Transfer of Fallout Strontium-90 to Cows’ Milk

Abstract

From 1962 through 1966, the monthly 90Sr fallout rate, the accumulated deposit, and the concomitant 90Sr levels in animal forage and milk were measured in samples collected at the Colorado State University dairy farm in Fort Collins. During the growing period, 42% of the total 90Sr deposit was retained on alfalfa hay at the first cutting, 29% at the second cutting, and 23% at the third cutting. The difference between the first cutting and the two following was significant. These deposition coefficients combined with values for the area of forage utilized by the animal can be used to predict the contamination level of alfalfa hay from the measured total deposit.The total intake of 90Sr and the amount secreted in milk were measured in several groups of dairy cows. The transfer coefficients of 90Sr from diet to milk ranged from 0.05–0.09% per liter of milk per day and were in agreement with tracer and field experiments reported by other investigators. By using the appropriate deposition coefficient, forage yield, forage intake, and diet to milk transfer coefficient, the amount of 90Sr in milk was calculated from the measured total 90Sr deposit over the growing period. The calculated 90Sr in milk levels agreed reasonably well with measured values. For example, during the 1963 growing season—a period of relatively high fallout rate–the calculated amount of 90Sr in milk was 18 pCi per liter based on a measured deposit of 2.66 nCi 90Sr per m2 Milk samples collected during the same period and analyzed for 90Sr showed 20 pCi per liter.