Resistance of cattle of various genetic groups to the tick Rhipicephalus microplus and the relationship with coat traits

This study evaluated the resistance of cattle of different genetic groups to the tick Rhipicephalus microplus and the relationship with traits of the animals' hair and coat. Cows of the Senepol×Nelore (SN), Angus×Nelore (AN) and Nelore (NX) genetic groups were submitted to four consecutive artificial infestations, at 14-day intervals, each one with approximately 20,000 tick larvae placed on the animals' lumbar region. From the 19th to 23rd day of each infestation five counts of the number of ticks were performed on each animal's left body side. The tick count data (TTC) were transformed into log(10) (n+1), and also into percentage of return (PR), where n is the total number of ticks counted at each infestation. Hair samples were collected 24h after the last infestation with flat-nosed pliers. Measures of the average hair length (HL), coat thickness (CT), number of hairs per cm(2) (NHCM2) and weight of the samples (SW) were obtained. Pearson's correlation coefficients were calculated within genetic group to measure association between PR and the hair and coat data. There was a significant difference among genetic groups for the number of ticks, with the AN group having higher counts than the SN and NX groups. For the hair and coat traits, the NX and SN groups had lower values of HL and SW than did the AN group. The SN genetic group had lower NHCM2 counts than the NX and AN groups. There were positive correlations between TTC and CT (P<0.05) and SW (P<0.05) in the SN group. No significant correlation was found for the AN genetic group (P>0.05).