It seems like growers frequently are trying to wait until they think all the weeds have emerged before they make postemergence soybean herbicide applications which can be late or at least less than optimum to maximize yield.
Extension Agronomist Bill Curran explains it is better to spray a little on the early side than risk later applications and potential yield loss and poor weed control. Assuming you start clean either with a burndown application or tillage, it is important to remove weeds by 5 to 7 weeks after soybean planting, which is generally when annual weeds are 6 inches tall or less. If more severe infestations of annuals exist or if perennials are the primary concern, applications may need to be adjusted one way or another by a week or so.
Glyphosate can be applied over the top to Roundup Ready soybeans up to 1.5 lb ae/acre. This is equivalent to 44 fl oz of Roundup PowerMax/WeatherMax, 48 fl oz of Touchdown Total or Durango DMA/Duramax or 64 fl oz of most generic types of glyphosate. You can use up to 66 fl oz of Powermax equivalent in your soybean crop (44 fl oz for corn). Use the higher rates for perennials and large or hard to control annuals. Glyphosate can be applied to RR soybean from emergence through flowering (R2 stage – ends when a pod is 5 mm or 3/16 inch long at one of the four uppermost nodes on the main stem). Include an appropriate surfactant if the product is not fully loaded and AMS for hard water or when tank mixing. You can tank mix a number of products with glyphosate to broaden the spectrum.