Herbicidal Activity of Glyphosate in Soil

Greenhouse studies were conducted using bentgrass (Agrostis tenuisSibth. ‘Highland′) as a bioassay species to measure herbicidal activity of 3.4 kg/ha of the isopropylamine salt of glyphosate [N-(phosphonomethyl)glycine] when applied directly to the soil surface prior to bentgrass emergence. Activity of glyphosate also was examined by applying glyphosate to a moist soil surface and placing alfalfa (Medicago sativaL. ‘Vernal′) and red clover (Trifolium pratenseL. ‘Kenstar′) seeds on the sprayed surface 3, 6, 9, and 24 h later. In two high-organic soils, glyphosate reduced bentgrass growth in each of four experiments, even when applied up to 5 days before bentgrass emergence. Significant glyphosate activity was measured in one of the experiments in another organic soil and in three of the experiments in a Chehalis sandy loam soil. Germination and growth of alfalfa and red clover were reduced when seeds were distributed on a sprayed soil surface up to 24 h after glyphosate application. These results show that glyphosate, at least under certain conditions, can cause significant crop injury when used prior to planting or emergence of some species.