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Time of Zeranol Implantation on Growth, Reproduction and Calving of Beef Heifers^1,2,

University of Nebraska, North Platte Center, North Platte 69101

Abstract

Zeranol implants were administered to 250 crossbred heifer calves at 1, 6 or 9 mo of age to evaluate growth, reproduction and calving performance. Heifers were assigned to eight treatment groups with 25 animals per group. Two additional groups of 25 heifers each were used to study the effects of multiple implants at two levels of nutrition on heifer performance. Implants at 1 mo of age (branding) increased heifer weights at 6 mo of age (weaning) by 5 kg (P=.08). Heifers receiving a combination of two implants gained faster (P<.05) from weaning to breeding (6 to 13 mo) than controls or heifers implanted three times. Implants at either 6 or 9 mo increased (P<.05) pre-calving pelvic areas (247 vs 241 cm² and 248 vs 240 cm² over controls, respectively). Implants did not affect the percent of heifers reaching puberty prior to breeding season. Conception rates in 62 d of breeding were comparable for implanted and control heifers (93 vs 96%), with the exception of heifers receiving implants at both 1 and 6 mo of age (56%). Calf birth weight, dystocia score, cow rebreeding rate and calf weaning weight were not affected by implant treatments. Heifers that received three implants and were fed at a high nutritional level (gained .62 vs .49 kg/d for regular level after weaning) tended (P;.10) to reach puberty at a higher rate prior to breeding and to have a higher total conception rate than implanted heifers on the regular nutrition level.

¹ Published as Paper No. 7555 Journal Series, Nebraska Agr. Exp. Sta., Lincoln.

² Appreciation is expressed to IMC Chemical Co., Terre Haute, IN for donation of zeranol and partial funding of study.

³ Animal Sci. Dept., North Platte Res. and Ext. Center.

⁴ Present address is 602 Bryant Court, Boulder City, NV 89005.