Pre-harvest glyphosate for weed control and as a harvest aid in cereals

Pre-harvest glyphosate application for cereals was introduced in the UK in 1980 to control perennial weeds, notably common couch. The application time enables very effective perennial weed control without delaying subsequent field operations. This recommendation has been so successful and that now only a small crop area needs to be treated.

Currently a much greater area of wheat is treated with glyphosate pre-harvest in order to aid harvesting. A survey of the scientific literature and experiments carried out by The Arable Group (TAG) and Scottish Agricultural College (SAC) suggest that the benefits of the treatment are variable and that careful targeting of this application is necessary. This may reduce the concern expressed in a recent report for the Food Standards Agency over the level of incidence of residues of glyphosate in UK bread.

The pre-harvest application of glyphosate as a harvest aid is easier to justify in barley than in wheat. It appears to reduce both grain and straw moisture content in a range of circumstances in barley and experimental data suggest that this reduces sieving and threshing losses and increases the throughput of the combine.

It is not clear why the treatments have different effects in wheat and barley. Barley grains may be more exposed to the spray and possibly treated wheat grains and straw may more readily reabsorb moisture from the atmosphere.

Further investigations are needed on the effect of post-application humidity and rainfall on the moisture content of grain and straw (including green stems of wheat ears that contain grain that is ‘harvest dry’) of wheat and barley treated with pre-harvest glyphosate. Early studies on harvester performance were undertaken in the 1980’s. Further studies are needed on the effect of current agronomy and harvesting machinery in evenly maturing and weed-free spring barley treated with pre-harvest glyphosate. Experiments on the effect on the wheat harvesting operation of pre-harvest glyphosate are not needed unless it can be shown that treated wheat grain and straw do not more readily reabsorb moisture from the atmosphere than untreated grain and straw.