Laboratory experiments were conducted to determine the effects of application factors and standing Triticum aestivum stubble on herbicide spray deposition and efficacy in a simulated no-till environment. Spray deposition on weeds was reduced in the presence of stubble, and deposition losses on Amaranthus hybridus were greater than those on Setaria faberi. Spray penetration through stubble was significantly enhanced with electrostatic charging of a fine hydraulic spray. The combination of 45 kV electrostatic charge and 50 cm nozzle spacing produced maximum spray deposition on weeds and resulted in a 96% and 345% increase in deposition on A. hybridus and S. faberi, respectively, compared to the uncharged controls. Deposit reduction from standing stubble was greater at travel speeds of 16 km h−1 (36 to 52%) than 8 km h−1 (9 to 38%). On a dry weight and plant density basis, weeds retained more spray than was retained by stubble, yet stubble, at average densities, was capable of capturing 9 to 12% of total applied spray dose per unit area. Bounce studies of individual droplets of water or imazethapyr plus adjuvant mixture demonstrated that T. aestivum straw had a general affinity for all spray droplets, exhibiting no rebound even for 800-µm water droplets. Setaria faberi foliage exhibited poor retention of droplets: both 350- and 800-µm water droplets as well as 800-µm droplets of imazethapyr plus adjuvant mixture rebounded. Only 350-µm herbicide mixture droplets were retained by S. faberi. Amaranthus hybridus retained all droplets. In broadcast spraying, British Crop Protection Council “Medium” quality sprays were poorly retained by S. faberi compared to “Fine” sprays, whereas A. hybridus retained both sprays equally well. However, imazethapyr spray deposits resulting from coarser sprays were more efficacious on S. faberi than fine spray deposits, a difference that was not observed for A. hybridus.
Nomenclature: Imazethapyr; metalaxyl, N-(2,6-dimethylphenyl)-N-(methoxyacetyl)alanine methyl ester; Triticum aestivum L., wheat; Amaranthus hybridus L. AMACH, smooth pigweed; Setaria faberi Herrm. SETFA, giant foxtail.
