Potato Review

28 POTATO REVIEW MARCH/APRIL 2022 HERBICIDES worth choosing products that will fit the crop, soil, the 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, so 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 for 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 prosulfocarb or aclonifen. Not all seasons go to plan, and when goes on to note that 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.” “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.” Herbicides forming the basis of the weed control strategy, their strengths and limitations Active ingredient Strengths Limitations Comments Metribuzin Good activity across a broad weed spectrum on a wide number of soil types Not much activity on polygonums Good basis for overall weed control strategy but care needed because of the sensitivity of some varieties (e.g. Innovator, Maris Piper and Lady Claire) Clomazone Good activity on groundsel and nightshade, Highly soluble in water and quite volatile so drift control may be necessary Contact and residual activity, and a good tank mix when used with metribuzin Pendimethalin Good activity on fat hen, cleavers and chickweed Not suitable for use on light soils or before rain Affects growing tip of crop and weeds, good activity when used in a tank mix and applied early Prosulfocarb Good activity on cleavers and black bindweed (but rates need to be kept up) Short persistence in soil, particularly if conditions are dry and warm Good crop safety and works well on light soils, so is a useful alternative for metribuzin-sensitive varieties Aclonifen Good activity on fat hen, brassicas and polygonums Less effective on black nightshade and fumitory A new mode of action and good in tank mixes Metobromuron Good activity across a broad weed spectrum Rates need to be kept up for efficacy Similar spectrum to linuron, safe on all soil types. Suitable for metribuzin-sensitive varieties Flufenacet Good activity on grassweeds e.g. Blackgrass and ryegrass, and black nightshade, moder- ate activity on cleavers Limited activity on fumitory, cranesbill and poppies Tends to be used too frequently elsewhere in rotations for black-grass control, so other chemistry should be used to prevent over-exposure. Not available as a stand-alone active, only supplied in mixes Cycloxydim(Laser) Good activity on grassweeds and volunteer cereals Good crop safety, but can be costly if trying to control couch Fluazifop-p-butyl (Fusilade max) Useful activity on volunteer cereals, bromes, wild oats, ryegrass Not as active on blackgrass as Cycloxydim Should be used before flowering Propaquizafop (Falcon) Useful for patches of couch, cost effective Can lead to variegated leaf colour on top of the crop (but does not affect yield) 7 week harvest interval on early crops, 8 weeks on maincrop Bentazone (Basagran) Foliar activity on black nightshade, cleavers, and many other weeds and brassicas Humidity dependent for efficacy, so weather conditions at spraying are critical Stewardship requirements Not for use on seed crops or earlies. Use before crop is 150mm tall. Rimsulfuron (Titus) Useful activity on leavers, brassicas Little activity on grassweeds, fat hen, fumitory, mayweeds Foliar activity and is not for seed crops because of bleaching