Crop Weeds

Weed control under drought conditions

Alvin J. Bussan, Crop Weed Extension Specialist, Montana State University 

To spray or not to spray?  That is the question many producers are dealing with across Central and Western Montana.  Dry spring conditions has led to delayed emergence of weeds in winter wheat.  Weeds that have emerged are potentially drought stressed making management with herbicides difficult.  In addition, decreased yield potentials dramatically impact the bottom line leading to questions about whether or not costs for herbicides are justified. 

Drought stress leads to thicker cuticles and a reduction or shut down in weed growth.  As a result, it is more difficult for herbicides to penetrate into the plant and/or once inside for translocation to growing points and inhibition of enzyme systems to occur.  As a result, overall herbicide efficacy is reduced

In addition, many grain fields in Central MT, face limited yield potential due to dry to extremely dry soils.  On fallow ground, there was anywhere from 20 to 30” of moist soil in mid-April, which is barely enough to ensure seed head formation.  Under recrop, there was as little as 10” of moist soil in some areas.  As a result, growers are reluctant to invest resources for weed management when the crop success is tenuous.  Recognize that weeds that are present are going to use moisture and can threaten crop yields even more given current moisture limitations. 

Weed management decisions and dollars invested in herbicide should be driven by the likelihood of the winter wheat crop producing a harvestable yield. In general, the first bushel of wheat takes 4” of moisture and each inch of rain after that results in 5 bushels of wheat.  Central MT has a 70% likelihood of receiving 4” of rain during the growing season.  If soil moisture is greater than 24” (4” of stored moisture) and a uniform decent stand of winter wheat has established (minimum of 5 to 8 plants per square foot), then chances are rather good the crop will produce 20 bu/A.  If some fields or field areas have less than 24” of moist soil or the stand is absent (i.e. damage by cutworms or winter kill) than the likelihood of producing even a 20 bu/A crop is remote. Rainfall patterns can and will change quickly which impacts yield potential so be flexible in your approach to weed management.

To justify weed management practices the cost of the herbicide needs to be made up for by an increase in yield in order pay for itself.  The exception is if one is strictly managing against future weed populations.  Minimal broadleaf control can be accomplished with $2.50 (2,4-D alone) to $5 (2,4-D + SU herbicide, 2,4-D + dicamba, low rate of Bronate, or numerous other options) per acre.  More aggressive broadleaf control tank-mixes will cost $7/A (Starane or full rate of Bronate) or more.  Wild oat, Persian darnel, and downy brome management with post emergence herbicides will cost $12 to $15/A.  All herbicide applications cost upwards of $3/A.  So at a minimum, it will take 2 to 6 bushels of wheat at $3/bu to cover the cost of the herbicide and application.  What is the likelihood that you will get a 2 to 6 bu/A increase in yield if current yield potential is 15 to 20 bu/A at best.  

Winter Wheat

Winter wheat was slow to start growing through all of March and most of April due to cold conditions.  With the sudden switch to record warm temperatures in early May, winter wheat started growing and is fast approaching the joint stage (point at which the growing point emerges above the ground).  Herbicide recommendations will depend on likelihood of harvesting a crop. During this same time period summer annual broadleaf weeds were slow to emerge, so many winter wheat fields have had light populations of small broadleaf weeds.  Do these weeds justify treatment?

The low cost of some selective broadleaf herbicides for wheat will justify an application for managing low to moderate populations of annual broadleaf weeds.  With the delay in emergence of broadleaf weeds and with dry conditions, low annual broadleaf weed populations will not cause significant yield impacts so management will be strictly for preventing harvest interference and future weed problems. If heavy to severe infestations of hard to control weeds such as kochia, wild buckwheat, or Russian thistle occur then costs of more efficacious herbicides need to be carefully considered with regard to the yield benefit from weed control and potential decline in future weed populations.  If you have decided to manage weeds than spray early to improve chances of control (small weeds are generally more susceptible) and minimize water use. 

Although conditions were tough on winter wheat and summer annual broadleaf weeds, downy brome in areas has survived the winter in good condition. Fall soil applications of Fargo and other herbicide were not effective for managing downy brome.  Thin winter wheat stands will be susceptible to large yield losses from heavy downy brome infestations.  No management practices are available to rescue winter wheat crops with heavy downy brome infestations at this point in the growing season (Maverick needs to be applied in fall or early spring).  As a result, consider spot treating heavily infested downy brome areas with Roundup to prevent seed set and future problems.

Spring wheat

Spring wheat yield potential can be compromised much more by July heat and drought stress than winter wheat.  At the same time, spring wheat will be able to take advantage of late spring – early summer moisture better than winter wheat.  Broadleaf weed management recommendations are similar to winter wheat and dollars invested should be based on the crop yield potential.  A major difference is that spring wheat does not have the jump on broadleaf weeds that winter wheat did changing the competitive relationships between the crop and weeds.

Decisions regarding post emergence wild oat and Persian darnel management need to be made early to minimize water use by the grass weeds.  MSU research has documented large impacts on spring wheat tillering by early season competition from wild oat and Persian darnel.  So if managing wild oat or Persian darnel post emergence, spray early to minimize water use by weeds and maintain spring wheat reproductive tiller density.

Field scale research conducted by MSU weed scientists in 1999 demonstrated that wild oat management with post emergence herbicides resulted in net economic losses greater than if no herbicide was applied when yield potential was less than 20 bu/A and heavy infestations were present.  Under current conditions low wild oat or Persian darnel populations will not cause yield losses to justify application of wild oat herbicide.  Wheat crops will not recover from resource use by high populations of wild oat or Persian darnel and should be managed with Roundup if considerable precipitation does not occur shortly.  If wheat is taken to yield and weeds are allowed to go to seed, future management plans will have to account for the potentially increased weed populations.

Growers should consult with a crop insurance representative and local FSA office before destroying any crops.  Soil moisture will change rapidly with significant precipitation (2” by May 31) and will alter crop and weed management decisions.  Most of Central MT will need 6 inches of precipitation or more to reach 30 bushel yield potentials.  It is raining outside my window as I write this article so that could be considered a start.

Do you have a 30 bu/A wheat crop?  If so you can justify intensive weed management program.  Spray early to conserve as much soil moisture as possible.  It will take at least 6” of precipitation in many areas to reach a 30 bu/a wheat crop.

Specific herbicide recommendations:

2,4-D ester       Ester formulations are more effective for annual weed management than amine formulations especially during drought conditions.  In drought conditions, 2,4-D ester efficacy can be improved by addition of non-ionic surfactant.  Addition of ammonium sulfate does not influence efficacy as salts in water do not interact with 2,4-D ester.  MSU and NDSU research has not seen large differences in efficacy between different formulations of 2,4-D ester.  2,4-D ester is inexpensive and will control or suppress many annual broadleaf weeds.

2,4-D amine     Unlike esters, 2,4-D amine activity can be dramatically reduced by presence of salts in the spray solution.  Addition of ammonium sulfate or other water quality treatments that tie up free cations can improve 2,4-D amine efficacy.

MCPA             Same as discussion on 2,4-D    ester and amine

Dicamba           Clarity, Banvel, Banvel SGF and other dicamba products have good activity on harder to control broadleaves such as wild buckwheat, Russian thistle, and kochia (outside of dicamba resistant kochia areas such as the Eastern Triangle).  Winter wheat is quickly advancing beyond the window of application for dicamba so stage the crop carefully to prevent injury.  Addition of N (28 or 32% liquid fertilizer or ammonium sulfate) fertilizer can improve dicamba activity during drought conditions.  We typically don’t recommend addition of N-fertilizer to dicamba because under typical conditions injury is more likely.  If the weather changes to cool wet conditions do not add N fertilizer as it greatly increases potential for spring wheat injury with dicamba.

SU herbicides   Ally, Amber, Canvass, Express, Finesse, Glean, Harmony GT, Harmony Extra, Peak and have good activity on Russian thistle, wild buckwheat, and other broadleaf weeds.  During drought conditions make sure to use non-ionic surfactants to maximize activity.  SU’s provide and inexpensive way to manage heavy infestations of difficult to control weeds.

Starane             Starane and Starane pre-mixes have good activity on kochia, bedstraw and other hard to control broadleaf weeds.  Starane is relatively expensive so weigh the potential economic gain versus the cost carefully.  Less expensive products are available with better activity on Russian thistle and wild buckwheat.

Buctril              Buctril and other bromoxynil containing products are contact herbicides that generally work better during drought conditions than systemic counterparts (those listed above).  To optimize activity, apply Buctril in a minimum of 10 GPA water to weeds that are 1” in size or smaller.

Aim                  Effect of drought on Aim activity is unknown.  Kochia escapes under normal conditions are initially burnt down by Aim, then outgrow the injury.  Drought conditions will make it difficult for initially injured plants to recover.  Aim also works best under competitive conditions, but crops will not be competitive during 2001 making it difficult to predict efficacy of Aim if the current drought persists.  Increase gallonage and use non-ionic surfactant and ammonium sulfate to increase activity of Aim during drought conditions.  The effect of injury by Aim on crop yield is unknown during drought conditions.

Curtail              Curtail is typically targeted for perennial weed control.  It is extremely difficult to achieve long-term management of Canada thistle and other perennials during drought because plant growth ceases and herbicide translocation to root systems is nearly impossible.

Roundup          Drought can slow down activity of Roundup.  Instead of 4 to 7 days, Roundup may take 10 or more days to burndown plants.  Addition of ammonium sulfate to Roundup is more important when drought conditions exist.  Increase the rate to manage drought stressed weeds.

With all wild oat herbicides avoid tank-mixing with broadleaf herbicides if possible.(Exceptions are presented).  Antagonism is much more common when trying to manage drought stressed annual grasses.  Due to the relative cost of wild oat herbicides it is unlikely current year reductions in yield loss will cover the cost of wild oat management.As a result, look to spot spray wild oat herbicides, or use Roundup as a cheaperspotspray alternative to manage the most heavily infested areas.

Achieve            Achieve does not work well under drought conditions.   Delay application until after rainfall to improve activity.  Make sure to add ammonium sulfate to the Achieve spray solution.  Increasing gallonage to 15 or 20 GPA can improve Achieve activity.  Achieve can be tank-mixed with MCPA ester or Bronate (activity may actually be increased).

Assert              Assert works well under dry conditions but the plants must be actively growing at time of application.  Apply Assert prior to tillering to obtain best results.

Discover           Discover has worked well over a number of environmental conditions.  Do not apply at less than 3.2 oz/a during drought conditions as unsatisfactory results will occur.

Everest             Everest also works well under dry conditions.  Failing to tank-mix Everest with 2,4-D or MCPA will result in severe crop injury.  Everest works best with some precipitation shortly after application to allow for root and shoot uptake.

Puma                Puma also maintained activity on wild oat during drought conditions in MSU research trials.  Puma should provide good control to wild oat with decent crop safety in Barley under dry conditions.  Drought stressed barley may still be injured by Puma.  

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