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Are Herbicides Still Effective After a Fall Frost?

Fall is a great time to control perennials such as Canada thistle, perennial sow thistle and dandelion. As temperatures cool, nutrients move from the leaves down into the roots and if a herbicide can be taken up and translocated with those nutrients, it can equal better control. Glyphosate and/or the group 4s are examples of herbicides that provide effective fall control of perennials.

A frost event though, can kill some weeds or can damage leaf tissue which will reduce herbicide uptake and reduce the level of weed control. Within the next few days after the frost, you need to assess the target weeds in the areas that you want to obtain control – are the weeds still growing?  How much leaf tissue has been damaged?

Light frost: A light frost (0 to -3°C) can actually improve weed control by increasing herbicide translocation to the root.  However, duration of the frost also plays a role.  Check your weeds for frost damage if you plan on a herbicide application after a light frost.  Herbicides can only be taken up and translocated by weeds that are healthy and actively growing.

If you do spray – spray in the afternoon when temperatures are warm and sunny, as this will help with herbicide uptake.   You’re looking for daytime temperatures of ~8 to 10°C for at least 2 hours. Use rates appropriate to the stage and time of year – fall applications of glyphosate are recommended at a higher rate than when controlling weeds pre-harvest.

Hard frost: Depending on the damage, a hard frost (≤ -5°C) can put an end to (effective) post-harvest weed control. However, if the plant leaves are still shiny green with minimal leaf tissue damage (i.e. not blackened/brown or brittle) or if less than 40% of the plant has more serious leaf tissue damage (i.e. blackened/brown or brittle) there may still be a window to make a herbicide application. Wait at least 48 hours before assessing frost damage after a hard frost.

If you do spray – read the ‘If you spray’ paragraph above, it still applies.  You need those daytime temperatures to hit ~8 to 10°C for at least 2 hours. In addition, consider your coverage – higher water volumes may improve uptake in more heavily damaged weeds.

One last thing – look at the forecast for the next week following the application.  If daytime temperatures are below 8°C and/or if night-time temperatures are forecasted to continually be below freezing, it may be too late to make the application to get the economic control you are looking for.

Submitted by: Jeanette Gaultier, Provincial Weed Specialist, Manitoba Agriculture

Visit Manitoba Agriculture Crops webpage for more current topics: www.gov.mb.ca/agriculture/crops/seasonal-reports/current-crop-topics.html#agronomy; or the Manitoba Agriculture Weeds webpage for more information on fall control of dandelion and quackgrass: www.gov.mb.ca/agriculture/crops/weeds/.

 

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ANNUAL OR WINTER ANNUAL?

Fall is the most effective time to manage certain weed species. The recommendations seem clear-cut:  winter annuals = fall herbicide application/tillage; annuals = no fall management.  But figuring out the life cycle of the weeds in your field this fall is the catch……

Bromes, cleavers, chickweed, night-flowering catchfly, narrow-leaved hawk’s-beard, shepherd’s-purse and stinkweed are all facultative winter annuals, meaning that they can germinate in either the fall or the spring depending on environmental conditions.  These weeds are often best managed in the fall, if populations warrant it.  In general, waiting until about this time of year maximizes fall-germinating flushes of winter annuals.  If using a herbicide, consider weed stage and the weather forecast, prior to application.

The problem is, given the right conditions – like the long falls and mild winters we’ve had the last few years –several of our annual weed species can also successfully overwinter:

Biennial wormwood – Despite its name, biennial wormwood behaves like an annual in agricultural fields.  When scouting, estimate the average growth stage of biennial wormwood populations in a field.  If the majority of the plants have already set seed, a fall herbicide application won’t help.  An application may be worthwhile only if there is a large flush of biennial wormwood that haven’t set seed and are less than ~3 inches tall.  Herbicide tank-mixes containing glyphosate + group 4 are more effective than glyphosate + group 2 on this weed.

Round-leaved mallow (RLM) – This annual weed can act as either a winter annual or a short-lived perennial, although it is more sensitive to freezing than our common winter annuals.  Mild winters in 2015 & 2016 provided the right conditions for RLM to overwinter, allowing it to become (even more) problematic in certain fields over the last few growing seasons.  Long range forecasters are predicting a harsh winter across the prairies this year, which should control RLM.  However, if you have little faith in forecasts and decide to apply a herbicide, glyphosate mixed with either Distinct or DyVel DSp has activity on this weed.

IMG_20150915_081215

Round-leaved mallow post-harvest


Stork’s bill –
Like biennial wormwood, stork’s bill tends to be predominantly an annual in Manitoba.  If this is a problem weed for you, scout affected fields to determine average weed stage.  Again, if most of your stork’s bill has set seed you’re better off working on a plan for next year.  Stork’s bill, especially larger plants, is relatively tolerant of many herbicides.  If you decide to apply a herbicide because of stork’s bill this fall, glyphosate + group 2 or glyphosate + group 2 + group 4 on weeds up to the 4 to 6 leaf stage is probably your best bet.

Submitted by: Jeanette Gaultier, Provincial Weed Specialist, Manitoba Agriculture

Information on more weeds and their life cycles is available at: www.gov.mb.ca/agriculture/crops/weeds/
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HOW DO I CONTROL VOLUNTEER CANOLA IN MY SOYBEAN?

First ask yourself if you need to control the volunteer canola in your crop.  Research by Dr. Rob Gulden and graduate student Paul Gregoire at the University of Manitoba (U of M) showed that volunteer canola had little impact on soybean yield when there are less than:

  • 3 plants/m2 in solid seeded or narrow row soybean, or
  • 1.5 plants/m2 in wide row soybean.

Although economic thresholds (ET) such as these don’t consider seed return, this is generally not a concern for canola given it’s prevalence in our crop rotations.

If your volunteer canola populations exceed the ET, the U of M researchers also assessed the effectiveness of various post-emergent herbicides (Table 1).  Control of volunteer canola by the herbicides listed in table 1 are based on comparisons of treated research plots.  It’s unlikely that any of these options will provide full control of bolting or flowering volunteer canola.

Table 1: Ranking and application timing of volunteer canola herbicides in soybean

Vol Canola Control in Soybeans

*Will not control CLEARFIELD canola volunteers

**Registered in the Red River Valley only

Another consideration: use of these herbicides on larger volunteer canola may only set plants back, resulting in later flowering canola that may cause issues during soybean harvest.

Previous research by Dr. Gulden has shown that one of the best ways to manage volunteer canola is by limiting weed seedbank additions from canola harvest losses. Slower combine speeds while harvesting this year’s canola is a good way to reduce volunteer canola populations in future soybean stands.

IMG_20160712_070736

Submitted by Dr. Jeanette Gaultier, Weed Specialist, Manitoba Agriculture

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GOT JIMSONWEED?

According to reports from across the Prairies, you’re not alone. In Manitoba, the weed has been found in fields from Swan River down through the south-central region.

Jimsonweed (Datura stramonium) is an annual broadleaf with white/off-white trumpet-like flowers on stalks that can grow over 5 feet tall. It reproduces by seed that is contained in spiny pods (see Figure 1). This weed isn’t new to the Province but it’s only occasionally a problem. Although seed contamination is being considered, it’s just as likely that warm temperatures and less competitive crop stands contributed to increased germination and establishment of existing weed seeds.

Jimsonweed seed pod

Jimsonweed seed pod

Recommendations for Control:

Crop Rotation: Jimsonweed is not overly vigorous, so rotating to competitive crops like cereals or canola will help with control. The weed tends to be a bigger issue in less competitive and later season crops, including corn and soybean.

Herbicides: Although Jimsonweed doesn’t show up on many product labels there are lots of pre- and post-emergent herbicides that should provide control including glyphosate, glufosinate and various group 2’s, 4’s 6’s, 14’s and 27’s (www.msu.edu/~zandstra/Weed-Chem/jimsonweed-Chem.htm**; 0= no activity, 1= good to excellent, 2= fair to good). **Only use labelled applications of herbicides registered by Health Canada.

Application timing: Like most weeds, herbicides are most effective on Jimsonweed when it’s small. Because it’s a later germinating weed, it might be ‘missed’ by early herbicide applications but be too large for some later herbicide applications.

Rouging or cutting: Hand-pulling or cutting Jimsonweed before it sets seed is also an option, depending on the number of plants and their distribution in the field. Seed growers may need to put this option into play this fall since Jimsonweed is a prohibited noxious weed seed under the Weed Seeds Order (http://laws-lois.justice.gc.ca/eng/regulations/SOR-2005-220/page-2.html#h-4). Jimsonweed seed can be similar in size to canola.

Submitted by: Jeanette Gaultier, Provincial Weed Specialist, MAFRD

Visit MAFRD Crop webpage for more current topics: www.gov.mb.ca/agriculture/crops/seasonal-reports/current-crop-topics.html#agronomy

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Importance of Preharvest Glyphosate Timing

Modified from Post Originally Published July 30, 2014

Staging a crop for preharvest glyphosate application for weed control can be difficult when there is variability of crop staging within the targeted field.  To go back to the basics, for wheat you want to apply the preharvest glyphosate when grain moisture of the wheat crop is less than 30%.  In terms of visual assessment, the wheat crop must be in the hard dough stage.  This is when the kernel has become firm and hard and a thumbnail impression remains on the seed (see Figure 1).  Remember….you can’t rely on the color of the field as an indicator.  Walk the field and hand thresh heads to determine kernel staging.

Figure 1:  Kernels at various times during grain filling: a) kernel at watery ripe, b) kernel at late milk, c) kernel at soft dough, d) kernel at hard dough showing loss of green color, and e) kernel ripe for harvest.

Source:  Growth and development guide for spring wheat. 1995.  S.R. Simmons, E.A. Oekle & P.M. Anderson.  Photographer:  Dave Hansen.

So what is special about this 30% moisture content?  At the end of the hard dough stage, the kernel has reached its maximum dry weight and the wheat is therefore physiologically mature, i.e. no more weight is added to the grain.  Therefore, final yield has been determined.

If application of a preharvest glyphosate occurs prior to the 30% moisture content, yield can be reduced, along with quality factors such as kernel weight, test weight and protein.  In addition, early application prior to the recommended timing may result in grain with glyphosate levels above maximum residue limits.  This could have implications depending upon target market.

So in timing an application on a variable field, this will be difficult but remember its likely better to apply when majority of the field is at the recommended timing, or even on the later side, than too early.  Also remember that depending on weather conditions, glyphosate can take up to 2 weeks for optimal weed control. However, under hot, dry conditions harvest could commence is as little as 7 days after application.  So keep harvest timing and weather forecasts in mind as well when planning your preharvest application.

Notes: Do not apply to wheat, or any crops, grown for seed.  Not all glyphosate products are registered for preharvest application on all crop species – always refer to individual crop labels for a list of registered uses and crop species. Check with malt barley or milling oat buyers prior to application to confirm acceptance of glyphosate-treated grain.

Submitted by:  Pam de Rocquigny, Provincial Cereal Crops Specialist & Ingrid Kristjanson, MAFRD Farm Production Advisor, Morris, MAFRD

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Tips for Cool Weather Spraying

In a long spring with periods of prolonged cool then flashes of hot weather, you could be combating advanced stage cool season weeds with outbursts of warm season weeds emerged as well. As seeding and spraying are colliding, here are a few tips to get the best performance out of your herbicides:

  • Spray when weeds are actively growing
  • Spray when the temperature is around maximum (15-25 0C) for the day
  • Apply herbicides when target weeds are most susceptible, timing is more important than application method
  • Use the water volume stated on the label
  • Use the surfactant or adjuvant included or recommended
  • Check the label and abide by rain free period
  • To improve control add at least one other tank mix partner for better results
  • Spray when wind is steady about 3–15 km/hr
  • Do not spray when temperature inversion conditions exist

Some of the herbicides that work well under cool conditions are:

  • Most of Group 4 herbicides
  • Most of the residual herbicides of any group
  • “Fop” group 1 herbicides (e.g. Horizon)
  • Carfentrazone (e.g. CleanStart, Aim)

Some of the herbicides that work well under relatively warmer conditions are:

  • Group 9 herbicide (e.g. Glyphosate)
  • Group 10 herbicides (e.g. Liberty (works better when it’s bright and sunny))
  • “Dim” group 1 herbicides (e.g. Select, Centurion)
  • Non-residual group 2 herbicides
  • Contact herbicides (e.g. Bromoxynil, Sencor, Basagran, Flexstar, Reflex, etc.)

What about morning dew? Dew may increase absorption and weed control by hydrating leaf cuticle, but may reduce weed control if there is too much and spray run-off occurs. If dewpoint is reached, it may be better to spray later in the morning and during day when the dew drying and leaf surface not so wet.

Frost? Do not spray if a frost is expected in the next 48-72 hours, also after frost has occurred, wait for 48-72 hours before resuming spraying.

Submitted by Nasir Shaikh, MAFRD Weeds Specialist

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Spray corn now – or fertilize more

Early season weeds in corn not only compete and reduce yield, but mean more nitrogen fertilizer is required to maintain good yields.  Many farmers recognize that today’s herbicides can provide excellent control – and some can control weeds at advanced stages. The tendency may be to delay applications so one application of herbicide gets all the weeds. But studies in Wisconsin and demonstrations in Manitoba show that even though weed control may be 100% in such fields, the nitrogen efficiency is much reduced.  Wisconsin studies showed that compared to weed-free fields, when control was delayed until weeds were 4”  and 12” tall, an extra 20 to 60 and 60 to 160 lb N/ac respectively was required for full yield.  So to optimize nitrogen efficiency in corn, control weeds early.

Submitted by:  John Heard, Crop Nutrition Specialist, MAFRD

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Caution with Weed Control in Cool Weather

Every year comes with its own challenges. When the challenges are cool/wet conditions and weeds, it’s important to understand weed biology, impact on crops and match the control measures accordingly.

Weed Emergence Timing Concerns:

  • Weeds emerging earlier than the crop are highly competitive.
  • Consider: If planting into fields with weeds already emerged, pre-seed control may be required (e.g. non-selective pre-seed herbicide or tillage)
  • Weeds with delayed emergence or over an extended period can allow escapes.
  • Consider: Selection of herbicides with residual activity or using a tankmix of herbicides with no residual and residual components. There may also be the need to consider multiple applications

Weed Control Management Issues:

  • Cool cloudy/rainy weather following pre-seed or in-crop herbicide applications can reduce control or slowdown the rate of weed death, with extra concern if temperature are below 5 0C.
  • Also large annual weeds, winter annuals and/or perennials control may be reduced
  • Consider: Using the higher recommended herbicide rate for pre-seed treatments to assist in overcoming negative impacts of these cool temperatures. Caution has to be practiced that you follow the label and do not exceed the limits as crop injury may occur. If using tank mixes of two or more herbicides mixes must be applied according to the most restrictive use limitations for either product.

Remember, in-crop, with cool temperatures, there is potential for increased herbicide injury to the crops as the plants have reduced metabolism and may absorb more of the herbicide than normal.

Timing is important! If necessary, hire a custom applicator or plane to achieve the earlier spray timing. Waiting for favorable temperatures to proceed with spraying should be balanced with the size of weeds at time of application. If you have to spray in cool temperatures find out more about your chosen herbicide and the response of crops and weeds under cool conditions, spray when the temperature is at the maximum for the day and avoid application too early or too late in the day.

 

A carpet of wild oats on May 21, 2014

A carpet of wild oats on May 21, 2014

 

 

Also see May 22, 2014 Canola Watch post on weeds at http://www.canolawatch.org/2014/05/22/a-weedy-week/

MAFRD posting on Weed Management in Wet Weather at http://www.gov.mb.ca/agriculture/crops/production/weed-management-wet-weather.html

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Weed Escapes or Potential Resistant Weeds?

Prepared by Nasir Shaikh, MAFRI Weed Specialist

The 2013 season created situations where weeds got ahead of the crop and field conditions were not always favorable at the proper time for application. The result was weed escapes due to missed applications, advanced size/development, or other unexplainable reasons.  It is the unexplainable escapes that cause concerns specifically when Herbicide Resistance has developed.

In cases where there are weed survivors, information on time of application, weather conditions, weed species and staging becomes important.  A buckwheat plant or redroot pigweed plant that has escaped the herbicide – is it resistant or was the plant just too big and developed to be completely uncontrolled?

Knowing the limitations of the products being used (i.e. weather, water quality, etc) and the limit on the size or developmental stage of the weed can assist in determining an escape and potential resistance issues.  A weed is considered ‘tolerant’ when effective herbicides fail to control the weed in subsequent years.  The developed resistant populations tend to be in patches or patterns across the field or fields.  Looking for the patterns and keeping notes on escapes is important in determining if there is an issue. This can be accomplished by scouting the field’s post-herbicide application to determine effectiveness. 

In cases where Herbicide Resistance is suspected, plants can be submitted to the following labs to be tested. 

Ag-Quest, Inc. Box 144, #10 South Railway Street, Minto, Manitoba, CANADA, R0K 1M0. Phone: (204) 776-2284; Fax: (204) 776-2250

Herbicide resistance testing services at Ag-Quest include:

  • soil bioassay for group 3 and group 8 herbicides in wild oat and green foxtail
  • petri-dish bioassay for selected group 1 herbicides in wild oat and green foxtail
  • Whole-plant bioassay for group 1, 2, 8 and 25 herbicides in many major weeds of western Canada and US

 

Crop Protection Laboratory   346 McDonald Street, Regina SK S4N 6P6.  Phone: (306) 787-8130; Fax: (306) 787-8803.  E-mail [email protected]

Herbicide resistance testing at Crop Protection Laboratory include:

  • Green foxtail (group 3) $100
  • Green foxtail (group 1) $100
  • Wild oats (group 1) $100
  • Any other herbicide resistance tests $100

 

University of Guelph   Attn: Peter J. Smith, University of Guelph, Plant Agriculture Department, Crop Science Building, Guelph, ON., N1G 2W1.  Phone: 519-824-4120 x 58372; Fax: 519-763-8933.  E-mail: [email protected]

 Herbicide resistance testing at this Laboratory include: Group 1,2,3,6,7,9 and 22. And the test is free!

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Are Herbicides to Control Dandelions Still Effective After a Fall Frost?

Fall is a great time to work on controlling perennials such as Canada thistle and dandelions. As temperatures get cooler, the nutrients start moving down into the roots and if a herbicide can be taken up and translocated with those nutrients, it can equal better control.

A frost event though, can kill some weeds or can damage leaf tissue which will reduce herbicide uptake and therefore reduce the level of weed control. Within the next few days after the frost, you need to assess the target weeds in the areas that you want to obtain control – are the weeds still growing?  How much leaf tissue has been damaged?

If the plant leaves are still shiny green with minimal leaf tissue damage (i.e. not blackened/brown or brittle) there still may be the window to control the perennial, biennial and winter annual weeds with glyphosate.  If you do spray – spray in the afternoon when temperatures are warm and sunny, this will help with herbicide uptake.   Use rates appropriate to the stage and time of year – fall applications of glyphosate are recommended at a higher rate than when controlling pre-harvest.

One last thing – look at the forecast for the next week following the application.  If the temperatures look like they are going to continually be below freezing each night, it may be too late to make the application to get the economic control you are looking for.

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