Genetic and environmental correlations among female fertility traits and milk production in different parities of Swedish red and white dairy cattle

The main objective of this study was to estimate genetic correlations betwe en fertility and production traits in first, second and third lactations as well as between fertility traits measured in the same way at different age s. The fertility traits studied were: number of inseminations per service p eriod, number of treatments for reproductive disturbances, interval between first and last inseminations, interval between calving and first inseminat ion, and interval between calving and last insemination. Early milk product ion was measured as the average of the energy-corrected milk yield at the s econd and third monthly test-days in a lactation. The number of records was approximately 450 000, 350 000, 180 000 and 75 000 in the heifer period, f irst, second, and third lactations, respectively. A linear, trivariate mode l that included the effects of herd-year, year, month, age and sire of the cow was applied. To reduce the effect of ongoing selection, 305-days kg pro tein production in first lactation was included as a variate in all of the analyses. Correlations between the herd-year effects indicated that factors of herd-year level conducive to increased production had a tendency to inc rease the number of inseminations as well as the number of reproductive tre atments, although there was an earlier start and termination of the insemin ation period. Genetic correlations between fertility traits and production were in the range of 0.2-0.4, all of them unfavourable and higher at later parities. The genetic correlations between fertility traits in the heifer p eriod and the same traits in first lactation were 0.7. Genetic correlations between the first and second lactation varied between 0.7 and 0.9, and bet ween the second and third lactation they were all 0.9 or higher. In conclus ion, fertility and production traits need to be selected for simultaneously if fertility is going to be maintained along further genetic improvement o n production, and such selection should include fertility results from lact ating cows.