5 April 1996

Early summer best time to spray grass

DONT rush to spray grass weeds in set-aside this year. For best control wait until early summer, says Monsanto.

"The enhanced performance of Roundup Biactive (glyphosate) means that as long as the plant is still growing, all viable seed will be killed," says Colin Stride of Monsantos product development department.

Trials over two years have shown early spraying to kill blackgrass is a waste of time. No seed is viable until late May and the level only climbs slowly to peak at 40% at the end of June.

Plants are also growing actively in April and May, reducing chemical translocation in the plant. Enough survived when treated with a reduced rate of 1.5 litres/ha on Apr 21 to ensure 31% of seed shed in summer was viable. At double the rate, the figure was still 22%.

Not until the start of June was 100% control of seed achieved. Both rates were equally effective and both maintained that level of control through the month.

Barren brome

Work with barren brome gave similar results, but about a month earlier, says Mr Stride. Complete seed kill was achieved on May 9, again with both half and full dose. Within three weeks the lower rate was slipping and by mid-June 20% of seeds were viable when shed later in the season.

But the full rate continued to work. "The 3 litres/ha rate gives the flexibility to control brome until mid-June. But the half rate is not man enough.

"The work shows that optimum control of grass weed species using Biactive is at full ear emergence, usually late May and June."

A useful spin-off is the amount of cover that is left for wildlife. But growers wanting clean fields can apply two doses at 1.5 litres/ha, the first in mid-April and the second mid-June. That is as effective as one well-timed single spray for blackgrass, although the latter dose should be doubled for brome.

The only other good reason to spray early is if volunteer potatoes are a problem, he says. "Treat grasses and cereal volunteers in April. This will remove cover, and allow potatoes to develop. These can then be sprayed with a 4 litres/ ha rate mid-July. &#42