Do cultural control strategies for blackgrass need to change in view of autumn weather patterns? Mike Abram reports from the Niab Blackgrass open day

Delayed drilling remains a technically favoured action for more effective grassweed management, but there is recognition that after two difficult autumns for establishing wheat crops it is one that might need adapting, according to a leading weed management expert.

There is no escaping that for blackgrass, ryegrass and even broad-leaved weeds that a longer period between weed seed shed and drilling the next crop gives more opportunity for predation, seed death and spraying off weed seedlings with glyphosate, Niab weed biology and management specialist John Cussans says.

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“You reduce weed pressure in blackgrass, for example, by 15% every week you don’t drill a crop in the autumn,” he points out.

“But we are very much in a new normal when it comes to autumn weather patterns.

“If you’ve had a once in a 100-year rainfall season, three times in five seasons, you need to rethink risk management calculations.

“That obviously has an impact on the drilling date message that’s been used as a mainstay for grassweed management.

“We need solutions that allow growers to drill when they want, rather than always saying I wouldn’t start now, which is unhelpful,” he admits.

Unfortunately, finding those solutions is not easy.

Later drilling also usually means more moisture, which increases residual herbicide activity, while cooler temperatures extend the herbicide’s half-life, he notes.

“It’s a double-whammy. Herbicide performance can double or triple in some trials [in later drilled crops].

“Drilling earlier inevitably means using more herbicide for the same outcome and in the driest conditions we just can’t achieve acceptable levels of control.”

But with higher residual herbicide loading comes an increased risk of crop effects.

Possible solution

One possible solution Niab has investigated this season, with the support of Adama at its new Hinxton blackgrass trials site in Cambridgeshire, is to use seed rate as a tool to increase crop competition and mitigate against herbicidal crop effects.

In the trial, 15% of crop plants were lost at the earlier drilling date following use of the most intensive herbicide programme.

The trial compares four seed rates from 75 seeds/sq m through to 600 seeds/sq m overlaid with three herbicide programmes plus an untreated control, at two drilling dates – 30 September and 7 November.

While the impact of crop competition on blackgrass populations was visible when doubling the lowest seed rate to 150 seeds/sq m, the best outcome was in the plots with 600 seeds/sq m.

“This is above the optimal level from an agronomic point of view, but are the sorts of levels you need to have traction on weeds,” John says.

“You’re wasting your time if you’re thinking you can just nudge up seed rates where you’ve got a patch of blackgrass.”

Even so, on this site with untreated blackgrass populations in the high hundreds per square metre, control with the best herbicide programme and highest seed rate wasn’t commercially acceptable.

“You can mitigate to some extent by just piling the crop on, but you cannot get away from the fundamentals that you have high blackgrass pressure because you drilled early and lower herbicide performance because of the conditions.

“One advantage of drilling earlier is opening more opportunities for herbicide applications, so using a sequence of pre- and peri-emergence applications, which we’ve shown to be quite effective.

“But it is going to be a challenge to find solutions within the crop itself.”

Earlier drilling also reduces varietal disease resistance by nearly two rating points when combined with high seed rates, increasing required fungicide input, while lodging with higher seed rates is also a potential challenge.

All those factors have input cost implications, which calls into question the tactic’s financial viability.

Last autumn

But in an autumn like last year, delaying drilling also has its challenges.

“Our next opportunity to drill was 7 November when conditions were marginal.

“We had an issue with blocked coulters, and with the same seed rates crop establishment dropped by 25% and the high intensity herbicide programme thinned the crop by 40%,” John says.

Herbicides worked better with a 70% reduction in blackgrass plant counts.

“But if we didn’t spray plots with knapsack sprayers, we wouldn’t have been able to spray.”

There’s also a seasonal element, following another spring not conducive for growth, which further reduces the suppressive nature of crop canopies.

“Even at 600 seeds/sq m, because you’re putting the crop in rows, there’s a limit to what you can achieve.”

However, he does think the trial demonstrates it is possible to use higher than optimal seed rates to facilitate drilling earlier, just not at the level of blackgrass pressure at the Hinxton site.

“If you start with a lower pressure situation, then it is an option.

“The power of the seed rate is phenomenal – it’s almost a straight line in how adding more crop is reducing seed return.

“But you have to accept your herbicide isn’t going to work as well, so to get the same level of control of blackgrass heads will require more spend.”

Ultimately the solution to the drilling date conundrum isn’t thinking about weed control in the crop itself but thinking about reducing the weed population round the rotation. Perhaps the best option is enabling earlier drilling is to extend rotations using more spring cropping.

“That takes the pressure off and can allow for earlier drilled autumn crops when they come round, but you need a spring crop that makes you money, which isn’t that easy to find.”

John Cussans and Jasper Kanomanyanga were speaking at a recent NIAB Blackgrass open day