Take home message

• Residual herbicides are an important part of weed management programs to provide alternative modes of action, and to reduce the numbers of weeds exposed to post-emergent herbicides
• It is important to understand the properties of the herbicide and associated plant back restrictions. Most of this information is contained on the label
• Be aware of high rates of herbicides (including post-emergent) used through sensor sprayers as there may be detrimental effects on the following crop
• Glufosinate can be a very effective double knock partner in the place of paraquat
• For best results, glufosinate needs to be applied in conditions that allow time for the herbicide to penetrate the leaf surface i.e. relative humidity greater than 50% and temperatures below 33^oC
• Glufosinate is best applied in full sunlight when the plant is actively photosynthesising.

Residual herbicides

Since the adoption of no- and minimum-tillage farming systems, the majority of weed management in northern cropping systems has relied on post-emergent herbicides, and predominately glyphosate. The development of glyphosate-resistance in several major weeds, and Group 1 resistance in grasses, has the grains and cotton industries looking for effective alternatives. Pre-emergent herbicides are one of those alternatives that when used correctly can enable the use of different modes of action, provide extended control of weed populations, and reduce the number of weeds exposed to post-emergent herbicides, such as glyphosate.

Some of the potential drawbacks of using pre-emergent herbicides include: long plant-backs to some crops; the necessity for some herbicides to be incorporated mechanically; requirements for incorporation by rainfall within set periods; sufficient moisture requirements for uptake; and prolonged persistence under dry conditions.

Unlike post-emergent herbicides, there are many factors to consider before including pre-emergent herbicides into the weed management program. These include:

• The product label should always be the first point of reference for plant-back periods and requirements for incorporation, mechanical or by rainfall.
• The planned crop and rotation. It is necessary not to consider only the immediate crop after the herbicide is applied, but also the crop after that when using some products.
• The label plant-back information assumes a ‘normal’ season. Plant-backs may be much longer than the label suggests in dry seasons.
• What weed seeds may be present (now and in the future) and where are they in the soil profile.
• Getting the herbicide to the soil so that it can be incorporated. Herbicides, such as trifluralin and pendimethalin, bind tightly to stubble. Large soil clods will limit the evenness of application of residual herbicides and result in uneven control.
• The mobility of the herbicide is determined by its solubility in water, its binding coefficient (how much is bound to the soil vs available in soil solution) and the soil type. Herbicides that are readily soluble in water and do not bind strongly such as imazapic and metsulfuron, require little rainfall for incorporation and move easily with soil water.
• The soil type. Heavier soils with a higher cation exchange capacity (CEC) will bind more herbicide than lighter soils with a lower CEC. For example, to achieve the same level of control, the applied rate of trifluralin needs to increase (as per the approved label) as the clay/CEC content in the soil increases.
• How best can the herbicide be kept where it is needed, away from crop seeds and near weed seeds. This can be done mechanically for some herbicides, such as prosulfocarb, with particular IBS (incorporated by sowing) requirements. Some herbicides take 2-3 days to bind to the soil. Therefore, it is important to avoid applications before heavy rainfall events, as rainfall can concentrate the herbicide around the seed and cause significant crop damage.
• It is also important to have knowledge of rainfall patterns in your region in general. Several herbicides have specific rainfall amounts and events on label to complement plant back timings.
• The rates of herbicides applied using sensor sprayers may be much higher than standard broadacre rates. Consequently, the plant-back period on these patches can be much longer than the periods indicated by the label when using broadacre application rates. Use of high rates of some products through sensor sprayers can cause patches of poor establishment or damaged crop throughout a field.
• Most herbicides are broken down by soil microbes which will be much more active in prolonged warm and moist conditions. Consequently, breakdown will be much faster in warm, moist soils, compared with cool, dry soils.  While prolonged dry periods with limited days of high intensity storm rainfall may lead to a rainfall target on a label being met, caution is advised as such conditions are not conducive to sustaining the prolonged periods of microbial activity needed to breakdown a herbicide residue. In such conditions, re-crop intervals may be longer than specified on the product label.

Plant-back periods

Plant-backs may be extended (longer than indicated by a label or misleading) when multiple herbicides are applied over time. The more herbicides used, the longer the plant-back period of the combination could be. For example, in an experiment at Narrabri repeated over 3 seasons cotton was planted after 8 in-crop (wheat or chickpea) herbicide treatments followed by 9 in-fallow herbicide treatments. None of the in-crop treatments damaged the cotton if glyphosate was the only in-fallow herbicide applied. Two of the in-fallow herbicides damaged the cotton, but several of the combinations also caused damage, where the same herbicides used alone didn’t cause damage. For example, Starane® Advanced herbicide applied in wheat followed by Sharpen® herbicide applied in the fallow prior to cotton (Table 1).

Table 1. Effect of the combination of wheat and fallow herbicides on subsequent cotton yield.
(Numbers in bold show values that are different according to the standard errors)

In another example (Table 2), there was a similar effect with the combination of Hussar® OD and Starane® Advanced in one season using the same design as the previous example. There was no problem with this combination in the other two seasons.

Table 2. Effect of a different combination of wheat and fallow herbicides on subsequent cotton yield.
(Numbers in bold show values that are different according to the standard errors)

A potential issue with sensor spray applications is that applied herbicide rates may be much higher than standard rates, especially if two or three nozzles are triggered by a large weed. Such situations may kill emerging cotton plants (trials have observed 99% loss of stand, resulting in a 90% yield loss). Patches with poor establishment in a crop could be a symptom of residual damage from high rates of herbicide applied via a sensor sprayer during the preceding fallow and will be very expensive. A 90% yield loss over 15% of a field equates to a 14%, or slightly more than 1 bale/ha, loss over the whole field. The loss could be even greater in a higher yielding crop.

Using glufosinate in a double-knock

The double-knock tactic has been widely adopted in grain and cotton systems for several years for control of glyphosate-resistant and difficult-to-control weeds in fallow situations. This tactic is best described as a sequential application of post-emergent herbicides with different modes of action, to kill any survivors of the first application to prevent weed seed production. The prevention of seed production is critical for resistance management and weed management in general.

The recent introduction of Xtendflex^TM cotton in Australia with resistance to glyphosate, glufosinate and dicamba has prompted an exploration of the usefulness of glufosinate as the second knock partner in the place of paraquat or paraquat+diquat. The recent development of paraquat-resistant weeds has also helped to generate interest in glufosinate which previously has had limited use in cropping systems.

The experiment

Two experiments were run in consecutive years in a shade house at the Leslie Research Facility examining combinations of glyphosate ± dicamba or clethodim followed by glufosinate at intervals of 1, 3, 7, and 10 days after the initial application. The weeds under investigation were glyphosate-resistant and glyphosate-susceptible populations of awnless barnyard grass, feathertop Rhodes grass, flaxleaf fleabane and sowthistle. Herbicides were initially applied when broadleaf plants had reached large rosettes with initiation of stem elongation and grasses were mid-tillering. Treatments were applied in a research track sprayer at 93 L/ha water with DG950015EVS nozzles at 2 bar. Rates are listed in Table 3. When interpreting results, it should be noted that these trials were applied to weeds larger than what is labelled for several of the herbicides, while also understanding that typically field results are likely to be significantly poorer than those achieved in pot experiments. As such, the relative performance of product combinations used is more informative than their performance at the rates used.

Table 3. Herbicides and rates used in all experiments.

Flaxleaf fleabane

Control of flaxleaf fleabane ranged from 61% to 100% across both experiments from the single or double knock applications, with 100% control achieved with dicamba alone or glyphosate+dicamba followed by glufosinate. Glyphosate alone followed by glufosinate gave inconsistent control, particularly on the glyphosate-resistant population. Control with glufosinate alone ranged from 80% to 99%. Control with dicamba followed by glufosinate was 100%. Following dicamba alone or glyphosate+dicamba, the timing of the follow-up glufosinate did not affect control in either experiment.

Sowthistle

Application of glufosinate alone provided greater than 99% control of both glyphosate-resistant and -susceptible populations in both experiments. Glyphosate alone followed by glufosinate 1-day later controlled 92% of the glyphosate-resistant population. Otherwise control with all other glufosinate double-knock treatments was greater than 99%.

Feathertop Rhodes grass

Consistent control of feathertop Rhodes grass was only achieved when the herbicides were used in double-knock treatments. In both experiments, 99% to 100% control was achieved with 7 day and 10 day intervals between initial herbicides and glufosinate. Clethodim alone provided greater than 90% control of both glyphosate-resistant and -susceptible populations. This was slightly higher than when clethodim and glyphosate were combined in a tank mix. This effect was less noticeable when the double-knock interval was 1 or 3 days for the follow-up glufosinate. However, clethodim alone followed by glufosinate was generally more effective than glyphosate+clethodim followed by glufosinate.

Awnless barnyard grass

All but one treatment controlled both glyphosate-resistant and -susceptible populations at 93% or higher. The exception was glyphosate alone on the glyphosate-resistant population where control ranged from 47% to 60% in each experiment.

Optimising glufosinate application

Glufosinate and paraquat have different modes of action, however they share some similarities in terms of being contact herbicides with limited translocation and requiring active photosynthesis for efficacy. Both herbicides have similar parameters for application and need good coverage with higher carrier volumes than required for systemic herbicides such as glyphosate. The major difference between the two herbicides is the rate of uptake into the leaf. Glufosinate is slow to penetrate the leaf surface, and as a result requires a considerable amount of time in solution on the leaf surface. Conditions that result in fast evaporation off the leaf such as low humidity and to a lesser extent high temperatures will not allow enough time for leaf penetration, and result in reduced control. Optimal conditions for glufosinate applications are full sunlight, warm temperatures and high humidity. Applications at night could allow plants to metabolise or compartmentalise glufosinate somewhere in the plant (vacuole, extracellular space) where it can no longer bind to the target site even after sunrise the next morning.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC and CRDC, the author would like to thank them for their continued support.

Contact details

Jeff Werth
Queensland Department of Agriculture and Fisheries
Leslie Research Facility, 13 Holberton St, Toowoomba, Qld, 4350
Email: jeff.werth@daf.qld.gov.au

Graham Charles
Weeds Research Unit, Invasive Species Biosecurity NSW Department of Primary Industries
Australian Cotton Research Institute, Locked Bag 1000, Narrabri, NSW, 2390
Email: graham.charles@dpi.nsw.gov.au

Date published
July 2024

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