ISSUE 11 July 15, 2004
FLOODING AND CROP GROWTH
Recent heavy rains in some production areas have caused some concerns. Prolonged soil saturation affects crop growth and yield. In early growth stages, corn or soybeans can survive for only two to four days under water in anaerobic conditions. Most dry bean classes or types are extremely sensitive to flooding. One or two days can completely destroy plants. Corn is also somewhat sensitive to flooding in the early vegetative stages (especially prior to the fifth or sixth leaf stage). It appears that soybeans are more tolerant to flooding that either corn or dry beans. Moderate water movement can reduce flood damage by allowing some oxygen to get to the plants, keeping them respiring and alive. Drainage within one to two days increases the chance of survival.
The injury extent to seedlings is determined by the plant stage of development at ponding, duration of flooding and the air/soil temperatures as well as if axillary buds are present on damaged plants. If temperatures are warm during flooding (greater than 77 F), plants may not survive 24 hours. Cool temperatures and cloudy conditions may prolong survival. However, wet weather favors disease development. Seed treatments are effective, but limited in protection. Seedling development slowed or delayed two to three weeks allow soil-borne pathogens a greater opportunity to cause damage. Seed rots, seedling blight, corn smut and crazy top affect corn plant 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. Carefully assess damage before tearing up or abandoning an existing stand.
Rotted seed or damped-off seedlings will reveal probable crop losses. On surviving stands, remember that favorable weather for plants after ponding is important. Cultivation, once soils are dry enough, will open and aerate surface soil and promote root growth. Be careful working the soil. Working wet soil causes compaction that varies crop growth. One must be patient in waiting for the soils to dry.
An additional nitrogen application in corn may be necessary in fields that show signs of yellowing or uneven growth. A late test for nitrate when corn plants are still six to twelve inches tall can determine if more nitrogen is needed.
Maintain a good weed control program so that crop plants are not robbed of nutrients and moisture later in the season.
SOYBEANS IN BLOOM
Many soybean fields are starting or are near early bloom in eastern North Dakota and in NW Minnesota. The beginning bloom stage or R1 stage in soybeans is marked by the plants having at least one flower on any node of the main stem. Soybeans are at 50% bloom when an open flower can be found on every other plant in a row. Flowering, unlike maturity on soybeans, begins toward the bottom of the plant (at the third to sixth node) and then progresses upward and back downward. Branches off of the main stem will flower a few days later than the main stem. While flowering begins at the base of the plant and proceeds to the top of the plant, physiological maturity of the beans will progress anywhere on the plant stem. Normally, soybean pods will be mature in the middle or top of the plant and down, thus remember to check pods toward the bottom of the plant when determining if harvest time has come. Flowering of soybeans is an important time in bean growth and development. At stage R2, full bloom, each plant has accumulated about 25% of its total dry weight and nutrients; it has attained about 50% of its mature height; and, it has produced 50% of its total mature node number. This later flowering stage begins the period of very rapid N-P-K and dry matter accumulation that will continue through R6. Also, during flowering the soybean plant gears up on its nitrogen fixation in order to provide for the demands of the plant. Scout for disease and insect problems (aphids) during this critical time period of flowering.
Duane R. Berglund
NDSU Extension Agronomist