HERBICIDE OPTIONS FOR PRE-HARVEST MANAGEMENT – KNOW THE DIFFERENCES!

Harvest 2017 is here as winter wheat and fall rye harvest has started in some areas of Manitoba. With favourable weather, crops are advancing quickly so now is a good time for a refresher on pre-harvest management!

As agronomists and producers, there are a few distinctions you need to be aware of between the pre-harvest herbicide products that are available for pre-harvest management. When done correctly, a pre-harvest application can provide a number of benefits including maximizing yield and quality, allow for direct combining of standing crops, perennial weed control and managing weed escapes from the growing season, and can speed up harvest timing.

However, it is critical to know the differences between the herbicide options to manage product expectations, and to ensure application is done correctly to make your crop export ready! The following are some key points to remember when considering your pre-harvest management.

  • The most commonly used active ingredients used for pre-harvest management are glyphosate, diquat (ex. Reglone), saflufenacil (ex. Heat), carfentrazone (ex. Aim) and flumioxazin (ex. Valtera). There are also products that are pre-packaged mixes of active ingredients (ex. CleanStart with glyphosate and carfentrazone).
  • Products such as Reglone or Heat are desiccants. GLYPHOSATE IS NOT A DESICCANT. Desiccants and glyphosate work very differently, usually require different application timings and parameters, and provide different benefits.
  • Desiccants that contain diquat (e.g. Reglone, etc.) have been registered the longest in many crops and are the gold standard to which all other desiccants are compared. Diquat rapidly dries down green plant material, with desiccation typically occurring within hours to a few days. In fact, diquat fast acting nature sometimes works against itself, by limiting uptake by drying plant material, which is why the labels recommends applying these products at dusk or on cloudy days.
  • Newer desiccants, such as Aim, Heat and Valtera, also result in the dry down of green plant material. Research has shown that Heat and Valtera are often just as or more effective than diquat for desiccating crops, and usually only take slightly longer to do so. Aim, on the other hand, is relatively slow acting, which makes it an ideal partner for glyphosate. And research has shown a synergistic effect when Aim and glyphosate are tank mixed (i.e. CleanStart), something not seen with other desiccants. Opposite to diquat, these desiccants are best applied on sunny, warm days.
  • Coverage is important for these contact products, so be sure to keep water volumes up!
  • GLYPHOSATE IS NOT A DESICCANT but can enhance dry down of crops. Research has shown that, compared with untreated crop, glyphosate can improve crop dry down after 7 to 14 days (depending on the weather). But glyphosate typically does not dry down crops as consistently or to the same extent as the true desiccants.
  • However, glyphosate is a popular pre-harvest choice for many growers since, as the only systemic, is provides control/suppression of weeds, including winter annuals and perennials. Desiccants will also dry down green weeds, but only provide top growth control.
  • Proper application timing is critical regardless of the product used. Always refer to and follow the product label for the correct timing and rates. Remember, desiccants or glyphosate neither bring about nor speed up crop maturity. However, it can decrease the time between when the crop has reached maturity and when the crop is harvested.
  • Proper application timing is especially important for glyphosate. When applied too early, glyphosate residues could accumulate in the grain and may exceed Maximum Residue Limits (MRLs) of important export countries.
  • Residues for the contact products (diquat, Aim, Heat and Valtera) are usually very low because of their contact nature. But that doesn’t mean that these products are without MRL concerns since, in many cases, residues tolerances can be exceeded simply because an MRL has not been established in all markets.
  • New for the 2017/18 crop year, the Declaration of Eligibility for Delivery form will ask farmers to declare if deliveries of wheat, durum, barley  and flax made by themselves or on their behalf have not been treated pre-harvest with any product containing saflufenacil, including the product known as “Heat LQ.” The application of saflufenacil is registered for use in Canada by Health Canada’s Pest Management Regulatory Agency (PMRA) as per its label, including the pre-harvest timing at the recommended rate and crop staging. However, maximum residue limits (MRLs) have not been established in certain export markets for wheat, durum wheat, barley and flax.
  • Cereals Canada, Canola Council of Canada and Pulse Canada have information available through their “Keep it Clean” initiatives. Please refer to the following websites for additional information for meeting export standards, why MRLs matter, pre-harvest interval information, and herbicides that should not be used.

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

Herbicide options for use as a harvest aid or desiccant before crop harvest are listed on Page 64 of the 2017 Guide to Field Crop Protection: https://www.gov.mb.ca/agriculture/crops/guides-and-publications/#gfcp

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STAGE CEREAL CROPS CORRECTLY FOR A PREHARVEST GLYPHOSATE APPLICATION

Modified from Post Originally Published July 30, 2014

Staging a crop for preharvest glyphosate application for perennial weed control can be difficult when there is variability of crop staging within the targeted field.  As well, kernels in the same spike will reach physiological maturity at different times, with the middle of the head maturing first. 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.

 

Another visual indicator for wheat is a change in color of the peduncle, which is the part of the stem located just below the head.  It will have turned very light green or yellow at physiological maturity (Figure 2).

wheat_spikes
Source: Topics Addressing Small Grain Crop Dry-down and Harvest . 2015. Jochum Wiersma, Small Grains Specialist; Doug Holen, Crops Extension Educator and Phyllis Bongard, Educational Development and Communications Specialist

 

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 test weightIn 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 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.

Following label instructions and keeping in mind pre-harvest intervals are also key component in Cereals Canada’s Keep It Clean initiative. More information is available at http://www.cerealscanada.ca/keep-it-clean/

Submitted by:  Pam de Rocquigny, Provincial Cereal Crops Specialist, Manitoba Agriculture

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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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