CONCENTRATIONS OF ESTRONE SULFATE DURING PREGNANCY IN MILK FROM JERSEY AND FRIESIAN DAIRY-COWS DIFFERING IN MILK-YIELD AND COMPOSITION

Oestrone sulphate concentrations were measured by radioimmunoassay in milk samples obtained weekly during pregnancy from Jersey and Friesian cows, with each breed grazed at two different stocking rates. Mean mi lk yields differed significantly (P<0.05) between the four herds, whil e mean percentage milk fat and protein values differed significantly ( P<0.05) between the two breeds. In all four herds, oestrone sulphate c oncentrations in milk rose progressively during pregnancy from a mean value of approximately 80-100 pg/ml at 60-80 days of pregnancy to a pl ateau value of approximately 1 ng/ml at 181-200 days. In non-pregnant cows, oestrone sulphate concentrations in milk ranged from non-detecta ble to 110 pg/ml, with a mean +/- s.e.m. value of 59 +/- 4 pg/ml. Ther e was considerable variation in milk oestrone sulphate concentrations between cows in each herd, and oestrone sulphate concentrations could also fluctuate markedly within cows from week to week. Despite this va riation, the concentration of oestrone sulphate in >98% of milk sample s obtained after 120 days of pregnancy was greater than the highest co ncentration found in milk from non-pregnant cows. Measurement of oestr one sulphate concentrations in milk samples taken at least 120 days af ter mating or insemination may provide an alternative, non-invasive me ans of determining or confirming pregnancy in New Zealand dairy cows.