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Abstract
The objectives of this study were to estimate genetic parameters of heat expression rate (HE) indirectly with simulated data and investigate the genetic correlation between conception rate (CR) and HE with Holstein data from commercial herds. Three different measures for HE were derived based on days open (DO), which is approximately equal to voluntary waiting period + [21/(CR*HE)] 21: HE1 = 21(number of services)/(DO + 21); HE2 = minimum of [21/service interval (SI)] or 1, where SI is the interval in days between two consecutive services; and HE3 = missing record for the first service record or, for other service records, 0 if SI >21 days or 1 if SI 21 days. In the simulation study, cows were assigned two correlated breeding values for CR and HE; the genetic correlation was allowed to vary from 0.9 to 0.9. Field data were milk yield and service records from 2000 through 2003, which were obtained from Dairy Records Management Systems in Raleigh, NC. Seven traits [DO, CR, HE1, HE2, HE3, days to first insemination (DFI), and CR at first service (FSC)] were analyzed by bivariate threshold-linear sire or animal models. The genetic correlation between CR and DO was estimated to be 0.99 regardless of the magnitude and sign of the assumed genetic correlation between CR and HE for simulation data as well as for field data. In the simulation study, correlations between CR and HE2 or HE3 from bivariate models agreed well with the assumed correlation between CR and HE. The estimated heritabilities for cow fertility traits from field data ranged from 1.4 to 5.9% which are comparable to literature estimates. The estimated genetic correlations between DO and HE2 and between DFI and FSC were 0.46 to 0.66 and 0.09 to 0.34, respectively. Based on estimates from field data and simulation data, the genetic correlation between CR and HE is moderately positive. In conclusion, the indirect HE measures paired with other fertility traits are moderately useful for estimation of the underlying genetic correlation between CR and HE.