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ISSUE 16  August 23, 2001



A killing frost occurs when the air at plant level reaches a temperature that kills the plant tissue. If you have frost damage, it is the result of plant tissue death.

Critical stages and temperatures for various major crops and described as follows:

Wheat, durum and other small grains: Air temperatures below 32E Fahrenheit (zero degrees Centigrade), the freezing point of water, will cause sterile spiklets if the plant is in the late boot through the flowering stage. In the milk stage, temperatures below 32E F will create shriveled kernels. Frozen immature spikes will turn white. After mid-dough stage, temperatures as low as 25E F will result in bran damage, some kernel shriveling and possible germination reduction. The bran damage will change test weight and probably will be discounted in the market. Grain that may be saved for conditioning for seed should be tested for germination and vigor.

Flax: Is most susceptible during flowering and early boll stage. Immature seeds can be killed by temperatures from 28-32E Fahrenheit. After flax seed reaches the dough stage it is more resistant to frost.

Sunflower: Most susceptible at bud and flowering stage. Temperatures of 28E to 30E F can result in damaged buds and sterile sections or rings in the flowering head. After pollination and petal drop sunflower can withstand temperatures as low as 25E F with only minor damage. Twenty-five degree temperatures at the bud stage will often damage the stalk below the bud and seeds will not develop.

Soybean: Easily damaged by light frosts in the 28E to 32E F range. Beans that are still green and soft will shrivel. Stalks rapidly turn dark green to brown and will not recover. Beans in pods that have turned yellow will mature normally. Some green beans will turn yellow after 30-40 days of storage.

Corn: Usually damaged by temperatures in 28E F to 30E F range or lower. Corn is usually physiologically mature 50-55 days after the 50% silking date. Colder temperatures will kill the entire stalk. If only leaves above the ear are frosted, kernel development will cease and soft shriveled corn will result. If corn is at around 35 percent moisture or if black layer has formed at base of kernel, the plant is physiologically mature and kernels will develop normally despite frost. Frosted immature corn is best used for silage or fodder.

Pinto and navy beans: Are very sensitive to frost (30-32E range). Earlier pods with yellow to tan color are sufficiently mature to escape damage. Late green pods or flowers are easily damaged by frost. Green beans will shrivel but should be left in the field until dry in order to separate them from mature beans.

Other crops: Most other crops such as buckwheat and proso millet are easily damaged in flower to milk stage. Buckwheat is reported by the Canadians to be very sensitive to frost prior to the mid-dough stage. Swath buckwheat immediately after a killing frost.

Potato: Tops will turn black but tubers are not usually damaged by light frost.

Sugarbeets: Are very resistant to frost.

Alfalfa: Can withstand light damage to tops but if frozen to ground level will not recover and should be harvested as soon as possible.

Remember, temperatures of 32E F at weather stations or the farmstead may result in temperatures of 28E F to 29E F in low-lying areas of fields. In addition, time of exposure to freezing temperatures will influence degree of damage. Two other factors that can influence critical frost temperatures are soil moisture, wind velocity and cloudiness.



Forages that are grazed or put up for hay as frost begins to appear my contain potentially toxic compounds that can cause problems for livestock producers, especially in areas that have been under drought stress or early fall frost.

The most common toxicity problems are associated with either nitrate, which is converted to nitrite in the body after consumption, or prussic acid (cyanide) poisoning, both of which impair the oxygen-carrying capacity of the blood.

Both of these potential problems can occur in forages during drought or when normal growing conditions have been interrupted or altered such as by frost, hail or rapid growth after a rain.

Prussic Acid

Prussic acid poisoning is mainly confined to sudangrass and forage sorghum, which both contain high levels of compounds that can release cyanide under certain situations. Sorghums and sorghum-sudangrass hybrids both contain more prussic acid than sudangrass. There are selected varieties, such as Piper and Trudan sudangrass, that tend to be lower in prussic acid.

If plants are damaged by freezing or trampling, or new growth is stimulated, cyanide concentration and release is increased, resulting in an increased chance of poisoning. New leaves and shoots contain from two to 25 times more prussic acid than stems, so new growth following a frost, dry conditions or when the crop has been grazed down should be considered suspect.

Before you regraze these pastures with cattle, new growth should be at least 18 to 20 inches for sudangrass and 24 to 30 inches for sorghum-sudangrass hybrids. As temperatures drop to levels where no regrowth takes place, you can wait five to six days for the forage to dry before turning cattle back in to graze.

The prussic acid content of sudangrass hay decreases by as much as 75 percent while curing, so well-cured hay is rarely hazardous for livestock.

Nitrate Poisoning

Nitrate poisoning is not limited to the sorghums and sudangrasses. Excess nitrate levels can occur in many plants because it is normally taken up by plant roots. Problems are generally found when growing conditions are disrupted, such as with low light intensity, drought, frost, hail, herbicide and nitrogen fertilizer applications, and disease.

Frost or hail damage can greatly reduce the leaf area of plant. This limits the ability of the plant to convert nitrate to protein, so the plant will accumulate nitrate. Temperatures of 70 degrees to help push nitrate into protein formation, but as temperatures cool down to 50 degrees protein formation is reduced and nitrates may begin to accumulate.

Some plants are more likely to accumulate nitrates, including pigweed, lambsquarter, kochia, wild sunflower, Russian thistle, oats, barley, millet, corn, sorghum and sudangrass. Grasses such as brome grass and legumes such as alfalfa are not normally high in nitrates.

Younger plants usually contain higher nitrate levels, but dry conditions and high soil levels of nitrate can lead to potential problems in mature plants. Plant leaves and grain do not contain appreciable levels of nitrate. The highest concentration of nitrate is in the lower stalk.

Unlike prussic acid, nitrates do not dissipate when forages are put up for hay. Harvesting at a more mature stage can reduce the concentration of nitrates, and ensiling forages may reduce nitrate concentrations by 40 to 60 percent during the fermentation process.



Harvest maturity is the point when beans can be combined with minimum of field loss or bean damage. At this point, beans are 13 to 14 percent moisture, and maturity is not always related to pod color. Growers and researchers over the years have tried color keys of yellow leaves, yellow pods, and brown pods to estimate maturity. Usually preciseness was lacking, as varieties will differ in color codes or symptoms regarding physiological maturity.

Studies in other states have reported that yellow pods sprinkled with brown are the best visual clue of physiological maturity. Open pods to check on shrinking of the beans and separation of the beans from the white membrane inside the pod. This indicates that the soybeans are at physiological maturity. Usually harvest follows about two weeks after soybeans reach physiological maturity.


Growth Stage Yield

Days After
Bloom Begins

Days to

of Total

Begin pod




Full pod




Begin seed




Full seed




Begin maturity




Full maturity




-Source: University of MN

If all leaves on a soybean plant are killed between full seed stage and beginning maturity, 53% or less of yield can be lost. A freeze before maturity has less effect on yield the closer the freeze date is to mature date.

Air temperatures of 29 degrees F are necessary to completely kill soybean plants.

Duane R. Berglund
Extension Agronomist




Winter wheat has the potential to be a highly productive crop in North Dakota. Advantages of winter wheat include; more efficient labor and machinery use, reduced weed problems particularly wild oat, and often a higher yield potential. Winter wheat is not without risk, winter survival is a limitation.

Well-developed winter wheat is less likely to suffer from early season moisture stress than spring-planted small grains, since a well-developed root system enables the winter wheat crop to use sub-soil moisture.

Earlier crop development also helps in avoiding some disease and insect pressure. Winter wheat is no more resistant to scab than spring wheat, however, with a three week developmental advantage it may avoid late infection periods that will affect spring wheat. Orange wheat blossom midge only lay eggs in developing heads between head emergence and flowering. Flowering in winter wheat will be complete before the midge emerge and it will effectively avoid the pest.

Well-established winter wheat is more competitive with summer annual weeds than spring cereal grains, resulting in less dependence on chemical weed control. A healthy stand of winter wheat has good ground cover which shades the soil and acts as a strong competitive force against weed growth. Wild oats is rarely a problem in vigorous winter wheat fields.

Planting and fertilizing the crop in the fall lightens spring planting work loads. Since winter wheat will be ready for harvest two to three weeks before spring wheat the harvest work load is also spread out. Spreading out planting and harvest provide more efficient use of machinery.

Winter survival - Winter survival is the most critical factor in successfully producing a winter wheat crop in the Northern Plains. In fact, the major limitation in the 2001 crop was winter survival. Cultural practices that help ensure winter survival are those that provide snow cover to maintain warmer soil temperatures in the crown area, just above the plant's root system. NDSU research indicates that a minimum of 3 inches of snow cover is necessary to prevent winterkill due to low temperatures. While three inches of snow is sufficient protection during most of our winters, 4 to 6 inches will further reduce the extent of crown injury and increase stand survival.

Several methods can be used to enhance snow cover. Winter wheat can be no-till seeded directly into flax, barley, mustard, sunflower, or other standing crop residues left to catch the snow. Seeding into wheat or durum stubble will increase the risk of some diseases, but even this practice is often preferred to seeding into clean-tilled fields, particularly in more arid areas where disease is less of a problem, since the stubble will enhance moisture conservation and protection from cold weather.

To avoid a "green bridge" for movement of wheat streak mosaic virus, volunteer wheat and grass weeds should be controlled two weeks prior to planting. Grain stubble should be left at least 6 inches tall to obtain the minimum snow cover required. Hoe drills, which permit deeper seed placement and trap snow in furrows over the seed row, are highly recommended for bare fallow, stubble mulch or chemical fallow fields to improve winter survival.

Stands of winter wheat are often reduced due to winter injury. Donít be hasty to destroy these stands. It may be mid April, or later, before recovery is evident. Remember winter wheat will readily tiller, stands of 8-10 plants per square foot will produce very good yields. Even when a stand of winter wheat is lost the cropping season is not lost, and generally another early season crop can be planted.

Planting - The recommended seeding dates for winter wheat are September 10 to September 30 in the southern half of North Dakota and September 1 to September 15 in northern regions. Planting after the recommended dates may reduce winter survival and grain yields and also delay maturity of the succeeding crop. Planting prior to the recommended date unnecessarily depletes soil moisture reserves, increases risk of disease and may reduce winter survival.

Winter wheat should be seeded at a rate of 1,000,000 viable seeds per acre or about 80 pounds per acre. Higher seeding rates are suggested for late seeding or for poor seedbed conditions.

Only the most winterhardy varieties available should be considered when growing winter wheat in North Dakota. Selection of a winter hardy variety should be given the greatest priority. See NDSU Extension circular A-1196 for detailed agronomic information on available winter wheat varieties.

Fertilizer applications for winter wheat should be based on soil tests and yield expectations. Winter wheat's nitrogen need in the fall is low and does not exceed the rate that can safely be applied in the drill row at seeding time. Nitrogen applications on snow are not recommended. Snow compaction under wheel tracks destroys snow insulation properties and results in winterkill beneath the track. Movement of N offsite is also a problem on deeply frozen soils when snow melts in the spring.

Phosphorus aids overwinter survival by stimulating root growth and fall tillering. The secondary root system that develops with tillering is essential for a healthy deep-rooted plant capable of withstanding stress. While important, the contribution of phosphorus to overwinter survival is secondary to varietal hardiness and soil temperatures at the growing point in the plant crown.

Additional information is available in NDSU Extension publications: Fertilizing hard red spring wheat, durum, winter wheat and rye, SF-712; Winter Wheat Production in North Dakota, EB-33; The North Dakota fertilizer hand book, EB-65.

Check for the most recent winter wheat yield data on the North Dakota small grains web page,

http://www.ag.ndsu.nodak.edu/aginfo/smgrains/ .

Michael D. Peel
Extension Agronomist

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