ISSUE 5 June 2, 2005
MICRONUTRIENTS AND GLYPHOSATE- AN UNHAPPY MARRIAGE
Soybeans in our region do not respond to most micronutrient applications. Zinc has not been found to increase yield in our area. Iron might make a plant green up on an occasional field, but research has illustrated the inconsistency of any yield response. Manganese has not been shown to be beneficial in our region. However, in Michigan and northern Indiana, manganese responses are common on some soils. In their efforts to combine trips across fields and save time and money, farmers in this area have added manganese and other micronutrients to their glyphosate spray mixture. Odd things started to happen. A researcher at Michigan State (Bernard) has conducted a series of studies on this technique and this is what has been found so far:
1. Control of velvetleaf (a common soybean weed in the mid-west) and lambsquarter control was reduced about 50% with addition of iron and zinc to glyphosate.
2. Control of weeds when metals were applied was increased if the glyphosate rate was increased and/or the ammonium sulfate concentration was increased, but not up to levels without the metal additives.
3. Most manganese formulations reduced weed control. The mechanisms for reduced efficacy varied depending on the source of the metal. Some reduced absorption, while some had no effect on absorption, and the effect was more physiological.
This research has been supported by an article from Virginia, where manganese fertilizers reduced the control by glyphosate of several common weed species.
The following are my recommendations:
NDSU Extension Soil Specialist