Influence of Synchro-Mate-B and breed composition on estrous response and pregnancy rate in spring- and fall-bred Brahman crossbred beef cows.

Estrous response, pregnancy rate, and the relationship between palpable corpora lutea (CL) and plasma progesterone concentrations were studied during the spring (n = 149) and fall (n = 146) breeding seasons using postpartum, crossbred beef cows consisting of 0-Brahman, 1 4 - Brahman , or 1 2 Brahman breeding. At the start of each breeding season, a jugular blood sample was collected for progesterone analysis; each cow was palpated for the presence of a CL and randomly alloted within breed to a non-treated control or Synchro-Mate-B (SMB) treatment. Fewer (P<0.01) fall than spring bred cows exhibited estrus within the first 21 d of breeding. Within 48 h of implant removal, 84 and 64% (spring and fall, respectively) of synchronized cows exhibited estrus. In both seasons, more SMB than control cows became pregnant by 7 d of breeding. Percentage of Brahman breeding did not influence estrous response or pregnancy rate. On the basis of 292 evaluations, palpation per rectum correctly assessed ovarian status 74% of the time. These data indicate that although season can influence response. SMB can be used effectively to synchronize Brahman crossbred cows. In addition, it was found that there is a close relationship between palpation per rectum and plasma concentrations of progesterone.