The objectives were to determine whether Zeranol (ZER) and its metabolites or pharmacological zinc levels could change the incidence of vaginal prolapse(bearings) in ewes. In the ZER experiment 100 twin pregnant ewes were controls, a further 50 ewes each received either two low (24 mg and then 12 mg) or two high (36 mg and then 24 mg) doses of zeranol (ZER) implants separated by a four-week interval with 65% of treated ewes receiving an additional 12 mg implant at set stocking. Urine samples were collected from 12 ewes, twice weekly, bulked into three samples and analysed for Zearalenone (ZEN), ZER and creatinine. Two weeks after receiving an implant, the ZER:creatinine ratio increased from 2.6 in C ewes to 21, 30 and 36 and the ZEN ratio from 3.25 to 6.7, 10.8 and 14.7 following treatment with 12, 24 and 36 mg of ZER (P<0.01). One month after treatment the ZER and ZEN ratios had decreased but were still higher in treated than control ewes (ZER 3.6 vs.11, P<0.01; ZEN 1.2 vs.5.2, P<0.01). The bearing incidence was 3%, 2% and 2% for control, low and high dose ZER respectively. In the second experiment 100 scanned twin or triplet pregnant ewes were administered with 72 g of zinc oxide (The Time Capsule ®) as a ruminal bolus 4-6 weeks before lambing on four farms with historical high incidences of bearings. Pre-treatment and all control serum zinc concentrations were low (9.4 ±0.2 mmol/L) but not considered deficient. Zinc supplementation increased serum zinc concentrations to 21.5 mmol/L (P<0.001). Bearing incidence was higher (P<0.001) in zinc-treated (4%) than in control ewes (1.9%). In conclusion, bearings are not caused by ingestion of ZEN in late pregnancy. The role of zinc in bearings warrants further study.
Proceedings of the New Zealand Society of Animal Production, Volume 67, Wanaka, 68-72, 2007
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