Serum and milk progesterone in Syncro-Mate-B treated postpartum beef cows

Fifteen post-partum (61.5+/-6.1 days) cyclic beef cows (4.1+/-0.3 years old) were randomly assigned to receive the Syncro-Mate-B (SMB) treatment on day 0 (estrus), day 9 or day 18 of their cycle with implant removal nine days later. An additional five post-partum (41.7+/-5.6 days) beef cows (3.2+/-0.4 years old) not detected in estrus and without corpora lutea also received the SMB treatment. Blood samples (10ml) were obtained via venipuncture on the day of implantation and one, three, five, seven and nine days after implantation and ten days after the post-treatment estrus. Milk samples (25 ml) were obtained by hand stripping at the same periods. Nineteen of twenty cows were synchronized, i.e. estrus occurred within five days of implant removal. Progesterone concentrations in both serum and milk were similar and followed similar patterns for each group. The day-9 treated cows had greater serum and milk progesterone concentrations than the other groups of cows on days 1, 3, and 5 after implantation. In conclusion, cows treated with SMB had similar serum and milk progesterone concentrations, endogenous progesterone secretion was inhibited by SMB treatment and the inhibition was influenced by prior luteal function.