Variability in the control of black-grass with herbicides in winter cereals - harvest years 1987-1989

Abstract

Relatively high incidence of poor control of black-grass (Alopecurus myosuroides) with soil-applied herbicides occurred in winter cereals during the harvest years 1987,1988 and 1989. In 1987, the incidences of poor control were not associated with soil texture but in 1988 and 1989 there as an association with heavy soils.

A review of the literature on the factors affecting the activity of the major soil-applied black-grass herbicides of the substituted (phenyl) ureal group along with specific studies was commissioned by the HGCA in 1989.

The pot studies indicated that waterlogging in autumn 1987 and enhanced degradation were not contributory factors. However, the common observation of the establishment of the weed root system at greater depths below the soil surface than normally occurs was proved to reduce the control achieved by soil-applied herbicides. This 'deep-rooting' has been associated with specific seedbed conditions and cultivation practices.

This report explains some of the factors associated with poor control in the years in question. Lack of rainfall after application was predicted not to have given sufficient herbicide movement in the heavier and more absorptive soils for optimal activity in 1988 and 1989 whilst in 1987 the cause was largely due to excessive degradation and leaching associated with warm and wet conditions. However, the role of the mild weather during the winters of 1987/88 and 1988/89 remains unclear as does the impact of cultivation practices on the weed root system and the influence of herbicide resistance.