Effects of timing and dose on levels of chlormequat in wheat, barley and oats

Abstract

Chlormequat is a very effective lodging control agent in winter wheat. It has been shown to reduce lodging risk by the same amount as improving varietal standing power by 1-2 units, delaying sowing by 2-4 weeks and establishing about 100 plants m-2 fewer. In the UK, plant growth regulator products containing chlormequat are applied to about 65% of the winter wheat area and almost 50% of the winter barley and oat areas.

Previous surveys have recorded chlormequat residues in the harvested grain of most cereal crops that have been treated with chlormequat. Whilst the residues found in cereal grains are almost always below the maximum residue limit (MRL), pressure may be exerted in the future to further reduce residues due to the Food Standard Agency's aim to minimise pesticide residues in food irrespective of the MRL.

Published literature has shown that the level of chlormequat residues may be determined by the time of PGR application and dose rate, and possibly by the crop's growing conditions. This study aimed to quantify the impact of management practices and crop conditions that affect chlormequat residues in wheat, barley and oat grain, whilst maintaining the ability of chlormequat to reduce lodging.

The experiments in this study were carried out at three UK sites during the 2002-03 growing season. Chlormequat residues were found in the grain of all of the wheat, barley and oat crops that were treated with a range of timings and rates of chlormequat. Residues ranged from 0.01 to 0.34 mg kg-1 for wheat, 0.01 to 0.46 mg kg-1 for barley and 0.01 to 0.77 mg kg-1 for oats. None of the residues exceeded the MRL set for chlormequat in wheat and barley of 2 mg kg-1, or oats of 5 mg kg-1. In winter wheat, the most effective method of reducing chlormequat residues, without significantly reducing lodging control, was achieved by applying chlormequat earlier in the plant's life-cycle.

The effect of changing the application timing from GS31 to tillering was to reduce the chlormequat residues in the grain by 60% and only cause a small and non-significant reduction in efficacy. In oats, the trials in this single season study indicated that chlormequat residues could not be reduced by changing the time of application or reducing the dose rate without greatly reducing the efficacy of lodging control. In winter barley, chlormequat did not reduce lodging or crop height, but did cause a non-significant increase in yield of 0.25 t ha-1 at one of the two sites.

Previous studies have also shown that, in certain circumstances, chlormequat can increase the yield of winter barley. Applying chlormequat at late tillering reduced residues by 33% compared with applications at GS30. Reducing the dose rate to ¼ only reduced the residues by 36%.