Quantitative Aspects of Aqueous-Spray Applications of 2,4- Dichlorophenoxyacetic Acid for Herbicidal Purposes

Abstract

1. An apparatus was set up, with which sprays of different droplet size could be applied at desired volume rates on plants for the purpose of determining the effect of growth-regulating compounds applied as herbicides. 2. Aqueous sprays, containing a constant threshold amount of the ammonium salt of 2,4-dichlorophenoxyacetic acid, were found to be most effective at a volume rate of 10-20 ml. per square yard when applied to young kidney-bean plants. Rates of 40-100 ml. per square yard were less effective, and there was a progressive decrease in magnitude of response as the volume was reduced below 10 ml. per square yard. 3. Further studies were made on sprays applied at the low rates of 8, 6, 4, and 2 ml. per square yard. It was shown that, as the volume was reduced, a progressively larger amount of growth-inhibiting substance was required to kill young soybean plants. Twenty mg. of compound per square yard was needed at the 8-ml. per square yard volume rate and at least 60 mg. per square yard (3 per cent concentration) at the 2-ml. per square yard rate. 4. Sprays of relatively large-droplet size (561-250 μ av. diam.) were more effective than those of smaller-droplet size (30 μ av. diam.). 5. The investigations on interception showed that more solution was deposited and retained on a plant when sprays of larger-droplet size were used. There was also an indication that a higher percentage of the spray was intercepted and retained when applied at volume rates of 10-20-ml. per square yard than in smaller or larger volumes. 6. For most efficient use of growth- regulating compounds, they should be applied in sprays devoid of finely atomized particles and in volumes of 10-20 ml. per square yard rather than at saturation levels of about 100 ml. per square yard. Rates below 10 ml. per square yard, while less efficient in terms of amount of compound required, are of interest. The fact that growth inhibitors can be effective when applied in such low volumes opens up new fields for large scale practical application of herbicides.