Residual Chemistry 101

Following suitable cultural controls, residual herbicides are the foundation of weed control in winter cereals, particularly against the twin menaces of black-grass and Italian ryegrass. Stacks and sequences containing multiple actives are used from pre-emergence until early post-emergence of the crop.  

With financial, environmental, regulatory and resistance problems all raising questions about the long-term pathway for crop production, it’s worth reviewing how this chemistry works and ways to improve performance. 

Which herbicides and actives are we talking about?

There are a number of overlapping terms that are used to describe these herbicides. They are commonly referred to as residual, soil-applied and pre-emergence / pre-em herbicides. Although used interchangeably these are all describing different aspects of the herbicide. 

Residual herbicides remain in or on the soil and are effective against target weed species for a useful length of time, this could range from a few days to several months. This characteristic allows them to be applied before weed germination.  

Soil applied is a description of where the herbicide is applied.  

Pre-emergence (pre-em), strictly speaking is the timing rather than the type of herbicide. Pre-emergence is the main timing for applying residual herbicides even though many of these products can be applied after germination of the crop until tillering in some cases to control subsequent weed germination.  

Contact-acting / post-emergence (post-em) herbicides are the other main type of herbicide These are applied to the foliage of the target weed. Please note, it is possible for the same herbicide to have residual and contact-acting activity.  

How Modes of Action assists with Weed Management 

A herbicide’s mode of action (MoA) is the chemical basis for how an active interferes with essential plant processes to kill it. Several herbicides can have the same mode of action and are put into groups to help farmers and advisors with resistance management. This is overseen by the Herbicide Resistance Action Committee (HRAC), an international body founded by the agrochemical industry to fight against herbicide-resistant weeds.   

For example, flufenacet, prosulfocarb and tri-allate all inhibit cells from making very long-chain fatty acids, disrupting cell division. They’re all in HRAC Group 15. Diflufenican and picolinafen are in Group 12 as they inhibit the enzyme phytoene desaturase, which is important in photosynthesis. 

You may hear the term ‘site of action’ which is the specific enzyme or binding site affected by the herbicide, also called the target site. Herbicides with the same mode of action may have different sites of action. Sometimes the terms site and mode of action are confused and used interchangeably.  

Site of uptake is another similar sounding term with a different meaning, The site of uptake is where the herbicide enters the plant, the roots, shoots or foliage.  

How Residual Herbicides Support Long Term Weed Management Goals 

Residual herbicides persist in the soil for a useful length of time, ranging from days to months. They control weeds following uptake by the embryonic roots and shoots of the weed, hence conditions conducive to germination are needed for the herbicides to work. They have little if any direct action on the seed itself. 

Some residual herbicides can provide limited control of established weeds with one or more true leaves. But in general, sequences of residual herbicides with a ‘top-up’ spray aim to control later germinating weeds rather than anything that survived the pre-emergence application.  

Physical properties 

The physical and chemical properties of residual herbicides determine how they behave in soil and water as well as their reaction to temperature, light and other external factors. Understanding how these differ between actives can help when planning a pre-emergence programme. But please refer closely to product labels as formulation and results from product trials will give guidance on the best way to use these products in the field. 

Solubility 

Solubility is a measure of how much herbicide can dissolve in water, it is usually quoted in mg/L of water at 20°C. Some residual herbicides depend on sufficient moisture to become incorporated in the soil and for root active molecules to reach the rooting zone for uptake. Only in exceptionally dry years like 2011 is a lack of moisture a barrier to residual herbicide performance in the UK. But patience is needed to wait for seedbeds to moisten up. 

Higher solubility herbicides tend to move more easily within the soil, but this also increases the risk of them reaching crop seeds or leaching away causing environmental impacts. 32mm is the recommended drilling depth for cereals to ensure seed remains physically separated from pre-emergence herbicides. At shallower depths, there is far more risk of the germinating seed encountering harmful levels of herbicide causing crop effects. 

Adsorption (mobility)  

Within the soil, a proportion of herbicide will adsorb (bind) to soil organic carbon and soil particles rather than be available in soil water for uptake by plants. Herbicides with a greater tendency to bind are less mobile in the soil.  

The level of binding is predicted by the Koc value which is the ratio of herbicide adsorbed onto the soil in comparison to the amount remaining in the soil water. As herbicide is lost from the soil water through uptake, degradation and leaching, an equal proportion is liberated from the soil keeping the ratio in balance. Higher Koc values mean the pesticide is more tightly bound and less mobile in the soil. 

For some pesticides, binding is reported as the Freundlich soil-water coefficient – Kfoc value. For the purposes of understanding binding Koc and Kfoc can be treated as the same. 

When planning a herbicide programme, adsorption and solubility need to be considered to ensure there is a balance of actives with complementary properties. 

Low solubility, low mobility herbicides tend to remain nearer to the soil surface; aclonifen, diflufenican and pendimethalin are all thought of as ‘surface acting.’ Pairing these actives with more soluble and mobile actives e.g. flufenacet or cinmethylin that can reach the rooting zone tends to improve weed control. For example, popular co-formulated products like Liberator (flufenacet + diflufenican) and Crystal (flufenacet + pendimethalin) both use this combination.  

Lighter sandier soils and those with lower organic matter tend to have freer movement of moisture and fewer binding sites for adsorption. Consequently, the risk of herbicides reaching germinating crop seeds and causing damage is greater. In these situations, take sensible precautions: avoid applications when rainfall is forecast, adjust application rates and choose actives with lower solubility and mobility.  

Degradation 

Herbicides degrade in the soil due to chemical reactions (hydrolysis) and microbial decomposition. How quickly this happens determines the duration of any residual activity. Speed of degradation is usually measured as the DT50 value or half-life; the number of days it takes for 50% of herbicide to break down in the soil.   

Soil and weather conditions have a huge effect on the half-life of any active in a field situation. In general, warm, moist conditions promote degradation whereas dry and cool conditions allow greater persistence. Hence, herbicides applied later in autumn, after delayed drilling for example, have better longevity because of cooler weather.  

In the herbicide information table, there are DT50 values taken from the University of Hertfordshire Pesticide Properties Database (PPDB). Caution is needed when using any DT50 values to predict duration of residual protection. But it does provide a useful guide to the relative longevity of different actives. 

When planning a herbicide programme, the extent of residual protection depends on the DT50, rate of herbicide application, speed of degradation (temperature and moisture), losses due to leaching and the sensitivity of the target weed. Delaying drilling date, choosing actives with greater reported longevity and applying a top-up application will increase the duration of protection. 

Volatilisation and photodegradation 

Herbicides on the soil surface can also be lost through volatilisation and photodegradation. Different actives are affected to different degrees, moreover, effective formulation tends to mitigate the risks of losses for more volatile molecules.  For example, liquid triallate used to require prompt incorporation to prevent volatilisation losses but the modern granular and liquid formulations mean this is no longer necessary. 

Overall, these two pathways for losses are not a major concern for UK farmers provided applications are made when there is sufficient moisture in the seedbed.  

Selectivity and crop effects 

 Residual herbicides are selective meaning they control specific set of weeds without causing serious harm to the crop. This selectivity can be based on species tolerance: the crop is able to rapidly metabolise the herbicide or physical separation, the crop is sown below the treated layer of the soil.

Consistently drill cereals to at least 32mm depth to ensure physical separation of seed from significant concentrations of herbicides. Smaller seeds such as grass-weeds, tend to germinate from near the surface in the soil profile. Larger seeds like cereals and perennial weeds can germinate from a greater depth.

Using stacks with multiple products increases the higher herbicide loading and risk of crop effects. Be particularly cautious on more sensitive crops like barley and oats, lighter soils and with rainfall forecast. Splitting out the programme into a sequence will also increase safety but at the cost of greater workload.

Resistance considerations

Herbicide resistance is a danger for any herbicide. Contact-acting herbicides generally have a higher resistance risk than residual herbicides, which is reflected with experience in the UK where HRAC Group 1 (fops and dims) and Group 2 (ALS) have both suffered from fairly rapid development of resistance in grassweed populations.

For residual actives, the development of reduced sensitivity and resistance has been much slower and isolated than for contact-acting chemistry. Also, the ability to use multiple actives in a programme means that an active can remain useful even if as a single active it is of limited value.

However, increasing reliance on residual chemistry for grass-weed control means that more cases of resistance are likely to appear. Stick to sound resistance management principles:

• Diversify your weed control programme

• Use cultural controls first

• Use actives from different HRAC MoA groups in your programme

• Follow up residual chemistry with contact-acting chemistry where appropriate 

• Rotate actives in the programme within and between seasons

• Use crop rotation to have a wider range of actives to choose from

• Monitor success and test if resistance is suspected

• Hand rogue / patch spray survivors with glyphosate to break the life cycle of resistant plants

Seedbed quality

Soil and seedbed quality has a significant impact on residual herbicide performance. In the worst cases, this could be open slots with exposed seed which is not safe for a pre-emergence application.

Less serious but still problematic are rough, cloddy seedbeds which have a much larger surface area and tend to suffer from inconsistent distribution of herbicides. Compaction and other damage to the soil close to the surface is likely to disrupt the movement of herbicides dissolved in the soil water affecting performance.

In low-tillage systems, well chopped and spread residues are not thought to cause significant problems for residual herbicide performance. However, thick patches or bands of straw could create a barrier and should be broken up before and spraying commences.

Rolling after drilling but before pre-emergence application is a helpful step to improve seedbed quality in most situations.

Pre, peri or early-post?

For residual applications in autumn, the following three timings are most commonly discussed.  

Pre-em: Pre-emergence a herbicide applied after drilling but before the crop emerges. Ideally, this is with 48 hours for efficacy and safety but could be within one week of drilling depending on the speed of germination.  

Peri-em: peri-emergence means a herbicide application just as the crop shoot becomes visible on the soil surface. It is a risky time to apply due to potential crop effects but can be a necessity if a pre-emergence application was delayed. For applications at this timing having a simple tank-mix – perhaps one co-formulated product only is a sensible precaution. In discussions, peri-em is sometimes used to refer to applications much later when the crop has fully emerged. Such applications are probably better labelled top-ups or early post-em. 

Early post-em: Also called a ‘top up’ Usually refers to residual type herbicides applied fairly soon after crop emergence (up to 4 weeks), most likely as the second part of a sequence following a pre-em.  

Tank mixing 

Pay attention to tank mix sheets and product labels when planning stacks and sequences to avoid antagonistic effects that can cause problems for the application equipment or the sprayer.  

Application technique 

Nozzles: Choose correct nozzle type, forward and backward angled nozzles alternated can improve surface coverage. 

Water volumes: 150 – 200L/ha gives better coverage than 100L/ha but at the expense of work rate. In time pressured situations, lower volume spraying which manages to apply a pre-emergence herbicide is still better than not applying at all. 

Boom height: The correct height above the soil surface is 50cm for 110-degree flat fan nozzle.  

Speed: Pay attention to forward speed of the sprayer, 12km/hr generally gives good, even coverage. Increasing forward speed beyond this can lead to uneven soil surface coverage 

Weed management strategies summarised

• Herbicide performance varies due to soil type, conditions and weed spectrum. Trial results and other farmers / agronomists experience can only tell you so much, look carefully at what happens when you use different products on farm.

• Understanding the physical properties of residual herbicides is helpful when planning and evaluating the herbicide programme but do not lose sight of the basics. Read product labels carefully and apply with good application technique at a suitable timing.

• Lighter soils need more care when using residual herbicides as herbicides are more mobile and can reach crops seeds or be lost entirely following rainfall.

• Achieving good results with residuals depends on soil condition, effective cultivation (where practised) and good establishment.