Effects of growth stage, weather factors and adjuvants on herbicide activity against the sterile brome - an aid to defining 'weather windows' for spraying

Summary

Sector:
Cereals & Oilseeds
Project code:
PR93
Date:
01 April 1989 - 31 March 1991
Funders:
AHDB Cereals & Oilseeds.
AHDB sector cost:
£78,174 From HGCA (Project No. 0044/2/88)
Project leader:
J.C. Caseley, P. Gendle, V. Down and H. Baggett Long Ashton Research Station, Institute of Arable Crops Research, Department of Agricultural Sciences, University of Bristol, Long Ashton, Bristol BS18 9AF

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About this project

Abstract

The aims of this project were to quantify the importance of weather factors before and after spraying on the activity of herbicides used to control sterile brome, to define 'weather windows' for optimum herbicide activity and to improve herbicide performance under adverse conditions by the use of adjuvants.

Sterile brome was planted in sandy clay loam soil contained in 10cm diameter pots and grown in defined environments where one factor was varied while the others were held constant so that the importance of individual factors could be identified. In outside and glasshouse situations the microclimate was closely monitored. Extensive use was made out of controlled environment enclosures and a rain stimulator.

The herbicides were most effective when applied early post-emergence of the weed and although reduced, activity was still acceptable at the 4 leaf growth stage. Recommended field doses of chlorotoluron, isoproturon and mtoxuron failed to control plants with 6 leaves and 3 tillers; only SMY1500 and, to a lesser extent, cyanazine were phytotoxic at this growth stage. Chlorotoluron, isoproturon and metoxuron performed best when post-spraying temperatures were cool (10/4°C day/night respectively and lower) and when 3 or 6mm of simulated rain was applied after spraying. Activity of these herbicides was reduced under warmer conditions (15/10°C day/night respectively and higher ) while SMY1500 and cyanazine performed well at higher temperatures.

Pre-emergence application of cyanazine and isoproturon to a 'dry' compared to moist soil surface (1 cm and 35 ± 1-% compared to 85 ± 10% field capacity) reduced activity whether post-spraying rain was applied or not. In contrast, tri-allate was unaffected by surface soil moisture.

Post-emergence applicatio of cyanazine and isoproturon to brome grown in dry soil (35% field capacity, FC) had little effect when low FC was maintained. However, watering the plants from above and increasing the soil moisture to 100% FC resulted in high herbicide activity due to movement of herbicide down the soil profile and rapid development of adventitious roots into this herbicide containing zone.

Adjuvants including Agral and Silwet L-77 significantly enhanced the activity of post-emergence herbicide treatments against brome, but the risk of crop damage was also increased.

Frost and relative humidity had little effect on herbicide activity.

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