Consider weed control targets very carefully

4 June 1999

Consider weed control targets very carefully

By Andrew Swallow

LATE emerging weeds in sugar beet can hit yields hard and cause harvesting problems. But control is costly, so chemicals need careful targeting and applying only where absolutely necessary, says IACR-Brooms Barns Mike May.

"If weeds are going to get above the crop canopy then they need controlling. If not forget about them," he says.

Thistles, fat-hen, amaranthus and volunteer potatoes and oilseed rape are the main culprits warranting control. Recent Swedish work shows one oilseed rape plant/sq m cuts sugar yield by 0.5t/ha (0.2t/acre).

Full-rate Debut (triflusulfuron-methyl) is the best answer on large oilseed rape, though rates may be cut if it is caught at the two to four-leaf stage, says Mr May.

Volunteer potatoes should be tackled in a two or three-hit programme delivering a total of 1 litre/ha of Dow Shield (clopyralid). Adding Debut speeds knock-down and improves control of hard-to-kill, big tuber varieties such as Cara. But that adds about £17/ha (£7/acre) per application to the £50-60/ha (£20-24/acre) cost of clopyralid.

Reduced rates of clopyralid are possible if growers dont aim to eradicate the potatoes. "Good suppression of foliage may be sufficient to prevent yield loss. But to prevent potatoes setting daughter tubers complete control is necessary," says Mr May. Mixing in ethofumesate increases clopyralid activity, especially on daughter tuber production, he adds.

For complete thistle control a total of 1.5 litres/ha of clopyralid are needed. A first dose of 0.5 litres/ha should be applied to the weeds at the 15cm (6in) rosette stage. That may be patch sprayed to cut cost and avoid unnecessary chemical use. But patch spraying second applications can prove tricky as partially controlled plants are not easy to spot.

For some growers partial control may be adequate if thistles can be tackled more cost effectively elsewhere in the rotation. But Mr May warns them not to over-estimate their ability to do this. "It often gets over-looked," he says.

Fat-hen demands extra vigilance until the crop meets across the rows. The weed emerges rapidly and can reach the four-leaf stage, the limit of effective controls, within 10 days. Phenmedipham (eg Betanal) at 2.5 litres/ha up to cotyledons, or 3.0 -5.0 litres/ha at two to four true leaves, should give good control at £10-40/ha dependent on rate. In gappy crops a late residual herbicide such as meta-mitron (Goltix), lenacil (Venzar) or ethofumesate plus chloridazon (Magnum) may be worthwhile.

Imported weed amaranthus is also late emerging and fast growing. Desmedipham mixes such as Betanal Compact (desmedipham + phenmedipham) should control both fat-hen and amaranthus. &#42


&#8226 Beware tall weeds, eg thistles, potatoes, fat-hen, osr, amaranthus.

&#8226 Clopyralid (Dow Shield) for potatoes and thistles.

&#8226 Phenmedipham for fat-hen, + desmedipham for amaranthus.

&#8226 Debut best bet on oilseed rape.

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