Well - it seems that Minnesota is once again the land of extreme weather conditions. The spring started off wet and cool and we couldn’t get the small grain crop planted. Now -many areas of northwest Minnesota need rain and the crop and weeds are under stress.

Drought stress complicates weed control in most crops, including small grains. Prolonged dry weather has a direct bearing on several aspects of weed management including (1) weed germination, growth, and hardiness, (2) weed and crop interactions, and (3) chemical weed control.

Weed germination is inhibited under dry conditions. Thus, these drought conditions have reduced weed infestations. You are probably seeing this in many fields. I have been receiving calls on the lack of foxtail in most fields. However, the cool and wet conditions this spring have given many of you a record crop of wild oats in many fields. These wild oats need to be controlled as soon as possible to prevent yield losses due to competition.

Drought that occurs after weed emergence toughens or hardens plants. Weed response to severe drought stress includes leaf cuticle thickening, reduced vegetative growth, and rapid flowering; making drought-stressed weeds more difficult to control with postemergence herbicides.

Weeds compete with crop plants for moisture, nutrients, and light. Many weeds are highly efficient at using available soil water. The combined effects of drought and weed competition can severely decrease spring wheat and barley yields.

Postemergence herbicides can be dramatically affected by drought. Efficacy of postemergence herbicides, particularly those that are translocated within the target weed, is highly dependent upon active plant growth. Typically, the better the growing conditions, the better the performance of postemergence herbicides. Good soil moisture, moderate temperatures, and high relative humidity are conditions favorable for maximum growth and herbicide activity. Essentially all postemergence herbicides have a statement on the label regarding weed growth and environmental conditions. A typical example is "Do not apply to grasses or crops under stress such as stress due to lack of moisture.as unsatisfactory control may result." To attain adequate control, weeds must be actively growing and must not exceed the maximum growth stage.

For some weeds, delaying postemergence control efforts until moisture stress is relieved is not feasible. Many postemergence treatments are effective only on small weeds. For weeds under drought stress, you should use the highest labeled rates for the herbicides and possibly alter the adjuvant type or rate. This is definitely not the time to be reducing postemergence herbicide rates.

Where product labels permit, addition of crop oil concentrate rather than nonionic surfactant usually results in greater herbicide activity. Some products also allow for N-based spray additives that tend to improve efficacy of certain products during periods of slight stress.

The problem we are facing now is making the decision of delaying herbicide and waiting for a rain or going ahead and spraying drought-stressed weeds. Control is generally unsatisfactory when weeds are drought-stressed, but delaying an application often allows the weeds to get too big. Making this decision is difficult, and must be done on a field-by-field basis.

I have already received some complaints regarding poor weed control this year. In many cases, the complaints have followed a postemergence herbicide application to drought-stressed weeds.

• if rainfall is in the immediate forecast, post-emergence herbicide activity will be maximized by waiting until after rain to apply, but
• large drought-stressed weeds can be extremely difficult to control, so
• when there appears to be no rain coming, apply when weeds are small for best activity.

This may result in the need for a later application if late rain stimulates a new flush of weeds, but is a more effective strategy than trying to kill large tough weeds with herbicide mixes that are excessively injurious to the crop. Also, removing weeds as soon as possible will result in less yield reductions due to crop competition.

Crop Injury: Remember, the crop is also under drought stress, so don’t be surprised if you see more crop injury this year than in past years. Also, be careful with additives.