Improving the efficiency of glyphosate allows to optimize costs and reduce its use. It is a foliar systemic herbicide, so the conditions at the time of application and during the following days (humidity, temperature, rain delay, delay before intervention) have a great influence on its effectiveness. One method to reduce its use is the practice of low-volume. Its efficiency is improved with a reduction in spray volumes: more concentrated, it penetrates better into the plant, which implies reducing the water volume. It is the active substance that responds best to low volume.
Recommended stages and doses for the destruction of crop regrowth and various weeds (ARVALIS, 2021)
Recommended stages
For annual weeds or biennial weeds, favor applications at young stages (e.g., rapeseed with fewer than 4 leaves).
For perennial weeds, favor applications on developed plants. The goal is to intervene when the sap flows back to the rhizome, allowing destruction of vegetative organs:
For thistles, treat when plants are at the "tight bud " stage. The plant is then 15 to 20 cm tall.
For bindweeds, treat on stems > 20 cm, if possible at flowering.
Optimal conditions for intervention
Being a foliar systemic herbicide, glyphosate's effectiveness is greatly influenced by conditions at application and in the following days:
Humidity > 70 % (key factor, improved if at 90 %), possible with light dew;
Temperature between 15 and 25°C: better to avoid during falling temperatures (especially upcoming frost), or northeast wind or windy weather;
Rain delay of 1 to 6 hours (variable depending on formulations);
Avoid water stress and prefer applications on moist soils;
Avoid applications on cold soils (for perennial weeds, soil temperature influences treatment efficacy);
For treatments on stubble, given low humidity and high temperatures in August, prefer morning treatments to benefit from dew;
Respect delays before soil work (⚠️ new regulation). Glyphosate penetrates the plant by foliar absorption, more or less quickly depending on plant physiology (annual: 1 day; perennial: 7 days).
Recommended doses
For grasses
Authorized products for this use (U 46 Pro ®, CHARDOL 600 ®) can be combined with glyphosate-based products for effective control of certain dicotyledons: bindweeds, crucifers.
For dicotyledons, it is preferable to combine with 2-4 D in intercrop. However, this may compromise glyphosate's effectiveness, as there is a risk of "burning" the plant too quickly and not allowing the systemic product time to migrate to the weed roots.
Recommended doses
The table summarizes the recommended doses for glyphosate and 2-4D according to the weeds to be destroyed. It dates from before the usage restrictions in force since 2021.
Recommended doses according to weed types before usage restrictions (source: ANSES, 2020)
Some crops are sensitive to 2-4 D, requiring a delay before planting. The following table summarizes the delays to respect according to crops.
Delays to respect according to crops for 2.4D use (source: ANSES, 2021)
Preparing a low-volume solution
It is crucial to properly prepare this solution:
Respect the order of incorporation of products and adjuvants.
Adjust the pH.
Pay attention to water hardness.
Use adjuvants.
Water hardness
Glyphosate efficiency without adjuvant on wheat depending on water hardness and applied product dose at T+28 days (source: Arvalis, 2013)
Information on water hardness is specified on water bills and at town halls. There are also tests with strips to measure it (available in water softener, aquarium, or pool sections). Only glyphosate is sensitive to this characteristic.
glyphosate is sensitive to calcium and magnesium ions present in so-called "hard" water, notably from the water network. Calcium ions inactivate glyphosate molecules. No problem with rainwater. Otherwise, it is better to neutralize water hardness to avoid loss of efficacy.
Adjuvants formulated with ammonium sulfate, have a role as water correctors in addition to their wetting role.
To correct water hardness: Add ammonium sulfate (an authorized product for herbicide spray use, such as Actimum): 100 g of ammonium sulfate neutralizes 100 ppm of calcium in 100 L of water. Addition can be done up to 24 hours in advance to make this hardness reduction very efficient. The sulfate, negatively charged, traps positively charged calcium ions, thus limiting their effect on molecules sensitive to water hardness.
Use of ammonium sulfate authorized as fertilizer for hardness correction is prohibited. It is necessary to use adjuvants formulated with ammonium sulfate. The usual dose for these adjuvants is 1 L/ha.
Magnesium sulfate has no effect on water hardness. It may even have the opposite effect: adding hardness ions to the water. The effect is humectant: Salts such as ammonium sulfate or nitrogen liquid also help reduce glyphosate's sensitivity to crystallization. Indeed, they absorb moisture from the air and slow down the drying of spray droplets on the leaf surface. This positive effect is especially interesting in low humidity conditions.
Adjuvants
Adding adjuvants improves penetration and regularizes efficacy, especially at low dose and low volume.
Vegetable oils
They are especially useful on poorly wettable leaves such as for poorly wettable grasses like ryegrass or blackgrass.
You can add 0.5 liters/ha of esterified vegetable oil.
Humectants
By their action on sprayed droplets, they allow better penetration of the product into the plant. The role of humectation is to allow sprayed droplets to dry more slowly on the leaf surface, thus allowing longer penetration of the product into the plant.
Humectants are recommended if humidity is limiting (around 60 - 70 %), in the form of sulfate (magnesium sulfate for example or ammonium sulfate as supplement).
Functions of so-called "wetting" products (source: Regulation (EC) No 1107/2009)
Wetting agents
The wetting agent or spreading agent (type silwett L77 at 0.02% of spray volume) allows to spread droplets by lowering surface tension on leaf surfaces and to make them stick to the leaf like a "glue".
List of "wettable" and "poorly wettable" crops (source: Arvalis, 2021)The efficacy of wetting agents depends on the plant/adjuvant/formulation trio of the product. Like all plant protection products, they have a marketing authorization. To be approved, they must have at least one of the functions in the table.
On "wettable" plants, the use of wetting agents is not justified, whereas it can be on poorly wettable plants (see table).
Intercrop usage conditions
Since 2021, new intercrop usage conditions have appeared. This notably concerns practices with plowing. In arable crops, these uses cover the destruction of cover crops in intercrop, crop regrowth, perennial or woody weeds, in order to sow or establish a new crop limiting competition with weeds.
Maintenance or withdrawal of glyphosate uses for "general intercrop weed control, fallows and crop destruction" in new marketing authorizations (source: ANSES, 2021)
