Estimates of Genetic Parameter for Tick Count and Infection Level of Babesia Bovis Traits in Braford and Hereford Cattle

Tick and tick-borne diseases, including babesiosis (B. bovis), constitute a major drawback to improve productivity of beef cattle in the tropics, especially for systems where Bos taurus cattle animals and their crosses are used. This study aimed to estimate genetic parameters for tick counts (TC) and infection level of Babesia bovis (IB) using Bayesian inference. Tick counts on one side of each animal were obtained from 717 Braford and 43 Hereford cattle in farms located at Rio Grande do Sul State, Brazil. The tick counts were collected when average individual infestation exceeded 20 engorged tick females with at least 4.5 mm in length. Blood samples were collected, in tubes containing EDTA for DNA extraction. B. bovis quantification was performed using both qPCR technique from genomic DNA of each animal with specific primers for this protozoary and the absolute quantification method. The TC and IB records were transformed in log base 10 (x + 1), the mean were 1.334 and 1.511 for TC, and IB, respectively. The model included fixed effects, random effects that represented additive genetic direct effects of each animal, and residuals. The fixed effects were composed by contemporary group (same farm, sex, year and season of birth, management, and tick count date). Also, the effects of Nelore breed proportion, heterozygosity, recombination loss, and linear and quadratic coefficients for animal age at counting in days (358 to 697) were included in the model as covariates. Contemporary groups with less than 4 individuals were excluded from analyzes. The Gibbs sampler in a bivariate animal model analysis using GIBBSF90 software was run for 500,000 iterations (burn-in period of 50,000). Moreover, the convergence of the Markov chains was checked based on tests by using the coda package of R software. The mean additive genetic variances were 0.018 and 0.080 for TC, and IB, respectively. The mean residual variances were 0.144 and 0.356 for TC, and IB, respectively. The mean h² were 0.109 and 0.182 for TC, and IB, respectively. The genetic correlation was 0.38 between TC and IB. The selection response of Braford and Hereford cattle for IB trait will have greater magnitude than for TC, and moderate correlated response between these two traits would be expected.