Factors affecting fertility in the lactating domestic rabbit
Lactational anoestrus and anovulation place limitations on the economic and biological efficiency of the three main domestic farm species ie cow, pig and sheep by reducing reproductive efficiency. The factors of Remating Interval, Nutrition and Suckling have been shown to influence fertility during lactation in these species. The objective of the present study was to examine the effects of these factors on fertility and identify the mechanisms by which they act, using the lactating domestic rabbit as an experimental model and so provide information which may help overcome the obstacles preventing concurrent pregnancy and lactation in the domestic farm species. Experiment 1 provided data on the fertility of primiparous crossbred lactating does in the immediate postpartum period. This information was used to decide on which day postpartum does should be remated in subsequent experiments. Sixty-four does each suckling 6 pups were mated on day 1 (N = 17), 2 (N = 17), 3 (N = 15) or 4 (N = 15) postpartum (where day 0 = the day of parturition) and killed on day 10 postcoitum (where day 0 = the day of mating) to assess fertility. Overall fertility levels were high during this period. There were no significant differences between does remated on day 1- 4 postpartum in their mating response (percentage of those does tested for mating, which mated) and, overall this was 97%, ovulation response (percentage of those does mating, which ovulated), overall 77%, implantation response (proportion of those does ovulating, which implanted ova), overall 83%, implantation rate, overall 8.7 or pre-implantation mortality (%), overall 24%. Ovulation rate was significantly increased in does mated on day 3 (13.3) and 4 (13.1) compared with those on day 1 (10.2) (P 0.05) and 2 (9.6) (P 0.01) postpartum. From these results day 1 postpartum remating was found to result in the highest level of absolute fertility (ie by multiplication of the estimated pregnancy rates (the percentage of does implanting ova of those mated) of' 82%, 60%, 60% and 53% with the implantation rate 7.9, 8.1, 9.4 and 9.6 of does mated on day 1 (N = 17) , 2 (N = 15), 3 (N = 15) and 4 (N = 15) postpartum respectively, giving a total of 111, 73, 85 and 77 pups on day 1, 2, 3 and 4 respectively) during this period postpartum and was therefore used in future experiments concerning the remating interval. In Experiment 2, the effects of the remating interval and nutrition on the fertility of lactating crossbred does was examined. Eighty-six does each suckling six pups were remated on either day 1 or 14 postpartum and offered diets with digestible energy (D.E.) contents of 8.5 or 15.4 M.J./ kg D.M. and digestible crude protein contents (D.C.P.) of 129 or 227 g/kg D.M. and which were otherwise nutritionally adequate for lactating does. The experiment was carried out over three consecutive lactations and does were killed at selected times in each lactation to assess fertility and body composition. Diet had no effect on any aspect of fertility, while in contrast the remating interval was shown to have a significant effect. Does remated on day 1 had a significantly greater ovulation response in the first (82% vs 63%, P 0.10) and second (100% vs 77%, P 0.05) lactation compared with those mated on day 14 postpartum. Pregnancy rate was also significantly greater in does remated on day 1 in the first (76% vs 51%, P 0.05) and the second (100% vs 67%, P 0.01) lactation compared with those mated on day 14 postpartum. There were no differences in pre or post-implantation mortality between does remated on day 1 and 14 postpartum and therefore pregnancy rate was determined by the initial ovulation response. Litter size at term was found to be related to the initial ovulation rate and both were greater in does remated on day 14 compared to day 1 postpartum. This reached significance for litter size at term in the first lactation (7.8: day 1 vs 10.1: day 14, P 0.01) and ovulation rate in the second lactation, which was 12.4 for does remated on day 1 and 17.8 for those remated on day 14 postpartum (P 0.01). There was no significant difference in the body composition of pregnant does on the various treatment combinations over the course of the experiment or in lactation performance between does which were pregnant and those which were not. Experiment 3 was undertaken to confirm the effects of the remating interval on fertility observed in does on Experiment 2 and also to record measures of doe lactation performance to assess if they could be related with the effects of this factor.