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Meta-analysis of the effect of estrus expression before fixed-time AI on conception rates in beef cattle
Expression of estrus after PGF and before fixed-time AI has been reported to change the uterine environment, increase fertilization rates, accessory sperm numbers, and overall embryo survival. Thus, expression of estrus can strongly impact overall pregnancy success. Because of variation in percentage of beef animals exhibiting 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 6,981 animals in 20 studies that utilized variations of the 5 most common fixed-time AI protocols (CO-Synch, CO-Synch+CIDR, 5-d CIDR, PG 6-d CIDR, and the 14-d CIDR protocols) to examine the effect estrus had on conception rates. A random-effects model was used to combine the studies. The overall model indicated a positive effect of estrus on conception rates with cows expressing estrus before fixed-time AI having a 27% greater conception rate compared with those not exhibiting estrus (P < 0.05; 95% CI = 22% to 32%). Next we determined factors that influenced expression of estrus. Data were available on 547 cows synchronized with one of the CIDR based fixed-time AI protocols and observed for estrus for 2 to 4 yr. Analysis of these cows indicated that days postpartum (P = 0.22) did not impact estrus expression. In contrast, BCS influenced estrus expression (P = 0.04) with cows in a BCS of ≤ 4 (51 ± 5%) having decreased expression of estrus compared to those with a BCS > 4 (≥ 70 ± 4%). Initiation of estrous cycles before the breeding season also influenced estrus expression (P= 0.03), with anestrous cows having greater expression of estrus compared with estrus-cycling cows (78 ± 5% versus 70 ± 5%, respectively). In conclusion, among all currently recommended fixed-time AI protocols, cows expressing estrus before fixed-time AI improved conception rates, and BCS and estrus-cycling status had the greatest influence on expression of estrus.
Keywords: fixed-time AI, estrus, pregnancy success