The transfer of tritium-labeled organic material from grass into cow's milk

Two lactating cows were given tritiated hay containing organically bound tritium (OBT) only for about 4 weeks. Tritium activity was determined in milk fat, casein, lactose, milk water, and whole milk. In one cow, milk was sampled for approximately 450 days, covering two lactation periods. At steady state, specific tritium activities in casein, lactose, and milk water were 58, 10, and 11%, respectively, of those in milk fat. Some OBT was converted into THO during catabolism and entered the body water pool. This 3H source accounted for nearly 40% of tritium in lactose, but in casein and milk fat about 97% of tritium was derived from ingested OBT. Comparison of the specific activity of milk constituents with the specific activity of ingested hay showed the following values: 0.84 for milk fat, 0.49 for casein, 0.05 for lactose, 0.10 for milk water. About one-half of the tritium transferred to milk was found in organic milk constituents and the other half in milk water. Decrease of tritium activity with time could be represented by three components with different half-lives for the organic milk constituents. Those for milk fat and casein were quite similar, with a slow component of nearly 3 months.