title: Timed artificial insemination early in the breeding season improves the reproductive performance of suckled beef cows.
abstract: The objective was to compare reproductive performance of breeding programs that used natural service (NS), AI after estrus detection (ED), and timed AI (TAI). In experiment 1, 597 suckled beef cows were randomly allocated to one of four groups. Cows in the TAI+NS group (N = 150) were bred by TAI at 11 days after the onset of the breeding season (BS). Bulls were placed with cows 10 days after TAI and remained together until the end of the 90-day BS. Cows in the TAI+ED+NS group (N = 148) received TAI, then AI based on ED for the next 45 days, and finally NS for the last 45 days of the BS. Cows in the ED+NS group (N = 147) received AI based on ED during the first 45 days of the BS, followed by NS for the last 45 days of the BS. Cows in the NS group (N = 149) were bred by NS for the entire 90-day BS. Cows in the ED+NS or NS groups had a decreased (P < 0.001) hazard of pregnancy compared with cows in the two groups bred by TAI at the onset of BS. Also, cows bred by TAI (TAI+NS = 92.7%; and TAI+ED+NS = 91.9%) had higher (P < 0.01) pregnancy rates at the end of the BS compared with cows not bred by TAI (ED+NS = 85.0%; NS = 83.2%). In experiment 2, 507 suckled beef cows were randomly assigned to one of two groups at the onset of a 90-day BS. The NS group (N = 255) received only NS during the entire BS, and the TAI+NS group (N = 252) received TAI at the onset of the BS, followed by NS until the end of BS. Cows in the TAI+NS group had 63% higher hazard of pregnancy (P < 0.001) compared with cows in the NS group, and reduced the median days to pregnancy by 44 (11 vs. 55 days). However, there was no difference (P = 0.31) in proportion of pregnant cows at the end of the BS (TAI+NS = 77.0% vs. NS = 71.0%). Therefore, incorporation of TAI programs early in the BS increased reproductive performance of suckled beef cows.
authors.affiliation: Department of Animal Reproduction, FMVZ-USP, Sao Paulo, Sao Paulo, Brazil. manoelsa@usp.br
authors.forename: Manoel F
authors.initials: MF
authors.lastname: Sá Filho
authors.forename: Luciano
authors.initials: L
authors.lastname: Penteado
authors.forename: Everton L
authors.initials: EL
authors.lastname: Reis
authors.forename: Tomás A N P S
authors.initials: TA
authors.lastname: Reis
authors.forename: Klibs N
authors.initials: KN
authors.lastname: Galvão
authors.forename: Pietro S
authors.initials: PS
authors.lastname: Baruselli
mesh_terms: D000818:Animals; D000833:Animals, Suckling; D001947:Breeding / Q000379:methods*; D002417:Cattle / Q000502:physiology*; D005260:Female; D007315:Insemination, Artificial / Q000379:methods / Q000662:veterinary*; D008297:Male; D011247:Pregnancy; D018873:Pregnancy Rate; D011258:Pregnancy Tests / Q000662:veterinary; D012098:Reproduction / Q000502:physiology*; D012621:Seasons; D014463:Ultrasonography / Q000662:veterinary