Potato Review

www.potatoreview.com POTATO REVIEW MARCH/APRIL 2022 29 HERBICIDES Making the most of herbicide efficacy 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 the 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 as 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 words around the obstacle,” he said. The spray operator can also use Omnia to pinpoint an area of interest in the field that he wants the farmer or agronomist to check, he adds. Timing crucial to effective herbicide management 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 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 idea to avoid actives such as pendimethalin and clomazone 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. Spray application techniques drive efficacy 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 facing nozzle. “For good foliar coverage, the water rate needs to be 200l/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 a wet season, because the tractor firms down the wheel-marks for the sprayer to travel on. Summing up, Andrew notes that having potato crops in the rotation can be a 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.” “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.”