Abstract

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 milk yields differed significantly (P<0.05) between the four herds, while mean percentage milk fat and protein values differed significantly (P<0.05) between the two breeds. In all four herds, oestrone sulphate concentrations in milk rose progressively during pregnancy from a mean value of ∼80-100 pg/ml at 60-80 days of pregnancy to a plateau value of ∼1 rig/ml at 181-200 days. In non-pregnant cows, oestrone sulphate concentrations in milk ranged from non-detectable to 110 pg/ml, with a mean ± s.e.m. value of 59 ± 4 pg/ml. There 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 variation, the concentration of oestrone sulphate in >98% of milk samples obtained after 120 days of pregnancy was greater than the highest concentration found in milk from non-pregnant cows. Measurement of oestrone sulphate concentrations in milk samples taken at least 120 days after mating or insemination may provide an alternative, non-invasive means of determining or confirming pregnancy in New Zealand dairy cows.