Targeted selective treatment with anthelmintic for New Zealand dairy heifers.

We investigated whether measurement of live weight gain (LWG) could be used to deliver targeted selective treatment (TST) with anthelmintic that maintained target live weight (LW) while reducing anthelmintic use. Dairy heifers (n = 546) from four commercial, seasonal calving, pastoral New Zealand dairy herds in the Canterbury region of New Zealand were recruited to compare effects of TST, against suppressive treatment (ST), on LW and LWG in their first year at pasture. Animals were enrolled at weaning (December) and weighed ± anthelmintic treatment every month until May and then in August and September, pre-mating in October. All ST calves were treated every month with an oral anthelmintic at 1 mL/5 kg LW, delivering 0.2 mg abamectin, 8 mg levamisole and 4.5 mg oxfendazole/kg LW, (Control). TST calves received the same anthelmintic at the same time but only if either their individual LWG fell below a pre-specified, time altered, breed specific target for that group (Group target) or if their individual LWG fell below an individual, time altered target for that individual (Individual target). Faecal egg count (FEC) and serum IgA OD levels were recorded from a sample of calves throughout the study. Compared to ST, anthelmintic use was halved in both TST groups (P < 0.001) and there was no evidence for differences in the proportion of calves reaching target LW pre-mating (Control = 80 (95 % CI = 79.1-87.2 %), Group = 78.9 (95 % CI = 59.6-98.2%), Individual = 78.2 (95 % CI = 58.4-97.9 %), P = 0.935). Control calves were heavier pre-mating (310 (95 % CI = 290-330 kg) than Group (300 (95 % CI = 280-320 kg) or Individual (298 (95 % CI = 278-318 kg), P < 0.001). An interaction between FEC and time meant calves with FEC > 200 EPG grew more slowly in the autumn (P < 0.001), but more quickly in the winter (P < 0.001). FEC and IgA OD levels were consistent with levels of parasitism sufficient to impact LWG, but there was no evidence for differences between treatment groups. Sensitivity and specificity of LW and LWG as predictors of reaching target weight varied month-on-month but these results suggest monthly LWG and LW were poor indicators for AHC treatment. In conclusion, TST can be effective in reducing anthelmintic use and maintaining group level performance. LW and LWG sensitivity was 57-85 % and 66-93 %; specificity 38-83 % and 32-79.