Grass weeds will be the main target for cereal crops in the autumn which will determine the herbicide strategy. Many of the herbicides used for grass weeds will also have some effect on broadleaf weed activity too.
The function of the autumn stack is to get the crop off to the best possible start. The pre-emergence application removes potentially competitive weeds before they have the chance to emerge, and sensitise those that do. For the pre-emergence treatment to not impede the growth of the crop it is very important to ensure seed is drilled to a depth of 32mm. This will ensure that any herbicides in the soil do not make contact with the seed even if there is some rain to wash it down, the addition of an adjuvant to the mix will bind the residual to the soil particles, further protecting the crop.
The pre-emergence application is the most valuable tool for combating black grass in the herbicide armoury; it targets the weed before emergence while protecting the crop from any contact with the herbicide. To get the most out a pre-em treatment a firm, moist, clod-free seedbed is required. For severe black grass situations it may be appropriate to apply Avadex granules (tri-allate) immediately after or while drilling. The pre-em applications are usually based around combinations with flufenacet. The application may not have killed all weeds, however this is not a failure as many of the weeds which have come into contact with the chemicals will now be sensitised and more susceptible to further herbicide applications. The activity of the pre-em starts to wear off after about 3 weeks, it is therefore important to apply a top-up of flufenacet and PDM. For example, if the crop was drilled on the 1st October, this initial application will begin wearing off by late Oct to early Nov. The second post-em application will give a boost for a further 3-4 weeks, taking the residual activity through to early December and into the spring. It is important to not overload the pre-em application to allow for further post-em top-up applications. Once in spring all contact herbicide routes are still available to us.
In order to maximise the efficacy of residual chemistry, it is advised to reduce sprayer speed and increase water volumes. Nozzle selection is also an important aspect of application. A nozzle which is angled and creates a medium to coarse droplet is preferred. All these actions result in larger water droplets which are less likely to drift into water courses, and therefore increase soil coverage of the active. Not only will this increase the efficacy and reduce the amount of wasted chemical, it will protect water courses helping to maintain the registration of actives for the future.