The long-term effects of using a higher amount of iodine supplement on the efficiency of laying hens.

1. Two different levels of dietary iodine supplement (K + 0, 3.57 mg/kg; K + I, 6.07 mg/kg) were used in a 52-week experiment using 32 ISA Brown laying hens. 2. The greater iodine content in the diet impaired the egg production (K + 0, 319.9 +/- 1.31 eggs/hen; K + I, 312.4 +/- 4.19 eggs/hen), the egg weight (K + 0, 64.4 + 0.66 g; K + I, 63.1 +/- 0.61 g) and the food to egg mass ratio (K + 0, 2.13 +/- 0.023 kg/kg; K + I, 2.22 +/- 0.030 kg/kg). 3. The greater dietary iodine content had significant (P<0.05) negative effects on Haugh units, yolk index and eggshell weight.