EFFICACY OF A SPRING STRATEGIC FENBENDAZOLE TREATMENT PROGRAM TO REDUCE NUMBERS OF OSTERTAGIA-OSTERTAGI INHIBITED LARVAE IN BEEF STOCKER CATTLE

Efficacy of a spring strategic treatment program with fenbendazole (FB Z) to reduce the accumulation of Ostertagia ostertagi inhibited early fourth-stage larvae (EL(4)) was investigated in two groups of crossbre d beef heifers which were 7-9 months of age and ranged in weight from 155 to 223 kg. The cattle were allocated to groups and treated on 27 A pril (Day 0). Group 1 calves served as nontreated controls. Group 2 ca lves were treated with FBZ 10% drench suspension at 5 mg kg(-1) on Day 0 and with 6-day courses of FBZ free-choice mineral (to provide 5 mg kg(-1) per animal) on Days 28 and 56, Each group grazed on a separate 4.9 ha pasture for 105 days to 10 August. On 10 August the cattle were taken off pasture and each original group was re-allocated to subgrou ps of three cattle which were treated orally with FBZ (5 mg kg(-1)), o xfenbendazole (OXF, 4.5 mg kg(-1)) or left nontreated (CONT), The catt le were necropsied on Days 120 and 121. Mean actual and cumulative fec al egg counts indicated near total suppression of egg output in the st rategically treated group (high of 5.2 eggs g(-1) feces (EPG) on Day 2 8), Egg counts of the nontreated group remained above 100 or 200 EPG t o Day 63 and then decreased to less than 100, Ostertagia was the predo minant genus, followed by Cooperia on most sampling dates. The largest O. ostertagi worm burdens were recovered from the CONT-CONT subgroup; numbers of EL(4) ranged from 18 922 to 5 1 137. Reduction in numbers of EL(4) in original controls, treated with FBZ or OXF in August, were low, being 60.2% and 74.3%, respectively. The numbers of O. ostertagi recovered from subgroups originally treated strategically with FBZ we re generally lower than in original controls. The largest reduction in O. ostertagi numbers was in the FBZ-CONT subgroup, which was not trea ted in August. Percent reduction values for O. ostertagi adults, devel oping L(4) (DL(4)) and EL(4) were 84.6%, 96.7%, and 99.0%, respectivel y. Percent reduction values for adults, DL(4) and EL(4) in the FBZ-FBZ and FBZ-OXF subgroups were 90.7%, 61.3%, 85.6% and 92.4%, 45.5%, and 73.0%, respectively. Variability in numbers of EL(4) acquired during g razing by original controls and strategically treated cattle and varia bility in efficacy of August treatments was evident as observed in hig h outlier numbers of worms in all subgroups except the FBZ-CONT subgro up. Liveweights and gains were nearly identical in the two groups on 1 0 August.