Beet the rising blackgrass threat
Blackgrass weed problems are spreading to sugar beet due to the build up of weed seed in long autumn-sown crop rotations. Adam Clarke looks at how growers can fight back
Herbicide-resistant blackgrass is becoming a growing problem in sugar beet so using the right products pre- and post-emergence is key to improving weed control.
Blackgrass populations are building to higher levels in long autumn-sown crop rotations, and a bulging weed seed bank is now beginning to emerge during the establishment of sugar beet.
This is being exacerbated by increasing herbicide resistance to ACCase inhibiting graminicides in blackgrass populations, says independent sugar beet agronomist Pat Turnbull.
“Growers have relied on ACCase inhibitors for their post-emergence grassweed control in broadleaved crops for many years, particularly in oilseed rape and sugar beet,” she explains.
This over-reliance on a single mode of action has led to a mutation of the target site that the herbicide binds to, rendering the active substances ineffective due to target site resistance.
The ACCase inhibiting herbicide group includes the “fop” and “dim” active ingredients such as propaquizafop (various products including Agil) and tepraloxydim (various products including Aramo).
Dr Turnbull points out that blackgrass is becoming a particular issue on heavier sugar beet land, as lighter land tends to be ploughed more throughout the rotation for crops such as potatoes.
“Although blackgrass is still at a relatively low level in sugar beet at present, it’s essential to stop populations building for the sake of the rotation,” she says.
Historically, spring cropping has escaped high levels of blackgrass infestation as it is predominantly an autumn germinating weed, while sugar beet is usually established after deep cultivation such as ploughing, which has given good grassweed control.
But now the major weed headache for growers of autumn-sown crops is spilling over into spring crops, and sugar beet farmers will have to be more aware of the emerging dangers from blackgrass.
Dr Turnbull says cultural controls will form the foundation of the strategy against blackgrass, and with much of sugar beet ground ploughed in the autumn and over-wintered, a stale seed-bed can be created.
This gives an ideal opportunity to use glyphosate herbicide pre-drilling to kill any germinated weeds.
Dr Turnbull urges growers to apply the glyphosate directly on to ploughed land before any secondary cultivations take place in the spring.
“We have seen much better results where ground has not been disturbed, as it can partially bury blackgrass and allow it to escape the application,” she adds.
Alternative chemistry
In the quest to use herbicide products with alternative modes of action where resistant blackgrass is present, growers should be using both ethofumesate and metamitron products pre-emergence.
Current guidelines from the British Beet Research Organisation (BBRO) suggest that ethofumesate should be included in the pre-emergence mix at a minimum of 500g active/ha and metamitron at a minimum 1,400g active/ha.
“It is important to hold some ethofumesate back for the post-emergence sprays, but be careful not to exceed label rates as they can vary between ethofumesate products,” says Dr Turnbull.
All ethofumesate products have a total limit of 1kg active/ha every three years, which growers should keep in mind, she adds.
As the sugar beet crop emerges it should be monitored for the presence of blackgrass and where there is continued growth post-emergence, at least one, but possibly two further split applications of ethofumesate and metamitron should be used in the mix.
“Although post-emergence programmes in beet are predominantly tailored for broadleaved weed control, we know these actives have some activity on blackgrass,” Dr Turnbull explains.
Making its Debut
There is anecdotal evidence that Debut (triflusulfuron-methyl) will improve the blackgrass activity of post-emergence sprays, but Dr Turnbull cautions that more evidence is needed to establish a direct link.
“It was used in mixtures during initial trials, so any effect could be attributed to the other actives, but visually it seemed to help with blackgrass control,” she adds.
Another useful tool for sugar beet growers could be Advadex (tri-allate), with the pre-emergence herbicide already used successfully against resistant blackgrass.
However, in the first year of blackgrass control studies included in BBRO herbicide trials during 2012, results were disappointing, notes Dr Turnbull.
“We have to be clear that these trials are still in their infancy, particularly on blackgrass control, so we need much more research on these actives.
“Next, we will be looking into the responses to different doses for pre-emergence use of ethofumesate and metamitron and a range of post-emergence treatments, so more robust advice will be forthcoming,” she concludes.
Sugar beet herbicides to tackle ACCase resistant blackgrass
| Chemical family | Active | Product |
|---|---|---|
| Triazinones | metamitron | Goltix Flowable, Beetix Flo |
| Thiocarbamates | tri-allate | Avadex |
| Benzofurans | ethofumesate | Ethosat 500, Ethofol 500 |
Know your enemy
Where ACCase herbicide resistance is suspected, growers are urged to collect seed for testing to improve decision-making for their herbicide programmes.
For more information on herbicide resistance testing visit the Weed Resistance Action Group (WRAG) page on the HSE website
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