Knocking out the imposters
Potato volunteers are a visible, and embarassing, reminder that more than a few tubers escaped the harvester. Tom Allen-Stevens reports on new research
THE return of potatoes the year following harvest varies hugely from 12,000 to over 300,000/ha. In fact youre easily capable of leaving more behind than you planted in the first place," Agrovistas John Keer told growers at a recent technical workshop in Norfolk. It was a chance to present the results of the BPC research, which studied the population dynamics and cultural control of volunteers, with some new herbicide work carried out by Agrovista.
Dr Keer carried out the new research himself which looked at chemical methods of controlling volunteers, and that in itself can be a daunting task: "A lot of them emerge in April, but senesce in July – often before the host crop has matured. With these theres no opportunity for pre- or post-harvest glyphosate," says Dr Keer.
But leaving them in the following crop is not an option. They can have a serious competitive effect which will hit yield, especially in open crops, such as onions, carrots and sugar beet and potato berries can contaminate vining pea crops.
Crucially they also act as a reservoir for disease: they are a primary source of blight, although ironically, blight itself is effective at controlling volunteers, and potato leaf roll virus often originates from volunteers. They can also halt the natural decline of potato cyst nematode (PCN), while helping tuber blemish diseases to thrive. "If youre aiming for a five- to six-year rotation and you have a lot of volunteers, youre effectively looking at a lot less," Dr Keer points out.
The volunteers are believed to come exclusively from daughter tubers left in the field. Although a potato crop can produce up to 150 million viable seeds/ha, theres a very low incidence of volunteers that grow from seed, and those that do are effectively controlled by residual herbicides. Even a very young, small plant that has no evidence of a tuber feed source will most likely have grown from a very small tuber, says Dr Keer.
Its the small tubers that predominantly cause the problem; 70% of returned tubers are 10-30mm in diameter. "Thats logical really, since theyre the ones that will fall through the web of the harvester. So the most sensible thing to do to reduce volunteers would appear to be to reduce the web spacing. But theres a conflict here with the need to get on and harvest a clean crop."
Instead Dr Keer advises growers to try to avoid growing tubers of a size that cannot be harvested. "Aim for an even tilth and even planting, and make sure theres a lack of compaction." Good agronomy will also help reduce chat-sized tubers. Measures include the appropriate use of nitrogen, well-scheduled irrigation and well-timed haulm desiccation. "Good PCN and rhizoctonia control is also essential. Were not talking about insurance-type treatments. Estimate if you need it, and then tackle at full rate."
Frost can have an effect, but should not be relied on to control volunteers, he warns.
Generally Dr Keer accepts that such methods of cultural control should be considered because they are politically correct, but believes they are not practical and commercial considerations should rule. This would also go for a favoured method of cultural control available to growers: to grow a competitive crop. "Its possibly better to follow with a more open crop to increase the effectiveness of your herbicides; information in this area is sadly lacking."
That said, control through herbicides is a route strewn with difficulties. Residuals do not work because a tubers food reserves are too large. In fact no herbicide is completely effective and multiple applications are often required because volunteers have a long period of emergence. This can also cause problems when it comes to crop restrictions; some herbicides cannot be applied to cereals after flag leaf, for example, but the volunteers can emerge after May.
In the potato crop itself growers can use Fazor (maleic hydrazide), a growth regulator designed to make any tubers returned unviable. This is probably the best start to overcome the problem, but Dr Keer has reservations about its effectiveness in every case: "Theres certainly a dramatic reduction in volunteer population in the next crop, but does it just make the seed go dormant, holding over the problem to subsequent years?"
In non-cereal crops following potatoes, Shield (clopyralid) was found to be reasonably effective in the trials, with early applications affecting numbers and later ones affecting the viability of the second generation seed; those that grew on were very distorted.
Shield would be a suitable chemical for sugar beet, and the addition of Betanal Progress (desmedipham + ethofumesate + phenmedipham) and Debut (triflusulfuron-methyl) would help the haulm control, says Dr Keer. For onions, Totril (ioxynil) with Starane (fluroxypyr), turned out to be the best treatment.
A range of selective herbicides were used on cereals. A full rate of Ally (30g/ha of metsulfuron-methyl) resulted in a complete recovery after two weeks. Two litres of Starane gave the best haulm control, but would be a highly expensive choice. Initially there was an effect when a full rate of Ally was mixed with a litre of Starane, but some regrowth was found.
An important finding and one that warrants further research is whether it is wise to give volunteers a knock early on, says Dr Keer. "It could hamper the translocation in the plant of glyphosate applied pre-harvest. If you dont have too high a population of volunteers, you may be better off leaving them and hitting hard with a non-selective."
Glyphosate was found to be by far the most effective form of control. Set-aside is the ideal situation, but it can also be effectively used either pre- or post-harvest, says Dr Keer: "Post harvest is best – you get a true flush after harvest which makes for more reliable control."
Rates should be kept high, however. Second generation daughter tubers were recovered from all of the plots in the cereal and set-aside situations and tested to see if they would grow on. None of the selective herbicides completely prevented production of viable tubers, while only measured success was achieved with glyphosate. "At 2 litres/ha some daughter tubers were still viable, while after four and six litres, there was little regrowth. It shows growers should never miss an opportunity to apply glyphosate."