title: Assessment of parasite control and weight gain after use of an ivermectin sustained-release bolus in calves.
abstract: OBJECTIVE
  To assess parasite control and weight gain after administration of an ivermectin sustained-release bolus over 135 days to calves grazing in the midwestern United States.
  
  
  METHODS
  Replicated pasture study.
  
  
  METHODS
  56 Bos taurus calves.
  
  
  METHODS
  Calves were matched for body weight and randomly allocated to remain untreated or to receive an ivermectin sustained-release bolus before turnout on day 0. Calves were grazed by treatment group on B pastures (4 replicates). Body weights and fecal egg counts were recorded on days- 1 and 28, and then at 28-day intervals until day 168.
  
  
  RESULTS
  Parasitism was not clinically evident prior to or during the study. In treated calves, mean fecal egg counts were at or near 0 at all posttreatment evaluations. Although the mean egg count exceeded 20 ova/g only once in control calves, the cumulative egg output was > 42 million/calf. For the treated group, it was < 0.1% of this number. Mean total weight gain was 33.9 kg (74.6 lb) greater for ivermectin-treated calves than for untreated control calves (P < 0.02): a 34% increase.
  
  
  CONCLUSIONS
  Fecal trichostrongyle eggs from calves can accumulate over a grazing season to provide enormous potential for augmenting pasture infectivity. An ivermectin sustained-release bolus (administered to calves being placed on pasture) controls parasitism, limits pasture infectivity, and can substantially influence growth by limiting the impact of subclinical parasitism.
authors.affiliation: University of Missouri, Southwest Research Center, Mt Vernon 65712, USA.
authors.forename: W G
authors.initials: WG
authors.lastname: Ryan
authors.forename: R J
authors.initials: RJ
authors.lastname: Crawford
authors.forename: S J
authors.initials: SJ
authors.lastname: Gross
authors.forename: D H
authors.initials: DH
authors.lastname: Wallace
mesh_terms: D000818:Animals; D000969:Antinematodal Agents / Q000008:administration & dosage / Q000494:pharmacology* / Q000627:therapeutic use; D002417:Cattle; D002418:Cattle Diseases / Q000453:epidemiology / Q000503:physiopathology / Q000517:prevention & control*; D003692:Delayed-Action Preparations; D005243:Feces / Q000469:parasitology; D007267:Injections / Q000379:methods / Q000662:veterinary; D007559:Ivermectin / Q000008:administration & dosage / Q000494:pharmacology* / Q000627:therapeutic use; D015146:Midwestern United States / Q000453:epidemiology; D010270:Parasite Egg Count / Q000662:veterinary; D014251:Trichostrongyloidea / Q000302:isolation & purification; D014252:Trichostrongyloidiasis / Q000503:physiopathology / Q000517:prevention & control / Q000662:veterinary*; D015430:Weight Gain / Q000187:drug effects* / Q000502:physiology
chemical_list: D000969:Antinematodal Agents; D003692:Delayed-Action Preparations; D007559:Ivermectin