18 February 2000

Control: Flexibility is key in in

sugar beet

BEST sugar beet margins do not necessarily come from crops with the most efficient weed control, and growers need flexible herbicide strategies to cope with seasonal demands.

So says Martin Lainsbury of Morley Research Centre in Norfolk who advises growers to avoid the trap of rigid "blueprints".

"Because of the two-tiered payments system it is vital not to let weeds compromise your ability to meet A and B quotas. But no-one wants excess tonnage because the C price may be insufficient to pay for luxury weed control."

The C beet safety margin is lowest after a hot, dry summer when yield is limited and effective weed control required to maximise output. In damper seasons with high yields, weeds need controlling as cheaply as possible.

"So it is a moving target. When profits are hard to find the emphasis must be on the most economic and not the most effective weed control. This can only be determined as the season starts to unfold."

10 tips on cost

effectiveness

1 Pre- or post-emergence?

Pre-emergence sprays are favoured by heavyland growers. But they could be more widely exploited to insure against weather-delayed spraying. Pre-em treatment gets the crop off to a clean start. By widening the application window for the first post-em it buys time and adds flexibility in difficult seasons. Weeds not killed may become sensitised and more vulnerable to follow-up sprays.

2 Timing.

Timing the first post-em spray is critical. Fast growing weeds are very competitive when the crop is at the cotyledon to two-leaf stage, but they are then also most vulnerable to herbicide. Phenmedipham works best when weeds are growing strongly. A pre-em can allow the first post-em to be delayed until conditions are ideal.

First spray should be broad-spectrum, the second more target-specific aimed at survivors and later germinators. The third provides final clean-up and completes residual build-up to seal the soil against further weed emergence.

3 Herbicide choice.

Failure to get the timing right can restrict product choice. Larger weeds are harder to kill and may need more costly materials. Post-em treatments should be matched to weeds present and prevailing soil conditions.

There are over 100 different products available, but based on only a handful of tried-and-tested actives. The only new one in the past 20+ years is Debut (triflusulfuron-methyl).

In cold springs, when weed growth is slow, some activity from phenmedipham-based products can be lost, so a more expensive mixture may be needed.

4 Weather.

Best results with most post-em herbicides come in warm, sunny weather where there is adequate soil moisture.

Even a slight breeze can cause trouble. Note wind direction and, particularly if using Debut, what is in adjacent fields. Drift could wreck neighbouring peas.

A single sharp frost can damage the beet leaves waxy protection making crops more herbicide-sensitive. But during a series of frosts metabolism slows so there is less risk.

5 Coverage.

Repeat low doses of 80-100litres/ha as a fine spray work well as small droplets reach both surfaces of target weed leaves.

Most pores are on the under-sides and chemicals gain easier access by by-passing the slower waxy upper leaf surface route. This is particularly important on species like fat hen and orache.

6 Time of day.

This can affect speed of chemical access. BASF-funded work at Morley shows early morning, when leaf pores open, is a good spraying time. Later on, particularly in hot spells, they may close to prevent wilting.

7 Nozzles.

Bubble jets tend to be unsuited to beet herbicides because their large droplets do not reach weeds lower surfaces.

Morley trials show a fine spray from a flat fan nozzle is best. Where there is risk of damage to other crops, reduced drift nozzles can be fitted. Some are as good as flat fans on some weeds. Wear can change droplet size, so nozzles should be replaced regularly.

8 Water volumes.

SBREF-funded work a few seasons ago identified differences in treatments when a range of volumes was used. But these could not be repeated, so the conclusion is there is no need for more water than currently used.

9 Pressures.

It is vital to use the nozzles recommended pressures to maintain droplet size.

10 Water quality.

Phenmedipham can become unstable in acutely alkaline water, so avoid high pH levels.

Plan but stay flexible to make the most of sugar beet herbicides, advises Martin Lainsbury who offers growers a 10-point check-list.