21 February 1997

Salty solution – but go carefully

Salt can be a useful herbicide in sugar beet crops. But getting the best results from this material requires care, says Morley Research Centres Mike May

THERE are records of salt being used since the 1930s. But under current legislation it needed to have approval before it could be applied as a herbicide to sugar beet.

Approval as a commodity product was published in last Octobers issue of the Pesticides Register, although authorisation was granted some time earlier.

Growers with volunteer potatoes will benefit most, but others who have trouble controlling weeds such as redshank and pale persicaria will also have a use for salt.

It should not be used as a matter of course. But it can be cost effective where large volunteer potatoes or weeds such as redshank and pale persicaria have been left by normal herbicide programmes. Such tall weeds are very competitive and cut yields substantially. A single uncontrolled tall weed/sq m can reduce yields by 10%.

Uncontrolled potato haulm smothers sugar beet and can cause severe crop loss. In these cases salt treatment is particularly useful. It can also help reduce trouble in following crops, although early volunteer control is needed to avoid problems such as eelworm.

The main herbicide used for controlling volunteer potatoes in beet is Dow Shield (clopyralid). Two, three or even four-way splits of 1litre/ha give good suppression. One of Dow Shields main benefits is the effect it has on regrowth of any daughter tubers produced. Most will not grow properly and are easily killed in subsequent crops.

Debut (trifluslfuron-methyl) also affects volunteers and can control them. But salt has no direct effect on daughter tubers already present. So if you intend to use salt after Dow Shield or a Debut programme, you should leave as long an interval as possible – possibly two weeks or more – to allow the Shield to work before the salt is applied. However, failure to apply enough Sheild will leave viable daughter tubers.

Best effects from salt are seen when it is applied under hot, humid conditions. It works by scorching the plant, so is best used when good cover of the weeds can be obtained. It will be less useful when beet are starting to smother weeds.

Treatment is usually 1100 litres/ha of saturated salt solution plus 0.1% non-ionic wetter. Most modern farms are not set up to spray at such high volumes, but it will work equally well when applied as two passes of 550litres/ha.

To obtain a saturated solution, 310kg of salt should be dissolved in each 1100litres of spray water. However salt is hard to mix and this concentration can be difficult to achieve in the farmyard.

In practice, as long as the concentration is about 25% (275kg/1100litres) the salt solution will work well. Unfortunately dendritic salt, a relatively easy to dissolve crystal form, is no longer made so many growers prefer to buy a ready mixed solution.

Growers mixing and preparing the solution themselves should ensure the spray water is as warm as possible.

If you are considering using salt this season, an investment in black storage tanks that can be placed in the sun to warm the water inside could save a lot of heartache.n

Salt can be useful to control large volunteer potatoes, redshank and pale persicaria which survive previous treatments, says Mike May.


&#8226 Approved as sugar beet herbicide.

&#8226 Useful as follow-up treatment on large volunteer potatoes, redshank or pale persicaria.

&#8226 Apply in hot, humid weather.

&#8226 Ensure good spray cover of weeds.

&#8226 Can be applied in 2 passes each of 550 litres/ha.

&#8226 Dissolve in warm water if possible.