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Background Fluazuron is a chitin synthesis inhibitor administered as a pour-on formulation in cattle for tick control. This study analyzes under endemic tick infestation, the incidence of the pour-on application pattern on the plasma levels of fluazuron in calves and cows in the lactation period of the beef cow. Two hundred and ninety-two beef cows around parturition were treated with a commercial pour-on formulation of fluazuron at a rate of 2.5 mg/kg of body weight. A total of 4 treatments were carried out on days 0, 32, 77, and 117. At each administration time, the cows were grouped according to the pour-on administration pattern: long (~ 60 cm pour-on application surface) and short (~ 30 cm pour-on application surface). Fluazuron levels in cows and calves plasma were determined before the third and fourth application for each subgroup (n = 10) by HPLC-MS/MS. During the entire study, cow-calf pairs were maintained under field conditions and qualitatively examined for tick infestation on the day of each treatment. Both treatments (long and short) schemes were designed to prevent the annual persistence of ticks. Results No animals with presence of ticks were identified during the first 117 days of the study, except for three cows and one calf at the time of the third application (day 77). There were no differences after 40 days (day 77) post-treatment of the second application (30 ± 5 ppb vs. 28.5 ± 12 ppb, p > 0.05) and 45 days (day 117) after the third application (147 ± 55 ppb vs 140 ± 46 ppb, p > 0.05) between groups of cows treated with the long or short pour-on application, respectively. Plasma concentration of fluazuron at second and third application was increased (3.3 and 2.9 times, respectively) in calves under free suckling compared to cows. Nevertheless, both groups of cows and calves showed a significant increase in plasma concentration of fluazuron between times (4.9 times, p < 0.0001 and 2.8 times, p < 0.0001, respectively). In both groups, tick prevalence was 0% throughout the trial, except for day 77, which reached 1%. Conclusions The main conclusions of this study were the following: 1) Different administration patterns (long vs. short) did not differ in plasma levels of fluazuron.; 2) Given that only the cows were treated and lactating calves presented higher plasma levels of fluazuron than cows, passage through milk appears to be relevant and possibly due to a cumulative effect and continuous drug intake.

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