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Secretion of I¹³¹ into Milk under Conditions of Environmental Contamination of Pasture

Department of Animal Science, Colorado State University, Fort Collins and Hazards Control Branch, Health and Safety Division, U. S. Atomic Energy Commission, Idaho Falls, Idaho

ABSTRACT

A series of studies has been conducted to investigate the steps of I¹³¹ transfer in the air—vegetation—cow—milk—human food chain. This paper deals with one aspect of these experiments: the uptake and secretion of I¹³¹ into the milk of dairy cows confined to a I¹³¹-contaminated pasture. The pasture was contaminated by releasing 1 c of I¹³¹, in the form of iodide gas, into the atmosphere over the pasture. Six Holstein cows were allowed to graze the contaminated pasture. Grass samples were collected daily, along with milk, urine, and feces samples, and analyzed for gross gamma activity.

Pasture activities declined daily from 1,450 to 270 pc of I¹³¹ per gram of dry grass over the 14-day period, with an effective half-time of five and a half days. The milk levels of I¹³¹ increased rapidly and peaked in two to three days. There were fourfold variations among cows in the I¹³¹ levels of their milk, and the percentage of the daily ingested I¹³¹ secreted in the milk ranged from 4.4 to 29.7 (mean 12.8). Feces and urine I¹³¹ levels paralleled milk, but the feces showed little variation between cows. When the cows were removed from the contaminated pasture, milk activity declined with an effective half-time of less than one day for the first three days, but then declined at a slower rate.