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Effect of iodine, selenium and cobalt rumen boluses given to dry dairy cows on the immunoglobulin and thyroid hormone status of calves.

The objective was to determine the effect of rumen boluses containing 6800 mg iodine, 1000 mg selenium and 1000 mg cobalt given to dry dairy cows on the efficiency of colostral immunoglobulin G (IgG) absorption in calves. Thirteen cows received the bolus approximately 58 days before calving. A further 12 cows received no bolus and were used as controls. The cows were housed as one group. Calves were prevented from suckling for the first 24 h of life, and were given three feeds of a fixed quantity of colostrum. At 24 h, the average plasma concentrations of IgG in the calves were 15.5 and 13.4 g/L for the control and bolus groups, respectively; these were not significantly different (P = 0.212). Bolus treatment was associated with higher levels of free and total tri-iodothyronine (T3) and thyroxine (T4) in the dams (all P < 0.001), although it had no effect on thyroid hormone levels in the calves. There were nevertheless positive and negative relationships between the efficiency with which colostral IgG was absorbed at 24 h and, respectively, total T3 at 24 h (P < 0.05) and total T4 at 1 h of age (P < 0.05). The underlying basis for these relationships remains to be established.