Potato desiccation requires a strategic rethink following the removal of diquat from growers chemical portfolio in February 2020. Hutchinsons agronomist Andrew Goodinson says decisions need to be taken now in order to plan how crops will be managed next year in terms of haulm management and destruction.
He says that several considerations will come into play as a result of these changes, including varietal selection, nitrogen rates, herbicide choice and headland management.
"Consider non-determinate and determinate varieties for canopy density and yield potential as this could directly influence nitrogen, potash and magnesium rates," says Mr Goodinson. "Can we reduce nitrogen levels on varieties that produce a lot of canopy without reducing yields? Yes we can and yes we have to without changing varieties. We have to manage the crops better going forward."
If organic manure is being applied take account of its nutrient value, because growers underestimate this and what is actually available from this source during the growth of the crop, says Mr Goodinson.
"With flail options for desiccation likely to feature more strongly for growers going forward, care must be taken on how the potato field is actually planted," he says. "If new to flailing consider what happens on the headlands for example. When turning at the end of the row more area must be left uncropped to ensure the flail is not lifted out of work too early, compromising the topping process."
Traditionally Mr Goodinson advised growers to apply low rate Reglone 1-1.51/ha plus a blight spray in 300 litres/ha of water. This slows down the growth rate and starts the desiccation process. It also opens the crop up to allow better penetration of the next desiccant applied 5-7 days later targeting the stem, which would have been Spotlight Plus (carfentrazone-ethyl) or Reglone (diquat) or a mix of the two.
"If flailing, the head tendency is normally to leave out the first spray, but there's not enough attention to detail given to the machine set up in terms of blade condition/height and forward speed, which can lead to a less than satisfactory operation," warns Mr Goodinson.
"What flailing should do is leave the stem 15-25cm long with leaves and trash deposited in the bottom of the row.
"Spotlight Plus or Reglone would be applied 24-48 hours after topping."
Losing the Reglone option means growers will either have to buy a flail, use a contractor or look to apply two applications of Spotlight Plus, he says.
Historically, carfentrazone-ethyl has been used as a stem desiccant rather than leaf desiccant, but in trials this year based on Spotlight Plus and Ranman TOP (cyazofamid) applied in water volumes of 300-350 litres/ha of water, crops were opened up nearly as well as Reglone if given enough time.
"Spotlight Plus, which can be used on both ware and seed potatoes, takes four to five days longer than diquat to open up a crop," explains Mr Goodinson.
"Making time allowance for carfentrazone-ethyl and it's just as effective a stem desiccant too, in my opinion.
"The wetter in Ranman TOP helps the efficacy of the desiccant however other blight sprays can also be used in mixture with Spotlight Plus.
"Growers must be mindful of maximum dose and of using a higher water volume to give coverage and penetration. Next season we recommend growers flail and apply Spotlight Plus at 0.61/ha followed by a second application up to a week later of Spotlight Plus at 1l/ha," he says. "lt provides better and faster stolen removal and prevents stem regrowth, which is an important consideration for green crop going into store.
"Faster stolon separation reduces crop stress too. The key is to apply it in bright and warm conditions to maximise speed of knock down. However, carfentrazone-ethyl is rainfast after an hour which also makes it more flexible in that it can be applied in catchy weather conditions. There are no following crop restrictions either with this product and unlike other potato desiccants there is no 5m buffer zone."