Effects of tactical late-season treatments with ivermectin on calves naturally exposed to trichostrongyles

This study was conducted to evaluate the effects of tactical treatments with ivermectin against trichostrongyles in first-season grazing heifer calves in the Danish marshland. A group of Black-Pied Friesian calves was turned out in early May on a permanent pasture naturally infected with trichostrongyle larvae. In late July, when high herbage infectivity started to appear, the pasture was divided into two plots of equal size, which from then and until housing in late October were each grazed by half of the original group of calves. One of these groups was given three anthelmintic treatments with ivermectin at 4-week intervals starting in late July. The other group served as non-treated controls. Ostertagia ostertagi and Cooperia oncophora were the predominant trichostrongyles. Nematodirus helvetianus was observed on few occasions. Although the animals were exposed to a high herbage infectivity from July onwards, the anthelmintic treatments conferred a significant reduction in trichostrongyle loads, as evidenced by an almost complete cessation of egg excretion and a significant lowering of pepsinogen and gastrin levels in the blood. This was reflected in higher weight gains.