Effect of time of progesterone supplementation on serum progesterone and the conception rate of cooled Holstein heifers during the summer

To investigate the effects of progesterone supplementation at two different times on serum progesterone (P-4) concentration, conception rate and resynchronization of cooled Holstein heifers in summer, 90 heifers were randomly assigned to two groups: (i) heifers subjected to TAI (timed artificial insemination) and progesterone supplementation from days 4 to 14 after TAI (S-1; n = 45); and (ii) heifers under the same TAI protocol as S-1 and progesterone supplementation from days 17 to 22 after TAI (S-2; n = 45). The groups S-1 and S-2 were cooled 10 days before and 21 days after TAI. Respiratory rate, body surface temperature, vaginal temperature and rectal temperature recorded during the experiment were not different (P>0.05) between S-1 and S-2 groups. Progesterone concentration was not different (P>0.05) in S-1 compared to S-2. The conception rates on days 30 and 55 were similar between groups (P>0.05). Progesterone supplementation did not increase either conception rate or concentrations of P4 in heifers during the summer. Heifers not pregnant to first service in the group S-2 were resynchronized (77.7%) for a second breeding.