Parasitism of calves in South Island large dairy herds

Abstract

Industry reliance on the frequent use of short interval anthelmintic and the effects this had on the prevalence of nematode resistance on South Island dairy heifer replacements were investigated. Initially first year cohort replacement heifers (<12 months) from ten large-scale South Island dairy farms were routinely monitored for faecal egg counts (FEC), from September 2008 to April 2009. Larval cultures identified the prevalence of nematode species throughout the calf-raising season. FEC reduction tests (FECRT) were conducted on four of the farms, to quantify the levels of resistance to a range of anthelmintics. Herd mean FEC in calves were consistently low, <100 eggs per gram (epg) for all properties sampled until 26 weeks of age. However results from FECRT revealed significant anthelmintic failure with three of the four farms recording resistance (<95%) to both levamisole and ivermectin drenches. All four of the farms tested recorded anthelmintic resistance to at least one of anthelmintic treatments. In view of widespread anthelmintic resistance found and the high frequency of anthelmintic treatment given, it would appear that the selection pressure for the development of anthelmintic resistance in these farming systems is likely to be high. It is concluded that current nematode control approaches in replacement stock in these large herds would appear to be unsustainable and without a reduction in drench usage anthelmintic efficacy will continue to fall.