Store managers may need to consider looking beyond the direct costs of the remaining approved sprout suppressants and towards adjusting store management strategies to keep the number of applications to a minimum, and therefore helping to keep costs down.
As sprout suppressant chlorpropham (CIPC) will not have its licence renewed in the EU, the hunt is on for cost-effective alternatives. Spearmint, marketed as Biox-M, and dimethyl naphthalene (DMN, marketed as 1,4 SIGHT) are both approved for use in the Netherlands, but both are more expensive.
Spearmint oil is one of the recent additions to the sprout suppression tool box, having received full UK registration in 2012. The active ingredient is the naturally-occurring substance R-carvone, a terpenoid, and is applied as a hot fog to burn back existing sprouts. DMN, is approved for use in the Netherlands and is likely to be submitted to the UK’s Chemical Regulations Directorate (CRD) for approval later this year.
Dutch perspective
However, while it can deliver robust sprout control, costs of using it can be as much as seven times higher than CIPC in application costs for each treatment, warns Jeroen van Kappel of Netherlands-based Mooij Agro, who has been trialling both spearmint and DMN across multiple sites.
Moreover, as spearmint oil is relatively volatile, stores need to be closed for 48-72 hours after application as otherwise it could be lost as a vapour, resulting in lower efficacy.
Application is recommended with an electrical fogger, although other approved equipment may also be used. Circulation fan speed needs to be at 100 per cent for best results, the ongoing trials have found.
Jeroen says: “When it is applied stores have to be closed for 48 hours or more, ideally with no natural ventilation.
During fogging, the supplier recommends running fans on forced internal ventilation. Because of the closure it is not possible to avoid the crop temperature rising as it is important not to run refrigeration units during the period of store closure.
Fry colour and CO2
“The problem with closing stores is that levels of carbon dioxide (CO2) can rise significantly. If this is for a short time, it seems like the fry colour remains acceptable, but if closure is for too long , it could become be an issue.”
Further research will be undertaken on the effects of the rise in CO2, and from 2020 the trial stores will have a new CO2 sensor capable of measuring 0-10.000 PPM. The effects of longer closure on weight loss and fry colour are also being explored.
“So far, fry colour has been acceptable, although only the one variety, Fontane, has been used in our trials to date. The decision to just use one variety was taken to keep the number of variables down and ensure comparable data,” explains Jeroen, adding that the fry quality work has been cross-referenced by specialists Lamb Weston.
Weight losses measured in February were around 2.1 per cent, and in June, they varied between 4.2 and 6.2 per cent; this variation may have been due to one of the farmers using only cooling, whereas the others used both cooling and outside air.
DMN is also applied as a fog, and the temperature of the tubers needs to be above 5 deg.C. For best results, fan speed needs to be reduced to 50 per cent, similar to the way it was for CIPC, he emphasises. As with spearmint, an electrical fogger is recommended but other hot fog equipment may be used. After application, stores need to be closed for 48 hours, although internal ventilation is permitted. Applications need to be made every four to six weeks.
Jeroen says: “It is important that stores are at least 40 per cent filled with potatoes for the use of DMN or Biox-M (air ducts and plenum needs to be included in the calculation). This could be a problem for growers with large stores in years with poor yields , so it would be better over time to move towards using smaller stores that allow more flexibility.”
Investing in fans which are capable of working at different air speeds on application may also prove crucial. “If you do not have the ability to slow fans down to 50 per cent, then your sprout suppressant will not be as effective, and may end up costing you more if further applications are necessary.”
Moving on to talk about storage of the future, he highlights the importance of stable temperature and the need for refrigeration to achieve it.
“One of the challenges is that all the alternatives to CIPC are relatively new, so there is still a lack of information on how often they need to be applied, and the best store management strategy to deal with them. We have found that the more efficient the store, the fewer applications that are needed.”