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Effects of breeding system of origin (natural service or artificial insemination) on growth, attainment of puberty, and pregnancy rates in crossbred beef heifers
The objective of this study was to compare growth, attainment of puberty, and pregnancy rates in beef heifers originating from two different breeding systems. One hundred and ninety crossbred Angus heifers were born to dams that were exposed to 1 of 2 treatments: 1) natural service (NS, cows were only exposed to herd bulls for the duration of the breeding season), or 2) fixed-time artificial insemination (TAI, cows exposed to ovulation synchronization and AI followed by natural-service bulls). Body weights were taken on day 0 and 189 with a mean age of 209 ± 1.2 d at the initiation of the trial (d 0). Blood samples were collected at day 0, 10, 112, 122, 219, and 229 and progesterone concentrations were used to determine the proportion of females that had attained puberty during the development period. On d 229 synchronization of ovulation was initiated (7-d CO-Synch + CIDR) and all heifers were inseminated with a single TAI at 54 h after CIDR removal. Clean-up bulls were placed in breeding pastures 10 d after AI and remained with heifers until 56 d after AI. Pregnancy rates were determined via transrectal ultrasonography on d 27 and 91 after AI. Body weight at initiation of the experiment was greater (P = 0.01) for heifers in the TAI treatment (239.9 ± 2.8 kg) compared with heifers in the NS treatment (229.6 ± 2.8 kg). However, no differences (P ≥ 0.14) between treatments were observed in weights of heifers taken at the time of pasture turnout (d 189; 345.1± 3.4 and 338.0 ± 3.4 kg for TAI and NS, respectively), or ADG (0.56 ± 0.01 and 0.58 ± 0.01 kg/d for AI and NS, respectively). At the initiation of the experiment a greater proportion of the NS heifers (11.6%) tended (P = 0.06) to be cyclic compared with TAI heifers (4.2%). However, no differences (P ≥ 0.40) were observed between treatments in the proportion of heifers cyclic at the interim evaluation (d 112 and d 122, 27.5% cyclic) or at the initiation of the breeding season (d 219 and 229, 85.5% cyclic). No differences (P ≥ 0.81) were present between treatments in either pregnancy rates to AI 32.9% or season-ending pregnancy rates 91.1%. Breeding system of origin did not influence growth rate during the development phase, attainment of puberty, or pregnancy rates in crossbred beef heifers.
Keywords:
artificial insemination, heifer development, natural service