Article taken from CPM Cover Crops Supplement, April 2018, written by Lucy de la Pasture.
Cover crops have established their position in the rotation on many farms over the past few years for a variety of different reasons. With the vast majority of these needing desiccation before cultivations or direct drilling the following crop, achieving a good kill of all the weed species present can prove problematic, according to FMC Corporation's agronomy manager, Jeff Fieldsend.
""The weaknesses glyphosate has on some weeds have really come to the fore, particularly since manufacturers have moved away from tallow-amine formulations of the product," he explains.
""A drop in the level of glyphosate performance has been widely observed since the change in its formulation, with generic formulations tending to be the most variable. In many situations, it's become necessary to use glyphosate in a mixture, either as the preform Kyleo (glyphosate+ 2,4-0), or by tank-mixing glyphosate and Shark (carfentrazone), a method that's gaining popularity," he says.
Although many people tend to think of glyphosate as a 'total wipe-out' herbicide, it can struggle to achieve complete kill of several important arable weeds, particularly those that have hairy or very waxy leaves, explains Jeff.
Wide spectrum
""Shark controls a wide spectrum of weeds and helps improve the performance of glyphosate on species such as speedwells, common nettle, brassica weeds, volunteer oilseed rape, as well as tricky weeds like rosebay willow herb. Its weaknesses are mayweed, larger groundsel and chickweed but if its used in conjunction with glyphosate, they're well controlled," he says.
Agrovista agronomist John Ball has been using Shark to pep up his cover crop desiccation for some time. Based in Lanes, he looks after a range of crops, including potatoes, carrots and other vegetable crops alongside cereals, maize, grassland and forage crops.
""It's become the norm to use cover crops as part of greening measures, but also because of the many benefits they can bring to soil health. They also help stabilise the soil and mop up nutrients over the winter period," he adds.
John agrees that glyphosate performance has become noticeably less reliable since the loss of the tallow-amine formulations. ""It definitely needs another active ingredient in the mix and I've moved to the more premium-type glyphosate products to be sure of the best activity," he comments.
Many of John's vegetable growers have been making good use of cover crops for many years, but there's been a change in the type of cover crop they're now planting. ""I'm seeing a move away from grass, which was the crop they traditionally planted, to black oats and vetch mixtures. Growers are finding these easier to get established quickly and they're finding them better able to withstand difficult conditions," he explains.
And it's where he's following a cover crop with vegetables that he really has to carefully consider any label restrictions on the interval needed before planting after it's been burnt off.
One of the reasons he opts for Shark as the partner to glyphosate is because it enables the following crop to be drilled fairly quickly after application, without any worries about possible residues from phenoxy-herbicides (2,4-0) which can be damaging to following vegetable crops.
""I prefer to use this mix in many circumstances because it's more flexible than Kyleo. The 2,4-0 component in Kyleo means there are different intervals between application and re-planting, and these depend on the following crop. For vegetables there's a 60-day requirement. In contrast, Shark has an interval of just 28 days, whatever the following crop.
""That's not to say Kyleo isn't a good product - it does help control some of the weeds glyphosate alone may struggle with. But I also find Shark a better alternative where waxy weeds are present, such as volunteer OSR and brassica species," he comments. Other weeds he often finds a problem include redshank, pale persicaria, knotgrass, black bindweed, groundsel and small nettle. John has gained plenty of experience with Shark, where he's been using it in combination with glyphosate as a stale seed-bed treatment. So it's been a natural progression to transfer this mix to cover crop desiccation," he explains.
""It creates an issue in the following crop if you don't deal with cover crop desiccation properly. All the positives the cover crop brings can soon turn into a negative, particularly if transplanted weeds become established in the following crop. If this happens then they're both difficult and expensive to control, so it's better to get the job right the first time," he adds.
John suggests that such problems can be avoided by understanding the characteristics of the different cover crop varieties and managing them accordingly, which is an area that Agrovista has invested a lot of resources in.
Understanding how the desiccant works is also vital to get the best from it, adds Jeff. ""The active ingredient in Shark, carfentrazone, works by inhibiting protoporphyrinogen oxidase (PPO), which is an enzyme important in maintaining the integrity of cell membranes. PPO inhibition causes cells to collapse and plants then rapidly die," he explains.
The activity of PPO inhibitors is best under conditions of high light intensity, so Shark will be at its best on a sunny day, advises Jeff. ""Temperature isn't as important when it comes to timing applications, but there does need to be a period of at least an hour post-application for maximum rainfastness.""
Cool conditions
John prefers to burn off his cover crops as soon as the weather starts to warm up in the spring as he finds the activity of a glyphosate mix isn't usually as good under cool conditions. ""My timing of burn-off will depend on the weeds present, but I like to allow at least two weeks before incorporating any crop residues to give the chemical time to work.
""Generally, cartentrazone gives a quicker kill because of its fast-acting contact activity, and this can potentially allow cultivations to take get underway a little earlier in the spring," he adds.
Getting the ratio of carfentrazone to glyphosate rate is key to getting a really good kill, stresses Jeff. ""It's important that the ratio of one part carfentrazone to 50 parts glyphosate (360g/l) is adhered to, so that the contact action of the carfentrazone doesn't inhibit the glyphosate from being translocated to the roots of plants.""
In situations where there are large weeds(> 15cm), he'd recommend 0.33 I/ha Shark plus 3 I/ha glyphosate. For stubble cleaning this can be reduced to 0.251/ha Shark plus 2.5 I/ha glyphosate and where there's a predominantly clean situation, 0.2 I/ha plus 2 I/ha will do the job. ""There's also no LERAP requirement for this tank-mix," he advises.
John tends to keep to the higher rates in cover crops. ""Where these have been grazed by sheep, it's important to allow a period for regrowth before trying to burn it off so there's enough green leaf to take up the chemical," he says.
Apart from improving the activity of glyphosate, Jeff believes that using glyphosate with a partner product is a sensible approach from a product stewardship point of view
""Although glyphosate resistance hasn't been reported in weeds within the UK, there are increasing issues with reduced sensitivity to the active around the world. Using it in a mixture with an active ingredient that has a different mode of action makes good sense as part of a resistance management strategy," he concludes.