INFLUENCE OF AGE, WEIGHT AND GROWTH RATE ON BASAL LH, GROWTH HORMONE AND CORTISOL, AND ESTROGEN-INDUCED LH RELEASE IN PREPUBERTAL GILTS

Abstract

Thirty-six Yorkshire × Landrace gilts were selected at 74 d of age and 32 kg body weight and assigned equally to one of six treatments. The first three treatments involved feeding gilts to achieve weights of 90 kg at 175 d, 90 kg at 195 d, or 70 kg at 175 d (treatments 1, 2 and 3, respectively). A further three groups of gilts were fed to achieve the same weights (90, 90 or 70 kg) 20 d younger than those above. Upon the achievement of their final weight the latter three groups of gilts were subjected to a feed restriction in order to achieve a zero net weight for 20 d until they achieved the required age (treatments 4, 5 and 6). At the achievement of the specified age-weight end point blood samples were taken from all gilts at 2-h intervals for 24 h (2400–2200 h) for the determination of plasma concentrations of luteinizing hormone (LH), growth hormone (GH) and Cortisol. Additionally, during a 4-h period on the same day (0800–1200 h) all gilts were sampled at 15-min intervals for determination of pulsatile LH secretion. Following the initial 24-h sampling, all gilts received an intramuscular injection of estradiol benzoate (EB; 15 μg kg−1 body weight) and blood samples were collected at 6-h intervals for 72 h for determination of plasma LH. No treatment effects were noted on the mean daily plasma levels of cortisol, LH or GH. Plasma concentrations of cortisol tended to decrease with increasing age or weight, but these differences were not significant. Significant elevations of plasma GH occurred in the early morning and evening, but rarely between 0400 and 1600 h. A similar nycterohemeral rhythm was noted for plasma LH levels. There were no treatment effects on LH pulse frequency but LH pulse amplitude was greater at a given age (P < 0.07), in the heavier gilts (treatments 1 and 4 vs. 3 and 6). There was no effect of treatment on the LH response to EB injection, although two patterns of LH release were observed, involving either a single or a double peak of LH. Key words: Gilts, age, weight, puberty