Risk factors related to resistance to Rhipicephalus (Boophilus) microplus and weight gain of heifers

Abstract

The aim of the present study was to evaluate the influence of age and genetics in dairy heifers on resistance to the cattle tick Rhipicephalus (Boophilus) microplus and correlate these parameters with weight gain. Twenty-two heifers were evaluated from birth up to two years of age. Resistance to the cattle tick was evaluated by counting the number of engorged female ticks and subjective qualification of the larvae and nymph infestation. The animals were weighted in the first 24 hours after birth and at six, 12, 18 and 24 months of age. The average tick count and weight gain were compared by Tukey's test at 5% significance. Subsequently, linear regression was performed to verify the strength of the association between the risk factors age and genetics and infestation by R. (B.) microplus. Age and genetics were both significant risk factors for R. (B.) microplus infestation in heifers. Between the third and sixth months of age, the animals showed a window of susceptibility to R. (B.) microplus. Regardless of age, Bos taurus heifers had higher infestations than Bos indicus, crossbred F1 (1/2 B. taurus x 1/2 B. indicus) and crossbred Gir-Holstein (Girolando) (5/8 B. taurus x 3/8 B. indicus) heifers. B. taurus heifers were heavier than B. indicus heifers at birth and had significantly greater weight gain (p < 0.01).