Polygenic control of Rhipicephalus microplus infestation in purebred and crossbred cattle revealed by GWAS

Selecting cattle for enhanced resistance to ticks is essential for improving herd productivity and safeguarding animal welfare. In Brazilian production systems, tick resistance is a key factor influencing competitiveness in domestic and export-oriented beef chains. This study evaluated tick resistance in taurine and indicine cattle and their crossbreds in Rio Grande do Sul, a subtropical region where Rhipicephalus (Boophilus) microplus infestation is highly prevalent and represents the only tick species parasitizing cattle. Breed-group differences in tick counts were quantified, and the potential of genomic tools to support the selection of more resistant animals was assessed. A repeated-measures mixed model was used to compare mean tick loads and to estimate direct, maternal, and heterotic effects. Nellore cattle exhibited the lowest mean tick counts (1.00 ± 0.24), followed by Angus × Nellore (3.66 ± 0.57), Nellore × Angus (4.34 ± 0.55), Caracu × Angus (8.24 ± 0.78), Angus (10.28 ± 0.94), Angus × Hereford (12.44 ± 1.37), Hereford (12.61 ± 1.38), and Hereford × Angus (12.92 ± 1.18). Heritability and repeatability for tick infestation were 0.165 ± 0.03 and 0.179 ± 0.05, respectively. A genome-wide association study identified genomic regions on chromosomes 3, 4, 5, 16, and 22 associated with variation in tick load. Candidate genes within these regions were characterized using MeSH enrichment analyses. The results reinforce the polygenic basis of tick resistance and demonstrate the relevance of genomic information for improving selection strategies in both purebred and crossbred cattle populations.