Heavy rains and cooler soils in May in many areas of the state delayed soybean planting or the application of preemergence herbicides. Early season weed control is important to avoid crop-weed competition and yield loss, particularly in soybean where there is a limited choice of effective postemergence herbicides. Heavy rains also created a situation where preemergence herbicides washed off and the duration of their weed control efficacy was reduced. With plenty of soil moisture and warm temperatures the last three weeks promoting weed seed emergence and growth, several soybean fields now have heavy weed infestation (Figures 1, 3).

It is important to consider soybean growth stage or preharvest interval when applying postemergence herbicides to avoid potential crop injury. Herbicide product labels provide a maximum soybean growth stage after which broadcast herbicide applications should not be made (Table 1). When using an herbicide tank-mixture, follow the recommendations for the most restrictive label language of products being used in the tank-mixture.

How to Determine Soybean Growth Stage

• Restriction of postemergence herbicide application in soybean is based on growth stage listed on herbicide labels or preharvest interval.
• Restrictions primarily are indicated as a particular soybean leaf stage or preharvest interval; sometimes both are listed (Table 1). The growth stage that is most restrictive should be followed when both are listed.
• After the first set of single leaves emerges, all new emerging leaves are trifoliates (compound leaves with three leaflets). The vegetative (V) stages are defined by the number of trifoliate leaves that have developed (unrolled) on the main stem, not the branches.
• The V1 stage occurs with the full opening of the first trifoliate (leaf has unrolled and edges no longer touch). Every three to five days, new leaves appear through the V5 stage. For example, a soybean plant with five trifoliates is at V5 stage. After this point trifoliates emerge every two to three days.
• At the reproductive one (R1) stage at least one flower appears on the plant on any node on the main stem, also known as bloom stage. The plant is at the V6 to V8 stage and generally 15 to 18 inches tall. At the reproductive two (R2) stage, the plant is beginning full bloom.
• When the first pod on one of the four upper nodes reaches 3/16 inch long, the plant is at R3 stage. Most postemergence soybean herbicides have restrictions up to R3 growth stage. Certain herbicides can be applied later in the season when the restriction is based on preharvest interval. For example, Select MAX can be applied as long as you can maintain 60 days of preharvest interval.
• At the R4 stage, the plant reaches the full pod stage.

Table 1. Application restriction of postemergence herbicides commonly applied in soybean.

Herbicidea,b	Crop Height Limit or Preharvest Interval (PHI)
Arrow	60 days, PHI
Assure II	Not after pod set, 80 days PHI
Basagran	30 days for forage or hay
Cadet	Full flower or 60 days PHI
Classic	60 days PHI
Cobra	45 days PHI
FirstRate	Bloom to 50% flowering, 65 days PHI
Engenia (only for Roundup Ready 2 Xtend soybean)	R1
Enlist Duo (only for Enlist soybean)	Up to full bloom (R2)
Extreme	Prior to flowering
Flexstar GT	Bloom
Fusilade	Bloom
Fusion	Bloom
Glyphosate	Consult specific label, generally through R2
Harmony SG	60 days PHI
Liberty (only for Liberty Link soybean)	Bloom
Marvel	Up to R3 or 60 days PHI
Phoenix	45 days PHI
Pursuit	Prior to flowering
Raptor	Prior to flowering
Reflex/Flexstar	Bloom
Resource	60 days PHI
Roundup PowerMAX	Consult label, in crop, generally full bloom (R2); harvest aid, 7 days PHI
Roundup WeatherMax	Consult label, in crop, generally full bloom (R2); harvest aid, 7 days PHI
Rowel/Rowel FX	Within 3 days after planting, prior to soybean emergence
Scepter	90 days PHI
Select MAX	60 days PHI
Targa	Podset, 80 days PHI
Touchdown	Consult label, in crop, generally full bloom (R2); harvest aid, 7 days PHI
Touchdown Total	Consult label, in crop, generally full bloom (R2); harvest aid, 14 days PHI
Ultra Blazer	50 days PHI
Warrant	R2
Warrant Ultra	R1
XtdendiMax (only for Roundup Ready 2 Xtend soybean)	R1
AWhen using an herbicide tank-mixture, follow the recommendations for the most restrictive label language of the product being used in the tank-mixture. BUse labeled adjuvants with postemergence application of herbicides. Refer to page 55 in 2017 Guide for Weed, Disease, and Insect Management in Nebraska.