Appropriate herbicide rates for cereal crops

Abstract

This report reviews the results of a project devised to examine whether the approaches developed by SAC and DANI through government and HGCA funded work for optimising herbicide use in cereals could be used on a wider basis, and to examine further the conditions which effect herbicide dose responses. The approach is to develop optimum herbicide rates and programmes consistent with conditions, weed size and crop situations ('appropriate rates'). Earlier trials series were confined to weed flora excluding some of the most competitive weeds, and did not look in detail into the effects of crop/weed relationships, conditions and timing on herbicide activity.

The objectives and conclusions of the two parts of the project are:

A. A series of trials in winter wheat examined whether such approaches have benefits in controlling a difficult weed, using Galium aparine (cleavers) as the model, and what are the important components determining the efficacy of such herbicide programmes.
Using an autumn-residual herbicide, such as diflufenican (DFF) + isoproturon (IPU), pendimethalin + IPU or isoxaben + IBU, improves the activity of the spring cleaver herbicide, fluroxypyr. The sequential herbicide approach improves weed control allowing dose reductions at least to half-doses.
Crop vigour affects the dose response curve. Use more herbicide in less vigorous crops, and in poorer conditions.
Crop density has a clear effect on cleaver growth and the capacity of fluroxypyr to control the weed. This was also confirmed for oilseed rape and perennial ryegrass, used as model weeds in a further trial series at QUB. Increased crop competition improves herbicide activity.
Fluroxypyr treatment after autumn use of a residual treatment has greatest benefit when used around GS32 of the crop in terms of effect on cleavers and yield benefits.

B. Futher trial series examined the impact of herbicide timing and weed/crop interaction on dose responses in terms of weed control and crop yield of spring barley:
Increasing crop density, though competition, reduces weed biomasss, and normally improves herbicide efficacy. A vigorous crop is the key to good weed control. Crop phytoxicity from herbicide use is most likely at highest crop densities in good growing conditions. Reduce herbicide doses on vigorous crops in good growing conditions. Sequential pre-/post- herbicide treatments give very variable weed control due to variability in soil moisture required for residual treatments tested. Use such an approach only when annual meadow-grass is likely to be a problem. 
Yield response to weed control is very variable in spring barley.
However, in general, early post-emergence treatments (2-4 leaves of weeds) give the best weed control.
Early treatment gives the greatest possibility of dose reduction, but dose reduction is more effective in competitive and denser crops.

Further trial series have shown the clear impact of crop cultivar in weed suppression and improving herbicide efficacy. Weed suppression and control is improved by use of cultivars giving relatively high levels of early ground cover.

The project emphasises the importance of timing and weed size on herbicide activity, and, in winter cereals, the usefulness of the sequential treatment strategy in allowing dose reduction with a high level of security. It emphasises the importance of crop vigour and competition in determining appropriate rates for herbicide programmes. There is a concern regarding crop phytoxicity with herbicides, which is greater in dense, vigorous crops, but it is also in these crops that herbicide dose reduction is most possible.