Effect of somatotropin and protein supplement on thyroid function of dairy cattle

Abstract

Somatotropin (ST) treatment may impair the hypothalamic–pituitary–thyroid axis in dairy cattle and may therefore exacerbate the low goitrogenic potential of canola meal (CM). Four cows and four heifers, in a double Latin square design, were treated with pituitary bovine ST (30 mg d−1) or excipient during the last 14 d of each 28-d trial period. Treatments consisted of barley-based diets containing either CM or soybean meal (SBM) with or without ST treatment. Milk production, milk composition, feed intake and body weight were determined regularly. On the final day of each period, blood samples were taken prior to and following a thyrotropin-releasing hormone (TRH) challenge (0.5 μg kg−1 BW) to determine basal and response levels of thyrotropin (TSH), ST, thyroxine (T4), and triiodothyronine (T3) resulting from each treatment. Somatotropin increased milk and fat corrected milk (FCM) production (9 and 11%, respectively). Milk composition, feed intake, feed efficiency and body weight were unaffected by treatment. Canola meal decreased milk iodide (I−) and increased milk thiocyanate (SCN−) without any effect of ST treatment. Somatotropin treatment increased plasma ST levels without dietary effects. Significant period × treatment interactions were associated with the TSH, T4 and T3 data. The levels of all hormones were within normal physiological ranges and indicated a euthyroid state. In addition, there were no differences between treatments in the responses of ST, TSH and T4 to the thyroid challenge test. We conclude that ST treatment at this dose, duration and stage of lactation does not impair the hypothalamic–pituitary–thyroid axis of dairy cattle. Key words: Somatotropin, dairy cattle, thyroid, canola meal, goitrogens