Arable News

  • Written by: Farmers Guide
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Protect spring pesticides from the effects of hard water

Water is the primary carrier for applying crop protection products constituting more than 95 per cent of the spray volume. It has many unique properties, but to gain the most from its potential to support pesticide performance means recognising what else it may be carrying and applying the appropriate remedy. In most cases this means adding a suitable water conditioner.

The solution to hard water is to properly condition it prior to adding the pesticide.

A crucial distinction of water conditioners is that they are not about enhancing pesticide performance per se, but about treating the water so that the pesticide is fully available. Hence, they are not classified as adjuvants by the Chemicals Regulatory Division (CRD) of the Health & Safety Executive (HSE).

What we overlook when thinking about water is that there is almost always something else dissolved in it. Reservoir and ground water often contain mineral salts of one form or another, be it carbonates or bicarbonates, which are typically associated with higher pH supplies (pH >8), or sulphates and nitrates of calcium (Ca), magnesium (Mg), iron (Fe), or zinc (Zn), which are typically found in lower pH sources (pH <6). In addition to pH, these mineral salts – called ‘cations’ and ‘anions’ after dissolving – give water the distinctive properties that affect taste and hardness.

These naturally occurring soil minerals are collected as ground water moves through the ground on route to the tap. A greater concentration of cations in the water leads to a higher percentage of herbicide lock-up. In a similar way that limescale forms in a kettle, these salts go through a change of state as the solution dries on the leaf surface taking on a solid form that is unable to penetrate the leaf surface.

It is important not to confuse the effects that pH and hard water have on herbicide and fungicide performance. Water with a pH of more than 7 can negatively affect some herbicides, principally carbamates such as phenmedipham, through a process known as alkaline hydrolysis. This is a gradual, but irreversible process and it occurs as soon as the pesticide is mixed and continues until it dries on the target when the active ingredient is rendered permanently inactive.

It is, however, hard water that is the greater threat to performance. Herbicides are the most widely affected, but certain PGRs are also vulnerable. Left uncorrected hard water can de-activate certain herbicides, ultimately reducing the amount of dose of the active delivered by as much as 30 per cent.

Popular springtime herbicides such as all those containing sulfonylureas, for example, Atlantis, Pacifica and Cintac (all containing mesosulfuron + iodosulfuron) for grass weed control, Centurion Max (clethodim) and glyphosate for stale seedbeds are all impaired to some extent by the cations in hard water.

Negate the chelates

The solution to hard water is to properly condition it prior to adding the pesticide by adding a water conditioner such as X-Change, AquaScope or X-Clude. The cations in hard water form complexes with the chelating agents in the conditioner leaving them unable to bond with the herbicide molecule. This allows the herbicide to remain unaffected in solution and be delivered to the target weeds in the form to which it will perform at its optimum level in a hard water situation.

Glyphosate manufacturers, for example, have long supported the use of a water conditioner, such as X-Change to maintain the required level of efficacy, and stewardship guidelines from the Weed Resistance Action Group (WRAG) endorse this position.

It’s hardest in the east

The extent of water hardness, particularly in the east of England, was identified in a 2017 survey. All samples collected in the region were classed as ‘very hard’ – found to contain calcium carbonate levels in excess of 300mg/litre of CaCO₃ – with Ipswich found to the town with the hardest water in England at 423mg/litre CaCO₃.

This is 44mg/litre higher than Colchester, the second hardest water town in the UK. Boston (363mg/litre), Luton (360mg/litre), Norwich (359mg/litre) and Watford (358mg/litre) are also heavily affected.

At these levels the effect of herbicide lock-up cannot be ignored. The extent to which herbicides and some fungicides are negatively affected by hard water can be determined by the strength of its acidity in solution. This is measured using the pKa scale with the lower the pKa value, the more acute the negative effects.

Those substances with a pKa value of less than 5 are expected to be at significant risk from lock-up (see Table above for product examples). If you plan to apply the herbicides listed in the table, it is worth considering the use of a water conditioner to ensure as much of the product is available as possible to do its job.

PGR

Herbicides are not alone needing a water conditioner to support biological performance; plant growth regulators, principally those containing prohexadione-calcium, namely Canopy, will also benefit because the calcium needs to be released from the prohexadione to work effectively. The addition of a water conditioner improves the efficiency of the release process and is why the inclusion of X-Change with Canopy is supported by BASF.

While the primary function of a water conditioner is to counter the effects of cations, products such as X-Change, AquaScope and X-Clude confer other benefits. All will lower the pH of the water to about 4.6 which is the median value of most herbicides and all include a humectant to prevent crystallisation on the leaf. Lastly, they include an anti-foam to ensure effective foam control and minimise the amount of time lost during filling.


  • Written by: Farmers Guide
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