Blackgrass may spread to more new ground than usual this autumn as a consequence of the early start to harvest, warns weed specialist Stephen Moss of Rothamsted Research.
Typically, blackgrass plants shed most seeds before the wheat harvest begins. However favourable conditions, both for blackgrass growth in many parts of the country, and for early harvesting, have allowed combines in before blackgrass plants have finished shedding.
This could mean weed seeds are more likely to contaminate straw and grain, thereby increasing the risk of blackgrass being spread to new areas, suggests Dr Moss.
“Blackgrass plants will continue to shed seeds into August as harvest proceeds. There will possibly be more potential for contamination than if they had all ripened and fallen below the crop canopy as usually happens in wheat.”
The seriousness of this threat shouldn’t be underestimated, with blackgrass infestations capable of increasing by more than tenfold in one season. It’s particularly worrying for growers in “non-traditional” blackgrass areas.
“One year blackgrass can be adequately controlled using herbicides, and the next it can be unmanageable simply due to different weather patterns.”
“I have received reports of blackgrass in Ireland, Scotland and parts of Yorkshire where there is a suspicion it has come in with crop seed. It is reportedly only growing in the row, which, despite other possibilities, is a strong indicator of contaminated crop seed.
“Since such blackgrass may already be herbicide resistant despite no herbicides having been used in affected fields, I would collect a seed sample for resistance testing,” urges Dr Moss.
- Greater spread of blackgrass seed.
- High blackgrass seed return = high 2015 populations.
- Low autumn dormanacy – moisture dependent germination
Hand rogueing, ideally done in June, could still be appropriate and worthwhile where spring crops are concerned. It is fairly straightforward in a year such as this when weed plants are tall, he adds.
Dr Moss believes the likelihood of next year being a bad blackgrass year is quite high due to substantial seed return, though as always, the outcome will depend upon the autumn weather. Many uncut wheat fields have high numbers of blackgrass heads (200-500/sq m), despite good control from pre-emergence herbicides last autumn.
“While pre-emergence herbicides applied before November generally worked well, the few survivors fared well and recovered during the mild winter. Then they took off in warm spring seedbeds, produced more tillers than usual and ended up towering above wheat.”
The situation varies from field to field. It is important to modify management, considering each case separately, where chemical control is failing due to herbicide resistance, he suggests.
The worst case scenario is where blackgrass populations exceed 500 heads/sq m despite high herbicide use. These “messy” fields are typical where minimum tillage has replaced ploughing and resistance to post-emergence herbicides is well developed.
Burying weed seeds to at least 10cm is still one of the best methods of control, provided they are not uncovered by that approach the following season. Dr Moss suggests “opportunistic” rather than rotational ploughing, that is, choosing the year it will have most benefit rather than routinely ploughing.
Options and Actions
- Delay drilling and make the most of stakle seed-beds.
- Opportunistic ploughing.
- Resistance testing
- Switch to spring cropping, fallow or grass leys for at least two years in problem fields
While careful choice of cultivation should ease the blackgrass burden, the principal solution is to modify cropping. Question whether early autumn sown crops such as oilseed rape are justified, he insists.
Based on the knowledge that average seed decline is 74% per year and just 3% survive three years, he would consider double fallow, double spring crop, or fallow for 18 months then grow a spring crop in the second year. Alternatively, a two to three-year grass ley is an option where this can be used.
>500 heads/sq m; herbicide resistance rife; crop lodged; yields halved.
Question whether you should still be autumn cropping. Consider spring cropping, opportunistic ploughing and even a fallow or a grass ley.
<50 heads/sq m; pre-emergence herbicides mostly effective; minimal yield loss.
Use stale seed-beds, delay drilling, use high seed rates and robust pre-emergence herbicides.
A few scattered heads; no yield effect.
Dont ignore consider how the blackgrass might have reached your farm in seed, straw, manure or machinery? Get a seed sample tested for resistance and prevent further spread. Hand rogue if still relevant.
“You must look long term and minimise seed return over at least two seasons. Good control for one year won’t be sufficient to bring bad populations of resistant blackgrass down to acceptable levels.”
Around 80% of blackgrass plants emerge in the autumn, depending on the level of dormancy, which he expects to be low, but not super low, this year. The period of dormancy, set in the seed by shedding, is mostly temperature driven. A hot and dry summer leads to a state of low dormancy, meaning seeds germinate quickly after shedding, provided there is sufficient moisture.
“This summer has seen above average temperatures coupled with localised showers in many regions. I wouldn’t be surprised if some weed seeds take longer to germinate than usual, though I’m not expecting high dormancy,” he says.
So that’s the worst case, but a more common scenario will be low blackgrass levels that aren’t yet seriously impacting on yield and generally good herbicide results. Dr Moss warns growers in this situation not to be lulled into a false sense of security. Seed should be tested for herbicide resistance and carryover proactively prevented, to avoid future problems.
The single most effective and least disruptive way to prevent blackgrass buildup is to delay drilling to allow time for weed emergence and subsequent kill using glyphosate (eg Roundup), says Dr Moss.
August and September rainfall will determine the speed of weed seed germination. Remember that if it remains dry throughout the autumn, blackgrass germination pre-drilling will be poor and pre-emergence herbicides won’t be as effective as you’d like them to be, he warns.
“Bear in mind that blackgrass has become successful for two principle reasons – too much September sowing of cereals and increasing herbicide resistance. You cannot ‘undo’ resistance, but you can sow later in the autumn or in the spring,” he concludes.