Weeds in winter cereals, April 2019

Grass weeds are a priority if they escaped autumn treatment, although bromes and black-grass are less easy to control in the spring.  There are no herbicide options in oats and very limited options in winter barley.  There are a number of options in wheat, although resistance issues and restrictions on sequences limit what can be done. Only one ALS grass weed herbicide is allowed per crop, so if an ALS treatment was applied in the autumn, there are very limited options in the spring.  Grass weed herbicides work best with good spray coverage on actively growing weeds. It is counter-productive to treat stressed crops too early in cold weather.

Rye-grasses

If in the past repeated application of ACCase inhibitor herbicides have been used to control rye-grass, there may be some resistance in the rye-grass population, resulting in patchy or variable control. Resistance is known in England, the extent of this resistance in Scotland is not known.  As in all cases with grass weeds, it is best to treat rye-grass when small, control is more effective and it helps reduce the risk of the development of resistance.

Wild oats

Spring germinating wild oats will continue to emerge over a protracted period, so wait until it is likely that all have emerged before treatment. If there are very large over-wintered wild oat plants, these can be treated with a split dose approach, with the second treatment catching the later germinators.

Black-grass

The main flush of black-grass is in the autumn, but there can be some emergence in the spring.  It is an increasing threat so it is always worth keeping an eye out for black-grass.  Once it becomes established it is extremely difficult and expensive to eradicate.
If it was present in a field last year, and no post-emergence treatment was applied in the autumn, then spray that area with a suitable herbicide even if the population is very low.  It is best controlled with herbicide then followed up with roguing in order to try to prevent any seed return. Speak to your agronomist or agri-consultant to discuss your strategy for black-grass control.

Mixtures and sequences of grass herbicides

Most of these grass weed herbicides have complex issues regarding mixtures and sequences with other herbicides, and, in some cases, also fungicides.  This is mainly because of effects on activity rather than safety to the crop.  Check the label carefully.