To avoid off label applications, producers should check the growth stage of the crop in each field before making late-season applications of glyphosate to Roundup Ready soybean.
Weed escapes can happen in Roundup Ready soybeans for several reasons. However, they may be more prevalent this year due to the weather-induced planting delays. The planting delays have led to less vegetative growth in the 2011 soybean crop. This has delayed canopy closure and allowed sunlight to reach the soil surface, creating favorable conditions for weed seed germination.
It is tempting to control the escaped weeds with a late-season application of glyphosate. However, some of these late-season applications may be off label and therefore illegal. Glyphosate product labels state that glyphosate can be applied postemergence to Roundup Ready soybean anytime from the cracking stage through flowering (R2 growth stage). Applications made after this are off label and illegal so it is important for soybean producers to be aware of the glyphosate application window and be able to identify the end of the R2 growth stage.
The R2 growth stage occurs when there is an open flower on one of the two uppermost nodes on the main stem with a fully developed trifoliate leaf. The end of the R2 growth stage occurs when the plants move into the R3 or beginning pod growth stage. The R3 growth stage is reached when a pod 3/16 of an inch long exists on one of the four uppermost nodes on the main stem with a fully developed trifoliate leaf. A fully developed trifoliate leaf is defined as a leaf that is unrolled so that the leaf edges are no longer touching. Fields are considered to be at a particular growth stage when 50% or more of the plants have reached that stage of development.
It is especially important this year to check the growth stages of your fields before applying glyphosate as the smaller plants may be deceiving and may have reached later growth stages than would be expected given the size of the plants.
(Source: Michigan Ag Connection)