20 February 1998

Two differing tactics to keeping peas weed-free

Weeds can devastate pea

yields and add to harvesting

costs. Elizabeth Mann

quizzes two growers with

differing opinions on how to

achieve best control

PRE-EMERGENCE herbicides in combining peas have been a waste of time at Wood Farm, Bluntisham, Cambs, says grower Philip Godfrey.

He prefers to hit weeds when they are small and actively growing in warm, moist conditions. Low herbicide doses minimise risk of crop damage.

"We learned how to use post-emergence herbicides during 15 years of vining pea growing." That preference was confirmed in 1994. "We tried a pre-emergence Stomp/Fortrol mix on combining peas. It was a complete disaster, with little weed control and significant crop damage, made worse by the need for post-emergence treatment. Yields dropped to 5.1t/ha, way down on our 6.5t/ha average."

For the past three seasons Mr Godfrey has refined a multi, low-dose, post-emergence approach.

Last years herbicides cost £46/ha (£18.60/acre); two-thirds was spent on controlling charlock, speedwell and cleavers, the main problems on his clay moraine soils. The first Fortrol (cyanazine) and Pulsar (bentazone + MCPB) mix, usually about half rate (2 and 0.2 litres/ha respectively), is applied around the end of April, when peas have at least two nodes (two fully expanded leaves). A second dose follows 14 days later.

"Good results last year with the first split meant we could reduce the Pulsar in the second dose to 0.75 litres/ha. For the first application we use 220 litres/ha with Lurmark 04-F110 jets at approximately 3.5 bar. For the second treatment we up water to 300 litres/ha with 05-F110 jets at 4 bar."

This season, Mr Godfrey hopes to trim chemical rates in the first application. "As long as we keep Fortrol rates up to deal with charlock, 1-1.5 litres/ha of Pulsar could be enough."

Pre-emergence sprays are considered necessary on JSR Farmings moisture retentive chalk wold and sandy clay soils in Yorks. Technical director Philip Huxtable favours half rate Opogard (terbuthylazine + terbutryn) to sensitise weeds and follows with a single post-emergence spray.

"No pre-emergence spray is totally reliable," admits Mr Huxtable. "But it will usually buy you a bit of time." They are not for dry soils and cannot be justified at full rate, he advises.

Opogard is cheap at about £15/ha (£6/acre), and is relatively reliable. Reflex T (fomesafen + terbutryn) disappointed in farm trials, and Stomp (pendimethalin) is avoided as it is widely used in cereals.

With volunteer rape and cleavers as the main weeds, and polygonums on the sandy clays, a follow-up half rate Pulsar/Fortrol mix is applied in 220 litres/ha of water as a fine to medium spray. That costs about £14/ha (£5.60/acre), says Mr Huxtable. If delayed, a three-quarter rate is used to control bigger weeds.

"Pulsar, particularly at higher rates, can be harsh on the crop. Well-waxed leaves, confirmed by the crystal violet test, are vital." Recent weather patterns with dry early springs have made pre-emergence spraying more of a lottery, says Cathy Knott of the Processors and Growers Research Organisation. But late frosts make post-emergence tactics more risky too.

"Too often growers dont give pre-emergence sprays the best chance. There is time, as crops can take up to 28 days to come through. Listen to the weather forecast and spray when rain is coming."

Better matching of herbicides to specific problems helps too. "Dont wait until the crop and weeds are through before deciding what to use." &#42

JSR APPROACH

&#8226 One pre-emergence spray to sensitise weeds.

&#8226 Half-rate Opogard favoured.

&#8226 Follow-up spray of Fortrol and Pulsar also at half rate.

&#8226 Cost – £29.50/ha.

GODFREY APPROACH

&#8226 Two post-emergence sprays kill broad-leaved weeds, one other used for grass weeds.

&#8226 Low doses applied in warm, moist conditions minimise crop damage.

&#8226 Fortrol/Pulsar at two nodes, repeated 2 weeks later.

&#8226 Cost – £30/ha + graminicide.

A bad experience with pre-emergence herbicides means Philip Godfrey prefers to wait until peas and weeds have emerged before spraying.

Two passes using low rates of chemical reduce the risk of crop scorch.