Effect of two prostaglandin injections on days 5 and 6 in a timed AI protocol after estrus expression on pregnancy outcomes in dairy cows during cold or hot seasons of the year

The objective of this study was to test whether prostaglandin (PG) injection on day 30 postpartum (pp) and detection of estrus can affect the efficacy of injecting PG on days 5 and 6 in the timed artificial insemination (TAI) protocol on pregnancy rate in a large dairy herd in hot or cold seasons. Out of 2235 cows, 1998 received an injection of PG at [Formula: see text] d pp and estrus was observed. Cows that displayed estrus during the estrous observation period after PG injection were classified as estrus (E), while those that did not show estrus were classified as nonestrus (NE). Cows in each group were assigned to two treatments: CO-72 (control treatment) (ECO-72 and NECO-72) (day 44 GnRH, day 51 PGF [Formula: see text] , day 54 GnRH [Formula: see text] TAI) or PG–PG (EPG–PG and NEPG–PG) (day 44 GnRH, day 49 PGF [Formula: see text] , day 50 PGF [Formula: see text] , day 52 GnRH [Formula: see text] TAI). Pregnancy was diagnosed on days 33 and 47 after artificial insemination (AI). The proportion of cows in estrus on the day of TAI was higher ([Formula: see text]) for cows that received two PG than for cows that received one PG. Pregnancies per AI ([Formula: see text] /AI) on days 33 and 47 for cows inseminated during and after a voluntary waiting period in the NEPG–PG treatment had higher rates than for cows in the EPG–PG, ECO-72 and NECO-72 treatments. Moreover, [Formula: see text] /AI were significantly ([Formula: see text]) affected by parity. Primiparous had higher [Formula: see text] /AI (37.0 %) than multiparous cows (31.6 %). Cows inseminated in cold months had higher [Formula: see text] /AI and reduced PL (35.6 % and 20.8 %) than cows inseminated in hot months (29.1 % and 30.6 %, respectively). In conclusion, treatments with PG on days 5 and 6 after the first GnRH injection increased [Formula: see text] /AI. Estrus detection before the beginning of TAI protocol did not affect fertility. To maximize [Formula: see text] /AI cows exhibiting heat at any time during the synchronization protocol should be inseminated.