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Long-Term Persistence of Picloram in a Sandy Loam Soil¹

ABSTRACT

Picloram (4-amino-3,5,6-trichoropicolinic acid) was applied to a ryegrass (Lolium perenne L. cv S23) sward at 0.05, 0.28, and 1.68 kg ai (active ingredient)/ha in June 1967 and again in August of 1969 and 1970. Small amounts of residue were found in soil samples 1 year after application of the low and medium doses. On the highest dose plots 5 to 6% of the amount applied was usually recovered 1 year after spraying. Following the final application this residue degraded slowly over the following 3 years. Analyses of the final samples (taken 222 weeks after spraying) indicated that the residue had declined to about 0.5% of the total amount applied. In stratified soil samples, residues were found to the maximum sample depth of 90 cm 1 year after the initial application, but 69 weeks after the final application no residue was found below 30 cm.

Crops grown across all plots in 1974 did not differ in yield. However, on the plots that had received the highest dose, leaf abnormalities were noted in beans and potatoes and color differences were observed in kale.

Key Words: Tordon • United Kingdom • herbicide leaching • phytotoxicity

¹ Contribution of the Agric. Res. Counc. Weed Res. Organ., Begbroke Hill, Yarnton, Oxford, OX5 1PF, UK. Project WRO 02010. Pap. 48/1977.

² Director, Higher Scientific Officer, and Senior Scientific Officer (retired), respectively.

Received for publication March 14, 1978.