the effect of iodine levels above of nrc recommendations on performance and thyroidal hormones in holstein dairy cows

in order to survey of using iodine levels above of nrc recommendations on performance and thyroidal hormones, sixteen holstein dairy cows with the average live body weight and daily milk production of 652 ± 43 and 32.9 ± 2.4 kg respectively, allocated to 4 treatments in a complete randomized design. the dietary treatments were 1) the basal diet (without potassium iodide) as control diet, 2, 3 and 4, the basal diet plus 2.5, 5 and 7.5 mg/kg dm potassium iodide respectively. the number of replications in each treatment was 4 cows. the dry matter intake (dmi), milk yield and composition were compared between treatments. the iodine concentrations in feed, water, urine and blood as well as thyroidal hormones (t3 and t4) were determined. there were no significant differences between treatments for dmi, milk yield and compositions as well as diet efficiency. iodine contents in blood and urine were affected by iodine supplementation and increased significantly (p<0.01). blood t3 and t4 concentrations were not differed between treatments. there were no clinical sings during the experimental period. it seem that using of iodine levels above of nrc recommendation, do not effect on performance and thyroidal hormones of holstein dairy cows.