A protocol of tick quantification to obtain an indicator of individual resistance to infestation by Rhipicephalus (Boophilus) microplus in Brangus cattle.

Accurate phenotyping of cattle resistance to tick infestation remains a major challenge for its incorporation into breeding programs. In this study, we evaluated a field-based protocol of tick quantification to derive indicators of individual resistance to Rhipicephalus (Boophilus) microplus in Brangus cattle. The protocol combined four repeated tick counts under natural infestation from late winter through late summer with a subsequent artificial tick challenge (approximately 50 bovines per herd) as a validation benchmark. Tick count data from 803 animals across six herds were analyzed using linear and generalized linear mixed models to estimate individual effects as indicators of resistance. These indicators showed concordance with rankings derived from artificial infestation, supporting their biological relevance. However, the performance of the protocol strongly depended on the level and stability of tick challenge. Herds with high and sustained infestation showed greater variability among animals and stronger agreement between natural and artificial challenge results, whereas low or inconsistent infestation levels reduced the discriminatory power of the protocol. These results highlight the importance of the epidemiological context for reliable phenotyping and support the use of a protocol consisting of repeated field measurements as a practical tool for genetic improvement of tick resistance in cattle.