PLASMA AND MILK PROGESTERONE CONCENTRATIONS AND EARLY PREGNANCY IN ZEBU COWS

In an effort to confirm true oestrus and to detect early pregnancy in Zebu cows (Bos indicus), sequential blood and milk samples were collected at the day of imsemination (Day 0) and days 14, 20 and 24 after insemination. Progesterone was determined in skimmed milk and plasma by solid phase radioimmunoassay (RIA). Of the cows thought to be in oestrus plasma, (n = 46) and milk (n = 58) samples demonstrated low progesterone concentrations ($0.99{\pm}0.11$ and $2.02{\pm}0.14nmol/l$) in 42 (91%) and 52 (90%) cases respectively. Thirty two (76%) and 30 (71%) cows were thought to be pregnant by plasma progesterone RIA ($20.23{\pm}1.03$ and $20.48{\pm}1.01nmol/l$) at days 20 and 24 respectively. At the same period, 40 (77%) and 37 (71%) cows were thought to be pregnant by milk progesterone RIA ($27.82{\pm}1.28$and $28.02{\pm}1.27nmol/l$). Assuming 100% accuracy for rectal examination of pregnancy diagnosis between days 60-65 postservice, the RIA was found to be 84% and 90% accurate for plasma and 84% and 92% accurate for milk at day 20 and 24, respectively. All cows thought to be non pregnant by progesterone measurement were correctly diagnosed. Progesterone assay at 24 days after oestrus may therefore be accurate for early diagnosis of pregnancy in Zebu cows.