One group of first-season calves was dosed with an oxfendazole pulse release bolus at spring turnout (April 30) and on July 15 a second group received the front-loaded oxfendazole pulse release bolus. The objective was to test the boluses for the prophylaxis or control of nematodiasis. The control group consisted of calves to which no bolus was administered. The three groups occupied separate but adjacent plots. For the first five weeks of the trial, three calves, artificially infected with Dictyocaulus viviparus grazed in each plot. Parasitic bronchitis severely affected the control calves, necessitating repeated emergency treatment, whereas administration of the bolus at turnout almost completely prevented this condition. D viviparus infection increased markedly on the control herbage in July and August but was eliminated by the end of June on pasture grazed by bolus treated calves. Treatment in mid-season with the front-loaded bolus brought an outbreak of parasitic bronchitis under control. Gastrointestinal worm egg output was satisfactorily suppressed after the administration of both boluses, resulting in reduced levels of herbage infection. Calves treated with a bolus at turnout gained significantly more weight than either the controls (P less than 0.001) or the calves treated with a front-loaded bolus in mid-season (P less than 0.01). The weight-gain of the calves treated with a front-loaded bolus was slightly but not significantly greater than that of the control calves. On the basis of faecal egg counts, the first pulse released from the standard boluses was delayed and one front-loaded bolus failed to release a dose.(ABSTRACT TRUNCATED AT 250 WORDS)
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