Although it is difficult to predict the weather we will be having this growing season, reports indicate that higher than normal temperatures and low precipitation are concerning producers across the region (see Warm weather and little moisture concerns Montana’s ag industry. The Prairie Start. April 2004). Weed management in most crops, including small grains, is severely complicated by high temperatures combined with low and variable precipitation. Not only hot weather and drought affect the germination, growth, hardiness, and competitive ability of crops and weeds, but they also complicate mechanical and chemical weed control practices.
Drought inhibits seed germination leading to decreased weed abundance. However, if weeds with well-established root systems are already present on fallow land or before crop emergence, the lack of soil moistures may enhance their competitive ability. Because the joint effect of enhanced weed competition and drought stress could severely harm crop yield, it is very important to control these weeds and save stored moisture for crops.
High temperatures and low humidity increase herbicide volatilization and degradation reducing the effectiveness of preemergence herbicides, particularly of those that have not been mechanically incorporated. A shallow mechanical incorporation with two perpendicular passes, 24 hours after application will reduce loss of soil-applied herbicides. Unfortunately, when soils are too dry it is very difficult to achieve uniform herbicide incorporation. Also, as soil moisture decreases, herbicide molecules tend to bound more tightly with soil particles reducing the effectiveness of soil-applied herbicides.
Weeds respond to moisture stress by thickening their leaf cuticle and reducing their vegetative growth. These drought-stressed weeds are more difficult to be controlled with postemergence herbicides because of reduced herbicide absorption and low physiological activity. If you are planning to use a systemic postemergence herbicide, do not reduce rates and check the label to see if it permits the addition of crop oil concentrates or N-based spray additives. These products can improve application efficacy in dry weather conditions. Consider applying herbicides in the early morning (expect glyphosate), as weeds are more susceptible to herbicides when they have been able to recover from the drought stress during the night. Delay herbicide applications only if rainfalls are anticipated for the next days. In most cases, waiting too much time will only allow weeds to get bigger, making them more competitive and more difficult to be managed.
If you are planning to mechanically manage your weeds, remember that tillage reduces soil moisture. When soils are dry, tillage could result in seedbed desiccation and reduced crop establishment. You can minimize moisture loss by applying a shallow tillage and using a harrow to fill in and seal the ground. In most cases, adopting minimum tillage or no-tillage may be the best solution to conserve soil moisture, but care should be taken to reduce the proliferation of perennial weed species.
Drought and heat not only complicate weed management, they also increase crop susceptibility to preemergence and postemergence herbicides. Dry weather reduces microbial and chemical degradation of soil applied herbicides increasing carryover injuries. If you applied a residual herbicide last fall, pay special attention to carryover problems. Also, drought stressed crops are more susceptible to postemergence herbicides and the combined effect of herbicide injury and drought may reduce crop yield.
Alvin J. Bussan, the former Cropland Weed Specialist at Montana State University, published an article entitled Weed Control Under Drought Conditions. This article provides a comprehensive review of herbicide recommendations for spring and winter wheat growing under drought stress. You can find it at MSU Weed Science website by visiting http://weeds.montana.edu/crop/weed_control_drought.htm. If we are luckily, it may start raining anytime soon and you won’t need to read it!
Categories: Weed, Herbicide Injury, Crop Injury
Date: 04/15/2005