Soil science Leave dry soils alone to
degrade blackgrass threat • Moisture needed to start germination • Cultivation unlikely to protect seed • Patience required aſt er hot summer
ing fi elds affected by blackgrass to ensure there is suffi cient mois- ture for weed seeds to germinate. Any cultivation in very dry conditions, even shallow till- age 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 fi rm Hutchinsons. “The most benefi cial 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 sun- light. In most areas there is sim- ply not enough soil moisture for germination.” This means any form of cul- tivation 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 fi rst sig- nifi cant soil wetting in November. “As soon as the moisture came,
G
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 pres-
rowers are being urged to wait following the dry harvest before cultivat-
sures on workloads and uncer- tainty 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 germi- nation, this status is almost irrel- evant when there is insuffi cient soil moisture, Mr Neale adds. Even where there is moisture,
peak blackgrass emergence is be- tween mid-September to the end of October, so there are still sev- eral months to go before the true extent of the threat to 2018/19 crops is known. “Soils are very warm, so any
signifi cant rain in August or Sep- tember is likely to spark a ma- jor fl ush of autumn-emerging blackgrass. But remember, you still need to allow around three weeks for seeds to emerge, so don’t rush in.”
“
Be patient and leave soil undisturbed to avoid blackgrass germination
38 ANGLIA FARMER • AUGUST 2018
You need to allow around three weeks for seeds to emerge, so don’t rush in
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 of- ten have the highest population of heads, which tend to be from ‘super weeds’ that survive herbi- cide 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.”
Phenomenal patience is essential for blackgrass control this autumn, says Dick Neale
Many soils will easily take
four inches of rain and still pro- duce good seedbeds, so there is no need to panic, says Mr Neale. Waiting for suffi cient soil mois- ture is also vital for pre-emer- gence herbicides to work effec- tively, he says.
Long-term NIAB/AHDB re-
search shows around 160mm of rain is needed during Septem- ber and October for optimum ef- fi cacy of residual chemistry. Soil has to be wetted to a signifi cant 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, herbi- cide-resistant and produce more tillers, which must be considered
Protect soil Acknowledging high straw pric- es may prompt more growers to bale and remove straw, Mr Neale says it may still be worth consid- ering sowing a catch crop to pro- tect bare stubbles and improve soil condition ahead of a follow- ing winter or spring-sown crop. Fast-growing crops such as lin-
seed, 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 man- agement.
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 equal- ly unlikely to germinate, so it will be better to cut stubbles slightly higher to provide some soil pro- tection.
“Don’t forget that winter oil- seed rape volunteers are a ready- made catch crop and should be viewed as such with a termina- tion 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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