Litter size and vagina-cervix catheter penetration length in gilts

As in other species, the reproductive tract in pigs increases in size with age and body weight, and the development of the reproductive tract depends on a balance between development of the pituitary-ovarian axis and the influence of metabolic hormones. Two experiments were conducted in prepubertal Duroc gilts, 150-180 days of age, to determine whether litter size is related to vaginal-cervix catheter penetration length during insemination. In experiment 1, oestrus was induced in 452 gilts with a combined dose of 400 IU Pregnant Mare Serum Gonadotrophine (PMSG) + 200 IU human chorionic gonadotropin (hCG). The gilts were classified into three catheter penetration length groups: Ih, < or = 21 cm; IIh, > 21 and < 28 cm, IIIh, > 28 cm. The litter size was lowest in group Ih (7.35 +/- 0.15) compared with groups IIh (7.81 +/- 0.12; p < 0.05) and IIIh (10.0 +/- 0.36; p < 0.001). In experiment 2, first oestrus was induced in 162 gilts by boar exposure. The gilts were classified into three catheter penetration length groups at insemination during their second oestrus: In, < or = 24 cm; IIn, > 24 and < 26 cm; IIIn, > 26 cm. As in experiment 1, the litter size was lowest in the group with the shortest catheter penetration length (8.32 +/- 0.19). The litter size was not different among gilts of groups IIn and IIIn (8.84 +/- 0.35 and 9.56 +/- 0.46, respectively), but litter size was lower (p < 0.05) in group In than in group IIn. Based on the combined data from both experiments, the correlation between the catheter penetration length and total number of piglets born was expressed as: y =5.346 +/- 0.104x; r = 0.361 (p < 0.05). Fertility rate was not different among the groups of gilts induced into oestrus by hormone treatment or inseminated in the second oestrus; however, the total fertility rate of boar-exposed gilts was higher (p < 0.0001) than PMSG/hCG treated animals. Thus, it is possible to conclude that litter size at first farrowing is associated with vaginal-cervix catheter penetration length during insemination of the gilt.