Outrageous shoulder pads and mullet haircuts are two 1980s introductions that haven’t stood the test of time, but ALS-inhibiting herbicides certainly have.

The herbicide group – which includes the widely used sulfonylureas – still provide growers with effective grass- and broad-leaved weed control in a number of crops 30 years on.

The active substances are extremely effective at very low rates – just a few grams/ha in some cases – and plant species can vary in their susceptibility up to 20,000-fold.

They are also absorbed through foliage, roots and have persistency in the soil. These characteristics make them extremely effective selective herbicides, but also mean there are significant restrictions on their use.

There are three key areas where use of ALS-inhibitors can have adverse effects on farm:

1. In following crops due to their persistence in the soil
2. Drift into sensitive adjacent crops
3. Residues left in the sprayer tank damaging subsequently treated crops.

As such, the Health and Safety Executive state that unless there is information contrary on the product label, growers should not apply ALS herbicides in tank-mixes or sequence with other ALS herbicides, including sulfonylureas.

Manufacturers of ALS herbicides are able to get mixes and sequences approved for use if they wish. To do so, DuPont – the founder and biggest manufacturer of sulfonylurea herbicides – told Crops the following criteria must be met:

• The sequence or mix has to be effective and crop safe.
• They do not pose a threat to following crops.
• They can be effectively removed from the sprayer tank.

What does this mean for growers?

Once an ALS mix or sequence has been approved, the recommendation will be displayed on the product label. This is done on a product-by-product basis, not by active, so two products containing the same active may have different guidelines.

The information will include compatible products, following crop restrictions and if any specific cultivation method must be used prior to drilling the next crop – sometimes ploughing is required to bury any herbicide residue that can persist within the surface layer of the soil.

In any case, you are never allowed to apply more than two ALS-containing products to a crop, either in a mix or sequence, and products with activity on grassweeds should never be used in sequence on a crop for resistance management purposes.

However, there is an exception to this rule, with flupyrsulfuron products such as Lexus permitted in sequence with Atlantis, providing it is used with a suitable residual partner, such as pendimethalin.

Restrictions can also be applied to product dose and an example of this would be Dow Agrosciences’ ALS-inhibiting active florasulam, contained in products such as Boxer.

The maximum total dose of the active is 7g/ha, but it can be applied in a split dose of 3.5g/ha, while only contributing to one of the permitted two ALS applications.

Due to the complex nature of ALS mix or sequence approvals, Georgina Clayton, senior registration representative at DuPont UK, advised growers to always read the label to be clear on the rules.

Using any ALS inhibitor outside  its label’s rules could result in crop damage and be a breach of cross-compliance. This may result in fines or prosecution if uncovered in a Rural Payments Agency inspection.

Pesticide watch

Crops is working with Gatekeeper agronomists to highlight key pesticide rule changes and the potential implications, keeping you on the right side of the law and aid with on-farm record keeping.

Sentinel is a decision support tool linked to Gatekeeper crop management software, a programme that helps arable farmers with record-keeping and legislation issues. For more details visit Farmplan or call 01594 545011