Altered secretion of LH does not explain seasonal effects on early pregnancy in gilts

This work was undertaken to study the effects of energy intake and gut fill on LH secretion in the early pregnant gilt to investigate the role of the hypothalamo-pituitary-gonadal axis in the seasonal disruption of pregnancy. Four groups of mated gilts (N = 23 in total) were individually housed and fed four different energy levels (23, 46, 47 and 55 megajoules digestible energy per day, MJ DE day-1) for two weeks after mating during the season identified as a period of increased rate of early pregnancy failures from January till April (summer-autumn). The energy content of base feed (13 MJ kg-1) was increased by means of adding a fat supplement (soybean oil) to the feed of two groups of gilts, while the other two groups were given different levels of the basic feed. The groups gained weight and backfat according to energy intake levels during the two-week period. Frequent blood samples (15-min interval) collected on day 14 revealed a significant treatment effect on LH pulse amplitude: the group on the highest energy intake level (55 MJ DE day-1) had higher LH pulse amplitude compared with the other groups (1.01 +/- 0.04, 0.74 +/- 0.04, 0.72 +/- 0.08 and 0.66 +/- 0.05 ng ml-1 in the descending order of energy intake, P < 0.01). There was no effect of energy intake on LH pulse frequency, mean level, area under the curve or mean nadir (P > 0.10). Plasma insulin concentrations tended to increase (P < 0.08) in each group with time after feeding, but no significant differences between the treatment groups were found. Plasma free fatty acid (FFA) concentrations were decreased 1 h after feeding (P < 0.05) in all but the group on 55 MJ DE day-1. This group also had higher postprandial FFA concentrations in comparison with other groups (P < 0.05). In conclusion, these results show that energy density of feed and energy intake have little effect on LH secretion in the early pregnant gilt. Protective effect of high feeding level against seasonal disruption of pregnancy appears to be mediated by mechanisms other than an alteration in LH secretion.