Possible fertility effects of zeranol in yearling heifers

Authors: Moller S
Publication: New Zealand Veterinary Journal, Volume 32, Issue 9, pp 157-158, Sep 1984
Publisher: Taylor and Francis

Animal type: Cattle, Livestock, Production animal, Ruminant
Subject Terms: Mycotoxicosis, Liver/hepatic disease, Disease/defect, Reproduction, Fungal/yeast, Reproduction - female, Pasture/crop, Reproduction - hormones
Article class: Correspondence

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Abstract: Female dairy herd replacement calves from a factory supply Jersey pedigree herd were injected with a subcutaneous implant of Zeranol 36 mg at 5-6 weeks of age (range 1-29 weeks) each spring for three consecutive years. The purpose was to improve the growth rate of calves. The calves were grazed away from the home farm at `runoffs` as yearlings (9-18 months) and run with a bull at mating time from early October till early January. Pregnancy diagnosis found in each year 23-47s of implanted yearlings to be not pregnant and others to be calving later than expected. Attempts to elucidate the reasons for poor fertility have been unsuccessful to date. The effect appears to be more than a delayed onset of puberty as some yearlings were `carried over` to the following season and remated as 27-month old heifers (total 19 in two seasons). The pregnancy rate amongst these was 5/19 (26%) after the same mating period. Clinical impressions were that overt oestrus did occur and corpora lutea did form in implanted cattle. It is not known whether the oestrus cycles were at normal intervals…

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