15 March 2002

Planning weed

control is vital

This weeks article in our

baseline advice series

dovetails with the potato

agronomy special, taking a

timely look at weed control

in this high value crop

PLAN potato weed control strategies well ahead of planting to avoid limiting herbicide options and falling into following crop catches, advises Mark Taplin, of Farmacy.

"Nine out of ten fields arent a problem. But be aware of the fields which need a different strategy."

With planting likely to get under way from mid-March in most areas, there is no time to be lost. "Where planting is taking place on rented ground, theres more need for forethought because such land is often an unknown."

Perennial weeds, such as couch, should be minimised before planting. "Glyphosate used either on stubbles or pre-harvest is cost-effective and will give good control," says Mr Taplin. With good management, theres no need for couch to be a problem."

Growers need to be aware of site-specific weed problems, especially cleavers, ryegrass and blackgrass, which may need additional or specialist herbicides. "Cleavers were a particular problem until Titus (rimsulfuron) came along. The alternative, Basagran, is less reliable and can be harsh on the crop.

"For fat hen, Basagran is a post-emergence option for specified varieties and it does have activity on cleavers and oilseed rape. Metribuzin, as in Sencorex or Lexone, will also control fat hen post-emergence, but only on specified varieties."

Linuron remains the mainstay of most herbicide programmes and will control late germinating weeds, but can only be used pre-emergence.

Metribuzin can be applied pre or post-emergence and offers broad-spectrum control, but some varieties react badly and it cant be used on sandy soils, warns Mr Taplin.

"You will also need to check the effect of herbicide choice on following crops because there can be restrictions on the use of some products."

Light and organic soils limit herbicide choice and soil variation within a field needs to be considered. "A repeat low dose programme can be a good option on organic soils.

"A mixture of Sencorex (metribuzin) and Titus (rimsulfuron), applied 2-3 times, gives good results. Remember theres a maximum of three post-em applications of Sencorex and two of Titus. A bonus of this approach is that it copes well with weed flushes."

Incorporation of a residual and mechanical weed control are also options that should also be considered in the planning process.

"Whichever course you choose, aim for maximum weed control before the crop gets in the way. There are some good post-weed, pre-crop options, such as paraquat, diquat and glufosinate-ammonium, which can be used very effectively."

Crop type can restrict herbicide choice, with some products only having approval for maincrop use, says Mr Taplin.

Proposed following crop can also have a bearing. "Some residual and post-emergence herbicides affect subsequent crops, which is why planning is essential.

"Similarly, the cultivation requirements of herbicides may be different to the preferred establishment method of the following crop. You may be forced into ploughing when min-till is needed to help with groundkeepers."

Match residual dose to soil type and the persistence required. "On lighter soils and with later applications, you can use a lower dose. Tank-mixing helps reduce the reliance on the residual, especially at the post-weed, pre-crop timing.

"Keep rates up where there are high weed populations and on heavier soils."

A competitive crop canopy assists weed control, making it doubly important to minimise checks to early canopy development.

"Poorly timed herbicides, especially those at the post-em timing made in the wrong weather, can affect the crop. And they will also exaggerate the effects of other stresses such as drought." &#42

1 Planning Essential to avoid limitations with herbicide use.

2 Perennial weeds Minimise before planting.

3 Site specific weeds Beware cleavers, ryegrass and black-grass. New herbicide Artist will help.

4 Soil type Light land limits choice. Repeat low dose on organic soils.

5 Pre-crop control Make the most of it before crop gets in the way.

6 Variety Restrictions apply to Sencorex and Basagran, so check labels.

7 Following crop Beware restrictions of some residuals and post-ems.

8 Cultivation requirements Herbicide use could prevent preferred establishment method for following crop.

9 Residual dose Match to soil type and persistence required.

10 Crop competition Minimise checks to early canopy development.

New herbicides

New for this season is Artist (metribuzin + flufenacet) from Bayer, combining old and new chemistry. Strengths include loose silky bent, meadow grass and ryegrass. Farmacys Mark Taplin says it is also good on blackgrass, brome, cleavers, shepherds purse and black nightshade. Applied pre-emergence of both the crop and the weed, it can be used on early and maincrop ware potatoes only. "Obviously it has the same varietal and following crop restrictions of Sencorex because it contains metribuzin." He anticipates it will have a place where greater activity on grasses is required, or where there are low levels of cleavers and the need for an overspray is borderline. "There will be specific sites where black nightshade is a problem where it will be useful."

What you dont want…beware site-specific weed problems, advises Mark Taplin.