Effects of a Single Zeranol Implant on Conception Rates and Dystocia in Primiparous Beef Heifers

Abstract

Beef heifers (374) were utilized to determine if zeranol implantation would influence pelvic development, dystocia and conception rates. One hundred fortyseven spring-born Simmental (SS) and 135 fallborn Angus (AF) heifers were allotted to two treatments: 1) controls (C); and 2) single zeranol implant at weaning (W). Ninety two fall-born Simmental (SF) heifers were allotted to three treatments: 1) C; 2) W and 3) single zeranol implant preweaning (PW) at 2 mo of age. Birth weights and weaning (200 d), yearling (360 d) and prepartum (700 d) weights and pelvic areas were measured. PW improved (P<.05) the gain of SF heifers (1.11 ± .20 vs 1.02 ± .02 kg) from birth to weaning but W had no effect on gains from weaning to yearling age. Pelvic areas at 360 d were increased (P<.02) for W compared with C (207 vs 195 cm2; 195 vs 188 cm2; and 175 vs 168 cm2; for SF, SS and AF, respectively). Prepartum pelvic areas were larger (P< .05) for SF and SS controls than for implanted heifers. Conception rates did not differ for PW, Wand C (89.5, 90.9, 85.7; 97.3, 94.5; and 82.7, 74.4 for SF, SS and AF, respectively). Percentages of heifers requiring calving assistance did not differ (21.1, 13.6, 19.1; 44.4, 40.5; and 35.5, 42.9 for SF, SS and AF, respectively). Assisted versus unassisted heifers at calving did not differ in pelvic area. Larger, zeranol induced, yearling pelvic areas were not present at calving. Zeranol had no effect on conception rates or dystocia.