Effects of excessive iodide administered in the dry period on thyroid function and health of dairy cows and their calves in the periparturient period

Experiments were conducted in three successive years in which iodide (I) doses of 1.25, 2.5, 5.0 and 7.5 mg/kg body weight were given to 47 dairy cows during the dry period, compared with 16 control cows on basal diets of 1 ppm I. Effects on cows dosed at 1.25 and 2.5 mg I/kg (50 and 100 ppm dry feed) were not different from controls in terms of vitality of calves, changes in plasma thyroxine (T4), plasma triiodothyronine (T3), thyroxine secretion rate and 10-mo milk yields of the lactation after treatment. Cows dosed with 5 and 7.5 mg I/kg (200 and 300 ppm dry feed) averaged 272.8 d gestation, which was significantly shorter than 279.5 d for all cows on lesser I intakes. Abnormal calves at birth were 25% from the two highest I dosages vs 8% from controls plus the two lowest I dosages. Average plasma T4 and T3 decreased on the day of calving by about 30%, while plasma total I increased about 20%. Changes were greatest in cows fed high I dosages for the longest period prepartum. Plasma I and T3 of calves at birth were about three times the concentrations in their dam's plasma and plasma T4 of neonatal calves was four to five times greater than their dams. Highest dosages of I for dams tended to depress plasma T4 and T3 in neonatal calves.