Abstract
A total of 71 primiparous, lactating rats were devided into 9 groups, with regard to the litter size and the duration of lactation. The number of suckling was adjusted to 4, 8 or 12 on the next day after parturition and the young were allowed to suckle for 7, 14 or 21 days, respectively. After weaning, each mother rat was placed with a fertile male and the intervals from weaning to the first post-weaning estrus, mating and conception were examined by daily inspection of vaginal smear. Results obtained are as follows:
1) The interval between weaning and the first post-weaning estrus (vaginal cornification) became shorter with increasing period of latctation. Multiple regression of the interval (Y) on the duration of lactation (X1) and on the litter size (X2) was Y=3.56-0.142(X1-15.0)+0.050 (X2-8.04), where b1=-0.142 was highly significant, while b2=0.050 was not significant.
2) The percentage of females mated up to the third post-weaning estrus was lower in 4-young nursing group, in comparison with 8 or 12-young nursing group. There was not significant difference between the percentage of females mated at the first post-weaning estrus and that of the second estrus.
3) Out of 55 mated females, 51 were conceived. Number of corpora lutea, implantation sites or living fetuses examined at the 19th day of gestation was not different significantly among groups.
