Dry Weather Weed Control

General Rules Of Thumb For Dry Weather

Dry weather means both weeds and crops shift gears. Weed spectrums can be different in hot, dry years, with green foxtail, kochia and redroot pigweed likely to be more abundant.

In addition, some herbicides withstand dry weather better than others so choose your product carefully. Here are some general guidelines on weed control during a dry period.

  1. Remove weeds early to minimize the amount of moisture consumed by weed populations. Where there is a leaf stage range for herbicide application timing, spray at the earlier end of the range wherever possible.
  2. Herbicides are most effective under ideal growing conditions, when both crops and weeds are growing rapidly. Conversely, conditions which slow plant growth, such as heat and/or moisture stress can reduce both crop tolerance and weed control. Review the "Effects of Growing Conditions" section for each product in the Manitoba Agriculture and Food 2002 Guide to Crop Protection. This section outlines the strengths and weaknesses of products under different weather conditions. For example, Group 1 products (such as Select, Poast, Assure) have performed reasonable well during drought conditions in the past but can still have performance problems. Growers contemplating spraying drought stressed crops can consult with manufacturers’ representatives on likely outcomes.
  3. Daytime temperatures of around 27oC can trigger crop injury in some products. Contact type herbicides eg Buctril M on flax, Basagran on beans, are most likely to be effected.
  4. Use full rates of herbicide. Reduced rates may work under ideal conditions, but will fail if weeds are drought stressed.
  5. Use split applications of broadleaf and wild oat herbicides rather than tank mixing if the Guide to Crop Protection warns that antagonism can occur.

Green Foxtail A Special Concern If Weather Stays Dry

Cool weather generally means that green foxtail does not compete well with cereals. Many producers have taken advantage of this in recent years and have opted to skip spraying their foxtail and let the crop take care of suppressing the weed.

This may not be the best strategy in fields (for example, failed winter wheat fields) that have had to be resown in early June. June temperatures are generally warmer than in May and this means that green foxtail will have a competitive advantage over cereals at an early stage in crop development. Thin, drought-stressed crops will also compete less with green foxtail. Foxtail plants which emerge at or about the same time as crops will receive sunlight through thin crop canopies.

Weed Control Strategies In Fields Used For Emergency Forages

In the event that a crop is being used for greenfeed, farmers should be aware that many herbicides used this spring may have grazing restrictions.

With most herbicides, a certain period of time must pass after application before greenfeed from a treated field should be fed to livestock. The amount of time varies from product to product. To determined what restrictions apply, refer to the "Restrictions: Grazing;" section for products listed in the Guide to Crop Protection.

In a field that is being utilised for greenfeed, growers can be less concerned about field cleanliness. As long as weed populations are low to moderate, it will not pay to spray for weeds: cattle will consume weedy plants with no problem as long as poisonous weeds are not a concern.