1 August 1998

BEFORE AND AFTER PAYS

A wet spring again highlighted the value of pre-emergence herbicides for getting an economic return on weed control in peas.

PEA growers might think that a belt and braces pre- and post-emergence herbicide programme costing £65/ha (£26/acre) is over the top, but it can pay off in most years.

Trials officer Cathy Knott, of the Professors and Growers Research Organisation, says that, apart from ensuring virtually nil yield loss through weed competition, such treatment removes the need for pre-harvest desiccant treatment at about £30/ha (£12/acre).

She has always advocated the use of a pre-emergence herbicide, unless the peas are drilled late into a dry soil, to eliminate weeds during establishment – when the crop is at its most vulnerable to competition.

Until this year, however, a run of dry springs limited the activity of residual products, discouraging their use and encouraging a swing towards post-emergence herbicides, notably Pulsar plus Fortrol.

"Ten years ago, about 75% of combining peas received pre-emergence herbicides and the rest post-emergence," says Ms Knott. "But by last year, the ratio was about 50:50 and even more growers were expected to go post-emergence this year due to the very dry spring in 1997."

Those opting for pre-emergence treatment this spring struck lucky thanks to plenty of rain in April to activate it. This is clearly demonstrated in the PGROs trials. All of the pre-emergence products worked exceptionally well, with cleavers more or less the only escapee.

"We need a herbicide with good cleavers activity to add to pre-emergence treatments for all pulses," says Ms Knott. "There are three new active ingredients that control this weed. For field beans, one of them could be tank-mixed with simazine and so it would be a fairly low-cost treatment."

The best results in the PGRO trials (with the combining pea, Elan, drilled mid-March) were produced by the pre- and post-emergence Rolls-Royce treatments, Opogard or Reflex T, followed by Pulsar plus Fortrol, all at full rates. Each sequence costs about £65/ha (£26/acre), which might make growers cringe but by the seasons end is more cost-effective than skimping on herbicides, she says.

Full-rate Pulsar plus Fortrol – 4 litres plus 0.4 litres/ha (0.16 litres/acre) – has also done a good job, apart from leaving annual meadow grass and a few cleavers that were a bit too advanced (beyond two whorls) when the mixture was applied at the crops third node stage.

Better still, nearly as good as the Rolls-Royce sequences are three different treatments with BAS 572H (bentazone plus pendimethalin). The Elan plots received a full or reduced rate of this product at the first node stage, and the third treatment was a split dose at the first and second node stages.

Another effective sequence is half-rate BAS 572H and half-rate Pulsar plus Fortrol applied at the first and second node stages respectively. All treatments proved safe to the crop.

"The loss of Skirmish, which also had residual and contact activity, has left a gap in the early post-emergence market that could be filled by BAS 572H which we hope will be registered within two years," says Ms Knott. "This season its controlled everything, including cleavers, poppy and black bindweed, and last year it also did a brilliant job on the oilseed rape we sowed in the plots."