) or ad libitum (H). From puberty to second estrus, one-half of the L gilts had their feed allowance increased to 2.8 kg d-1 (LH). Gilts in the H group continued to be fed ad libitum (HH). Gilts in the LH and H groups showed a greater increase in ovulation rate between first and second estrus than LL gilts (P < 0.05). Based on sampling every 4 h through the peri-estrous period of 28 gilts, feeding level did not affect mean plasma follicle-stimulating hormone (FSH), luteinizing hormone (pLH), estradiol-17-beta or progesterone concentrations (P > 0.05). During the follicular phase, however, LH and HH gilts had not only a greater proportion of samples > 1 ng mL-1 but also showed an increased mean pLH daily sample variance compared with LL gilts (P < 0.05). The latter parameters were interpreted as indicators of an increased frequency of episodic pLH release. Furthermore, HH gilts had greater (P < 0.05) plasma insulin concentrations through the peri-estrous period and greater insulin-like growth factor I (IGF-I) concentrations during the pLH-surge period than LL gilts. These results confirm that increased plasma insulin may be an important metabolic cue, which in an independent or synergistic action with gonadotropins and IGF-I mediates the ovulatory response to flush-feeding.