Pre-emergence herbicide strategy – Potato Review
In the second in our series or articles, Andrew Goodinson, agronomist and potato specialist, offers insights to help growers develop a cost-effective weed control strategy ...
Weed control in the early stages of crop development is vital for achieving high yields. Knowing your field, including soil type, layout and topography, as well as previous cropping and current weed burden, is the keystone to staying ahead of the game by devising a made-to-measure herbicide strategy, according to Andrew.
One of the challenges many growers face is that they are growing on rented land, so the knowledge of the previous history of the field can be limited, so he likes to walk the field, inspect stubbles and map out and identify patches of weeds, identifying control priorities.
Andrew said: “There are a wide range of soil types and weed spectrums, and as you cultivate you can bring up buried weed seeds, some of which can be more of a threat than others.
“Weeds such as bindweed and cleavers can grow through the crop and smother the canopy, while others, such as fat hen, oilseed rape, and thistles also compete with the crop for nutrients, light and water.
“Those of a similar family to the crop, such as cleavers and black nightshade (Solanum nigrum), not only compete, but they can also host pests such as Rhizoctonia which can hit quality and yields.”
Black nightshade, which is prevalent where Andrew is based in the west, is one of his priority weeds. According to the AHDB, seeds can remain viable in the soil for over five years, and may also be eaten by cattle when grazing, so seeds are then passed out and applied back onto the field.
Fortunately, we have still some useful tools, such as Defy (prosulfocarb), Artist (flufenacet) and Inigo (metobromuron), which help keep on top of black nightshade control, although it is an area of weakness for aclonifen.
Fat hen is another priority weed, as it has a high number of seeds and grows rapidly and can cause shading of young crop plants. “One of the problems of this weed, which likes high nitrogen loams and sandy soils, is not only does it compete for nutrients but it can also grow higher than the spray boom and can affect blight spray efficacy. It can also create a micro-climate where blight spores can proliferate.”
In areas where fat hen is prevalent, Andrew recommends aiming for pre-emergence control, and then if the weed is present at the time of spraying, using protoporphyrinogen oxidase (PPO) inhibiting herbicides as part of a tank mix to remove any which have emerged. Other weeds high on Andrew’s priority list include black bindweed, cleavers, fumitory and thistle.
Input costs are going up, but it is well worth choosing products that will fit the crop, soil, harvest interval, and work on the shape of the beds, Andrew said.
Soils in the area are sandy, or sandy loams, and can be quite challenging when it comes to pre-emergence herbicides, because applications of some residual active substance can cause crop damage in certain potato varieties. As such, any susceptibility should be taken into account in the decision-making process, he advises.
“Herbicides move through sandy soils more quickly than heavier ones, so persistence is shorter. However, as these soils tend to be used for growing earlies or salad potatoes, which are lifted earlier, you do not need activity for such a long time.
“In addition, if you are growing first earlies, a second crop may be planted, so you need to take into consideration whether it will be affected – some residuals such as clomazone can persist in the soil and affect following crops such as onions.”
Andrew recommends basing strategies around metribuzin, but, as every field and situation is different, adding others to the mix depending on weeds and soils. Soil type can affect activity of some of the herbicides, but one which is popular and can work across different soil types is a base mix of metribuzin and aclonifen.
“Metribuzin with metobromuron is another useful tank mix far pre-emergence control, and aclonifen can make a useful addition to controlling your weed burden,” he said.
“If the weeds have already emerged, then you may find that adding in a PPO inhibitor such as pyraflufen-ethyl, which disrupts the cell membrane, at a rate of 0.4l/ha, is the best way forward.
For priority weeds such as cleavers, he recommends a mix that includes metribuzin and prosulfocarb. “If you are growing varieties that are susceptible to metribuzin, then a good alternative is to opt for metobromuron and prosulfocart or aclonifen.
Not all seasons go to plan, and when unexpected weather events result in a flush of weeds well before emergence, and a pre-planting application of glyphosate was not put on, there is an option to use Roundup Flex immediately after planting.
“This is a particularly useful get-out-of-jail strategy if you have volunteer cereals or grassweeds, as the PPO herbicides are weak in this area,” said Andrew.
Bentazone is another useful but more expensive option if pressure from black nightshade or cleavers is very high but he cautions that care needs to be taken to ensure compliance with stewardship guidelines and prevent the product getting into groundwater.
With bentazone, getting the timing right can be difficult he said. If for any reason, pre-emergence herbicides do not go on at all, Andrew recommends going in with Rimsulfuron. “It is better to go in with something rather than leave the weeds to proliferate, he said.
He also reminds growers not to spray when heavy rain is forecast, because of the risk of run-off into water courses. We must not allow ourselves to become complacent about run-off.
Once the weed burden has been identified and decisions taken on which actives to use, Andrew then likes to spend time thinking about how to ensure maximum efficacy from spraying, reducing potential run-off and keeping booms level. If the farmer has Omnia technology, plans can be drawn up where the tramlines should be, before going into the field.
“I use the system to map where structures such us trees and pylons are placed, and plan tramlines so the operator can fold in one section of the boom, preventing spray misses or overdoses as the sprayer works around the obstacle,” he said.
The spray operator can also use Omnia to pinpoint an area of interest in the field that they want the farmer or agronomist to check, he adds.
The withdrawal of cheap and flexible herbicide diquat has meant agronomists and farmers have had to rethink timings and management, Andrew said.
“If time and workload allows, the optimal time to apply herbicide is now between seven and ten days after planting.”
Residual and contact herbicides are normally applied together seven days before emergence 70 percent of the time and recommends using contact herbicides until the crop is at five percent emergence, although this can be stretched to 10 percent in maincrop.
“If the crop is at 50% emergence, it can hit the plants and slow development by a week while it recovers. Whatever the old wives’ tales say about the crop making up for it, you really don’t want to stop it growing. But it can be a difficult decision whether to hold crop growth for a week or a potential hit on yield because of weeds.”
Strategies such as re-ridging after planting can affect herbicide application timings Andrew points out, as the herbicide should always be applied when the ridge has had time to settle.
“If the crop is about to emerge, any delay in application due to weather changes may result in a contact herbicide going on late with the subsequent risk of crop damage.
Over the past few years, dry spells during spring have made it difficult for pre-emergence herbicides to work effectively, as they rely on moisture for the weeds to uptake them. There can be added problems too, as dry ridges lose soil from their sides, so when moisture arrives, weeds germinate and can grow away unchecked.
“If spraying is delayed, it is a good ides to avoid actives such as pendimethalin and clomaznne because of their effect on the growing point of the crop. In this case, if possible, I advise opting for a mix of metribuzin + Rimsulfuron. A sulfonylurea herbicide such as Rimsulfuron will work most effectively against small actively growing weeds, so an application can target weeds at cotyledon four-leaf stage.
“Together these two do a better job than you would think, but before you go ahead, it is a good idea to make sure the variety is not sensitive to either of these actives.”
He adds that any spraying should be done before the crop height reaches 25cm, because afterwards the crop will find it difficult to repair the damage, noting that both Adama and NuFarm have useful information on sensitivities on their websites.
Phytotoxic effects of Rimsulfuron include paling and blotching of leaves, which look similar to those caused by virus, so he advises seed growers to avoid this herbicide if possible, as it could lead to confusion.
Water rates, boom height, forward speed and nozzle choice all contribute to efficacy, says Andrew. Getting the right nozzle choice is the first step to getting the spray on target and reducing drift, and Andrew’s nozzle of choice is the Defy forward/backward faring nozzle.
“For good foliar coverage, the water rate needs to be 2001/ha and the forward speed should be between 10-12km/h. Any faster and you will fail to get cover on the ridge slope which is where the weeds germinate.
The outside of the headland is often missed when spraying but if you miss it with a herbicide, you are also likely to miss it when you go in with the blight spray.”
Going through too fast can reduce efficacy as it is not hitting the target, which wastes valuable product and can go against environmental guidelines, he warns.
To help speed up spray times, he advocates marking out tramlines before going in with the tractor, so the paths are clear for the spray operator. This is particularly useful in a wet season, because the tractor firms dawn the wheel-marks for the sprayer to travel on.
Summing up, Andrew notes that having potato crops in the rotation can be useful to an overall strategy because it offers a way forward with weeds difficult to control in combinable crops, such as ryegrass and blackgrass.
Careful consideration of all the areas needing attention to detail and discussing the risks with your agronomist will help need to ensure the best strategy for your particular conditions.
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