THE GOITROGENIC POTENTIAL OF TOWER AND MIDAS RAPESEED MEAL IN DAIRY COWS DETERMINED BY THYROTROPIN-RELEASING HORMONE TEST

Abstract

The goitrogenic potential of a low glucosinolate rapeseed meal (RSM) (Brassica napus L. ’Tower’) in dairy cows was evaluated by a thyrotropin-releasing hormone (TRH) test. The TRH test consisted of measuring the plasma concentrations of thyroid-stimulating hormone (TSH) and thyroid hormones after an intravenous injection of 300 μg TRH. A high glucosinolate RSM (B. napus L. ’Midas’) and soybean meal (SBM) served as positive and negative controls, respectively. Tower and Midas RSM were fed at 0, 5.7, 13.2 and 18.9% of the total diet (air-dry basis) and SBM was added to bring the protein level in the supplement for all diets to 18.9%. Tower and Midas RSM were evaluated in two separate 4 × 4 Latin squares. When Midas RSM was fed at levels of 13.2 and 18.9% in the diet, plasma TSH concentrations were increased (P < 0.05) in response to TRH. Similarly, plasma thyroxine (T4) concentrations were decreased (P < 0.05). This indicates that diets with 13.2 and 18.9% Midas RSM contain sufficient quantities of goitrogens to affect thyroid function. Similar trends were observed when the 5.7% Midas RSM diet was fed. Triiodothyronine (T3) concentrations and T3:T4 ratios were not affected by Midas RSM in the diet. The plasma TSH, T4 and T3 concentrations in response to TRH were similar for cows fed the Tower RSM and SBM diets. The use of Tower RSM up to 18.9% of the total diet is without goitrogenic effect as measured by the TRH test.