Two groups of yearling suckled beef calves born between January and May of the preceding year and another two groups of lighter and slightly younger calves born between the previous March and July were grazed during their second year on four separate paddocks known to be contaminated with infective larvae of gastrointestinal nematodes. One of the heavier and one of the lighter groups were treated with a topical formulation of ivermectin three weeks and eight weeks after turnout. The treatment of the heavier group had only relatively minor parasitological effects and no effect on weight gains, whereas the treatment of the lighter cattle resulted in increased weight gains due to effective nematode control. The paper highlights that small differences in previous performances, age and exposure to parasites can have a substantial impact on the benefits accrued from anthelmintic treatment.
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