The reproductive performance of female goats treated with melatonin is not improved after introduction to bucks displaying springtime sexual activity if these does are experiencing decreasing body weight/condition score

•Reproductive activity of bucks is reduced in spring when male effect is used.•The response to the male effect decreases when females have an inadequate food supply.•Melatonin might reduce the negative effect of decreasing body weight-condition score.•Greater fecundity, fertility and productivity were observed in gain-melatonin does.•Plasma glucose and IGF-1 concentrations could explain the better reproductive results.

The aim of the present work was to determine whether treatment with melatonin modifies the reproductive response of female goats experiencing increasing or decreasing body weight (BW)/body condition score (BCS) when introduced to bucks displaying springtime sexual activity. During natural anoestrus, 53 does were isolated from bucks for a period of 42 days and distributed into two groups: 1) low BW/low BCS animals (N = 24) (LLg group), which were fed 1.9 times their maintenance requirements so they would experience increasing BW and BCS; and 2) high BW/high BCS animals (N = 29) (HHl group), which were fed 0.4 times their maintenance requirements so they would experience decreasing BW and BCS. Half of each group was treated, or not, with melatonin (LLg + Mel N = 12, HHl + Mel N = 15, LLg-Mel N = 12 and HHl-Mel N = 14). On 6th May they were introduced to six males, showing natural sexual activity, fitted with marking harnesses (thus permitting the detection of oestrous activity). The ovulation rate was assessed by transrectal ultrasonography and confirmed via the plasma progesterone concentration (measured twice per week in blood samples). Plasma glucose, IGF-1 and non-esterified fatty acid concentrations were also determined, along with the conception rate, fertility, prolificacy and productivity of the does. LH concentrations and LH pulsatility were also recorded in the hours around introduction to the males. 'Oestrous plus ovulation' was observed only in does treated with melatonin. A higher conception rate and greater fertility and productivity were observed among the LLg + Mel does. These females showed higher glucose and IGF-1 concentrations after the introduction of the males. LH concentrations increased after male introduction independent of all other conditions. In conclusion, the present results show that treatment with melatonin does not enhance reproductive performance in does experiencing decreasing BW/BCS, but can improve it when does are experiencing increasing BW/body fat reserves - even when exposed to males displaying only springtime sexual activity. This might be explained by the higher blood glucose and IGF-1 concentrations of the LLg + Mel females.