NEMATODE BURDENS AND PRODUCTIVITY OF GRAZING CATTLE TREATED WITH A PROTOTYPE SUSTAINED-RELEASE BOLUS CONTAINING IVERMECTIN

One hundred and twenty four-month-old Hereford-Friesian cross heifers weighing from 88 to 130 kg were divided into two equal groups. One gro up acted as a control with each animal receiving one placebo bolus, th e other animals received one prototype intraruminal sustained-release bolus designed to deliver approximately 8 mg ivermectin/day for 100 to 120 days. The boluses were administered the day before turnout in mid -May. Each group was grazed separately for 167 days on pastures contam inated,vith parasitic nematode larvae including the lungworm Dictyocau lus viviparus, and the gastrointestinal worms Ostertagia ostertagi, Co operia oncophora and Nematodirus helvetianus. Parasitic disease did no t occur in the ivermectin-bolus group, but the control group required anthelmintic treatment to control parasitic gastroenteritis at 111 and 154 days after turnout. Up to the 111th day after turnout, the peak m ean nematode egg and larval counts per gram of faeces in controls was, respectively, 564 epg and 0.5 lpg. Based on faecal nematode egg count s and worm burdens in bolus-treated cattle removed from pasture at 119 days after turnout and bolus function studies, it was concluded that ivermectin delivery from the prototype bolus ceased between 95 and 98 days after administration. However, unlike the controls, the treated c attle did not develop parasitic gastroenteritis at any time. Their fae cal nematode egg output was significantly (P < 0.01) lower (<1 epg) co mpared to the controls and lungworm larval output zero during the func tional life of the bolus. The faecal egg and larval outputs continued low until the end of the trial. In addition, the nematode contaminatio n of the pasture grazed by the bolus-treated cattle remained low throu ghout the grazing season compared to the control pasture, even after c essation of ivermectin delivery. By 119 days after turnout, the iverme ctin-treated cattle had a significant (P < 0.01) mean weight gain adva ntage of 28.6 kg over the controls, which was maintained until the end of the grazing season.