Comparison of parasite load and average daily weight gain in suckling beef calves treated with macrocyclic lactones in either an extended-release injectable or a pouron formulation

The objective of the study was to compare the effect of macrocyclic lactones anthelmintic in either an extended-release injectable (ERI) or a pour-on (PO) formulation on parasite load and average daily weight gain in suckling beef calves. Spring born Angus cross beef calves (n=1228) grazing on 17 pastures from 4 cow-calf operations in Washington State during 2017 were included. Calves in each pasture were randomly assigned to ERI (n=611; Long Range®, 50 mg of eprinomectin/ml, Merial Inc., Duluth, GA, USA; @ 50 mg or 1ml/50 kg BW) or PO (n=671; Ivomec®, 5 mg ivermectin/ml; Merial Inc., Duluth, GA; @ 500 mcg or 1 ml/10 kg BW) treatment 105 days prior to weaning (approximately at 8 months of age) at the time of pasture turn-out in mid-May. Calves were weighed initially, at mid-trail (approximately 55 days after start) and at weaning. Faecal samples were taken from a subset of calves (n=30/pasture; 15 in each treatment group, a total of 510 calves) for faecal egg counts (FEC) at the start of the study and again at 15, 30, 60 and 90 d later. The data were analyzed using a mixed model. The FEC did not vary between calves in treatment and control groups on day 0 (P>0.1). However, accounting for temperament by treatment interaction, ERI treated calves had lower FEC compared with PO treated calves on day 90 (P<0.01). Accounting for gender (P<0.05), temperament (P<0.05) and treatment by age (P<0.01), ERI treated calves gained more weight compared with PO treated calves (P<0.01). In conclusion, treatment of nursing calves at late-spring with ERI experienced a reduction in GI (gastrointestinal) parasite load and a measurable but modest increase in weight gain compared with calves treated with PO.