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  • Title: Timed artificial insemination strategies with or without short-term natural service and pregnancy success in beef heifers.
    Author: Kasimanickam R, Kasimanickam V, Kappes A.
    Journal: Theriogenology; 2021 May; 166():97-103. PubMed ID: 33721682.
    Abstract:
    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.
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