Burndown and Residual Herbicide Issues

 

Source:  Mark Loux, OSU

Depending upon where you are in the state, it’s possible right now to be experiencing delays in getting anything done, progress in planting but delays in herbicide application, weather too dry to activate residual herbicides, and/or reduced burndown herbicide effectiveness on big weeds due to cold weather.  What’s become a typical Ohio spring.  Some information relative to questions that OSU Extension educators have passed on to us:

1.  Residual herbicides and rainfall.  Residual herbicides do vary in the relative amounts of rain needed for “activation”, or adequate movement into the soil to reach germinating seeds.  Most growers are applying mixtures or premixes of several products, so we’re not sure these differences are as important as the overriding principle here.  Residual herbicide treatments need to receive a half to one inch of rain within a week or so after tillage or an effective burndown treatment, to control weeds that can will start to emerge at that time.  This varies with timing of application and weather.  Summer annual weeds are the target here, and their emergence ramps up in early May, although cold weather can slow this down.  So residual herbicides applied in mid-April, prior to most of the summer annual weed emergence, may not need rain as soon after application, compared with herbicides applied in May.  Aside from this, residual herbicide activity is not really dependent upon soil reaching a certain temperature.  Under more marginal rainfall conditions, it’s possible that herbicides may control the small-seeded weeds that emerge at or just below the soil surface, but be less effective on larger-seeded weeds that can emerge from deeper.  In a tilled situation, a timely rotary hoe can be used to remove some of the weeds that are about to emerge (the “white stage”) and buy some time for rain.  The good news here is that we have effective POST herbicides to remedy many situations where the residual herbicides are not completely effective.

2.  Residual herbicides and crop injury. The concerns here seem to be more about soybean herbicides, which may partly reflect the overall greater safety of residual corn herbicides.  Several residual soybean herbicides can cause injury, depending upon when they are applied relative to planting, rainfall, soil type, seeding depth, etc.  These include products that contain metribuzin, sulfentrazone, flumioxazin, and chlorimuron.  One of the things that has reduced our risk of injury from all of these herbicides is that in no-till soybeans they have usually been applied a week or more prior to planting to accommodate restrictions on 2,4-D ester and dicamba.  Application at or after planting increases the risk of injury, as does use in tilled situations.  We have increased metribuzin use substantially over the past decade, but injury has been extremely rare due to application prior to planting and use of relatively low rates in combination with other products.  We hear more about injury or suspected injury from flumioxazin and sulfentrazone when wet weather delays planting and forces application of residual herbicides after planting.  It’s worth noting here also that the Xtend and Enlist soybean systems do away with the wait to plant soybeans for dicamba and 2,4-D, respectively, and more growers may be waiting until after planting to apply burndown/residual herbicides.

In brief, symptoms of these are as follows:  chlorimuron – slowed development, stunting, yellowing; flumioxazin and sulfentrazone – necrosis on young leaves and stem, stunting; metribuzin – usually delayed until first trifoliate, yellowing and possibly necrosis on margins of older leaves.  Cloransulam, imazethapyr, and imazaquin are generally safer on soybeans than chlorimuron, in situations where injury is a concern.  Activity of metribuzin varies considerably with soil texture and organic matter content, so using the labeled rate for soil type is important.  Injury from any of these may be more likely when herbicide application is delayed for several days after planting, followed by substantial rain as the soybeans are about to emerge.  Labels for products containing flumioxazin state that soybeans should be planted 1 ½ inches deep and herbicide should be applied no later than three days after planting, in an attempt to avoid this situation (does not always work).  The good news here is that early injury to soybeans usually does not reduce stand, but may slow early growth and rate of crop canopy development and leave soybeans open to the effect of other stresses.  In some of these situations, it can be difficult to sort out how much of the damage is due to herbicide and how much is due to other factors.  Yield loss is probably infrequent based on the soybean plant’s ability to compensate for these types of factors.

3. Cold weather and burndown herbicides.  We had a fairly warm winter and early spring, followed by the recent month of colder than normal weather.  The net result of this is large winter annual weeds, and weather that is currently not terribly conducive for burndown activity.  There is not much specific guidance on herbicide labels about cold weather, just general statements about how effectiveness can be reduced under adverse conditions that include cold weather.  We expect many experienced applicators may have their own set of rough guidelines on this, or at least gut feelings.  Under cold conditions, the rate of herbicide activity declines and also the overall effectiveness.  It’s more difficult to define the weather conditions when herbicide should not be applied.  These would certainly include periods when frost or freeze is occurring overnight and daytime weather is cool and cloudy (less than about 50).  One night of frost followed by a warm sunny day may still allow for decent herbicide activity, if weeds appear sufficiently recovered from the frost.  Aside from this we could make a general recommendation to keep applying as long as night and day temperatures are at least 40 and 60 to 70, respectively, although this is still not ideal compared with day temperatures higher than 70 with sun.  One way of dealing with this problem is to just wait for a return to warm, sunny weather before applying burndown herbicides.  Another is to increase herbicide rates and use a more comprehensive herbicide mixture.  For example, adding Sharpen to a mixture of glyphosate plus 2,4-D or dicamba.  As with the less than effective residual herbicides under dry weather, burndown herbicide problems can sometimes be resolved with an effective POST treatment of glyphosate, 2,4-D, or dicamba, depending upon the trait system.

4. Reminder about the value of fall herbicides.  Fall herbicides are an essential tool for marestail management, but given our current situation of dense, big weeds in no-till fields and potential problems with burndown herbicide effectiveness, it’s worth reminding all of us why fall herbicides started being used in the first place.  In the late 1990’s, growers were experiencing problems with dense stands of winter annual weeds such as chickweed that interfered with tillage and planting.  One contributor to this was the occasional reduced activity of spring-applied burndown herbicides in cool weather, which resulted in too slow death and dry down of weeds to prevent the problems the weeds caused.  Fall-applied herbicides became a solution to this, since they result in almost weedfree spring seedbeds up until the point when giant ragweed and other summer annuals emerge (early May for most of these).  As anyone knows who has used fall herbicides, their effectiveness reduces the overall importance of the spring-applied burndown, since it does not have to control a mess of large, overwintered weeds.  It’s all just way easier.    And issues with cold weather and spring-applied burndown herbicides are therefore less important.  For as little as $6 worth of fall-applied herbicide.  Something to think about moving forward.

Spraying with Drones

By John Fulton (Associate Professor), Chris Wiegman (graduate student), Erdal Ozkan ( Professor), and Scott Shearer (Professor), Ohio State University Department of Food, Agricultural and Biological Engineering

Drones or Unmanned Aircraft Systems (UAS) have become a common technology in agriculture. As of early 2019, there were around 1.3 million registered drones in the U.S. and over 116,000 registered drone operators within the commercial sector. Within agriculture, drones have been mainly used for scouting purposes. Today, uses of drones include collecting remotely sensed imagery, tissues samples, and water samples. Spraying with drones is also available through some manufacturers.

Drone spraying has been used Southeast Asian countries such as China, Japan and South Korea for several decades. In fact, the use of this type of spraying in Japan can be traced back to the 90’s. Currently, we are seeing a significant increase in the number of drones used in these countries, mostly in rice production that requires applications done when the field is flooded with water, making entry of motorized vehicle to the field impractical. Drone spraying has also been considered as the most effective and safe way to treat crops grown in steep hills.

Drone spraying is becoming increasingly available for specialty crops and row-crop production. Here is the U.S., drone spraying was approved in 2015, but under strict policies in the state of California. The Yamaha RMAX from Japan was the first drone sprayer tested in California prior to approval. Most recently, drone manufacturers such as DJI (https://www.dji.com/) have started offering high payload rotor drones that include sprayers. Spray applications using drones has arrived in Ohio as well.

Spraying with drones is a unique practice since it is conducted autonomously. Drone sprayers are equipped with almost all the parts of any other sprayer: a tank, a pump to push liquid through the hoses to the nozzles, filters and a pressure gauge. But there are limitations, mostly on the size of these components because of the power required to keep the drone sprayer in flight mode for a reasonable time.

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Omitting residual herbicides in soybeans – really – we have to have this argument again?

Source: Dr Mark Loux, OSU Extension

According to our network of sources, the effectiveness of new soybean trait systems has some growers once again thinking about omitting preemergence residual herbicides from their weed management programs.  Some people apparently need to learn the same lessons over and over again.  Having gone through this once in the early 2000’s when Roundup Ready soybeans had taken over and we all sprayed only glyphosate all day every day, we think we’re pretty sure where it leads.  We’re sensitive to concerns about the cost of production, but the cost-benefit analysis for residual herbicides is way in the positive column.  We’re not the ones who ultimately have to convince growers to keep using residual herbicides, and we respect those of you who do have to fight this battle.  Back in the first round of this when we were advocating for use of residuals, while the developers of RR soybeans were undermining us and telling everyone that residuals would reduce yield etc, we used to have people tell us “My agronomist/salesman is recommending that I use residuals, but I think he/she is just trying to get more money out of me”.  Our response at that time of course was “no pretty sure he/she is just trying save your **** and make sure you control your weeds so that your whole farm isn’t one big infestation of glyphosate-resistant marestail.”  And that answer probably works today too – maybe substituting waterhemp for marestail.

We need to state here that a good number of growers kept residual herbicides in their programs through all of this, and we assume they aren’t tempted to omit them now either.  For everyone else – maybe interventions are called for.  Where the recalcitrant person is repeatedly thumped with a stick while being reminded of what happened last time, until they change their minds.

Weed scientist:  so you’re going to use residual herbicides right?

Soybean grower:  no

Thump

WS:  remember what happened last time – lambsquarters became a problem when every residual herbicide would have controlled it.  Change your mind yet?

SB:  no

Thump

WS:  remember when the weather didn’t cooperate and you ended up spraying 2 foot tall weeds because of no initial control?  Do you want this again?

SG:  no

WS:  so you’re going to use residuals?

SG:  not sure

Thump

WS:  and you expect your local dealer to clean up whatever mess occurs when you don’t use residuals?

SG: yes

Thump

WS:  remember when you burnt out the FirstRate on marestail and then the glyphosate wouldn’t work?  Do you want this to happen with dicamba, 2,4-D and glufosinate?”

SG:  no

WS:  well then

SG:  maybe

Gentler persuasive tap

WS:  You know how bad a weed waterhemp is right?

SG:  yes

WS:  what if residuals will help prevent waterhemp infestations

SG:  Ok then – yes

WS:  ok then

Note:  we considered a number of sound effects here – thump, zap, whack…. Thump won out for no particular reason.  We could not decide whether getting hit by a stick was more or less acceptable than getting shocked in this context.

The bottom line is that residual herbicides provide both short- and long-term risk management in weed management for a relatively low cost.  A non-inclusive list of these:

– reduces weed populations overall and slows weed growth, resulting in more flexibility in the POST application window.

– Reduced risk of yield loss if weather interferes with timely POST application.  In the absence of residual herbicides, soybean yield loss can occur when weeds reach a height of 6 inches.

– increases the number of different sites of action used within a season, slowing the rate of resistance development

– reduces the number of weeds that are treated by POST herbicides, which also slows the rate of herbicide resistance development

– residuals control lambsquarters which is not well-controlled by POST herbicides

– the most significant weed problems in Ohio soybean production – waterhemp, giant ragweed, and marestail – cannot be consistently controlled with POST herbicides alone.  They require a comprehensive herbicide program that includes residual and POST herbicides.  It may be possible to make a total POST system work some years or for a while, but in the end this approach will result in problems with control and speed up the development of resistance.

This whole subject of omitting residual herbicides makes us cranky because we don’t have to guess what will happen.  We’ve made our best case here.  It’s up to you of course, but we suggest that we not have to come back and have this discussion again.  Because next time we’re bringing a few friends, a bigger stick, and a gorilla.

Disclaimer:  Parts of this article are meant in pure jest.  We would certainly never advocate in earnest the use of physical harm or other methods of persuasion to change the behavior of herbicide users.  This goes against everything that the discipline of weed science stands for, and also OSU.  Plus – we don’t even know where to rent a gorilla.

 

Fall Herbicide Treatments – Even More Important This Year?

Source: Dr. Mark Loux, OSU

If you have never applied herbicide in fall to burn down winter annuals, or done it only infrequently, this might be the year to make an investment in fall herbicides.  Fall treatments are an integral component of marestail management programs.  They also prevent problems with dense mats of winter annuals in the spring, which can prevent soil from drying out and warming up, interfere with tillage and planting, and harbor insects and soybean cyst nematode.  2019 was a generally tough year for weed control, leading to higher end of season weed populations in some fields.  A number of acres were never planted, and growers got to experience the difficulty in obtaining season-long control in the absence of a crop.  Reminds us all how important the crop canopy and shading of the soil is during the second half of the season.  Bottom line – there was substantial production of weed seed in some fields, and a replenishment of the soil seedbank by both winter annual and summer annual weeds.  The seed of winter annuals and marestail lacks dormancy so above-average weed seed production can lead to an immediate increase in fall-emerging weeds.  Applying herbicides this fall can compensate for increased weed populations and make life easier in the spring.

We have published information on fall herbicides fairly frequently, and our suggestions for fall treatments have not really changed much.  There is plenty of information on fall herbicide treatments in the C.O.R.N. newsletter archive and on other university websites.  Our philosophy on this has not changed much over the past decade.  A few brief reminders follow:

1.  When to spray?  Anytime between now and Thanksgiving will work, and possibly later.  We have applied into late December and still eventually controlled the weeds present at time of application.  Once hard freezes start to occur, there is usually a substantial change in the condition of certain weeds, such as dandelion and thistle, that renders them less sensitive to herbicides.  We discourage applications during periods of very cold weather which can occur starting about Thanksgiving, and also (obviously) when the ground is snow-covered.  The generally dry conditions we are experiencing have limited weed emergence so far this fall.  We anticipate that rain occurring now that leads to some sustained soil moisture near the surface will likely result in germination and emergence of the weeds that have been missing until now.  Our recommendation is to wait for rain and the additional weed emergence before applying any herbicide this fall.  The risk in this is that the weather turns wet, making it difficult to apply herbicide.  So it’s also possible to apply now and include a residual component to help with later fall emergence (which is the exception to the “no residual” recommendation in #4 below), such as simazine, a low rate of metribuzin or Canopy, or a Sharpen rate higher than 1 oz.

2.  What about all of the crop residue on the ground after harvest – won’t that cause problems?  We have not worried about this, and the herbicides seem to work regardless.  Most agronomists I have asked have the same impression.  On the other hand, it probably wouldn’t hurt to wait a while after harvest to let the residue settle down, and the weeds to poke through.  Dense crop residue usually prevents marestail from emerging anyway. Continue reading