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 and yellow foxtail, barnyard grass, kochia, lamb’s quarters and redroot pigweed likely to be more abundant.
Keep in mind that many soil-active pre-emergent applied herbicides rely on soil moisture for activation, and under dry conditions may not be as effective. Post-emergent herbicides can also be less effective because weeds may have smaller leaves, so less herbicide hits the weeds. As well, leaves may have thicker cuticles (waxy layer) that slows the penetration of herbicides.
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.
- 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.
- Know your crop stage. Drought may impact on the height of the crop, but if anything, it reduceds the time for the crop to move from vegetative to reporductive staging.
- 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 of each product in the Manitoba Agriculture Guide to Field Crop Protection. This section under each herbicide, outlines the strengths and weaknesses of products under different weather conditions. For example, Group 1 products (such as Select, Centurion, Arrow, Poast Ultra, Assure II, Yuma, Contender) have performed reasonably well during drought conditions in the past but do note that they are less effective when plants are stressed by a lack of moisture. Growers contemplating spraying drought stressed crops should consult with manufacturers’ representatives on likely outcomes.
- Daytime temperatures of 25⁰C or higher can trigger crop injury with some products. Contact type herbicides eg: Buctril M on flax, Basagran on beans, are most likely to be effected.
- Use full rates of herbicide. Reduced rates may work under ideal conditions, but are more likely to fail if weeds are drought stressed.
- Use higher water volumes. Higher water volumes will help to ensure good coverage of the weeds and will minimize the risk of crop injury.
- Use split applications of broadleaf and grassy herbicides rather than tank mixing if the Guide to Field Crop Protection warns that antagonism can occur.
- Check the forecast for rain. Shallow, stressed crops roots may be impacted by herbicides moving into the root zone.
- Compare the risk of crop injury (i.e. how stressed is the crop, what proportion of the field is stressed, etc.) to the risk of yield loss due to weed pressure. If the stressed crop is limited to a few low spots in the field, it’s likely worth risking a few acres of injury to protect yield from weed competition. However, if most of the field is water-stressed, it might be worth moving on to a different field and returning after a day or two.
Green Foxtail A Special Concern If Weather Stays Dry
Even though green foxtail was the #1 weed on the 2016 Manitoba Weed Survey, cool weather generally means that it does not compete well with most crops. Many producers have taken advantage of this in recent years and have opted to skip spraying their green 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 and thrive in a row crop situation.
Weed Control Strategies In Fields Used For Emergency Forage
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 determine what restrictions apply, refer to the "Restrictions: Grazing Restrictions;" section for products listed in the Guide to Crop Protection.
In a field that is being utilized 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.