NDSU Crop and Pest Report

Plant Science

ISSUE 7   June 17, 2004


A buckwheat planting date trial was conducted at the NDSU Carrington Research Extension Center in the mid-90's. The varieties 'Mancan' and 'Manor' were planted at four dates: May 25 or 26, June 6, June 21 or 23, and July 6 (one year). Across years and varieties, seed yield averaged 1410, 960, and 810 lb/acre with late May, early June, and late June plantings, respectively. Seed yield across varieties in second year averaged 1435, 970, 865, and 705 lb/acre with late May, early June, late June, and early July planting, respectively. The data indicates that seed yield may be satisfactory with late-planted buckwheat, but yield is substantially reduced compared to late May planting and the crop is at risk if an early killing frost occurs.

An established stand of buckwheat should have a density of about 800,000 plants per acre or 16 to 20 plants/sq ft. Buckwheat has the ability to compensate if stand density is less than desired. In a planting rate study at NDSU Langdon Research Extension Center no significant differences in yield were reported between low and high plant populations of six and 24 plants per square foot ( 261,000 vs 1,045,000 plants per acre). Therefore, stands as low as 6 plants/sq ft have produced adequate yields. An average of 9.2 plants/sq ft yielded 1280 lb/acre or only about 10 percent less than a stand of 14.4 plants/sq ft that yielded 1430 lb/acre.

Buckwheat begins to flower four to five weeks after seeding and continues to flower and produce seed until freeze up. The crop should be swathed prior to a killing frost at a stage when in appears the crop will produce the largest seed yield. A killing frost on a standing crop results in considerable harvest difficulty plus yield loss due to shattering and lodging. Swathing is sometimes made difficult by the succulent nature of the stalks. Research conducted in Canada indicates the optimum time for swathing to obtain maximum yields is when flowering is near completion and about 25 percent of the seeds are still green. If shattering becomes severe, the crop should be swathed immediately. The crop should be swathed in the morning when the crop is wet with dew or in damp periods to avoid seed shattering. A frost while the crop is in the swath speeds drying.



Dry soil surfaces accompanied by warm dry days are optimum conditions to control weeds with a harrow or rotary hoe. Populations of shallow rooted seedlings of green and yellow foxtail (pigeongrass), pigweed, nightshade and kochia can be severely reduced by timely harrowing. Light spring tooth harrows should be set shallow (1/2 inch in depth) and operating speeds monitored to avoid crop injury. Tines should always be slanted back when harrowing for weed control.

Either 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) up until 2-3 trifoliate leaf, sunflower in the 2 to 6 leaf stage and corn beyond the spike stage to 3 leaf stage can safely be harrowed. 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 2-6 leaf stage of development. If this type of weed control is planned, sunflower should be planted at rates 5-8% higher than normal. It is highly recommended not to use a harrow or rotary hoe on small seeded crops such as flax, canola, mustard, proso millet or buckwheat.

For more information on post-emergence weed control with tillage see NDSU Extension Circular W-1134 Mechanical Weed Control with a Harrow or Rotary Hoe. Web address:




Soybean plants that are 3 to 5 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 of 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 lateral roots will completely reach between 30-inch rows, making any row cultivation at V5 needing to be very shallow to reduce or limit any root pruning.

When checking soybean plants 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
Extension Agronomist



Corn growth has been slow this spring as temperatures have been below average for most of the state. In Fargo for example, assuming a May 1 planting date, corn growing degree days are running about 50 fewer this spring than average. However, with temperatures warming, corn growth is beginning to accelerate. Many management practices are growth stage dependent, therefore, being able to properly identify the growth stage of your corn crop will be important to ensuring that management practices are applied at the appropriate time. This is particularly true of the application of herbicides during early vegetative development. In this article I will briefly review how to determine the growth stage of your corn crop.

The procedure that I describe here is referred to as the leaf collar method and is the method most often used in recommendations related to the timing of herbicides. When growth staging your crop you should begin by obtaining a representative sample of plants from the field or part of the field of interest. Ten plants should be selected at random. If emergence has been uniform, you can probably get by with fewer plants. Remove any soil attached to the plants so that you are able to observe the roots and crown. Vegetative growth stages of corn are defined by the number of leaves. Counting leaves in corn is fairly straight forward as the process is not encumbered with tillers and leaves on tillers as is the case in small grains. However, care must be taken to ensure that the earliest leaves are included when counting leaf numbers. The first leaf is small and often dies and is torn from the plant early in the growth of the plant. The first leaf has a blunt tip. Look for sheath remnants at the crown of the plant if you suspect that the first leaf (or second for that matter) is missing. Include only those leaves that have a collar. Include all leaves, even those that have been damaged by hail or frost. The total number of leaves that a plant will developed is more or less fixed for a given hybrid; leaves that are stripped from the plant will not be replaced by additional new leaves.

In order to determine the growth stage of older plants that have lost their lower leaves, uproot the plant and split the stem with a knife through the root ball. At the very base of the stem, identify the first visible internode. Internodes are the white area between the more yellow bands of the nodes. The first obviously visible internode should about Ĺ to 3/4 inch in length. The node directly above this internode will be the fifth node, and the leaf arising from this node will be the 5th leaf. Find that leaf and continuing counting leaves from that point.

Corn height

In corn, management recommendations can be made based on the height of the plant, rather than growth stage. For example, certain herbicides can only be applied to corn less than 12 inches tall. The plant height in this case is measure from the base of the plant to where the upper most leaf reaches without stretching it out.

Predicting Leaf Stages in Corn

Sometimes it is useful to be able to predict when a certain growth stage will be reached (i.e. how many days do I have before a herbicide application will be off label). Plant development is more closely related to the accumulation of heat units (growing degree days) than it is to calendar days. Corn growing degree days (GDDs) are calculated using a base temperature of 50 degrees F and are therefore not the same as small grain growing degrees. You can obtain GDDs for NDAWN sites nearest your farm ( http://ndawn.ndsu.nodak.edu/corndd_form.html ). Up until the 10th leaf stage, corn requires about 85 GDDs to produce a new leaf (and only 50 GDDs after the 10 leaf stage). Depending on your location in the state and the season, daily GDDs in the last two weeks of June will average between 10 and 20. Based on these averages, corn should add a new leaf about every 4 to 8 days during this period, depending on where you are in the state and the weather that predominates during that period.

Joel Ransom
Extension Agronomist - Cereal Crops

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