Issue # 13, September 20, 2000

• Frost Effects on Late Seeded Crops
• Desiccants Can Speed Sunflower Harvest
• Wildlife Services Cattail Management Program

Frost Effects on Late Seeded Crops

    Fall and frost go hand in hand and also questions concerning possible frost damage to unharvested crops.  If the crops have reached physiological maturity, frost can be helpful to desiccate any green material and speed drying.  The following will explain how to determine physiological maturity in various crops

Flax: Is most susceptible to frost damage during flowering and early boll stages of development.  Flax is near physiological maturity when 75-80% of the bolls have turned brown.  At this time flax can be swathed, a desiccant can be applied or is a safe time for a frost.  If the plants are not at this stage, considerable frost damage occurs to immature flax seeds when temperatures drop to the 27-23 degree F.  Leaves are severely damaged at the 25-23 degree F range.  The recent warm weather has not been conducive to hardening off of immature flax and preparing late planted flax for frost tolerance.
Soybeans:  Physiological maturity first occurs when the seed (and generally the pod) turns yellow, or has completely lost all green color (R7 growth stage).  Although not all pods have on a R7 plant have lost their green color, the plant is essentially at physiological maturity because very little additional dry weight wll be accumulated.  The soybean seed is at physiological maturity is about 60 percent moisture and contains all necessary plant parts to begin the next generation soybean plant.  The best clue of physiological maturity is to open pods and check shrinking of beans and look for separation of beans from the white membrane inside the pod.  This indicates the soybeans are physiological mature and fairly safe form frost injury. 
    Soybeans are easily damaged by frost in the 28-32 degree F range.  Temperatures of 28 F for any extended period of time can completely kill soybean plants (stems and leaves).  In the event of a leaf killing frost when pods are still light green or yellow, wait until the pods are mature in color before combing. Green soybeans can cause market discounts.  If the soybeans are only slightly green, there is a good possibility that drying and storage will fade out the green.  
Corn:  Physiological maturity is reached when the hard starch layer has advanced completely to the cob and a black or brown absicission layer has formed.  This black layer formation occurs progressively from the tip ear kernels to the basal kernels of the ear.  
    Substantial frost damage of leaf, stalk, and husk tissue will occur when temperatures remain below 32 F for 4 to 5 hour or when temperatures decline to 28 F for several hours before increasing above 32 F.  If all leaves on a corn plant are killed at the late dent stage when 85 percent of the yield has been produced, the yield reduction due to the complete killing frost is 15 percent.  At the one-half milk line, 8 percent yield losses can occur.  Frost damaged corn will have a higher moisture content which may take longer to dry down.

Desiccants can Speed Sunflower Harvest

    Using desiccants on oilseed and confection sunflower crops to speed drydown is not a common practice but can be useful when the crop is mature and an early harvest would be an advantage, according to Duane Berglund, extension agronomist at North Dakota State University.  Sunflower planted early last spring are starting to show signs of physiological maturity, and persistent rains in some areas have resulted in sclerotinia head rot, Berglund says. Early drydown of sunflower plants may slow or stop development of head rot and reduce sclerotia and destruction of seeds.  However, he says, reduction of head rot depends somewhat on weather following desiccation. Wet weather may actually increase the problem in desiccated sunflowers.  White mold does attack dead plant tissue, and, while little data is available, Berglund says grower experience indicates enhanced head rot when wet weather follows desiccation compared to green fields left untreated.  "Growers need to weigh the weather probabilities and the advantage of early harvest compared to the risk of wet weather following desiccation," he says.  There are other reasons desiccation might be desireable. "Bird pressure is always a concern for many sunflower producers.  Applying a desiccant narrows the window of opportunity for those migrating birds that feed on a sunflower crop," says Berglund.  Reduced heat shattering is another factor favoring desiccation. "The quicker harvest time comes, the less time large sunflower heads will spend knocking against each other. Accelerating the harvest season will also prevent further deterioration of crops affected by disease," he says.  Using a desiccant to control weeds, especially large weeds like kochia and marshelder, will result in less dockage, less wear and tear on combines, and easier crop drying with reduced drying costs.
    In North Dakota, Gramoxone (paraquat) and Drexel Defol (sodium chlorate) are labeled as sunflower desiccants. Both have a seven-day preharvest application interval.  Producers need to apply either desiccant after the backside of sunflower heads have turned yellow and the bracts are turning brown. Physiologically mature sunflower plants have a seed moisture content between 33 and 35 percent.  "Some of our sunflower hybrids now have a stay-green stalk characteristic, so go by the heads or seeds," Berglund says.  "Another way to tell if physiological maturity has occurred is to rub the chaffy material on the front of a sunflower head. If it rubs off easily, the plant is physiologically mature."  Producers planning to apply a desiccant should check the herbicide label or consult the current edition of NDSU Extension Service publication W-253, 2000 North Dakota Weed Control Guide.

Wildlife Services Cattail Management Program

    I have been receiving many calls concerning blackbird problems.  At this time there is only one program that is available through USDA Wildlife Services.  The Cattail management program uses Rodeo aquatic herbicide to reduce large stands of cattails used by roosting blackbirds.  By reducing cattail densities, blackbirds can be dispersed from roosting marshes leading to a reduction in damage to sunflower, small grains and corn crops.  The cattail management program is a cost-free program offered to agricultural producers on a first come, first serve basis.  To be eligible for spraying, wetlands must meet certain requirements:

1. Each individual wetland must contain at least 15 acres of cattails
2. Wetlands must have standing water
3. Wetlands must currently or have historically been used by blackbirds
4. Wetlands must be near sunflower growing areas

Wetlands will be sprayed by certified aerial applicators contracted by Wildlife Services.  For best results, wetlands will only be sprayed after July 1st and spraying will cease after the first hard frost.  
    To sign up for this program, contact the Griggs County Extension Office for the necessary forms.  This should be done this fall in order to be on the list for next year.  For more information contact Wildlife Services at 701-250-4405.

Please Contact Our Office For Additional Information
E-mail: griggs@ndsuext.nodak.edu
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