7 October 2000

A-teams mission to beat blackgrass

Cultural control forms the A-teams cornerstone for keeping blackgrass in check. Tom Allen-Stevens seeks their advice.

BLACKGRASS. Its the arch villain of weeds – the cunning combination of highly prolific yield robber thats difficult to tackle. Herbicide-resistant blackgrass has been found in every county where the weed occurs. Target site resistance can occur rapidly and has been found in over 25% of samples sent in for testing this year. Its far less common and harder to deal with than enhanced metabolic resistance. But even so, if other weeds pose a problem then this one poses a complex managerial challenge.

Ready and willing to take up the challenge is the A-team – the team of experts who have come together on a mission to achieve the optimum yield from a low cost input regime. Blackgrass is the first hurdle for the team and this is where specialist knowledge from ADAS James Clarke comes to the fore. His rule of thumb is to find out first what youre dealing with: "Its about knowing your resistance status. All growers should now be considering the resistance risk as part of their blackgrass control strategy."

Resistance test

So a key feature of this strategy is to get a resistance test done. This should be repeated at least every three to four years on problem fields. Growers should also study their records – cultivations, crop choice, herbicide use – on a field by field basis to assess resistance risks and past results of control strategies, says Mr Clarke. "Blackgrass doesnt occur on its own, so growers also need to be determining their strategy in conjunction with controlling other grasses like wild oats and barren brome."

As well as offering advice our experts will be putting their gems of wisdom into practice in a series of trials, sponsored by Novartis. A site at ADAS Boxworth has been specially chosen to study the various blackgrass control strategies. The site is badly infested with the grass, and a provisional resistance test from within the same field this year has shown it to have some target site resistance (RR). Two different cultivation regimes – the plough versus minimum tillage – will be tested in these plots, along with seed rate and drilling date.

And these are the factors that growers should manipulate to get the majority of their blackgrass control, insists Mr Clarke. "The critical period is between harvest and drilling. Harvest has been delayed by two weeks in some parts of the UK, so this key window for control is already smaller. Were also predicting a protracted emergence of blackgrass seed this year." It is believed that when blackgrass seed matures in wet conditions it has a longer dormancy, so this year Mr Clarke is predicting there will be no sudden flush of weeds, reducing the numbers that might have emerged before drilling.

All this adds fuel to the argument to delay drilling where there are high populations. "Your grass weed control strategy should form a large part of deciding when to drill. If thats before the end of September, youll have missed valuable opportunities in your worst fields to reduce the population of blackgrass in the crop, hence putting more pressure on herbicide choice within the crop. By delaying drilling to around mid-October you will maximise the numbers controlled by non-selective options. Make the most of your performance by choosing a variety that is best suited to drilling in mid-October."

As for cultivations, Mr Clarke insists there is definitely a place for the plough: "If theres been a high weed seed return this year, but not much in previous years, getting the plough in could be a good idea. Thorough inversion via ploughing remains an effective control of a wide range of grass weeds."

It also gives you the opportunity to spread drilling dates, he points out; ploughing fields with few new seeds coming from the seedbank will result in low blackgrass levels which means fields can be drilled earlier. Minimally cultivated fields, where you must get full control by encouraging a flush of weeds and using non-selective herbicides before drilling, can be drilled later. Bear in mind other rotational considerations however, such as take-all, advises Mr Clarke.

Weed population should also be taken into consideration when deciding seed rate, he believes. "The final step of getting good cultural control is to make use of crop competition. This will both help herbicides and minimise the numbers of seeds produced on any survivors. Adequate plant populations can result from reduced seed rates, but you must tailor seed rate to the conditions and drilling date."

Once all the cultural methods have been put in place, the final step is chemical control. "Herbicides should be regarded as the icing on the cake. If you use them as your sole method of blackgrass control, you will be likely to get resistance," warns Mr Clarke.

"You should be looking to maximise activity and use a strategy that brings a high level of control. That means timing of herbicide applications will be very important," he continues. The time when there is most scope for chemical control is relatively early – when the plant has between one and three leaves, he maintains.

Mixture

As far as product choice is concerned, the key is to get a variety of herbicide groups both between and within years, says Mr Clarke. He suggests a sequence or mixture of different chemicals as a good strategy to maximise the efficacy of different modes of action. One product often used in sequence is tri-allate (Avadex). Its most effective if used pre-emergence of the blackgrass with the follow-up timing of the main spray being at the one-to two-leaf stage again. "If you fail to apply it pre-emergence, it might still be worth using it very early post-emergence because of added residual value," says Mr Clarke.

Pat Ryan, Novartis head of development and technical support, also strongly advises the use of a mixture: "Reliance on one active ingredient alone can lead to resistance. Also a two-product mixture gives better results because of factors that affect efficacy at the time of application." Hawk (clodinafop-propargyl and trifluralin) is an example. Spraying conditions may be ideal for one active ingredient but bad for another. A mixture of the two spreads the risk, he says.

But Mr Clarke warns against over-reliance on fops: "Our work has shown this can be foolhardy. We have had incidences where amount of resistance relates closely to amount of fops used. My strategy would be to try to take the pressure off the fops and use them less. Id be particularly worried about their use year after year in a min-till, continuous cereals situation."

So its the leading mixtures and sequences on trial in the plots. Three post-emergence mixtures will be compared at growth stage 11-12 of the blackgrass: Hawk + IPU, Lexus (flupyrsulfuron-methyl) + Stomp (pendimethalin) and Hawk + Lexus. Pre-emergence treatments will also be trialled with these, some existing and some not yet approved. Cost range will be £20-25/ha without a pre-em, £35-40 with.

Its not a big consideration in the plots, but Mr Clarke stresses growers will need to take the whole weed spectrum into account: "Some treatments, while excellent blackgrass killers, will not do anything for wild oat control. Many people have found that when they have used Lexus + Stomp, they discovered they had wild oats they had forgotten about when using previous treatments, such as IPU."