How Weeds Deal with Drought
Currently, Ohio has experienced the third worst drought, only surpassed by 1930 and 1933. Fall is the driest time of year (August-November) and about 60% of Ohio is still in a severe to exceptional drought. That means that field capacity, the amount of soil water held in the soil after the excess water has drained away, is still very low for next year. After a rain, it generally takes 2-3 days for excess soil water to drain away. The small amount of precipitation (rain, snow) we currently received may help, but it will take considerable moisture to fill up our available soil water holding capacity reserve for growing crops next year.
In Nebraska, a graduate student (Mandeep Singh) studied how weeds handle drought conditions. He studied 89 journal articles with 1,196 paired observations over 30 years. The data was collected all over the world; mainly North America, Europe, Asia, and Australia. Here are his findings on how drought and water stress affects weed production and weed growth.
First, weed still produce weed seed when soils are dry, just not as much. Weed seed production dropped 50% under moderate water stress (30-60% of field capacity) and 88% under severe drought stress (less than 30% field capacity). While it may not seem like it, weeds are hurt by dry conditions too. Grasses, with their multiple fine roots generally grow better than broadleaf weeds during a drought. Weed seed still germinate under dry conditions because they do not require a lot of water, they just do not grow as well or produce as many weed seeds.
On growth, above-ground growth or plant height was decreased by 24% with a moderate water stress and 37% with severe water stress. Leaf area decreased, 24% with moderate drought stress and 37% with severe water stress. Total weed biomass decreased 43% under moderate and 44% under severe water stress. Water stress reduces plant growth by reducing the amount of plant available nutrients. How do weeds and plants cope with water stress? Generally, they grow less roots, but grow their roots deeper to access deep soil water.
Here are strategies to deal with water stress expected next year, possibly even 2-3 years yet. First, reduce tillage and keep the ground covered. Each tillage pass loses 0.5 to 1.0 inches of water in the soil surface. Second, keep residue on the soil surface intact. An Ohio study showed that the no-till mulch reduces field evaporation by 18-35%, conserving soil moisture.
Third, kill cover crops early in the spring to conserve moisture. Is it beneficial to not plant cover crops to conserve moisture? Not really. Bare soil loses 6 inches of water while cover crops take up about 6.5 inches of water yearly. However, the cover crop also adds soil organic matter (SOM). Each 1% SOM adds .25 up to 0.5 inches of water to the soil profile. Stopping tillage results very quickly in about a 1% gain in surface SOM. Soils high in SOM store more soil water and tolerate drought much than low SOM fields.
Other strategies: Plant crops early to get good fast root growth before the hot dry weather sets in during the summer months. Consider planting earlier maturing corn. Early corn planted early may mature before moisture stress gets severe. Consider shorter corn varieties and shorter, bushier soybean plants to maximize water usage for crop yield.
A big factor in improving crop yields in dry conditions is reducing soil compaction. Vertical tillage especially, creates several layers of soil compaction zones that roots need to penetrate to access soil water. The best solution is to plant deep rooted cover crops to access potential soil water 20-30 inches deep. Deep rooted brassica cover crop species (radish, kale, and rape) are good. However, oats, Sorghum Sudan, and even cereal rye roots may penetrate soil 30 inches deep. Usually cover crops can penetrate severely compacted areas no more than 0.5-1 foot per year. If tillage is performed, deep wide-spaced subsoilers actually do the least amount of damage in dry conditions, because the soil will not smear.
Scouting for weeds during the growing season is critical. Weeds compete for moisture with the grain crop. Weeds under drought stress take up herbicides less effectively, so killing weeds early and right after a rain when they are actively growing is your best alternative. Avoid glyphosate late in the season, because it enhances drought stress and increases disease on your crops. Investing in irrigation is another strategy. Farmers may need a wetter than normal winter and a little extra snow to improve crop yields next year. Weed Study Source: No-till Farmer.