After the recent stretch of rainy weather in Minnesota, we are currently in a period of excellent conditions for finalizing postemergence weed control in soybeans. As crop stages progress, postemergence soybean herbicide options decrease due to growth stage or days before harvest restrictions.
To illustrate, products such as Flexstar, Ignite, Pursuit, Raptor and Extreme must be applied prior to flowering and FirstRate must be applied prior to 50% flowering. Postemergence grass herbicides such as Assure, Poast and Select can be applied up to 80, 75 and 60 days before harvest, respectively. Harmony SG and Synchrony XP can be applied up to 60 days before harvest, however, in conversations with DuPont, they indicate that as beans begin to flower the risk of crop injury becomes greater and they often don't recommend Harmony and Synchrony applications after July 4.
In the interest of crop safety it is also important to read product labels to note use of the proper adjuvant, appropriate tankmix partners and application restrictions based on environmental conditions.
For many growers the postemergence decision is focused on when to apply the second pass of glyphosate. Glyphosate can be applied through the R2 (full bloom), but weed species present, density and size should be the main factors in timing your second glyphosate application. Application to small weeds at low weed densities (often as a result of using pre-emergence herbicides) increases the probability of effective weed control.
Delaying weed control in anticipation of another weed flush or to save a trip across the field by tankmixing another product, such as an aphid insecticide, with glyphosate does not necessarily save you money at harvest. From a weed science perspective, delaying the glyphosate application increases the uncertainty of weed control, increases the risk of weed/crop competition and increases the amount of weed seed produced for future growing seasons. Remember: The weeds that emerged with the crop are the most likely to reduce your crop yield and produce the most weed seed – not the weeds that emerge later in the growing season.
Controlling weeds currently present in a field is a known problem; waiting to spray for weeds in anticipation of another problem (e.g., aphids) creates a greater risk for lost profits and future weed problems. Concurrently, spraying insecticides before the economic threshold is reached, in order to tankmix with glyphosate, often results in another insecticide treatment later in the season, negates the positive influence of natural predators and parasites on the aphid population and increases the probability of secondary pest infestations such as spider mites.