NDSU Crop and Pest Report

Plant Science

ISSUE 7  June 13, 2002



Soybean plants that are 5 to 6 inches tall should have their first unfolded trifoliolate leaflets (V2 stage).

Nodulation, the symbiotic relationship of bacteria on the soybean roots, can be seen shortly after emergence, but the plant is not actively fixing nitrogen until the V2 to V3 stages. The number and nodules formed on the soybean roots along with the amount of nitrogen fixed increases until the R5.5 stage. Nodules actively fixing nitrogen for the plant are pink or red inside. White, brown or green nodules indicate that nitrogen-fixation is not occurring. Nitrogen fertilization after planting is not recommended as nitrogen fertilizer applied to active nodules will render these nodules inactive or inefficient, depending on the amount of nitrogen applied.

Soil nitrogen is utilized over fixed nitrogen, if available in large amounts. Check the health of your soybean nodules and check root proliferation. At V2, soybeans should be rooting down six inches into the soil

and by V5 will completely reach between 30-inch rows, making any cultivation at V5 needing to be very shallow.

When checking soybean seedlings for nodules, donít pull the roots directly from the soil since that process will slough off the nodules and result in an inaccurate count. Its best to use a small shovel, spade or trowel and dig the soybean roots carefully and shake gently or place in a bucket of water to wash soil off the roots. Nodules then can be counted and examined for viability. Its suggested to check at a minimum of five sample sites in a field to make a good assessment.

Duane R. Berglund
NDSU Extension Agronomist



Recent heavy rains in the northern 2/3rds of the Red River Valley in both North Dakota and Minnesota have promoted inquiries of flooding and ponding in corn and its effects.

The extent to which flooding injures corn is determined by several factors including plant stage of development when flooding occurs, the duration of flooding, and air/soil temperatures. Prior to the 6-leaf stage (when the growing point is near or at the soil surface), corn can survive only 2-4 days of flooded conditions. Once corn is taller and has reached the silking stage shallow depths of flooding will not cause any noticeable amounts of damage. If temperatures are warm during flooding (greater than 77 degrees F) plants may not survive 24 hours. Cooler temperatures prolong survival. Iowa studies found that flooding when corn is about 6-inches in height for 72, 48, and 24 hours reduced corn yields by 32, 22, and 18%, respectively, at a low N fertilizer level (50 lb N per acre). At a high level of N (350 lb N per acre)these yield reductions ranged from 19 to 14% in one year to less than 5% the following year.

Research indicates that the oxygen concentration approaches zero after 24-hours in a flooded soil. Without oxygen, the plant cannot perform critical life sustaining functions, such as nutrient and water uptake which is impaired, root growth is inhibited, etc. Even if flooding doesnít kill plants outright it may have a long term negative impact on crop performance. If excess moisture in the early vegetative stages retards root development, plants may be subject to greater injury during a dry summer because root systems are not sufficiently developed to access available subsoil water.

If flooding in corn is less than 48 hours, crop injury should be limited. To confirm plant survival, check the color of the growing point (it should be white and cream colored, while a darkening or softening usually precedes plant death) and look for new leaf growth 3 to 5 days after water drains from the field.

Cold, wet weather conditions also favor development of seed rots and seedling blights. Seed treatments are usually effective but can provide protection only so long; if seedling development is slowed or delayed 2-3 weeks, soil-borne pathogens have a much greater opportunity to cause damage. Other disease problems which may become greater risks due to flooding and cool temperatures are corn smut and crazy top. The fungus that causes crazy top depends on saturated soil conditions to infect corn seedlings. There is limited hybrid resistance to these diseases and predicting damage is difficult because disease symptoms do not appear until later in the growing season.

Duane R. Berglund
NDSU Extension Agronomist



Dry soil surfaces accompanied by warm dry days are optimum conditions to control weeds with a harrow.

Wheat and Barley into one-three leaf stage can be harrowed with minimal injury. Wheat can be harrowed twice, while barley should be harrowed only once. It is not recommended to harrow oats. Corn can be successfully harrowed starting at the second leaf stage. Populations of shallow rooted seedlings of green and yellow foxtail (pigeongrass) and kochia can be severely reduced by timely harrowing.

Harrows should be set shallow 1/2 inch in depth and operating speeds monitored to avoid crop injury.

A light spring tooth harrow or rotary hoe can be used for weed control in soybean, corn and sunflower, but don't use these tools when the seedlings start to emerge as injury can be severe at this time. Soybeans with the first true leaves (unifoliate stage), sunflower in the 4-6 leaf stage and corn beyond the spike stage to 3 leaf stage can safely be harrowed. Tines should always be slanted back when harrowing for weed control. Trashy conditions which result in trash build-up in the harrow should also be avoided. Use of harrows and rotary hoes have been most effective at midday or on afternoons when temperatures are warmer and soil surfaces are dry, since these conditions promote weed drying and better kill. Also, less physical crop injury has been observed compared to early morning field operations.

Sunflower can be harrowed 2 to 3 times during the 4-6 leaf stage of development. If this type of weed control is planned, sunflower should be planted at rates 5-8% higher than normal.

Duane R. Berglund
NDSU Extension Agronomist

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