21 February 1997

Early-sown peas facing trouble from big weeds

This winters prolonged sub-zero temperatures could break the dormancy of more weeds than usual, producing big weed flushes on winter pea and linseed crops. Brian Lovelidge reports

TIMING of winter pea herbicide treatments will be critical this spring, particularly in crops which were drilled early.

Those growers who drilled around the third week of October, when soils were still fairly warm, were caught out by rapid crop emergence, says PGRO trials officer, Cathy Knott. Many missed the window for pre-emergence herbicides and will now have big weeds to control with contact herbicides.

Crops drilled later were generally treated with Opogard (terbuthylazine + terbutryn), Peaweed (prometryn + terbutryn) or Remtal (simazine + trietazine). These work better in the autumn when soils are moist and weed growth is slower, says Richard Peake, Harlow Agricultural Merchants chemical manager.

Some peas drilled in early November had not emerged by late December when the Arctic spell began. But even these will need spring herbicide early, possibly in the second week of March, reckons Ms Knott. Flowering occurs in April in the south and cut-off point for most herbicides is enclosed flower bud stage, she says.

But Mr Peake reckons most of his firms growers can usually omit a spring broad-leaved weed herbicide if their autumn residuals have worked well. Crops race away in March and smother weeds. How-ever, this spring he expects about 25% of crops will need spraying due to the cold spell, which will have broken the dormancy of black bindweed seed in particular.

Although cleavers survive autumn treatment, the weed is unlikely to be troublesome, as most will have germinated before drilling. Usually only about 5% of his customers crop need spraying for the weed, he says.

Ms Knott maintains cleavers is a common problem in winter peas. Although the crop can smother out less aggressive weeds, cleavers and black bindweed can compete.

"It is a case of waiting to see what appears and then going in quickly with the appropriate herbicide or mixture," she advises. "All contact products approved for spring peas can be used on winter varieties, the main ones being Pulsar plus Fortrol and Trifolex-Tra plus Fortrol."

Pulsar (bentazone + MCPB) plus Fortrol (cyanazine) controls cleavers up to one-whorl. Basagran (bentazone) will take out larger weeds up to about 5cm (2in) tall.

Volunteer oilseed rape can be a problem in winter peas. Autumn residuals, and Pulsar plus Fortrol or Basagran, offer reasonable control. Trifolex-Tra (MCPA + MCPB) plus Fortrol suppresses it.

Autumn cereal volunteers may be eliminated by seed-bed cultivations. But more can emerge in the spring, says Ms Knott. Last year she took no action. "It was the wrong decision."

Four graminicides – Checkmate (sethoxydim), Falcon (propa-quizafop), Fusilade (fluazifop-P-butyl) and Laser (cycloxydim) are approved. They have a wide application window from the crops two leaf pair stage up to early pod set. But growers must leave a gap of at least seven days before applying other herbicides, she warns.

Much winter linseed was drilled into dry soils, which caused patchy emergence and some weedy crops due to the impaired activity of trifluralin, the most widely used residual herbicide.

Mr Peake is no great fan of the product. It leaves the crops main problem weeds, cleavers, volunteer OSR and charlock, intact. But it is cheap, at about £7.40/ha (£3/acre).

"There will be some fairly big weeds to control in the crop this spring. The job should be done as early as possible," he urges. "Ally takes out most common weeds, including oilseed rape and charlock. But it has gaps in its spectrum such as cleavers, speedwells and annual meadow grass."

United Oilseeds agronomist, Richard Alsdon, agrees Ally is useful, though not for the significant number of crops plagued by volunteer OSR and cleavers. The best alternatives are Basagran plus Bromotril P (bromoxynil) or Basagran plus Vindex (bromoxynil + clopyralid) which control those two weeds and speedwells, charlock and others.

All crops should be within the recommended size range, generally from 2.5-30cm (1-12in) tall, for herbicide treatment, he advises. &#42

lHerbicides special continues on p67.

Large volunteer oilseed rape plants are hard to control in winter peas. Cleavers, black bindweed and cereal volunteers can also be a problem in the crop.



&#8226 Large weeds where autumn treatment missed.

&#8226 Treat early – enclosed flower bud is cut-off stage.

&#8226 Watch for cleavers, black bindweed, OSR and cereal weeds.


&#8226 Many autumn treatments poor.

&#8226 Big weeds – early treatment.

&#8226 Watch for cleavers, volunteer OSR and charlock.