Written collaboratively by Eric Jones, Philip Rozeboom, Stephen Robertson, Jill Alms, and David Vos.
The summer solstice roughly marks the date when soybean plants switch from vegetative growth to reproductive growth. The first sign of the switch from vegetative to reproductive growth is flower blooms on the first growth node, signifying the R1 stage. While some early planted soybean are larger in size compared with later planted soybean, the day length (i.e., photoperiod) is what induces the growth switch – not plant size (Figure 1).
Another important aspect of reproductive stage soybean is that many herbicides are labeled to no longer be applied after plants reach any bloom stage. However, 2,4-D choline (Enlist One) and glufosinate (i.e., Liberty Ultra, Interline, Cheetah, and others) can be applied at R1 stage (glufosinate) to the R2 stage (full bloom) (Enlist One). While the critical period of the weed period in soybean is between 1 to 4 trifoliate (V1 to V4), the cool, drought conditions of this growing season, some soybean field may require weed management if soybean plants are still small to protect yield potential. While herbicides can still be used to manage weeds, check specific labels and the most-recent South Dakota Pest Management Guide - Soybean for more information on application restrictions.
If plants are too mature for herbicide application, do not apply the herbicides! While late season applications are not injurious to the crop, the herbicide residues are translocated to the grain which will result in rejection from selling. Everyone’s favorite nonchemical tactic of “walking beans” can still be implemented to manage weeds before flowering to produce more seeds. If weed infestations cannot be feasibly managed by hand weeding, make notes of weedy fields that will likely require more extensive management.
