Preharvest Effects of Applying Glyphosate to Soybeans (Glycine max)

Abstract

Over a 4-yr period, glyphosate [N-(phosphonomethyl)glycine] was applied over-the-top to soybeans [Glycine max(L.) Merr.] at five rates, from 0.56 to 3.36 kg/ha, at four periods of time prior to harvest, 23 to 29 days, 15 to 21 days, 7 to 12 days, and 1 to 4 days. Soybean yields were reduced when glyphosate was applied at 2.24 and 3.36 kg/ha 23 to 29 days before harvest. Soybean seed were discolored following all five rates of glyphosate applied 23 to 29 days before harvest, following glyphosate at 1.12 to 3.36 kg/ha applied 15 to 21 days before harvest, and following glyphosate at 3.36 kg/ha applied 7 to 12 days before harvest. Germination of soybean seed was reduced at 30 C only when glyphosate was applied at 2.24 and 3.36 kg/ha 23 to 29 days before harvest, or at 3.36 kg/ha applied 15 to 21 days before harvest. At alternating temperatures (10.6 C for 72 h followed by 30 C for 72 h), all treatments applied 23 to 29 days before harvest reduced soybean seed germination; glyphosate at 1.12, 1.68, and 3.36 kg/ha applied 15 to 21 days before harvest also reduced germination. Seed from plots treated with glyphosate 23 to 29 or 15 to 21 days before harvest produced atypical soybean plants. Glyphosate at 2.24 and 3.36 kg/ha applied 23 to 29 days before harvest reduced the oil content in soybean seed, but none of the treatments consistently affected the protein content.