Fertility data from 673 cows in five dairy herds with a moderate to high seroprevalence of microscopic agglutination titres (MAT) of ≥1:10 against Leptospira interrogans serovar hardjo were collated to assess the relationship between pregnancy rates and antibody titres to serovar hardjo. A significant proportion of failures of conception (10 to 13 per cent, P<0.001) were associated with MATS of 1:10 to 1:100; the overall pregnancy rate of the seronegative cows was 28.5 per cent higher (P<0.001) than that of the cows with MATS of ≥1:100. First service pregnancy rates showed a similar trend but the differences were significant (P<0.05) only in cows with antibody titres ≥1:30. An assessment of other possible risk factors for reduced pregnancy rates, such as lactation number and calving to first service interval, had no effect on the statistical results of the study or its conclusions.
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