Defoliation options need considering
How best can potatoes be
defoliated? Peter Hill finds
out why one Norfolk contract
spraying company thinks
sulphuric acid remains the
best bet in many cases
ANYTHING with "acid" in its title surely runs the risk of a poor public image and would be avoided by an industry increasingly sensitive to consumer values and opinion.
Yet sulphuric acid remains the most popular means of destroying potato foliage to bring tuber development to a halt.
The reason, believes Les Sykes of Sands Agricultural Services, is simply that it does the quickest and most effective job. Nor does it have the fearsome environmental impact its name suggests.
"Sulphuric acid is patently something that has to be respected and applied with great care and consideration," he says. "But as long as it is used by professional contractors, who follow the industrys code of practice and know what they are doing, there should not be – and rarely are – any problems."
The chemical desiccants -diquat (Reglone) and glufosinate-ammonium (Harvest and Challenge) – clearly have their place, as does thermal treatment when chemicals have to be avoided. But none are as quick or thorough as sulphuric acid, says Mr Sykes.
"Flail topping gives the most immediate result, of course, but you get a lot of re-growth which causes an unfavourable reaction in the tubers with starch being converted into sugars," he says. "Also, as an operation it is relatively slow and you end up with a lot of macerated green-matter, potentially carrying blight spores, laid across the crop.
"Foliage sprayed with Reglone takes four to five days to die back, Harvest 10 days typically, depending on variety and conditions, and acid just 10 minutes," says Mr Sykes.
The relative slowness of the chemical desiccants, especially in unfavourable conditions, has implications beyond the speed at which tuber development is slowed to keep within the target grade, initiate skin-set and stabilise starch content.
"When activity of desiccants is expected to be slow, either because of product choice or conditions, a blight fungicide programme should be maintained while the desiccant spray takes effect to minimise the risk of tuber blight," notes Mr Sykes. "The rapid knock-down you get with acid minimises that risk and eliminates the extra cost."
So under what circumstances are chemical desiccants more likely to be used in place of acid?
"Not all growers, especially outside the main potato areas, have contractors available to do the job, and there is not always the capacity for contractors to fulfil everyones needs. It is also fair to say that many growers like the flexibility of being able to desiccate when they choose rather than relying on a contractor who may be delayed by bad weather or heavy workload."
For growers determined to avoid any form of chemical application, thermal treatment of haulm is one of the alternatives to plain flailing. Drackedon Agricultures Greenburner uses a propane burner to heat a ceramic grill. This generates a temperature in excess of 800C which ruptures the cells of any plant matter it passes over.
"It is a better solution than just flailing, and in fact the two treatments are complementary because topping improves exposure of the stems to the burner," says Mr Sykes. "But it is painfully slow compared with spraying and in trials we have seen too much re-growth."
Technicrop believes its Green Dragon "thermal cultivator" addresses both criticisms. This device has been developed from US technology.
With an operating width of up to 6m (20ft) giving an output of 4.8ha/hour (12 acres/hour), it also has work rate and consequent operating cost on its side. The two-row/single bed Greenburner manages just 0.4 to 0.8ha/hour (1 to 2 acres/hour).
"We also believe it is more effective because it works in a different way," says Stuart Goodinson of Technicrops Green Dragon Flaming division. "With this design, the propane does not ignite until it is several centimetres away from the nozzle. So the high temperature heat – of about 1000C – is generated within the crop canopy, making it more effective against the stems. In fact, we do not recommend topping first because we want the canopy to hold in the heat."
A non-replicated trial of different haulm destruction techniques carried out by Greenvale AP and Sands Agricultural Services provides some useful guidance on their effectiveness, although Mr Sykes stresses that the amount of top present (which usually relates to variety) and prevailing conditions are big influences.
A sequence of treatments invariably proved the most effective in terms of eliminating (or at least minimising) regrowth. *
HAULM DESTRUCTION TECHNIQUES – PROS AND CONS
Chemical desiccants Pros: Quick to apply; no specialist knowledge, equipment or safety measures needed; little risk of adverse public reaction.
Cons: Slower foliage knock-down delays halt to physiological ageing; some risk of blight infection moving to tubers unless fungicide also applied.
Flail topping Pros: Immediate effect; involves no chemicals.
Cons: Slow and therefore relatively expensive; spreads macerated foliage (potentially infected) over crop; too much regrowth.
Sulphuric acid Pros: Quick to apply; very fast acting.
Cons: Have to rely on contractors; risk of unfavourable public reaction.
Thermal treatment Pros: No chemicals involved; no specialist knowledge or safety measures needed; no risk of unfavourable public reaction.
Cons: Not always effective (too much regrowth); can be slow; requires specialist fuel storage.
Potato haulm destruction trial
Treatment 1 Treatment 2 Regrowth (% Cost of
Aug 25-27 Sept 6 groundcover treatments
at Sept 23) £/ha**
Flail – 35 75
Flail + Greenburner – 20 112
Diquat 4 litres/ha – 20 46
Harvest 3 litres/ha – 15 48
Flail + Harvest 3 litres/ha – 10 113
Harvest 3 litres/ha Harvest 3 litres/ha 5 96
Diquat 4 litres/ha Diquat 2 litres/ha 5 73
Acid 280 litres/ha – 5 78
Flail + Harvest 2.5 litres/ha – 5 107
Harvest 2.5 litres/ha Flail + Greenburner 5 154
Dropleg Harvest 3 litres/ha – 5 48+
Flail + Greenburner Harvest 3 litres/ha 0 160
Flail + Greenburner Acid 150 litres/ha 0 154
Flail + Acid 150 litres/ha – 0 117
Harvest 3 litres/ha Flail + Greenburner 0 160
Acid 150 litres/ha Flail + Greenburner 0 154
Dropleg Harvest 3 litres/ha Harvest 3 litres/ha 0 96+
Harvest 3 litres/ha Acid 150 litres/ha 0 90
Acid 150 litres/ha Acid 150 litres/ha 0 84
*Cara planted Apr 24, 1999, in sandy loam soil. **Includes chemicals, flail @ £75/ha; Greenburner @ £112/ha; overall spraying @ £10/ha, but not additional costs associated with dropleg sprayer. NB: diquat used at 4 litres + 2 litres for experimental purposes only. Source: Sands Agricultural Services/Greenvale AP.