Abstract Two areas of bare ground were treated with 5% heptachlor granules at rates of either 1/2 lb or 1 lb active ingredient per acre and sown to pasture. Maximum residues of heptachlor epoxide found in the fat of sheep and in the milk of cows grazed on these pastures, after withholding periods of up to 45 weeks and 33 weeks respectively, were about 0.10 p.p.m. Established pastures were treated directly with heptachlor granules at rates of 2–3 oz active ingredient per acre. The fat of ewes and wethers grazed on these pastures after withholding periods of five to 11 weeks contained up to 0.18 p.p.m. heptachlor epoxide. The fat of lambs running with the ewes contained up to 0.50 p.p.m. heptachlor epoxide. Heptachlor treatment of soil prior to sowing to pasture resulted in lower residues in stock than did the treatment of established pasture. Neither treatment would comply with a “nil” tolerance for heptachlor and its principal metabolite in animal fats.
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