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1 Department of Animal Sciences, and
2 Department of Large Animal Clinical Sciences, University of Florida, Gainesville 32611
Corresponding author: Carlos Risco; e-mail: riscoc{at}mail.vetmed.ufl.edu.
Effects of artificial insemination (AI) and natural service (NS) breeding systems on pregnancy rates (PR) by stage of lactation, season, and changes in milk production over time were examined using lactation and herd DHIA records of Holstein cows in dairy herds located in Florida and Georgia. The reported genetic profile of service sires of the herd was used to determine the percentage of cows bred to natural service bulls (%NS). Two seasons were considered: winter (November–April) and summer (May–October) from 1995 to 2002 (16 periods). Herd-periods were assigned 1 of 3 breeding systems: AI (0 to 10% NS), mixed (11 to 89% NS) and NS (90 to 100% NS). Seventy percent of the herds used NS bulls as a component of their breeding system during the study period. The PR during winter (17.9%) was greater than that during summer (9.0%). During winter, PR for AI herds (17.9%) did not differ from that for mixed (17.8%) and NS herds (18.0%). During summer, PR for AI herds (8.1%) was slightly less than that for mixed (9.3%) and NS herds (9.8%). During winter, PR for cows at 71 to 91 d, 92 to 112 d, and 113 to 133 d in milk were 1.4% lower for mixed herds compared with AI and NS. Pregnancy rate for NS herds was 2.6% lower during late lactation compared with AI and mixed herds. During summer, PR for cows at 71 to 91 and 92 to 112 d in milk were 2.6 and 1.8% greater, respectively, for NS herds compared with AI. However, from 260 to 364 d in milk, PR for NS herds was less than that for AI and mixed herds. No significant interaction was detected between breeding system and lactation number. Rolling herd average milk production during the study period was less in the NS herds (7180 kg) compared with AI (8513 kg) and mixed herds (8176 kg), but the annual change in milk production was not different among breeding systems. The results indicated that use of NS bulls did not result in meaningful disadvantages in terms of PR and changes in milk production over time.
Key Words: pregnancy rate • milk production • natural service • artificial insemination
Abbreviation key: DRMS = Dairy Records Management Systems, NS = natural service breeding system, %NS = percentage natural service breedings, PR = pregnancy rate, VWP = voluntary waiting period
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