Take a zero tolerance approach to blackgrass

Date:12-04-2013

UK cereal growers with blackgrass that has escaped autumn control strategies need to seriously consider re-drilling badly affected areas rather than placing undue pressure on herbicides next spring, warns leading agronomy firm Hutchinsons.

Seedbed conditions this year generally suited residual herbicide chemistry and stale seedbeds, so growers who still did not achieve adequate blackgrass control need to decide sooner rather than later what to do with the worst areas, says the firm’s technical manager Dick Neale.

“Every year there are more cases in bad blackgrass areas, where pre- or post-emergence control hasn’t worked properly, where the most appropriate route forward is to spray-off cropped areas with glyphosate and start again in the spring.

“It’s a big decision to make, but one that is increasingly being faced up to, especially among early drillers.

“If you’re heading into spring with a large blackgrass population, then it’s no use relying on Atlantis (mesosulfuron + iodosulfuron) to solve the problem. Atlantis can still control blackgrass, but it’s got to be used in the autumn when blackgrass is actively growing at the one to two-leaf stage.”

Time is of the essence

The sooner any re-drilling decision is made the more time growers have to plan what to do with land in the spring to get the best crop possible and maximise blackgrass control, Mr Neale says.

He acknowledges there is no clear “threshold” blackgrass population that dictates when the decision to re-drill should be made, as much depends on the yield potential of the crop and what level of competition it can sustain.

“If it’s down to pure numbers then zero tolerance on blackgrass is the best solution in an ideal world, as relatively low levels of blackgrass have a high impact on yield.”

For example, just 12 blackgrass heads per square metre at harvest can result in a 5% yield loss in wheat. Depending on when that blackgrass establishes that equates to a maximum of around five blackgrass plants per square metre, he explains.

“A lot of earlier drilled crops this season have good yield potential so may be able to sustain higher blackgrass populations,” he says. “A 10-15% yield loss on a 12t/ha crop still gives a good yield at the end of the day.”

But growers also need to consider total herbicide expenditure and the longer-term accumulation of weed seeds in the soil profile.

Don’t rely on herbicides alone

Mr Neale says there is little doubt that existing chemistry is failing with the increased incidence of herbicide resistance, but the main actives can still deliver good efficacy if given favourable conditions to work.

A key part of that requires getting blackgrass populations down to a level that gives autumn residual chemistry a decent chance. “In most cases that’s less than 100 plants per square metre, but we’re still seeing people trying to control populations of 400-600 plants/m2, which isn’t sustainable.”

If the decision is taken to spray-off a cropped area because of blackgrass, he says it is vital to make sure the replacement crop helps weed control as much as possible. He believes spring barley, or spring oats offer the most effective answer due to their competitive nature. Spring barley also has a wide range of suitable effective herbicides approved for use on the crop, but for spring oats this is restricted to DFF.

“Don’t just pick a spring crop that’s cheap to establish and offers a high price or big gross margin. Consider it as part of your longer-term blackgrass control strategy as well. It would be a good tactic to spend some of the intended in-cereal black grass herbicide budget on higher seed rates in spring peas or beans to provide a more competitive plant stand.”

Lincolnshire-based Hutchinsons agronomist Phil Vickers works across some of the country’s worst blackgrass land and also believes re-drilling can be an answer to getting on top of the problem in the longer-term. “It’s certainly not an ideal solution, but bigger stacks of chemistry simply won’t work and if we don’t do something, the problem is only going to get worse.”

For him, delayed drilling and getting in as many stale seedbeds as possible is the key to effective blackgrass control.

“A lot of farms have managed three kills of blackgrass before drilling this autumn, which has probably taken out 2,000-3,000 blackgrass plants per square metre. But you’ve got to have the nerve to hold off drilling until at least mid-October or early to mid-November on the more problematic land.”