Effect of including canola meal and supplemental iodine in diets of dairy cows on short-term changes in iodine concentrations in milk

The dietary requirement for iodine is based on thyroxine production, but data are becoming available showing potential improvements in hoof health when substantially greater amounts of I are fed. Feeding high amounts of I, however, can result in the milk having excessive concentrations of I. Canola meal contains goitrogenic compounds that reduce the transfer of I into milk. We hypothesized that including canola meal in diets would allow high supplementation rates of I without producing milk with unacceptable concentrations of I. Thirty midlactation Holstein cows were fed a diet with all supplemental protein from soybean meal (0% of diet dry matter as canola meal) or with all supplemental protein from canola meal (13.9% canola meal). A third treatment has a mix of soybean meal and canola meal (3.9% canola meal). Within canola-meal treatment, cows were fed 0.5 or 2.0 mg of supplemental I per kilogram of diet dry matter from ethylenediamine dihydroiodide. Cows were maintained on the canola treatment for the duration of the experiment but were changed from one I treatment to the other after 13 d of receiving the treatment. Milk I concentration before the treatments started (cows fed 0.5 mg/kg of I) averaged 272 mu g/L and increased within 22 h after cows were first fed diets with 2 mg/kg of I. As inclusion rate of canola meal increased, the concentration of I in milk decreased linearly. After 12 d of supplementation, milk from cows fed 0.5 mg/kg of I had 358, 289, and 169 mu g of I/L for the 0, 3.9%, and 13.9% canola-meal treatments. For cows fed 2.0 mg/kg of I, milk I concentrations were 733, 524, and 408 mu g/L, respectively. Concentrations of I in serum increased with increased I supplementation, but the effect of canola meal was opposite of what was observed for milk I. Cows fed the highest canola-meal diets had the highest serum I whether cows were fed 0.5 or 2.0 mg/kg of I. Feeding dairy cows diets with 13.9% canola meal maintained milk I concentrations below 500 mu g/L when diets were supplemented with 2 mg/kg of I.