Spraying strategy needs to be simple and smart

As more products are withdrawn from usage, contractors and farmers will need to follow best practice to retain the herbicide options available to them

Chemical weed control revolutionised cereal production. Nature hit back with the development of resistance by a small number of weeds to our commonly used herbicides.

Our earliest herbicides were known as hormone herbicides because they disrupted the functioning of normal plant hormones, resulting in death or restricted growth of our most common weeds.

Hormone herbicides had/ have to be used during periods of active growth and with soil temperatures in excess of 8C. Hormones were only used to gs 30 and are prone to vaporisation during warm weather as the active ingredient can lift and drift to adjoining sensitive crops.

Tank mixing of the different hormonal herbicides was the norm as each had a limited number of plants which it would control. A mix of MCPA/2 4 D/CMPP backed up with dicamba handled most situations.

The sulfonylureas (Sus) were introduced in the 1990s and became the backbone of herbicide programmes.

We started with Ally mixed with CMPP and moved to Harmony M for fire-brigade action. I suspect that it was the latter that first brought chickweed resistance to north Tipperary. We used a range of pre-formulated SU tank mixes generally mixed with CMPP until such time as it lost control of chickweed.

The advent of halauxifenmethyl (Zyphar and Pixxaro) has brought a valuable alternative mode of action to our weed control programmes and will control most of our problem weeds.

Weed control

Control is guaranteed for difficult weeds such as chickweed, fat hen, fumitory, hemp nettle, poppy, red dead nettle and shepherds purse. Reading back over that list, it is strange to see weeds we saw as simple to control in the past now being described as difficult.

Pixxaro has the new active combined with flurorphyr and is therefore the ideal choice for resistant chickweed; Zyphar with flosasulam active is applied on mayweed and Volunteer on oilseed rape.

It must be combined with either an approved SU or dicamba to fill up gaps in the weed control spectrum. The absence of chickweed, groundsel, pansy or common field speedwell might be regarded as a mechanism to ensure that it will always be tank mixed.

Sprayer operators

While the addition of new chemistry is most welcome, we are always under threat of more products being withdrawn.

The reduction in the permitted rate of Galaxy leaves the rate so tight that if spraying conditions are less than ideal we will not get successful control.

The sprayer operator now has to get everything right to ensure weed control and protect the chemicals that are available.

■ Do not rely on one active to control problem weeds. Rotate chemicals from year to year.

Do not remove one herbicide from a tank mix, with the intention of going back in later with it, in order to make the “mix” less hot. Remove one of the other pesticides/trace elements.

The one sure way of rotating chemicals is to practise a good crop rotation. We have all seen how fast sterile brome became established on continuous winter barley ground.

■ Timing of application must be right — annual broadleaf weeds must be at seedling stage and perennials must have large leaf area.

■ Prioritise weed growth stage when timing herbicide application and do not delay and wait to tank mix fungicides.

■ Spray under good weather conditions. Change nozzles if necessary. Spray penetration depends on droplet size. Poor contact with the target weed will help build resistance problems.

■ If forced to spray in less than ideal conditions, use an adjuvant. Specific adjuvants are necessary for specific purposes and can be used to stick pesticide to leaf, penetrate waxy leaves, improve leaf spread, and correct water pH.

Many budget pesticides have inferior-quality adjuvants. While the active ingredient and its concentration in the product may appear satisfactory, its availability for uptake is not.

■ Inspect crops two to three weeks after herbicide application to find if weeds are developing resistance.

Look for healthy weeds beside dead weeds of the same species; this will indicate the development of resistance and the need for a different herbicide to prevent the development of resistance; failure to control a susceptible species while a different susceptible species is killed is evidence of fully established resistance.

Practise good combine hygiene and avoid cutting smaller areas where problem weeds have become established. Clean your combine thoroughly before moving from problem to clean fields. Harvest clean fields first.

While we have the option of using herbicides we need to mind them, and the best way to mind them is to do all the simple things right.