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In early lactating cows changes in energy balance (EB) and subsequent mobilisation of body reserves result changes in milk yield (MY) and milk composition. These variations could be used as indicators of changes or problems in feeding, health and fertility. Recently it is postulated that changes in milk fat to protein ratio (FPR) may be associated with a negative EB. A negative EB, typical of the early phase of lactation impairs cows fertility whereas a recovery in EB from its most negative state, signals the initiation of ovarian activity indicating a direct relationship. Therefore, since measuring EB in large populations is difficult and expensive, assessing the genetic association between ratios of milk components and fertility traits, especially at different stages of lactation may provide an inexpensive indicator of EB. Besides, in selection programs, such information could be used to identify sires and cow families that have chronic energy deficiency and poor fertility in early lactation. The objectives of this study were to estimate covariance components for testday FPR, MY and fertility traits and to assess the genetic associations between these traits during lactation using random regression models (RRM). Genetic parameters of test-day FPR, MY and fertility were estimated using bivariate RRM that combine traits with different data structures employing a meta-model analyses. Fertility traits considered were days from calving to insemination (DFI), days open (DO), number of inseminations (NI), non-return rate to 56 days (NRR). Data was from a total of 22422 first lactation Finnish Ayrshire cows. The sire pedigree file had 638 males of which 509 sires had daughters with data. Heritability of test-day FPR during lactation ranged from 0.08 to 0.17 while the heritability of DFI, DO, NI and NRR were 0.06, 0.03, 0.01 and 0.02, respectively. Genetic correlations between test-day FPR and MY during early lactation ranged from 0.10-0.28. The positive genetic correlation between these traits indicates that genetically high producing cows tend to have high FPR during early lactation. Genetic correlations between test-day FPR and DFI, DO, NI, and NRR during early lactation were from 0.05-0.28, 0.03-0.24, 0.01-0.03, and -0.01-0.03, respectively. Of the fertility traits, the strongest genetic association was between test-day FPR and DFI or DO. The relatively low correlations between test-day FPR and the other fertility traits (NI and NRR) could be due to they are measures of fertility that are recorded after cows recovered from the most negative state of EB and started cycling. The positive and relatively higher genetic correlations between test-day FPR and DFI or DO indicate cows with high test-day FPR in early lactation tends to take longer from calving to first insemination and successful conception. The results from this study indicate that high FPR in early lactation could be used as an indicator of negative EB and cows of poor fertility, which take longer time from calving to first insemination and successful conception.

Genetic association between test-day milk fat to protein ratio and fertility traits in dairy cows: A random regression model analyses