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Fate of 2,4-D Iso-octyl Ester after Application to a Wheat Field¹

ABSTRACT

Dissipation of iso-octyl ester of 2,4-dichlorophenoxyacetic acid (2,4-D) and its acid metabolite in air, crop, and soil components were measured in a wheat (Triticum aestivum L.) field during and following application. Drift losses during application were only 0.2% of the amount applied. Air samples, collected at six heights ranging from 30 to 200 cm above the crop canopy during the first 7 d after application showed distinct ester gradients in the air, with concentration highest in the samples closest to the crop canopy. The highest concentration was measured during the afternoon of day 1 when 1604 ng m^–3 were recorded 30 cm above the crop canopy. The vertical flux of the ester showed distinct diurnal variations with maxima reached in the early afternoon of day 1 and 2, followed by a rapid decline of the ester flux thereafter, corresponding with the depletion of the ester from the crop canopy. The total or cumulative vapor losses of the iso-octyl ester over the 5-d sampling period were estimated to be 93.5 g ha^–1 or 20.8% of the amount applied.

The crop canopy intercepted 52% of the applied ester and acted as the major source of vapor losses. The magnitude of vapor activity was controlled primarily by the atmospheric stability and air temperature following application. On entry into the crop, the ester was hydrolyzed to the acid metabolite, which reached its maximum level on day 3. There appeared to be a rapid initial metabolism of the acid followed by a slow decline.

Ester losses from the soil surface occurred only when the soil surface was moist, i.e., after a rainfall event or in the early hours of the morning following the deposition of dew. In addition, both the hydrolysis of the ester and the subsequent degradation of its acid metabolite in the soil were dependent on the availability of surface soil moisture. No detectable 2,4-D remained in the soil after 34 d.

Key Words: residues • soil • air • crop • herbicide vapor flux • application losses • postapplication losses

¹ Contribution of Environmental Chemistry of Herbicides Section, Research Station, Agriculture Canada, Regina, SK S4P 3A2, Canada.

² Section head, meteorologist (Sask. Res. Council, Saskatoon, SK), residue chemist, residue chemist, and agronomist, respectively.

Received for publication May 14, 1984.