Arable News

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Address hard water issues for better weed control

Reasons for poorer than expected weed control are often varied and complicated but a factor in their performance is spray water quality, as Farmers Guide finds out in this special report. Resistance and loss of actives are well known impediments to successful herbicide programmes in all crops, but a factor that’s often overlooked is spray water quality.

Many of the active ingredients that are routinely used to control key weeds in all crops are highly susceptible to the properties of hard water. Addressing this issue will improve control rates by ensuring the active ingredient is delivered in its active chemical form and functions as it should inside the target weed species, says De Sangosse technical manager, Rob Suckling.

“When you consider that left uncorrected, hard water can de-activate certain herbicides, ultimately reducing the dose of active delivered by as much as 30 per cent, you realise what an impediment to performance it can be. Glyphosate manufacturers have long supported the use of a water conditioner, such as X-Change, to maintain the required level of efficacy and new stewardship guidelines from the Weed Resistance Action Group (WRAG) endorse this position,” says Mr Suckling.

Water hardness is often confused with pH, but it is important to separate these two properties as their effects on pesticides are very different, and it is the former that causes most concern.

Calcium carbonate

Hard water is primarily the result of high calcium carbonate levels, but other soil minerals such as magnesium and iron also contribute to hard water. These mineral salts dissolve in water and release ‘cations’ that can bind with the herbicide in a chemical reaction that renders the herbicide less biologically active. 

A recent survey of water hardness has highlighted just how severe the issue is, particularly to those in the east. Approximately 60 per cent of the UK is affected by ‘hard’ (more than 200mg/litre calcium carbonate (CaCO₃)) or ‘very hard’ (more than 300mg/litre CaCO₃) water but it is the east of England that is most heavily affected (see map).

Using data obtained from local water authorities, Ipswich was identified as the town with the hardest water at 423 milligrams of calcium carbonate per litre. 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. In contrast, water in Scotland and Cornwall is considered soft with a calcium carbonate concentration typically below 100mg/litre.

These naturally occurring soil minerals are collected as surface or ground water moves over or 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.

The extent to which a herbicide is negatively affected can be determined by the strength of its acidity in solution. This is measured using the pKa scale. The lower the pKa value, the more acutely affected it is by hard water. Herbicides with a low pKa value are generally accepted to be at risk of significant lock-up.

For instance, in independent trials involving pyroxsulam, grass weed control was improved by 18 per cent with the addition of a water conditioner. This is a significant boost to performance, says Mr Suckling. 

Grower understanding

For Frontier Agriculture eastern region agronomist Neil Leech, hard water is an issue he takes seriously, but convincing growers it needs addressing is largely a case of ‘seeing is believing’. 

Frontier Agriculture supplies AquaScope, a blended water conditioner containing a range of complexing agents to nullify the effects of cations, a pH buffer, a humectant to promote herbicide uptake, and an anti-foam.

“I regard addressing the threat posed by hard water as part of my obligation to the grower. It is inexpensive to correct and is fundamental to achieving a net reduction in weed seed return. In trials we have seen a 15 per cent increase in black-grass control when glyphosate is used with AquaScope,” he says.

“AquaScope has been heavily tested and its contribution to performance has been recognised in independent trials. When added to Atlantis (mesosulfuron + iodosulfuron) and BioPower (an adjuvant), black-grass control increased from 68.3 to 86.7 per cent. This is significant and goes a long way to reaching the necessary 97 per cent control needed to beat black-grass,” he says. 

Promoting the benefits of water conditioning is largely an educational issue, explains Mr Leech. 

“The effect cations have on spray performance can be difficult to explain because it is often hard to see the effects of a gradual fall in efficacy. We know glyphosate is heavily affected so urge growers to apply it with AquaScope to see the benefits for themselves. Once they see the gains in control, any scepticism vanishes.”

Not just the east

While those in the east may have to suffer the hardest water in England, it is far from soft elsewhere. For Hutchinson’s agronomist Kieran Walsh, the chalky soils of the Cotswolds, Chiltern Hills and North Wessex Downs mean high calcium levels are a constant barrier to spray performance. 

“Calcium carbonate levels vary from 210–340mg/litre in the Cotswolds. This cannot be overlooked. Once concentrations pass 200mg/litre the effect on performance of products such as glyphosate and clethodim is evident,” he says.

“Independent trial data show that use of a water conditioner can lift the performance of clethodim by 30 per cent one month after application. In situations where enhanced metabolic resistance to dim herbicides is a known concern, delivering a higher dose of active can mean the difference between control or failure,” says Rob Suckling.

Where possible Kieran Walsh encourages rainwater harvesting, but for those where clean rain water is not an option, he advises the use of X-Clude, another blended water conditioner containing complexing agents, pH buffers, a humectant, and anti-foam. 

“Many growers know they have hard water but haven’t considered the extent of its impact. Come the early spring the weed control benefits from having conditioned the water in the previous autumn are highly visible and attitudes change,” says Mr Walsh.

Understanding hard water effects

To promote understanding of the effects hard water can have on pesticide performance, De Sangosse has launched ‘H₂knOw’, an initiative that seeks to help growers and agronomists understand how best to support efficacy. Contact De Sangosse on 01223 811 215 for more details.


  • Written by: Farmers Guide
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