Timed artificial insemination strategies with or without short-term natural service and pregnancy success in beef heifers.

Objective was to compare estrous response, and first service and breeding season pregnancy rates in Angus cross beef heifers that received four progesterone (CIDR, vaginal insert) -based estrous-synchronization treatment regimens for timed artificial insemination (TAI) with or without short-term natural service (NS). Heifers (n = 1744; 4 locations) were assigned a reproductive tract score (RTS: 1 = immature, acyclic to 5 = mature, cyclic), body condition score (BCS: 1 = emaciated to 9 = obese) and temperament score (0 = calm to 1 = excitable). Within location, heifers were assigned to either of four treatment regimens. Heifers in Select-Synch + CIDR (C) + short-term NS (SSC + NS) treatment (n = 438) received 100 μg GnRH im + CIDR on Day -7 and CIDR removal + 25 mg PGF2α (im) + estrus-detection patches in the morning of Day 0 and were subjected to NS (bull to heifer ratio, 1:25) from the morning of Day 2 (48 h from CIDR removal) to evening of Day 8. Heifers in CO-Synch + C (COSC) +TAI + NS treatment (n = 433) received 100 μg GnRH + CIDR on Day -7 and CIDR removal + 25 mg PGF2α im + estrus-detection patches in the morning of Day 0 + 100 μg GnRH im + TAI on Day 2, 60 h from CIDR removal, and were subjected to natural service (bull: heifer, 1:25) from the evening of Day 2 immediately after AI through Day 8. Heifers in COSC + TAI treatment (n = 443) received 100 μg GnRH + CIDR on Day -7 and CIDR removal + 25 mg PGF2α im + estrus-detection patches in the morning of Day 0 + 100 μg GnRH im + TAI on Day 2, 60 h from CIDR removal. Heifers in SSC + split time AI (STAI) treatment (n = 430) received 100 μg GnRH + CIDR on Day -7 and CIDR removal + 25 mg PGF2α im + estrus-detection patches in the morning of Day 0. Heifers determined to be in estrus were inseminated at 64 h after PGF2α (25 mg, im) administration and non-estrous heifers determined to be in estrus 20 h later (at 84 h) were inseminated at that time, whereas remaining non-estrous heifers at 84 h received GnRH (100 μg) and were inseminated concomitantly. In SSC + NS and COSC + TAI + NS treatments, natural service bulls were removed and reintroduced (@ bull: heifer, 1:40) on Day 22 and kept until Day 63. In COSC + TAI and SSC + STAI bulls (bull: heifer, 1:40) were introduced on Day 14 and retained until Day 63. All heifers, irrespective of treatment regimens, were observed for estrus three times a day from Day 2 through Day 8. Pregnancy diagnosis was performed on Day 93, 30 days from end of breeding season. Accounting for temperament (P < 0.0001), BCS (P < 0.0001) and RTS (P < 0.0001), the breeding program influenced the estrous response (P < 0.0001). Heifers in COSC + TAI + NS (86.8%), SSC + STAI (84.9%), and SSC + NS (86.5%) treatments had greater estrous response compared with heifers in COSC + TAI (75.8%) treatment. Accounting for temperament (P < 0.01), BCS (P < 0.05) and RTS (P < 0.01), the treatment regimens influenced first service pregnancy rate (P < 0.05). Heifers in COSC + TAI + NS treatment had greater first service pregnancy rate (60.3%) compared with COSC + TAI (54.2%) (P < 0.05). First service pregnancy rate for heifers in SSC + STAI (59.3%) and SSC + NS (57.3%) treatments did not differ from COSC + TAI + NS and COSC + TAI treatments. Accounting for temperament (P < 0.001), BCS (P < 0.0001) and RTS (P < 0.0001), the treatment regimens influenced breeding season pregnancy rate (P < 0.05). Breeding season pregnancy rate for heifers in COSC + TAI + NS (95.4%) and COSC + STAI (94.5%) treatments were greater than that of heifers in COSC + TAI (90.8%) treatment (P < 0.05), and heifers in SSC + NS (94.1%%) treatment did not differ from heifers in other treatments. In conclusion, progesterone-based CO-Synch timed artificial insemination with short-term natural service treatment regimen resulted in proportionately more pregnancies than without short-term natural service treatment regimen. In addition, 64/84 h split-time AI or natural service following Select-Synch treatment regimen could be implemented as an alternative as these treatment regimens resulted in similar pregnancy rate as progesterone based CO-Synch timed artificial insemination with short-term natural service treatment regimen.