As the country’s single biggest weed challenge, black-grass is our first focus in weed expert, Barrie Hunt’s series of articles on the best integrated control.
Just 25 black-grass plants/m2 typically cut wheat yields by 10 per cent and produce 50,000 seeds/m2. These can survive in the soil for up to 10 years, and control levels of more than 95 per cent are essential to prevent populations increasing.
Add the fact that herbicide resistance is now widespread even in areas where the weed has not traditionally been problematic and it isn’t surprising black-grass is ‘Public Enemy Number One’ for so many of us.
However, it has several key weakness we can exploit. As an annual, it relies exclusively on seed to survive. The bulk of this germinates readily in the autumn under the right conditions. Seedlings cannot emerge from a depth of more than a few inches. And the weed positively hates competition.
Focus is the first essential we need in our efforts. Not all fields require the same level of attention. So coding each field every summer on the basis of its black-grass risk enables us to concentrate the most rigorous controls on where they are most needed.
Knowing exactly where the main black-grass problems are within our fields and the resistance status of these populations are equally important if we are to most effectively target our controls.
How and when we cultivate makes all the difference with black-grass. Ploughing the seed down to a depth from which it cannot emerge is a very useful ‘reset’ for badly infested fields where the bulk of it has been kept near the surface by years of shallow tillage. But our ploughing needs to be good enough to totally invert the ground and bury all the trash, otherwise it won’t do the job.
If we plough again within 4–5 years we can also bring viable seed back to the surface again. So we need to follow the plough with several years of shallow tillage to keep any remaining seed near the surface for the best glyphosate control ahead of drilling.
More than anything else, bad black-grass fields require patience. Delaying wheat drilling until mid-October at the earliest allows us to encourage weed seed germination with one or more stale seedbeds for glyphosate treatment.
If conditions don’t give us sufficient autumn weed emergence or enable us to produce reasonable seedbeds we must be prepared to switch to a spring crop. As well as the best margin-earning opportunity, this gives us more time for pre-planting control; something the worst black-grass fields almost certainly demand in most cases anyway.
Whenever we do drill, we should use the most competitive winter cereal varieties and spring crops at higher than normal seed rates for the greatest crop competitiveness. It’s also important we move as little soil as we can at drilling to wake-up the least amount of weed seed.
This means avoiding seedbed cultivation immediately prior to drilling as well as drills that cultivate more than the immediate area into which the crop is sown.
Even though most black-grass can no longer be controlled with herbicides alone, we simply must integrate our cultural controls with the most effective spraying programmes.
As well as removing all weed growth with glyphosate before drilling, we can also include a permitted glyphosate with any pre-em applications to tackle seedbed weed emergence where drilling has to be delayed by more than a few days after our stubble spraying.
Robust residual pre-em and peri-em herbicides are crucial for bad black-grass fields, as are firm and fine seedbeds with sufficient moisture for the greatest efficacy. And if all else fails, we should be prepared to spray-off badly-infested patches of the crop in the spring so we get the least possible seed return.
Finally, we need to keep bearing down on black-grass culturally and chemically, throughout the rotation, especially in cereal breaks which give us the best opportunities for selective in-crop grass weed control.
*As Roundup technical manager, Barrie Hunt co-ordinates the national on-line resource (www.monsanto-ag.co.uk/grassweed-action) providing the best information and guidance for growers keen to control grass weeds in the most sustainable way possible.