Volume 6, Issue 5 (May 2009)
Glyphosate Weed Control Enhancement with Ammonium Sulfate and Commercial Water Conditioning Agents
Glyphosate herbicide labels generally recommend addition of ammonium sulfate (AMS) to the spray solution, which often improves weed control, especially when mixed with hard water. AMS in solution disassociates, and the sulfate binds with cations in the spray solution, preventing the development of glyphosate-cation complexes that tend to have lower absorption into plant leaves. In addition, the ammonium ion can associate with the glyphosate molecule, which helps facilitate glyphosate absorption into the leaf. The recommended AMS rate is 1–2 % w/w, and AMS is available in both dry and liquid formulations. Applicators generally find AMS inconvenient to use because of the high use rate and handling issues. Several companies are marketing low-rate water conditioner products to be used as an alternative to AMS with glyphosate. Pesticide applicators like the convenience of low-rate water conditioners, but performance of these products has been inconsistent. Field experiments were conducted near Manhattan, KS, from 2005 to 2008 to compare the efficacy of glyphosate with AMS and various commercial water conditioners on velvetleaf, sorghum, corn, and sunflower. Each experiment consisted of a sublethal (0.31 or 0.43 kg ae/ha) dose of glyphosate applied in combination with the recommended application rate of each adjuvant. Water hardness, environmental conditions, and plant growth stages varied by experiment. Control of all assay species with glyphosate was enhanced by the addition of AMS, unless control was near complete in the absence of AMS. Commercial water conditioner products that included an AMS component at the equivalent rate of 1 % w/w gave equal or slightly better control than glyphosate plus 1 % w/w AMS. Commercial water conditioners that did not provide an equivalent amount of AMS gave less control than glyphosate with 1 % or 2 % w/w AMS and were often no better than glyphosate alone for the low-rate products.