Lipophilic Chemistry Affects Surfactant Phytotoxicity and Enhancement of Herbicide Efficacy

Abstract

Nonionic surfactants are comprised of a lipophilic and a hydrophilic moiety. Experiments were conducted to determine the effect of the lipophilic moiety on droplet spread; surfactant phytotoxicity, and surfactant enhancement of herbicide phytotoxicity. Six different lipophilic moieties and two herbicides were evaluated on four plant species. Each lipophilic moiety was represented by two surfactants: one with a low hydrophilic:lipophilic balance (HLB) value, near 12.0, and one with a high HLB value, near 16.0. Droplet spread was greater with low than high HLB surfactants and was greatest with trimethylnonanol ethoxylate (TMN); intermediate with secondary alcohol ethoxylate (SAE) and octylphenol ethoxylate (OPE), and least with linear alcohol ethoxylate (LAE), nonylphenol ethoxylate (NPE), and oxysorbic (TWN). Surfactant enhancement of droplet spread was less on redroot pigweed (Amaranthus retroflexus) than on barley (Hordeum vulgare), green foxtail (Setaria viridis), or kochia (Kochia scoparia). Isopropylamine salt of glyphosate [N- (phosphonomethyl)glycine] and the ammonium salt of imazethapyr {2-[4,5-dihydro-4-methyl-4-(1-methylethyl-5-oxo-1H-imidazol-2-yl] -5-ethyl-3-pyridinecarboxylic acid} had little effect on droplet spread. Lipophilic chemistry and HLB affected surfactant phytotoxicity to green foxtail, kochia, and redroot pigweed but not to barley. Foliar injury ranged from 1 to 23% with green foxtail, 0 to 17% with kochia, and 0 to 14% with redroot pigweed. Injury to barley was similar for all surfactants and ranged from 3 to 8%. Injury generally was greater with low than high HLB surfactants. However, SAE and OPE caused greater injury to green foxtail at high than low HLB. Green foxtail was the only plant species injured more than 8% by high HLB surfactants. Droplet spread did not correlate with surfactant phytotoxicity regardless of plant species. Glyphosate phytotoxicity generally was enhanced most by high HLB LAE to barley, green foxtail, and kochia and by high HLB TWN to redroot pigweed. Imazethapyr phytotoxicity was generally greatest when applied with high HLB SAE to barley and redroot pigweed, with high HLB LAE to green foxtail and with low HLB LAE to kochia. Neither droplet spread nor surfactant phytotoxicity correlated with glyphosate or imazethapyr efficacy.