Do glyphosate additives improve efficacy?

FAR trials investigating the efficacy of a number of commercially available glyphosate additives have shown little, if any, benefit from adding any of the products to the tank mix.

WORDS AND IMAGES SUPPLIED BY FAR

FAR’s Phil Rolston says a number of products are used as additives to improve glyphosate efficacy, to provide alternatives to chemical additives, or to provide environmental benefits, but most of the evidence around their benefits is anecdotal. “For example, some growers claim fulvic acid allows them to reduce glyphosate rates while others say they use it to mop up residual glyphosate. In order to check out the first claim, in spring 2019 we set up a trial to compare the efficacy of glyphosate with different additives used by growers. “The trial was setup in what had been a ryegrass seed crop, to remove the ryegrass before transitioning to a following crop. Treatments included fulvic acid powder (50, 100 and 150 g/ha), fulvic acid plus citric acid, fish oil at 4.4 L/ha and Pulse 100 mL/ha, ammonium sulphate 2 kg/100 L water. This trial showed no benefit of using any additive with glyphosate, whether at label rate, or at either of the reduced rates. For example, glyphosate used alone, at half label rate (735 g ai/ha), reduced the relative average number of tillers to 100, significantly fewer than any treatments where fulvic acid was added (248 – 316 tillers) (Table 1). Fulvic acid trial at Chertsey Arable site 2020 This trial has been repeated with modifications for the 2020/21 season. A different fulvic acid product, a liquid, has been tested in this experiment as well as the powdered product that was used in last year’s trial. This year’s trial also has two application timings; 23 October 2020 and mid-November. The trial showed that fulvic powder reduced glyphosate efficacy, and that while liquid fulvic slightly improved glyphosate efficacy, the increase was not sufficient to recommend its use. The key learning from both years’ trials is that glyphosate rate is the most important factor in determining the level of ryegrass control. A glyphosate application of 1470 g/ai provided better control than any other treatment, whether or not any additive was included. What does impact on glyphosate efficacy?

IMAGE: The glyphosate efficacy talk drew a lot of interest at FAR’s 2020 CROPS field day.

TABLE 1. RELATIVE AVERAGE RYEGRASS GREEN TILLER COUNT 71 DAYS AFTER TREATMENT (DAT) AT CHERTSEY, 2019.

Glyphosate rate (g ai/ha) 735

980

1470 AVG Fulvic rate (g/ha)

0 50 100 150 AVG

100 316 296 248 241

44 184 79 225 133

16 44 57 21 35

ENVIRONMENTAL FACTORS (i) At low humidity, droplets dry quickly, reducing plant uptake; (ii) Glyphosate is best applied during daylight as photosynthesis aids its movement through the plant; (iii) Temperature can affect both absorption and translocation. Optimum air temperature: 15–25°C.

WATER QUALITY (i) Glyphosate works best in acidic conditions. The ideal pH for a glyphosate + water solution is pH 4.0 - 5.0; and for most water sources the acidity of glyphosate will bring the pH into this range. (ii) Use clean water: clay or organic matter can bind to the glyphosate molecules and reduce efficacy; (iii) Hard water reduces efficacy as calcium and magnesium bind to glyphosate molecules.

BIOLOGICAL FACTORS (i) Glyphosate is more effective on young, actively growing plants; (ii) Glyphosate is not as effective on stressed plants due to reduced translocation; (iii) Understand the biology of the target weeds, not all weeds respond the same way (e.g. poor control of rhizomatous and deeply rooting weeds in early spring— Californian thistle).

Remember Glyphosate is a valuable tool for vegetation management in direct drill and reduced tillage of arable crops. However, public concern about its potential negative impacts is increasing, and several countries have restricted or banned its use. Alongside this, there is growing evidence of weed species developing glyphosate resistance. Maintaining both glyphosate efficacy, and the social license to use it, relies on appropriate and responsible use.