Growers should act soon to destroy heavy blackgrass infestations in their cereal crops and ensure that viable seed return is minimised.

This year has been a mixed bag for control of the problem grassweed, with some growers achieving acceptable control where well-timed pre-emergence herbicides were applied.

However, many later-drilled crops missed out on this important chemical application and those cases are now visible across the arable areas of England.

Rothamsted Research blackgrass expert Stephen Moss says he has received calls from growers concerned about the early appearance of blackgrass heads above the crop this year.

The mild winter resulted in forward crops and blackgrass plants, prompting fears of an earlier shed of viable seed.

“In my many years of studying blackgrass, it hasn’t changed much though and invariably shedding occurs from mid-June onwards,” he explains.

Dr Moss says that visible heads aren’t necessarily carrying viable seeds and growers should get out and check the blackgrass plants in the field.

“If you squeeze the spikelet and an orange caryopsis, or mature seed, comes out and not mush, it is viable,” he adds.

“Also, if rubbing the head doesn’t remove the seed, it’s unlikely that it’s viable,” says Dr Moss.

Good strategy

Using glyphosate to kill bad blackgrass patches or cutting a heavily infested crop for whole-crop silage are under-used methods to reduce viable seed return, he believes.

“Both are effective, particularly spraying. With the mapping methods now available it is easier to have an accurate, localised fallow year on year,” says Dr Moss.

Timing is critical for both, but more so for spraying with the herbicide glyphosate and technical manager at manufacturer Monsanto Manda Sansom says trying to kill blackgrass too soon can give variable results.

“If the plants are sprayed while still in stem extension, you can often get regrowth once the effects of the herbicide has worn off”.

The best kill is achieved when blackgrass is sprayed after heads have fully emerged, so growers should be guided by growth stage and not calendar date.

“However, the last week in May to the first week in June is a good guide,” explains Mrs Sansom.

She recommends 3 litres/ha of Roundup 360 or equivalent to deliver 1,080g active/ha with a water rate of 100-200 litre/ha. Growers should check that the product is approved for “crop destruction” on the label.

Low drift nozzles are also encouraged to minimise the risk of damage to surrounding crop or other non-target fauna and good weather conditions with little wind and no rain forecast are important.

Whole-cropping

The increased number of anaerobic digesters has made whole-cropping cereals, such as wheat and rye. to remove blackgrass a more viable option for many growers.

Dr Moss says although there is no danger of seed return when cutting the crop early, there is a question of how late to leave it.

“Growers should plan to remove the crop no later than the first week of June. After that the risk of machinery knocking viable seeds to the ground increases.

“With both spraying and whole-cropping, the later you go the less effective they become, so timing is everything,” says Dr Moss.