Expression of estrus before fixed-time AI affects conception rates and factors that impact expression of estrus and the repeatability of expression of estrus in sequential breeding seasons.

Expression of estrus after PG and before fixed-time AI has been reported to change the uterine environment, increase accessory sperm numbers, fertilization rates, and overall embryo survival. Thus, expression of estrus can strongly impact overall pregnancy success. Because of variation in percentage of beef females detected in estrus and number of animals per study, it can be difficult to detect a significant effect of estrus on pregnancy success. Thus, a meta-analysis was conducted using data from 10,116 beef females in 22 studies that utilized variations of the 5 most common fixed-time AI protocols (CO-Synch, 7-day CO-Synch+CIDR, 5-day CIDR, PG 6-day CIDR, and the 14-day CIDR protocols) to examine the effect of detection in standing estrus on subsequent fixed-time AI pregnancy success. A random-effects model was used to combine the studies/herds. The overall model indicated a positive effect of estrus on conception rates with cows detected in estrus before fixed-time AI having a 27% greater (P<0.05; 95% CI=22-32%) conception rate compared with those not detected in estrus. Next we determined factors that influenced expression of estrus. Data were available on 547 cows synchronized with a CIDR based fixed-time AI protocols and observed for estrus before AI during 2-4 breeding seasons. Analysis of these cows indicated that days postpartum (P=0.22) did not impact estrous expression. In contrast, BCS influenced estrous expression (P=0.04) with cows in a BCS of ≤4 (51±5%) having decreased expression of estrus compared to cows with a BCS>4 (≥70±4%). Initiation of estrous cycles before the breeding season also influenced estrous expression (P=0.03), with anestrous cows having greater expression of estrus compared with estrus-cycling cows (78±5% vs. 70±5%, respectively). In conclusion, among all currently recommended fixed-time AI protocols, cows detected in estrus before fixed-time AI had improved conception rates, with BCS and estrus-cycling status having the greatest influence on expression of estrus.