CORN
Crop Observation and Recommendation Network

October 9 - October 15, 2000
C.O.R.N.  2000-34

In This Issue:

A. Fall/Winter weed control in alfalfa
B. Fall weed control strategies in wheat
C. Prussic Acid Toxicity in Forage Caused by Frost

A) Fall/Winter weed control in alfalfa - Mark Loux and Jeff Stachler CORN Questions

Fall and winter can be a good time to work on a number of weed problems in alfalfa fields. Common weed targets include winter annuals (chickweed, mustards, purple deadnettle, henbit, etc), dandelion, and curly dock. In established alfalfa stand, one of the most effective strategies for control of these weeds is the late-fall through late winter application of Sencor or Velpar. These herbicides can be applied anytime in winter when alfalfa is dormant and soil conditions allow sprayer traffic. Sencor is labeled for use in alfalfa and alfalfa/grass mixtures, although some injury to grasses may occur. Velpar is labeled for pure alfalfa only, and may actually help control some non-desirable perennial grasses. Velpar may be more effective than Sencor for control of dandelion. Both herbicides will suppress or help control curly dock, in combination with a healthy forage stand and proper management of cuttings. Velpar should only be used where the alfalfa stand will be kept for at least two more years due to its persistance in soil and 2-year recrop restriction.

Weed control in new seedings (less than one year old) is somewhat more difficult, since Sencor and Velpar cannot be used. Herbicide options for fall broadleaf weed control in new stands include Pursuit, 2,4-DB (Butyrac), and Kerb.

• Kerb controls primarily chickweed, henbit, and some perennial grasses, and can be applied to alfalfa that has at least one trifoliate. The rate is 1 to 1 1/2 lbs/A for most weeds, and 2 to 3 lbs/A for quackgrass and other perennial grasses. Kerb should be applied after soil temperature falls below 60 degrees but before the ground freezes.
• Pursuit controls a number of winter annual weeds when small (1 to 3 inches), including chickweed, henbit, field pennycress, shepherdspurse, and yellow rocket. Apply a rate of 1.44 to 2.16 oz/A with nonionic surfactant (1 qt/100 gal) and fertilizer solution (1-2 qt/A). Alfalfa should be in at least the 2nd trifoliate stage of growth.
• 2,4-DB can be applied when alfalfa, clover, and trefoil have at least 2 trifoliates. Apply 2-3 qts/A when weeds are no more than 3 inches tall. 2,4-DB controls pennycress, wild mustard, shepherdspurse and yellow rocket, but is not effective for control of chickweed, purple deadnettle, or henbit.

Pursuit and 2,4-DB should be applied during periods of relatively warm weather if possible, when air temperatures are above 45 degrees at night and 60 degrees during the day for several days prior to application. Of these three herbicides for new stands, only 2,4-DB can be used on legumes other than alfalfa and will not affect grasses in the forage stand. A final note: do not waste time and money to control any summer annuals (foxtails, ragweed, lambsquarters, etc) that may be in fields still. A hard frost or freeze will kill these weeds, if it hasn't already by the time you read this.

B) Fall weed control strategies in wheat - Mark Loux and Jeff Stachler CORN Questions

Winter annual weeds seem to have increased over the past few years, and have interfered with spring wheat growth in some fields. A spring herbicide application can control winter annuals, but may not be timely enough to prevent reduced wheat growth in dense stands of winter annuals. Winter annuals should be more easily controlled in fall compared to spring, due to their smaller size. Fall applications may also more effectively prevent seed production, since some spring applications occur after plants go to seed. Three herbicides are labeled for late-fall application to emerged wheat - Harmony Extra, Peak, and Sencor. While a fall-applied strategy may minimize competition from winter annuals, it does mean an extra trip across the field compared to spring application of herbicide plus nitrogen for top-dressing. Separate applications minimize the risk of crop injury, though, which occasionally occurs due to application of herbicide in nitrogen fertilizer solution. The following guidelines assume application in water.

• Harmony Extra can be applied at a rate of 0.3 to 0.6 oz/A with nonionic surfactant (1 qt/100 gallons) when wheat is in at least the 2-leaf stage. Weeds should be past the cotyledon stage, and less than 4 inches tall. Harmony Extra controls wild garlic and most winter annuals, but is weak on marestail and large chickweed.
• Peak can be applied at a rate of 1/2 oz/A with crop oil concentrate (1-4 pts/A) after wheat is in the 3-leaf stage. Weeds should be 1-3 inches tall. Peak controls wild garlic, wild mustard, chickweed, and pennycress. Do not apply Peak if double-crop soybeans will be planted following wheat harvest.
• Sencor can be applied postemergence to certain wheat varieties, but we have no experience with this treatment and it can cause wheat injury if not applied properly. Bayer does have a regionwide label for this use of Sencor, but does not actively promote it. The risk of wheat injury decreases as wheat gets larger, allowing for higher rates. Risk of injury is greater on high pH (>7.3) soils, when very wet weather occurs following application, where spray passes overlap, when wheat is planted less than one inch deep, or when a triazine was used in a corn crop within the same year. Sencor controls most emerged winter annuals and downy brome, and provides some residual control. See the label for a list of tolerant and non-tolerant varieties and additional information.

We suggest the following strategy with regard to fall herbicide use in wheat. In no-till wheat, apply glyphosate a day or so before planting to control any emerged weeds (control when applied after planting may be reduced due to soil disturbance with the drill). In tilled fields, tillage should also kill all emerged weeds. Check fields in early to mid November to see if winter annual populations are high enough to justify a herbicide application at that time. Check fields again in early spring to plan herbicide use and nitrogen application.

C) Prussic Acid Toxicity in Forage Caused by Frost - Mark Sulc CORN Questions

The cold weather this past weekend in Ohio has raised concerns and questions of feeding certain forage species to livestock. These are very legitimate concerns with the sorghum species. Prussic acid poisoning can occur when feeding frost-damaged sudangrass, sorghum-sudangrass hybrids, forage sorghum, or grain sorghum. These species contain varying concentrations of cyanogenic glucosides, which are converted to prussic acid, also known as hydrogen cyanide (HCN). As ruminants consume forage containing high levels of cyanide-producing compounds, prussic acid is released in the rumen, absorbed into the bloodstream where it binds hemoglobin and interferes with oxygen transfer. The animal soon dies of asphyxiation.

Prussic acid acts rapidly, frequently killing animals in minutes. Symptoms include excess salivation, difficult breathing, staggering, convulsions, and collapse. Ruminants are more susceptible than horses or swine because cud chewing and rumen bacteria help release the cyanide. Generally, any stress condition that retards plant growth may increase prussic acid levels in plants. Hydrogen cyanide is released when leaves are damaged by frost, drought, bruising, cutting, trampling, crushing, or wilting. Plants growing under high nitrogen levels or in soils deficient in soil phosphorus or potassium tend to have high levels of cyanogenic glucosides.

The management practices described below can reduce the risk of prussic acid poisoning from forage sorghum, sudangrass, and sorghum-sudangrass hybrids.

Minimizing Risk of Prussic Acid Poisoning When Grazing:

1. Graze or greenchop only when sorghum grasses are greater than 18 inches tall. Never graze or greenchop sorhum grasses less than 18 inches tall, at any time of the year.
2. Do not graze on nights when frost is likely to occur. High levels of the toxic compounds are produced within hours after a frost occurs.
3. Do not graze after a killing frost until the plants turn brown and are dry. Wait 5 to 7 days to allow the released cyanide to dissipate.
4. If plants begin to grow after being frost damaged (non-killing frost), do not them until the regrowth is 18 inches tall or the entire plant is killed and turns brown by a later killing frost. To be on the safe side, never graze for two weeks after a non-killing frost.
5. Do not graze wilted plants or plants with young tillers.
6. Split applications of nitrogen decrease the risk of prussic acid toxicity, and proper levels of phosphorus and potassium in the soil will also help.
7. Don't allow hungry or stressed animals to graze young sorghum grass growth.
8. Do not graze plants during or shortly after a drought when growth has been reduced.

Minimizing Risk of Prussic Acid Poisoning When Greenchopping:

When feeding sorghum grasses as greenchop, use the same precautions for harvesting as outlined above for grazing. Cut plants after they are at least 18 inches tall, and cut down to about 8 inches.

Minimizing Risk of Prussic Acid Poisoning When Making Hay or Silage:

1. Cyanide potential is less if the sorghum grasses are put up as hay or silage. Fresh forage is generally higher in cyanide than in silage or hay because cyanide is volatile and dissipates as the forage dries. However, hay or silage that likely contained high cyanide levels at harvest should be analyzed for HCN content before feeding. Delay feeding silage for 6 to 8 weeks after ensiling.
2. It is very difficult to field cure these grasses adequately for safe storage as hay, especially this time of the year.

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C.O.R.N. is a summary of crop observations, related information, and appropriate recommendations for Ohio Crop Producers and Industry. C.O.R.N. is produced by the Ohio State University Extension Agronomy Team, State Specialists at The Ohio State University and Ohio Agricultural Research and Development Center. C.O.R.N. Questions are directed to State Specialists, Extension Associates, and Agents associated with Ohio State University Extension and the Ohio Agricultural Research and Development Center at The Ohio State University.

Editor: Barry Ward        Web Editor: Tom Rosati

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