Brassicas are an important forage species on many New Zealand sheep farms. However, glucosinolates present in some brassicas can cause iodine deficiency in lambs when pregnant ewes are fed such crops during gestation. To investigate the effect of differences in glucosinolate profile of fodder radish and a brassica on ewe thyroid function and incidence of goitre in lambs born to ewes grazing these crops during pregnancy, a 3 x 2 factorial study utilising three diets (Italian ryegrass, fodder radish, rape) and two rates of iodine (I) supplementation (with, without) was conducted. Thyroid hormone (T3, T4) response to a thyroid releasing factor (TRF) challenge in ewes and incidence of goitre in newborn lambs was determined. A mild goitre, based on thyroid weight:lamb weight ratio (g/kg) of >0.4, was recorded only in newborn lambs from ewes fed rape and the incidence was higher (75% vs 11%) in those without I supplementation. Pregnant ewes grazing radish or rape had the lowest T3 and T4 responses to TRF, except in the case of T4 where there had been I supplementation. Differences in glucosinolate content and composition may explain the differing results for sheep fed radish or rape.
Proceedings of the New Zealand Society of Animal Production, Volume 71, Invercargill, 66-70, 2011
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