Wednesday, February 20, 2019

Leave dry soils alone to degrade blackgrass threat

July 31, 2018 by  
Filed under Crops

Growers are being urged to wait following the dry harvest before cultivating fields affected by blackgrass to ensure there is sufficient moisture for weed seeds to germinate.

Any cultivation in very dry conditions, even shallow tillage for “stale seedbed” control, risks burying blackgrass seed under the surface, where it will remain protected until there is enough moisture for germination, warns Dick Neale of agronomy firm Hutchinsons.

“The most beneficial thing you can do when conditions are hot and dry is to leave blackgrass seed on the surface after harvest to let it degrade naturally in sunlight. In most areas there is simply not enough soil moisture for germination.”

This means any form of cultivation is likely to protect seed from the sun’s radiation until rain does arrive. A similar situation last occurred in 2011 when a lot of blackgrass got buried in dry soils and didn’t grow until the first significant soil wetting in November.

“As soon as the moisture came, blackgrass emerged within crops and swamped them,” says Mr Neale. “Growers must learn from this to avoid a repeat.”

Playing the waiting game can be hard in practice given pressures on workloads and uncertainty about when weather will break, acknowledges Mr Neale, so phenomenal patience will be essential for effective blackgrass control this autumn.

Dormancy status

Although warm, dry conditions during June and into July are likely to mean relatively low blackgrass dormancy, typically favouring earlier autumn germination, this status is almost irrelevant when there is insufficient soil moisture, Mr Neale adds.

Even where there is moisture, peak blackgrass emergence is between mid-September to the end of October, so there are still several months to go before the true extent of the threat to 2018/19 crops is known.

“Soils are very warm, so any significant rain in August or September is likely to spark a major flush of autumn-emerging blackgrass. But remember, you still need to allow around three weeks for seeds to emerge, so don’t rush in.”

Many soils will easily take four inches of rain and still produce good seedbeds, so there is no need to panic, says Mr Neale. Waiting for sufficient soil moisture is also vital for pre-emergence herbicides to work effectively, he says.

Long-term NIAB/AHDB research shows around 160mm of rain is needed during September and October for optimum efficacy of residual chemistry. Soil has to be wetted to a significant depth for residual herbicides to work well, but even then control still only averages around 62% of plants.

Plants that survive treatment are likely to be stronger, herbicide-resistant and produce more tillers, which must be considered when evaluating further controls and weed risk, says Mr Neale.

“We’ve seen at Brampton that plots with the highest percentage control of blackgrass plants often have the highest population of heads, which tend to be from ‘super weeds’ that survive herbicide treatment and are capable of producing 25-30 tillers per plant.

“So even if you achieve 90% control of plants, that probably only gives you 60% control of heads given the weed’s ability to produce more tillers at lower plant populations.”

Protect soil

Acknowledging high straw prices may prompt more growers to bale and remove straw, Mr Neale says it may still be worth considering sowing a catch crop to protect bare stubbles and improve soil condition ahead of a following winter or spring-sown crop.

Fast-growing crops such as linseed, phacelia, oats or a legume provides surface protection, but the roots also play an equally, if not more vital role in improving soil structure and moisture management.

This is particularly important where growers are looking to drill late in the season – or in spring – on heavy land. But if conditions remain dry, catch crops are equally unlikely to germinate, so it will be better to cut stubbles slightly higher to provide some soil protection.

“Don’t forget that winter oilseed rape volunteers are a ready-made catch crop and should be viewed as such with a termination date in late September when blackgrass germination really gets going. Growers should also avoid growing winter wheat on land where blackgrass is a big problem.”

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