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Effects of artificial insemination and natural service breeding systems on calving characteristics and weaning weights of resultant progeny
Angus crossbred females (n = 1,067) were used to evaluate effects of two breeding systems on calving characteristics and weaning weight of progeny. In two years females were stratified by age and BCS, then assigned randomly to 1 of 2 breeding systems: 1) exposed to natural service bulls for duration of the breeding season (NS; n = 541), or 2) exposed to ovulation synchronization and a fixed-time AI (7-d CO-Synch + CIDR) on d 0, followed by exposure to natural service bulls for duration of the breeding season (TAI, n = 535). Bulls were introduced to all females on d 1 and both treatments were managed as a cohort in the same pastures. Calving date, calving ease (scale of 1 to 5; 1 = no assistance and 5 = caesarean), calf vigor (scale of 1 to 5; 1= normal calf and 5 = stillbirth), and birth weights were recorded within 24 h of calf birth and weights were also collected from each calf at weaning. Binomial data were analyzed using Proc GLIMMIX of SAS, whereas continuous data were analyzed using Proc GLM. More (P < 0.01) TAI females (53.2%) gave birth in the first 21 d of the calving season compared to the NS treatment (41.3%). From d 22 to 42 more (P < 0.01) females in the NS treatment (32.3%) gave birth compared to females in the TAI treatment (21.6%). No differences (P ≥ 0.34) existed between treatments in the proportion of females that calved after d 43 or failed to have a calf. Overall mean calving date for females in the TAI treatment (d 16.7± 0.05 d) was 7 days earlier (P < 0.01) than that of females in the NS treatment (d 23.6 ± 0.93). However, no differences (P ≥ 0.37) were present between treatments in calving ease (1.13 ± 0.02) or calf vigor (1.13 ± 0.04). Calves from the TAI treatment (37.9 ± 0.32 kg) were lighter at birth compared with calves from the NS treatment (39.5 ± 0.35 kg). In contrast, calves from the TAI treatment (207.3 ± 1.51 kg) had greater (P = 0.01) weaning weights compared with calves from the NS treatment (202.4 ± 1.50 kg). Incorporation of TAI into beef cattle breeding systems resulted in calves born earlier in the calving season and heavier calves at weaning compared with natural service breeding systems.
Keywords: artificial insemination, calving characteristics, natural service