PSXVI-1 Impact of Breed and/or Season on Effectiveness of Anthelmintic Treatment in Weaned Calves

Antiparasitic resistance has become a major concern in the livestock industry and can have a tremendous impact on calf performance. The objective of this study was to determine resistance to different anthelmintic treatments in Brahman and Brahman/Hereford (F1) calves at weaning. Purebred fall weaned Brahman calves (n=95) and spring weaned F1 calves (n=45) were assigned to one of six treatment groups at weaning: Control (received no anthelmintic), received a benzimidazole (Synanthic 22.5% or Safeguard), a macrocyclic lactone (Dectomax or LongRange), or a combination of the two (LongRange and Synanthic). Fecal samples were collected on d 0, 14 or 28 (dependent on treatment), and 42 to determine fecal egg count (eggs/g; EPG). Body weights were collected at all time points. Data were analyzed as an ANOVA using the GLM procedure, or as an ANOVA with repeated measures using the MIXED procedure in SAS. There tended to be an effect of treatment (P = 0.0865), and there was an effect of breed (P < 0.0001), time (P < 0.0001), treatment by breed (P = 0.0458), treatment by time (P = 0.0067), and treatment by breed by time (P = 0.0214) on EPG. Overall, Brahmans had increased EPG compared with F1s (P < 0.0001; 178.83±11.73 vs. 79.81±17.08). Benzimidazoles had a greater reduction in EPG (99.29%; P = 0.0011) compared with macrocyclic lactones (45.36%). There was no effect of treatment (P = 0.5184), or treatment by time (P = 1.0) on body weight. Body weight increased similarly among all treatment groups throughout the study. Furthermore, there was no difference in ADG during the study among treatments (P = 0.73). These data indicate that antiparasitic resistance occurs in all breeds, and resistance varies between benzimidazoles and macrocyclic lactones. Furthermore, there were differences between breeds and/or seasons among treatments even within a single operation.