Written by Michael Macpherson – Imtrade National Technical Manager.
During my travels through the WA wheat belt this year, I have been astounded by the number of paddocks that have had glyphosate failures from early knockdown sprays. This problem does not seem to be isolated to one particular location, but a widespread occurrence. The response from agronomists and growers has been varied, with many reasons given as to why the failure occurred. In my opinion, a lot of the failures seen in 2015 have been due to environmental conditions. A lot of weeds germinated on the early rains in March, which was followed by a month of warm weather on drying soil profiles. Many growers took the opportunity to apply early knockdown sprays throughout this period, with glyphosate being the chemistry of choice. The visual symptoms of the weeds in paddocks where the knockdown failed are consistent with those expected from spraying plants that had shut down.
In order to explain the many possibilities as to why a translocating herbicide may fail, I have compiled a few notes on glyphosate, its translocation and how these relate to potential failures.
Glyphosate is classified as a Group M herbicide, being the only active currently in use from this group. Glyphosate demonstrates moderate cuticular absorption (movement of herbicide from exterior to interior of the leaf), which can be assisted via the addition of surfactants and/or ammonium sulfate (AMS).
Once inside the leaf, glyphosate follows the same translocation stream as sucrose elements, diffusing out of the leaf via the symplastic system into the phloem. Once inside the phloem, it is transported to ‘sink’ areas of the plant (areas using carbohydrates), typically meristematic tissue. Under ideal conditions, translocation out of the treated leaf will occur for 48-72hrs post application with up to 70% of the absorbed active moving into the vascular stream. Noticeable effects may take 7-14 days post application dependent on species and environmental conditions.
Reasons for sub-standard results:
Glyphosate, along with all systemic herbicides, has a number of reasons why it may fail to provide adequate results, which can be classified under the following.
With the increase in imported, fully formulated generic products, it has not been uncommon to hear of products that have tested under-strength. By inadvertently using a product with the incorrect g/L active constituent at the stated label rate, you are effectively providing a sub-lethal dose. To avoid risk, it is best to source your glyphosate products from a reputable source that offer a quality assured process and back-up sales support for their products.
A quick test can be conducted with a set of quality scales and the product MSDS. Multiply the volume to be tested (use L for units) by the specific gravity (SG) listed on the MSDS; the resultant number should equal the weight of the product.
Adverse environmental conditions impact markedly of the activity of most translocatable herbicides and spraying under such should be avoided.
Glyphosate requires good leaf surface contact, with many studies showing the larger the contacted surface area, the more efficacious the herbicide will be. Additionally, glyphosate is bound by various cations which negatively affect its efficacy. The presence of a dust layer on the target surface can cause issues due to these separation and binding effects. Spraying weeds coated in dust or in dry, dusty conditions should be avoided where possible.
– Stress inducing climatic conditions
As previously stated, glyphosate relies on the vascular stream to translocate within the plant to the ‘sink’ areas where it is most efficacious. Under adverse environmental conditions, plants shut down the vascular flow within the vascular tissues and thus the ability to translocate glyphosate to the sites of activity is reduced. The combination of the lack of translocation, limited window for glyphosate to actively translocate (48-72hrs) and the often dry conditions (preventing absorption to the leaf due to drying droplets), causes a significant problem. Typical symptoms of environmentally induced failure include, tip reddening/burning, twisted apical meristems without tissue desiccation, multiple re-shoots from the crown and general failure to desiccate.
Conditions considered to be adverse are:
- High/Low air temperature
- Pest damage
- Very low humidity
- Low fertility
- Combinations of the above.
Spraying under these conditions, especially where plants are showing signs of stress, should be avoided at all costs. Most plant species exhibit peak vascular function in the early morning or evenings in summer and during the middle hours of the day in winter (location dependent). Spraying at these times is considered best to maximize glyphosate efficacy.
Rainfall within 4 hours of application can reduce the efficacy of glyphosate due to general removal and/or dilution of the active on the leaf surface prior to absorption. Avoid spraying if rainfall is imminent.
Applying herbicides as sub-lethal (or sub rate) doses should be avoided at all costs. Not only is the likelihood of failures increased, but the rate of potential resistance development increases exponentially. Under-dosing an active can be inadvertently caused by a number of issues.
– Inappropriate use rate
Label rates for the usage of glyphosate are broad in most cropping situations. Ensure the rate of use is selected based on the hardest-to-kill weed being targeted. Use rates are often listed as a range; this is to account for varying climatic conditions, water rates and nozzle types. As a general rule of thumb, the more advanced the weeds, the more robust the rate should be. Similarly, as environmental conditions become more adverse, an increase in rate is warranted.
– Inappropriate water rate
A lot of work has been conducted on the concentration of active in the droplets contacting the target surface. The generally accepted principle for glyphosate is that the more concentrated the active is in the carrier (the more active/L water), the more efficacious the herbicide will be. This is a balancing act, as reducing water rates can negatively affect coverage, which is also vitally important to efficacy. Carrier volumes of 70-100L/ha are generally considered standard practice in broad-acre situations. Under this rate, a lack of coverage will cause issues and over this rate, without an increase in use rate, can cause reduced efficacy.
There are many recorded incidences of glyphosate resistance in numerous weed species across Australia via multiple mechanisms. Generally resistant weeds will show little or no effect to robust rates of glyphosate in the field. Resistance is often blamed for poor results from spray application, but it is not always the case. The best way to be sure of the resistance profile of weeds on your property is to send samples to an applicable institution for testing. Contact your local agronomic consultant for the service that is applicable in your situation.