By Angie Peltier
The Northwestern Illinois Agricultural Research & Demonstration Center (NWIARDC) received 6.24 inches of rain in April, more than 3 inches of which fell over the last 5 days of the month. Relatively speaking the Monmouth area lucked out as further South, both Perry and Snicarte accumulated more than 5 inches, Belleville more than 6 inches, Rend Lake nearly 8 inches and Carbondale more than 10.5 inches during that same 5 day period (WARM).
Before the rains producers in the Monmouth area were able to get much of their corn crop planted. Some of the crop had emerged over the past couple of weeks but much had not, however most soils are either water saturated or submerged. In many fields it might just be one or two small ponded areas, while fields in the river- or creek-bottoms may be completely submerged.
What saturated or flooded soils do to developing plants. The dangers to roots from flooded soils are many. Flooded soils quickly become devoid of oxygen - which is essential for proper root function. We know that plant leaves are able to use the sun's energy to convert CO2 and water to oxygen and glucose through a process known as photosynthesis. Respiration is sort of the opposite of photosynthesis, where oxygen and glucose are converted back into energy (and CO2 and water) that is used to run the machinery of cells. On a plant's typical day there is a balance between respiration and photosynthesis. On sunny days more photosynthesis than respiration occurs, allowing plants to make the building blocks essential for growth and development, eventually contributing to yield.
All of the organisms that live in soil need to respire to live and function. This includes many bacteria, and soil-living fungi, nematodes, insects and plant roots. Flooded soils quickly become oxygen-free (anaerobic) environments that do not support aerobic respiration. In the absence of oxygen, respiration still continues to occur in the soil and in roots, but this anaerobic respiration leads to the build-up of substances toxic to cells (ex. ethanol, organic acids). Additionally, while an anaerobic soil environment certainly does not favor normal cellular functions, root growth or development, prolonged oxygen deprivation can lead to cell death and death of roots or the whole plant.
Corn. According to the Illinois Agronomy Handbook, corn is very vulnerable to damage from flooding when it is younger than the six-leaf (V6) growth stage and the growing point is still below-ground. Only 3 to 4 days of being submerged in floodwater can be fatal to these young plants. If plants survive the flooding, root growth and function can continue to be reduced even after the flood waters recede. If root development is retarded, they may be unable to access the subsoil moisture needed to meet water and nutrient demands of plants in the reproductive growth stages.
Additionally, there can be concerns regarding soil nitrogen retention as flooding can lead to nitrogen loss through denitrification and leaching. Stay tuned for information about a season-long soil nitrogen monitoring that is taking place throughout the state this year. Information is likely to be shared in the Illinois Pest Management and Crop Development Bulletin.
Soybean. Flooding can also be detrimental to soybean root growth function and nodule formation and function. Without proper nitrogen fixation, soybean leaves can begin turning yellow. Research has shown that photosynthesis can be reduced by 1/3rd with 48 hours of flooding (Oosterhuis, 1990). This reduces dry matter accumulation both during and after flooding and can reduce seed yields. One bright spot is that after water drains away, photosynthesis and dry matter accumulation can resume.
Potential for crusted soils after rainwater drains. The strong winds and sunny weather today can cause soils to dry to form a hard crust that can interfere with can cause uneven seedling emergence (Figure).
Yield losses can occur when some corn plants emerge significantly earlier than their neighbors. Those plants that emerge earlier than their neighbors are often taller, have larger root systems and more photosynthetic capacity due to their later stage of development. Later emerging plants must compete with those that have already emerged for finite sunlight, moisture and nutrient resources and are often barren.
To determine how much yield loss may be associated with uneven emergence of corn plants, over seven location-years researchers in Illinois and Wisconsin (Nafziger et al., 1991) hand planted seeds at different times within rows to force plants to emerge at different times. Seeds were sown at three different times: 1) early: late April to early May, 2) middle: 10 to 12 days after early, and 3) late: 21 to 27 days after early.
Uneven emergence, 1 ½ week delay. When a portion of the plants emerged 1 ½ weeks later than others yield was reduced by 6 to 9 percent compared to a uniformly emerging early stand. Yields of these uneven stands were comparable to planting the entire stand 1 ½ weeks later.
Uneven emergence, 3 week delay. Delaying emergence of some plants in a field by 3 weeks resulted in significant yield reductions. Yield was decreased by approximately 10 percent when 25 percent of the plants emerged 3 weeks later than the rest of the stand. Yield loss reached 20 to 22 percent when 50 to 75 percent of the stand emerged late. These yield losses were greater than had the entire stand been planted 3 weeks later (Carter et al., 1989).
One must weigh the costs of replanting with the potential benefits of establishing a uniform stand by replanting. Communicating with your crop insurance adjuster to determine whether replanting costs are covered is important. Other resources that might help include the University of Illinois has a handy corn replant calculator and Purdue University's "Thin Soybean Stands: Should I Replant, Fill In, or Leave Them Alone?".