Graduate Thesis Or Dissertation
 

Efficacy of morantel sustained release bolus against gastrointestinal nematodes in first season grazing Holstein calves

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/m039k7953

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  • The efficacy of the morantel sustained release bolus (MSRB) in reducing gastrointestinal parasitism in first season grazing calves was evaluated during the summer-fall grazing seasons of 1982 and 1983 in western Oregon. Thirty-eight calves (1982) and 40 calves (1983) were randomly assigned to a control and treatment group each season. All animals in the treatment groups received the MSRB on the day of turnout onto pasture. To monitor burden of infective larvae, worm-free tracer calves (16 total in 1982 and 18 total in 1983) were introduced onto the pastures every four weeks. Mean worm burdens from tracer calves grazed with treated animals in 1982 and 1983 showed overall reductions of 86.4% (not significantly different) and 84.3% (P < 0.01) respectively. Ostertagia ostertagi, Cooperia oncophora, and Nematodirus helvetianus were the primary nematodes recovered at necropsy. Twelve full-season tracer animals (six treated and six control) necropsied at trial termination (1982) indicated an 88.1% (P < 0.05) overall reduction in mean worm burdens. Mean fecal worm egg counts of eggs per gram (EPG) of feces of treated animals reflected a reduction of 69% (P < 0.05) in 1982 and 90% (P < 0.05) in 1983. Based on the EPG's (1982, 1983) and the immature larvae recovered at necropsy from the final set of tracer calves (1982), infection levels were higher in the fall season. Between October 26 and December 7, 1982, the largest increase in worm counts was due to inhibited L₄ (pre-Type II) ostertagiasis. In 1982, there was a 7.9 kg weight gain advantage of control animals compared to treated animals at day 160; however, on day 202 the treated animals showed a 4.6 kg weight gain advantage over the controls. In 1983, the treated animals gained an average of 13.5 kg (P < 0.05) more than the control animals. All animals remained clinically healthy during the trials and no boluses were lost from treated animals. These trials demonstrated that the MSRB was an effective anthelmintic for reducing gastrointestinal parasitism in first season grazing calves and in decreasing pasture larval contamination.
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