The crop protection seminar and technical hubs at this year’s CropTec event highlighted some of the key challenges growers are facing, as well as providing practical measures for improved weed control. Heather Briggs reports.
There are a number of threats and opportunities in crop protection, said Fiona Burnett, professor of applied plant pathology at Scotland’s Rural College (SRUC) and sector lead for agriculture at Scotland’s Plant Health Centre.
Speaking at the CropTec Crop Protection seminar, she noted that when offered a binary choice between food with or without pesticides, public opinion comes down
on the side of no pesticides.
Other threats include legislation, sub-optimal practices and knowledge gaps. Pesticide resistance is also a growing problem, especially as fungicide resistance is eroding established actives and threating new ones.
Moreover, estimations are that it will take between five and eight years for new technologies to be able to ameliorate such losses for farmers to get back to the yield levels of the previous full pesticide cupboard.
However, the picture is not completely bleak as there are some opportunities.
New active ingredients are coming on the market, albeit slowly, and better stewardship will help retain existing ones, Prof Burnett said.
“Heavy usage of an active confers a massive advantage to resistant individuals, so stewardship measures based on reduced reliance are very important, as is the advice to use all measures to reduce pressure on chemistry; this includes mixing, alternating and using low-risk multi-sites.”
Prof Burnett noted that advice should be based on understanding when crops respond best and when other sprays can be stripped back or minimised.
For wheat, protecting the flag leaf is key, while for barley the most important application is at T1 and later sprays after T2 tend not to respond well.
Sustainable practices with market premiums and good public support could provide an important way forward when managing with less and protecting what is left.
“Do everything to reduce risk, such as managing rotations, choosing resistant varieties, using certified seed, monitoring crops and tailoring sprays according to the threat.
“Keep abreast of developments and follow the best technical advice. Challenges are real and we need to prepare for change,” advised Prof Burnett.
Weeds are plants out of place in a crop, but on a headland they can provide cover and food for wildlife, said ADAS weed specialist Dr Sarah Cook.
However, some weeds are highly competitive, and can have adverse effects on yields; for example just two cleavers, five wild oat plants, Italian ryegrass or sterile brome per m² can each result in a five per cent loss in yields.
“Moreover, charlock can raise erucic acid levels in oilseed rape, so there is a clear drive to control them,” Dr Cook said, adding that this used to be difficult as they are a species related to the crop, but is now possible by using Clearfield technology.
“Using herbicides for weed control is expensive and are the highest input cost for growers,” she commented further.
She expressed concern that glyphosate may be under threat – not from any scientific evidence but from pressure applied because of public opinion.
“This will make cropping both more difficult and expensive, and rather than using a single pass, there will have to be a return to annual ploughing, and it may even result in the need to cultivate larger areas to achieve the same output of crop.”
Dr Cook noted that with grass weed control there are options for cultural control because they mainly emerge from the top 5cm of soil. While cultivations bury some seeds, they also bring others towards the surface where they can then germinate.
“The good news is that grass weeds have a persistence of around five years, but broad-leaved weeds can persist for over 10 years.”
There is also speculation, but as yet little evidence, that resistant strains of broad-leaved weeds are appearing.
“As we lose more herbicides, choices are decreasing so this increases the pressure on those we have left, and we may see new weeds coming into crops.
“Couch may also reappear from the margins and there could be challenges from sulfonylurea resistant poppy, mayweed and chickweed.
“We need to learn the lessons from other resistant species such as certain grass weeds.”
A new case of resistance has been discovered in prickly sow-thistle, which is resistant to imazamox, metsulfuron-methy and thifensulfuron-methyl
Climate change may also play a role by facilitating new and different weeds to thrive in Britain, such as common ragweed, which originated in North America.
“Populations are already established in Hungary, France and Croatia, and, as this plant has highly allergenic pollen, it is causing significant health problems.
“These problems are likely to become worse in response to higher carbon dioxide levels over a longer period.
“Once it becomes established, control measures are both labour intensive and expensive.
“In the end it is not the weeds that you can control that is important, it is those that you cannot control.”
Seed bank dynamics play a crucial role in long-term black-grass control, said NIAB weed management specialist John Cussans, speaking at the Black-grass Hub.
This is because seedlings are just five per cent of the total black-grass populations and the remaining 95 per cent lurking underneath the surface need to be managed to keep on top of the weed. But it is not necessarily just a numbers game; the location where the seeds are in the bank is key to understanding a field’s weed burden, he said.
This is because seeds need to be in the top 5cm of soil to be able to germinate and once buried, their viability declines at between 75–80 per cent each year.
A recent weed seedbank assessment led by Mr Cussans as part of Bayer’s Black-grass Task Force in Action looked at weed seed distribution in soil samples from a range of depths on two farms using different cultivations strategies; deep non-inversion and no-till.
“There was a clear difference as no-till had the highest concentration very close to the surface,” he revealed.
However, germinated weed seedling numbers were the same on both farms.
Mr Cussans said: “The proportion of shallow seedbank recruited seedlings is lower in no-till and these observations showed that no-till resulted in a two- to three-fold reduction on how much of the surface reservoir is stimulated to germinate.
“Nevertheless, even if you spray off what is on the surface the absolute total size of the seedbank is higher so you still have the reservoir in the seedbank which will need managing to keep on top of the weed.
“Otherwise you could be just one poor establishment event away from becoming a black-grass victim.”
Getting boom height, water volume and forward speed right is key for effective pre-emergence weed control, said Syngenta eastern application specialist Harry Fordham, speaking at the Syngenta Spraying Hub.
Pre-emergence herbicides are now the backbone of autumn black-grass herbicides, so ensuring the best possible efficacy from application technique is crucial.
“Boom height has a huge influence on drift and making sure the herbicide hits its target,” he said.
This is partly because of reduced boom stability, but also the time of year can affect accuracy, especially when higher speeds are used.
“In the autumn, the warm soils create rising currents, so there is more turbulence behind the boom which impacts on small spray droplets, so fewer hit the soil in the right place.”
This is even more noticeable on bare soil, where there is no crop to drag down and capture smaller spray droplets.
“Moreover, when the sprayer operator is travelling at 16kph, drift can be 10 times more than when travel is 6kph, trials have shown.
We recommend not going over 12kph when applying pre-emergence herbicides,” he emphasised.
Getting water volume right can make a real difference, too, he added, noting that more than 200 trials have shown 200-litres/ha gives the best combination of coverage and efficacy.
Mr Fordham said: “One of the challenges, particularly this autumn, has been the limited window of good spray days. Between the beginning of September and the end of
November there have been just five.
“We know the 3D nozzle works extremely well under perfect conditions but, whereconditions may be compromised, the 90 per cent drift reduction nozzles really come into
their own,” he added.
“In trials they have matched or exceeded the 3D nozzle, with the best performance so far from the 90 per cent Teejet drift reduction nozzle.”
Average seed number stands out as the main determinant of performance success with OSR, according to ADAS analysis of the first three years of the Oilseed YEN competition reported at CropTec.
Speaking at the event’s Dekalb OSR hub, ADAS head of crop physiology, Dr Pete Berry explained that this single factor has accounted for 66 per cent of the 4.8t/ha yield variation recorded between the 150-plus crops involved in the benchmarking competition to date.
“To have a chance of getting 5t/ha you need more than 100,000 seeds/m²,” he stressed. “And since seed number is determined by the amount of light intercepted in the 2–3 weeks after flowering, in our experience this means a well-structured canopy with a green area index (GAI) of between 3.0 and 4.0.
“The highest yielding 25 per cent of our YEN crops have also come from noticeably lower seed rates than average, as well as receiving more fungicide applications.
We’ve seen a clear and consistent association between yield and the number of days from flowering to desiccation too. On average an extra 11 days of seed setting and pod filling has accompanied a yield difference of more than 1t/ha.“
“On the nutritional side, both soil and post-harvest seed analyses have highlighted a relationship between the highest yields and higher-than-normal magnesium contents which we are investigating further to establish the extent to which this link could be causal,” he continued.
Dr Berry agreed that the vagaries of the season have an important influence on OSR performance. Indeed, long-term ADAS studies show dry Decembers, warm Marches, dry and sunny Aprils and cool, wet Mays are associated with the highest yields.
However, he stressed that weather factors only account for less than 40 per cent of the variation recorded in annual crop yields. Which obviously leaves more than 60 per cent associated with farms and farming factors, many of which can actually be controlled.
“As it develops and grows, the Oilseed YEN competition is yielding better and better intelligence on the most important farm limitations to OSR performance to inform national improvement efforts.
“Every bit as important, it is allowing the growers involved to benchmark their production systems with others across the country for around 70 individual parameters so they can identify the elements they can most usefully focus on to improve their own particular enterprise performance.
“With the YEN approach we look forward to providing increasingly accurate guidance on the best ways of improving the level and consistency of UK OSR production, and to working closely with forward-thinking growers to explore a range of the most promising improvement options and ideas under field conditions where it really matters.”