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A study was undertaken to determine why DDT residue levels in milk produced on dairies in Northeast Louisiana was much higher than from dairies in other areas of Louisiana. The intensive agricultural practices and interrelationship of dairies and cotton farms in this area were thought to contribute to the contamination of milk.

A relationship expressed as a time/DDT-milk residue effect due to the seasonal application of DDT on cotton was examined. The relationship between milk contaminated with DDT and feed ingredients contaminated with DDT was also investigated.

Four dairies were selected for study. Herd milk samples from these four dairies revealed that all had DDT present in milk, ranging from 1.60 parts per million to approximately 6.00 parts per million (fat basis). All dairies investigated practice a combination of dry lot and pasture feeding.

A positive correlation between the seasonal use of DDT on cotton and maximum levels of DDT in herd milk of each dairy was found. Based on a corrected mean residue value for a twelve-month period, the range in total DDT residues in milk from the four dairies studied was as follows: Dairy A from 1.38 in May to 5.54 parts per million (fat basis) in September; Dairy B from 0.81 in May to 3.77 parts per million in November; Dairy C from 1.45 in June to 9.90 parts per million in October; and Dairy D from 0.39 in June to a maximum of X2.42 parts per million in August.

Residue analyses of feed ingredients revealed hay, silage and pasture grass to be highly contaminated with DDT considered a result of drift of DDT applied to adjacent cotton fields. Total DDT residues in hay fed to producing animals ranged from 0.17 to a maximum of 28.67 parts per million on Dairy C. The average concentration of DDT in hay fed to dairy animals on Dairy C during the course of this study was 5.24 parts per million.

Total DDT residues in silage fed as part of the dairy ration on Dairy C throughout the course of this study ranged from 0.03 to 6.50 parts per million. The average concentration of DDT in all silage fed as part of the dairy ration on this dairy during this study show that interrelated farming activities such as dairying and cotton production cannot be practiced without the occurrence of illegally excessive residues in the milk produced if chlorinated hydrocarbon insecticides such as DDT are used for cotton insect control. Where pastures, forage crops and silage crops for dairy and beef cattle feed are grown in close proximity to areas planted to cotton, inadvertent contamination of milk and meat cannot be avoided because of the contamination by drift of these insecticides onto the crops.