Effect of Soil pH on Imazaquin and Imazethapyr Adsorption to Soil and Phytotoxicity to Corn (Zea mays)

Abstract

Adsorption of14C-imazaquin {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid} and imazethapyr [2-(4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl)-5-ethyl-3-pyridinecarboxylic acid] to soil increased as soil pH decreased from 8.0 to 3.0 in laboratory studies. Significantly more imazethapyr3(AC-263,499) than imazaquin was adsorbed at soil pH levels 3.0 and 5.5, while the greatest difference in adsorption behavior between the two herbicides was observed at a soil pH of 5.5. In greenhouse studies, phytotoxicity to corn (Zea maysL.) was greater for imazaquin than AC-263,499 applied at 26 and 53 g ai/ha. There were significant pH by herbicide and pH by rate interactions, but in trend analysis only a small proportion of the corn response (r2= 0.01 to 0.35) was attributed to increasing soil pH values. In field studies where imazaquin was applied to soil pH levels of 4.2 to 4.8, 5.4 to 5.5, and 5.8 to 6.2, injury to corn across all pH levels decreased as the time delay between herbicide application and corn planting increased. There was no significant effect of soil pH on imazaquin injury to corn planted in July or August. Decreased injury from imazaquin was observed in 1985 on corn planted in June on the soil pH of 5.8 to 6.2. Imazaquin injury was less for June-planted corn in 1984 than in 1985, across all soil pH levels.