Optimizing postemergence herbicide deposition and efficacy through application variables in no-till systems

Laboratory experiments were conducted to determine the effects of applicati on factors and standing Triticum aestivum stubble on herbicide spray deposi tion and efficacy in a simulated no-till environment. Spray deposition on w eeds was reduced in the presence of stubble, and deposition losses on Amara nthus 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 5 0 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, res pectively, compared to the uncharged controls. Deposit reduction from stand ing 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 retaine d more spray than was retained by stubble, yet stubble, at average densitie s, was capable of capturing 9 to 12% of total applied spray dose per unit a rea. Bounce studies of individual droplets of water or imazethapyr plus adj uvant mixture demonstrated that I: aestivum straw had a general affinity fo r all spray droplets, exhibiting no rebound even for 800-mum water droplets . Setaria faberi foliage exhibited poor retention of droplets: both 350- an d 800-mum water droplets as well as 800-mum droplets of imazethapyr plus ad juvant mixture rebounded. Only 350-mum herbicide mixture droplets were reta ined by S. faberi. Amaranthus hybridus retained all droplets. In broadcast spraying, British Crop Protection Council "Medium" quality sprays were poor ly retained by S. faberi compared to "Fine" sprays, whereas A. hybridus ret ained both sprays equally well. However, imazethapyr spray deposits resulti ng from coarser sprays were more efficacious on S. faberi than fine spray d eposits, a difference that was not observed for A. hybridus.