NDSU Crop and Pest Report

ISSUE 7  June 13, 2002



**NOTE** Even though light to excess rain has occurred over some of the state there are still areas that are dry. Depending on future rainfall the following may apply to much of the state.

In addition to extreme weather conditions, the spring started off wet and cool and we couldn’t get the small grain crop planted. Now, most areas 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.

Germination of some weeds may be inhibited under dry conditions. However, the cool and wet conditions this spring have produced a record crop of wild oats. 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, especially wild oat, are highly efficient at extracting even limited amounts of 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 and small weed are always easier to control than larger ones. For weeds under drought stress, you should use the highest labeled rates for the herbicides and possibly alter adjuvants if label allows. DO NOT reduce postemergence herbicide rates in dry conditions.

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 which tend to improve efficacy of certain products during periods of slight stress.

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

Regarding application strategies in dry weather consider the following suggestions:

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

A later application may be required 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.

Remember: Puma and Discover have greater efficacy in cool, moist conditions and Assert and Everest have greater efficacy in hot, dry conditions.

Bev Durgan
U of MN Extension Weed Specialist

Richard Zollinger
NDSU Extension Weed Specialist



The EPA has granted a specific exemption for Poast on safflower for grass control including wild oats in no-till or reduced tillage fields. The exemption allows ground or aerial application of a maximum of 1.5 pints of product per acre on a maximum of 35,000 acres.

Applicators must follow all instructions, precautions and warnings on the product label and have a copy of the exemption use directions in their possession during application. The exemption takes effect June 15, and expires July 31, 2002.



Perhaps if crop quality and quantity is poor and in jeopardy and may not justify complete fence to fence spraying, consider spot spraying. Weeds may never take off and those that do may be controlled by spot spraying. This may be applicable for wild oat patches.

Most soil applied herbicides may not perform optimally, especially those applied preemergence. Those incorporated will work better. Weed control from most POST applied herbicides, especially POST grass herbicides (Discover, Puma, Achieve, Poast, Assure II, Select, others) will be lower in dry conditions.

For best results, use the highest recommended rate and use superior adjuvants if allowed on the label. Follow label direction and remember every product is different. Applying herbicides in the early morning (except glyphosate) may improve weed control in dry conditions. During the night, the plant has recovered some from the heat and stress of the previous day. The leaves may not be wilted as much, increasing the leaf’s surface area. The dew may help to keep spray droplets hydrated longer allowing better absorption.

Outside of proper calibration, good working components, and quality nozzles, there is not much to improve performance from your sprayer in dry conditions.

Why do weeds escape in dry weather? Weeds do not grow as quickly in dry conditions and metabolism is slowed. Many weeds are shallow rooted and stop growing in dry soil. Perennial weeds may handle drought better because underground roots can tap into moisture deeper in the soil profile. This does not mean perennials will not be affected by drought. Look at leaf composition. Plants grown in dry conditions have smaller leaves and develop a thicker cuticle which amounts into less droplets intercepting leaves and less herbicide absorbed. Superior adjuvants can help overcome leaf barriers like thick cuticle. Most POST herbicides used are translocated so movement through the cuticle and then through the plant to growing points are critical for adequate control.

Dust is another problem and can inactivate many herbicides. That is why poor control can be seen behind tractor tires and along gravel roads. Most important DO NOT cut herbicide rates. We have been spoiled the last several years with ideal weed killing conditions the last several years. Many have found, especially with wild oat herbicides, excellent weed control with reduced rates. Not a good idea to use that ideology this season during prolonged dry weather.



Two ND Section 24(c) SLN registrations were recently granted by the North Dakota Department of Agriculture. Supplemental labeling for shelterbelts & windbreaks and on mint. Goal is a product by Dow AgroSciences and labels can be obtained by a Dow representative, from the ND DOA, or from CDMS web site.

Richard Zollinger
Ext. Weeds Specialist

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