Abstract
Fertility of AI bulls is evaluated by non-return (NR) rates. Usually these NR rates are calculated as averages, uncorrected for effects of environments. An NR rate (0,1) is measured on the cow, but is affected by the bull and by the technician who carried out the insemination. The objective of this paper was to develop a system to adjust NR rates of AI sires and insemination results of technicians for systematic environmental effects. Of 875 sires, who accounted in total for 1.8 million first inseminations during one year, NR rates were evaluated. NR rates were adjusted for the effects of herdseason, season of insemination, age of the inseminated cow and technician. The correlation between adjusted and unadjusted NR rates was 0.83 for the NR rate on 56 days. Large changes in NR rates after adjustment occurred by sires, who were frequently used on heifers. Technicians within AI stud differed more than 9% NR, corrected for other effects. It was concluded that an NR evaluating system should adjust for the effects of herd-season, season of insemination, age of the inseminated cow and technician, to provide unbiased NR rates for management and research purposes.
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