THE EFFECTS OF THE LEVEL OF IODINE IN CANOLA MEAL CONCENTRATE ON MILK IODINE AND THIOCYANATE CONTENT AND THYROID FUNCTION IN DAIRY COWS

Abstract

Six lactating heifers were used in a randomized block design to measure the effects of different levels of iodine supplementation in a canola meal (CM) based concentrate on milk iodine and thiocyanate content and thyroid function. The treatments were a pre-experimental control diet (diet C) and a concentrate containing 20% Tower CM (air-dry) with 1.2 (CM-1.2), 2.2 (CM-2.2), 3.3 (CM-3.3) and 4.5 (CM-4.5) mg supplemented iodine/kg concentrate dry matter. The same roughage mixture was fed with all diets. Cows were maintained on diet CM-1.2 for 5 wk, after which they were changed sequentially to diets CM-2.2, CM-3.3 and CM-4.5 at 3-wk intervals. Milk samples were taken and a thyrotropin-releasing hormone test was conducted before the start of the experiment and at the end of each period. Milk samples were also taken daily between days 2 and 6 after diet change-over during treatments CM-2.2, CM-3.3 and CM-4.5. Concentrations of iodide in milk (mean ± SE) for diets C, CM-1.2, CM-2.2, CM-3.3 and CM-4.5 were 42.0 ± 10.9, 9.2 ± 0.4, 11.1 ± 0.3, 14.5 ± 1.0 and 18.7 ± 1.2 μg/100 mL, respectively. Milk iodide content for the CM concentrate with different iodine levels was lower (P < 0.01) than the pre-experimental control value. However, diets CM-3.3 and CM-4.5 successively increased (P < 0.05) milk iodide content compared to diets CM-1.2 and CM-2.2. During the adaptation periods of diets CM-3.3 and CM-4.5, milk iodide content increased (P < 0.05) rapidly compared to the previous treatment mean. Milk thiocyanate concentrations for the CM-1.2, CM-2.2, CM-3.3 and CM-4.5 diets were 134.8 ± 4.6, 135.3 ± 7.4, 117.3 ± 5.0, and 156.0 ± 6.7 μmoles/L, respectively. Thyroid function of the cows was not different among the different diets, being influenced by neither the CM nor level of iodine. Iodine supplementation of CM based concentrate at 3.3 and 4.5 mg/kg concentrate DM increased milk iodide concentrations, but not to pre-experimental levels. Milk iodide concentrations were normal during any treatment.