Importance of some methods of pasture feeding, of pasture type and of seasonal factors on ⁸⁵sr and ¹³⁴cs transfer from grass to milk

The activity of ⁸⁵Sr and ¹³⁴Cs was investigated in the milk of three groups of cows after deposition of the radionuclides on pasture under field conditions by a spraying technique at four different moments of the grazing season. The animals were kept according to three different methods of feeding: green soiling, rotational grazing and free grazing. ⁸⁵Sr milk levels were found to be significantly lower in the free grazing group which also showed the greatest variations in ¹³⁴Cs milk levels. No differences in milk activity could be demonstrated according to whether the cows grazed on an old permanent or a newly sown, temporary pasture. A highly significant seasonal effect on ¹³⁴Cs levels in milk was seen to occur in all groups, their milk activity increasing by a factor of about 1.5–3 for the late summer experiments whereas ⁸⁵Sr levels remained fairly constant. This different behaviour of the two radionuclides could be explained for the animals of the green soiling group by changes in transfer coefficient and variations in ingested radioactivity. The transfer coefficient from grass to milk was determined in the green soiling group. It ranged from 0.31–0.45% for ¹³⁴Cs and from 0.05–0.07% for ⁸⁵Sr, the lowest transfer of ¹³⁴Cs and the highest of ⁸⁵Sr being recorded at the beginning of the grazing season. The importance of some factors which might have influenced milk activity in the other groups, is discussed.