Growth-Dilution as a Factor in the Decline of Pesticide Residues on Alfalfa-Grass Forage

The persistence of endosulfan, Guthion, malathion, methoxychlor, Sevin, and toxaphene applied to alfalfa-grass forage for spittlebug control was studied by chemical analysis of deposits from application to harvest. Applications were made when the crop was 9 to 12 in. high. Malathion residues declined from 51 p.p.m. to trace amounts (0.26 p.p.m. or less) in 1 week, endosulfan from 18 to 0.36 p.p.m., and Sevin from 77 to 0.33 p.p.m. in 2 weeks, and Guthion from 74 to < 0.75 p.p.m. in 3 weeks. Methoxychlor and toxaphene treatments resulted in harvest residues from 1 to 9 p.p.m.To assess the rate at which the residues would have declined if diluted by growth alone, forage was cut from unit areas and weighed at each residue sampling time. At harvest the theoretical dilution by growth would have reduced the residues of methoxychlor and toxaphene to one-half their original level, whereas the actual residues were from one-fourteenth to one-eighth of the original deposits.