Recent changeable weather is a sting in the tail of this year’s growing season and it has highlighted the benefit at harvest of effective weed control.

Pre-harvest desiccation with glyphosate does help, but effective weed control from the start is a more sustainable solution. It’s fortunate that crops have ripened earlier than in some years, but still the pressure is on to get the fields cleared, autumn crops sown and herbicide applied.

Despite this, it’s worth remembering that if time can be found to take extra care over spraying residuals, control is more effective. Essentially, what research shows is that higher water volume applied at slower speed, together with the spray boom as low as possible and low drift nozzles greatly enhances weed control. But obviously at a slower work-rate.

This attention to detail, while not everybody’s idea of fun, is worthwhile particularly where there are problem grass weeds. It’s all about optimising control and preventing future hassle.

Brome is the most common problem, but black-grass is an increasing threat and Italian rye-grass also is something not to be complacent about.

With these weeds, the ideal is to stop them before they develop into a significant problem. Once established, they are very hard and expensive to eradicate.

Weed control is a lot easier and problem weeds less likely to develop if a mix of different winter and spring crops are grown, and there is a mix of cultivation and control techniques.

It is mainly in more intensive winter cropping situations that problems develop – this is where particular attention needs to be paid to weed control and herbicide programmes.

Essentially, it is about choosing the most appropriate active ingredients and dose to optimize control of the problem weeds. For winter cereals a residual spray applied pre-emergence of weeds is most cost-effective. At the pre-emergence timing there is greater weed control from a given dose.

Pretty much all the residuals control annual meadow-grass, a particular nuisance in a wet harvest. It is less easy to control annual meadow-grass post-emergence.

There are numerous residual products to choose from, usually being a formulated mix of pendimethalin and/or flufenacet which are particularly good on grass weeds, and diflufenican or picolinafen for broad-leaved weeds. Amounts vary depending on target weeds.

Popular products for broad-leaved weeds and grasses include pendimethalin + flufenacet (eg Crystal) and diflufenican + flufenacet (eg Liberator). There is also pendimethalin + picolinafen (eg Picona) which controls annual meadow-grass and is particularly strong on broad-leaved weeds.

Most of these active ingredients are also available as stand-alone products such as Hurricane (diflufenican) and flufenacet (eg Sunfire). These are useful to top up the amount to target a specific problem weed.

There are also other active ingredients that can be used in tank mix to enhance control of problem weeds – for example prosulfocarb (Defy) and newly-introduced aclonifen (Proclus). There is also tri-allate (Avadex) which is particularly strong on grass weeds and can be applied in tank mix, or as a separate granule application.

A special applicator is needed for the granules. They can be targeted at the worst parts of a field, for instance around the outer parts of the field for brome. Tri-allate has an alternative mode of action which enhances control. Generally, it is black-grass or brome grasses that need extra active ingredients to try to achieve control.

There are gaps in the weed spectrum of all these herbicides. For instance, fumitory is sometimes not that well controlled. So it’s horses for courses – matching the herbicide to the weeds.

There are follow-up options in the autumn, although many of the post-emergence cereal herbicides can only be used in the spring, so care is needed in choosing which product to use.

For a later autumn application for broad-leaved weeds, an active ingredient that has activity in cooler conditions, such as florasulam, is a good choice.

In fields that have a grass weed problem, delayed sowing can help control. For black-grass and sterile or barren brome the advice is to very shallow cultivate after harvest to encourage germination, then spray off once germinated. For rye, meadow or soft brome and for wild oats, the field is best left un-cultivated after harvest.