Residual Herbicides: Precipitation Requirements For Activation And The Likelihood To Receive It

Planting season is getting fully underway across the state, and with that we should all be getting our residual herbicides applied. Residuals are critical to successful season-long weed control as we have less documented herbicide resistance to these chemistries compared to postemergence products, as well as generally we see more consistent control from these herbicides because we’re targeting weeds before they’re even out of the ground. However, for residual herbicides to be successful, they require precipitation to be activated. I frequently get asked how much rainfall is required to fully activate these herbicides, how long can the herbicide wait until we’d receive this rainfall, and if a shallow tillage event would help the situation. These answers can be highly variable across herbicides (Tables 1 and 2), as they are normally dependent on water solubility and soil adsorption of the active ingredients; however, there are some general estimates that can be made.

  1. Generally, 0.5” of rainfall is required for many of our residual herbicides to be fully activated. Some herbicide formulations, such as encapsulations, will often require greater amounts of rainfall than non-encapsulated counterparts for activation.
  2. Most labels indicate activating rainfall should occur within 7 to 10 days following the application. In most situations, this is not necessarily due to the herbicide breaking down within this timeframe, but rather it is expected that if no activation has occurred within 7 to 10 days, weeds have already emerged and will not be controlled from a residual herbicide.
  3. Shallow (<2”) cultivation is often recommended on many labels between 7 to 14 days after application if an activating rainfall did not occur; however, in few situations does this truly aid activation of the residual herbicide. Rather the shallow cultivation is typically recommended to control emerged weeds and hopefully buy more time for an activating rainfall for the residual herbicide.

In addition to understanding these activation concepts, it is also important to have an idea of our chances at getting these residual herbicides activated. Figures 1 and 2 present the likelihood of receiving 0.5” of rainfall within 7 days for each calendar day within the months of April, May, June, and July, based on 20 years of weather data (2004-2024) from the Throckmorton Purdue Agricultural Center (TPAC). Similarly, Figures 3 and 4 present the same information for the Southeast Purdue Agricultural Center (SEPAC). Overall, as the summer progresses, our likelihood of receiving an activating rainfall within a week of application decreases significantly. This may be an indication that earlier planting dates, at least within April, could aid our weed management by providing a greater likelihood of activating residual herbicides both in our preemergence application and in an overlapping residual pass.

Based on the historical data, SEPAC and southern Indiana has a greater likelihood of receiving an activating rainfall with monthly averages above 64% for April, May, and June. In contrast, TPAC and central-northern Indiana never exceeded a 63% likelihood of activation even in April. Even more interestingly at TPAC, in the second half of May (blue bar and arrow, Fig. 1) and beginning half of June (yellow bar and arrow, Fig. 2), the likelihood of receiving 0.5” of rainfall within a week drops at or below 50%. This means that for an entire month (mid-May to mid-June), it is a coin toss on whether we can receive an activating rainfall for our residual herbicides.

With these reduced likelihoods for activation, it can be difficult to effectively use our residual herbicides. But here are a few recommendations to give us our best shot at activation.

  1. April planting dates will increase our chances for an activating rainfall, particularly if we can make an overlapping residual application at the beginning of May.
  2. If you have pivot irrigation, use that to your advantage to activate residual herbicides following an application.
  3. Pay close attention to the weather forecasts. Although forecasts often shift and change on us, if we can try to target an application close to projected rainfall events, we may just get lucky.
  4. Get selective in the order that you might apply your residuals. What I mean by this, since some herbicides require less moisture for activation, these herbicides may be candidates for your overlapping residual pass later in the growing season when the likelihood of receiving a greater rainfall event decreases. For example, since S-metolachlor requires more than 0.5” particularly on medium to fine soils, but pyroxasulfone only requires 0.5” at a maximum, it may be wise to apply S-metolachlor earlier in the season (greater chance at receiving a higher activating rainfall) and overlapping with pyroxasulfone.

Overall, residuals continue to be a powerful tool in our weed management toolbox. If we put the little things together mentioned above, we can hopefully maximize our use of this tool. And who knows? Maybe this will be the year that we all receive the perfect amount of rainfall at just the right time for success. We can hope, right? With that, please let us know if we can help, and good luck out there!

Table 1. List of commonly used soybean herbicides with the precipitation required for activation, days in which the precipitation needs to occur, and whether cultivation can be used for activation according to their respective labels.

Herbicide (active ingredient) Herbicide (Trade name) Precipitation Needed for Activation Days Required for Activation Is cultivation recommended?
chlorimuron Classic 0.5″ – 1″ for wet soil and 1″ – 2″ for dry soil Not specified. Only if herbicide is not activated. Do not cultivate within 7 days of application.
cloransulam FirstRate 0.5″ Not specified. Shallow incorporation can be used preplant  (1-3″).
dimethenamid Outlook 0.5″ – 1″ Within 10 days. Shallow (<2″) incorporation can be used.
flumetsulam Python 0.5″ – 1″ Within 10 days. Preplant incorporation at 2-3″. Shallow incorporation can be used if herbicide is not activated.
flumioxazin Valor > 0.25″ Not specified. Not recommended.
fomesafen Flexstar > 0.25″ Not specified. Not specified.
imazethapyr Pursuit Not specified. Within 7 days. Shallow (<2″) incorporation can be used if the herbicide is not activated.
metribuzin Tricor 0.25″ – 0.5″ Within 10 days. Not recommended.
pendimethalin Prowl H2O Not specified. Not specified. Recommended.
pyroxasulfone Zidua 0.5″ Not specified. Shallow (<2″) incorporation can be used.
saflufenacil Sharpen 0.5″ Before weed seedling emergence. Shallow incorporation (<2″) can be used.
sulfentrazone Spartan 0.5″ – 1″ Within 10 days. Shallow (<2″) incorporation can be used.
S-metolachlor Dual Magnum 0.5″ – 1″ Within 10 days. Shallow (<2″) incorporation can be used.

 

Table 2. List of commonly used corn herbicides with the precipitation required for activation, days in which the precipitation needs to occur, and whether cultivation can be used for activation according to their respective labels.

Herbicide (active ingredient) Herbicide (Trade name) Precipitation Needed for Activation Days Required for Activation Is cultivation recommended?
acetochlor Harness 0.25″ to 0.75″ Within 7 days. Shallow (1″ to 2″) incorporation within 14 days.
acetochlor (encapsulated) Warrant 0.5″ to 0.75″ Not specified. Not recommended.
atrazine Aatrex Not specified. Within 10 days. Shallow (not specified) incorporation within 14 days.
flufenacet Flufenacet 4SC Not specified. 7 to 10 days. Shallow (<2″) incorporation can be used.
halosulfuron Permit 0.25″ to 0.5″ 7 to 14 days. Not recommended.
isoxaflutole Balance Flexx Not specified. Within 14 days. Shallow (<2″) incorporation can be used.
mesotrione Callisto 0.25″ 7 to 10 days. Rotary hoeing is suggested to activate product 7 to 10 days after application if no rainfall is received.
simazine Princep Not specified. Not specified. Shallow cultivation suggested especially under relatively dry conditions and if weeds emerge.
rimsulfuron + others Steadfast Q/Realm Q 0.5″ 5 to 7 days. Cultivation recommended 7 to 14 days after application to control emerged weeds.
saflufenacil (encapsulated) + pyroxasulfone Surtain 0.5″ Before weed seedling emergence. Shallow (1″ to 2″) incorporation can be used.
thiencarbazone-methyl + tembotrione Capreno Not specified. 7 to 14 days. Not recommended.

 

Fig. 1. Probability of receiving 0.5” of rainfall within 7 days for April (top) and May (bottom) from 20 years of historical weather data at the Throckmorton-PAC located outside of Lafayette, IN.

Fig. 1. Probability of receiving 0.5” of rainfall within 7 days for April (top) and May (bottom) from 20 years of historical weather data at the Throckmorton-PAC located outside of Lafayette, IN.

 

Fig. 2. Probability of receiving 0.5” of rainfall within 7 days for June (top) and July (bottom) from 20 years of historical weather data at the Throckmorton-PAC located outside of Lafayette, IN.

Fig. 2. Probability of receiving 0.5” of rainfall within 7 days for June (top) and July (bottom) from 20 years of historical weather data at the Throckmorton-PAC located outside of Lafayette, IN.

 

Fig. 3. Probability of receiving 0.5” of rainfall within 7 days for April (top) and May (bottom) from 20 years of historical weather data at the Southeast-PAC located outside of Butlerville, IN.

Fig. 3. Probability of receiving 0.5” of rainfall within 7 days for April (top) and May (bottom) from 20 years of historical weather data at the Southeast-PAC located outside of Butlerville, IN.

 

Fig. 4. Probability of receiving 0.5” of rainfall within 7 days for June (top) and July (bottom) from 20 years of historical weather data at the Southeast-PAC located outside of Butlerville, IN.

Fig. 4. Probability of receiving 0.5” of rainfall within 7 days for June (top) and July (bottom) from 20 years of historical weather data at the Southeast-PAC located outside of Butlerville, IN.

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