NEWS for North Dakotans
Agriculture Communication, North Dakota State University
7 Morrill Hall, Fargo, ND 58105-5665

May 20, 1999

Growth Stage Determines Water Injury in Corn, Soybeans

Prolonged soil saturation affects the growth and yield of corn and soybeans. The extent of water-related injuries are determined by the plant stage of development when flooding occurred as well as the air and soil temperatures, says an agronomist at North Dakota State University.

"Cool temperatures may prolong seedling survival," says Denise A. McWilliams, extension crop production specialist at NDSU. "But cold, wet weather favors disease development. Seed treatments are effective, but limited in the protection they offer."

Seedling development slowed or delayed two to three weeks allow soil-borne pathogens a greater opportunity to cause damage, McWilliams says. Seed rots, seedling blight, corn smut and crazy top affect corn development later even though ponding occurred earlier. Delayed soybean growth allows diseases such as Fusarium root rot, Phytophthora rot and Pythium rot to establish and weaken or destroy seedlings. Limited hybrid and variety resistance to those diseases and difficulty in predicting damage make evaluation difficult.

"Producers should carefully assess the damage before they decide to replant," McWilliams stresses.

In its corn-loss instruction booklet, the National Crop Insurance Service says that yield loss from early season stand reduction (up to the 10-leaf stage) can vary. With a 100-percent plant stand, yield loss is negligible. A 75-percent stand can equate to a 10-percent yield loss. A 50-percent early corn stand is likely to experience at least a 26-percent yield loss, and a 25-percent stand will usually result in at least a 43-percent yield loss, if seeding rates were reasonable and remaining stands were healthy.

In soybeans, research through various areas of the United States has shown that yields are not affected by population reductions until stands drop below 125,000 plants per acre, but yields can be lower if large gaps are present, McWilliams says. These skips can rapidly reduce yields in soybeans. According to research from Indiana's Purdue University, two-foot skips in soybeans in 50 percent of each row can decrease yield by 6 percent. Three-foot skips in 50 percent of each row will drop yields by 13 percent. Four-foot skips in 50 percent of each row will decrease yield by 15 percent in healthy stands.

"Rotted seed or damped-off seedlings will reveal probable crop losses," McWilliams says. "Evaluate the uniformity of the original stand, consider replant pest control and seed costs, and then factor in projected crop prices. Weigh these costs and price projections against crop injury and replanting yield gains."

On surviving stands, favorable weather for plants after ponding is important. Cultivation, once soils are dry enough, will open and aerate surface soil and promote root growth, McWilliams says. But working wet soil may also cause compaction that produces uneven crop growth. An additional nitrogen (N) application in corn may be necessary in fields that show signs of yellowing or uneven growth. A late-spring test for nitrate when corn plants are 6 to 12 inches tall can determine if more N is needed.

"Scout more intensively in previously ponded areas of corn and soybean fields," McWilliams concludes. "Maintain a good weed control program so that crop plants are not robbed of nutrients and moisture later in the season."

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Source: Denise McWilliams (701) 231-8160

Editor: Dean Hulse (701) 231-6136