Effects of Zeranol® upon luteal maintenance and fetal development in peripubertal gilts

Eighty gilts were utilized to determine whether zeranol implants could maintain hCG-induced corpora lutea (CL) in peripubertal gilts and to examine the effects of a Zeranol® implant on fetal development. Crossbred gilts (171 ± 0.3 days of age, 109.1 + 1.4 kg) were blocked by weight and ancestry to control (n = 40) or treatment (n = 40) groups. To induce ovulation and CL maintenance, treated gilts received 500 IU of hCG i.m. and a Zeranol® ear implant (Ralgro®, 36 mg; day 0). All gilts were checked once daily for estrus with a mature boar from days 3–58 of the experiment. On day 42, treated gilts received two 10 mg injections of Lutalyse (PGF2α) spaced 6 h apart. Treated gilts not displaying estrus within 7 days of PGF2α received two additional 10 mg of PGF2α spaced 6 h apart on day 49. On days 44–58, gilts detected in estrus were inseminated twice, 24 h apart with pooled semen via AI. Blood samples were obtained on days 0, 7, 18 and 42 and analyzed for serum progesterone (P4). Bred gilts were slaughtered on days 58–62 of gestation. Ovulation, as determined by serum concentrations of P4 on day 7 of the experiment, was induced by hCG in 79.5% of treated gilts. Zeranol® implants, however, failed to increase (P > 0.05) the proportion of gilts available for breeding (treated, 21/39; control, 18/40). Of gilts inseminated on days 44–58, 16/21 treated gilts and 16/18 control gilts were pregnant at slaughter on days 58–62 of gestation. Number of fetuses (7.5 versus 12), fetal weight (83 versus 121 g), fetal length (117 versus 132 mm) and fetal survival (45% versus 78%) were reduced (P < 0.001) by Zeranol® implants. These data indicate that treatment of peripubertal gilts with a 36 mg Zeranol® implant did not increase the proportion of gilts available for breeding while causing deleterious effects upon the fetuses.