How will an early frost affect forages?

An early frost in many parts of the country has dairy producers asking, “What effects will the frost create for livestock feeds?” says Dave Fischer, University of Illinois Extension dairy educator. Following frost, producers should be cautious when grazing cattle with crops in the sorghum family. Grazing other grasses and legumes will not be a problem.

• Grazing sudangrass, sorghum-sundangrass hybrids, grain/forage sorghum. These crops are members of the sorghum family and will produce a glucoside called dhurrin that breaks down to release a toxin called prussic acid. A crop stress, such as frost, causes the prussic acid to be released at a rapid rate and the intake of high levels may be lethal to cattle. Clinical signs of prussic acid poisoning include rapid pulse, labored breathing, and eventual suffocation. Producers should move cattle away from grazing these forages for several days following a frost. If the crop was safe to graze prior to frost it, will again be safe to graze 3 to 5 days following the frost. Remember, sudangrass should be greater than 18 inches tall or sorghum-sudangrass greater than 24 inches before it is safe to graze under any conditions.

• Baling or ensiling sudangrass, sorghum-sundangrass hybrids, and grain sorghum immediately following or shortly after a frost is safe because the prussic acid will break down and dissipate during the harvesting process. Therefore, waiting 7 to 10 days after harvesting those crops made into hay or silage will be safe to feed. However, allowing the feed to cure or ferment in the silo or bale for 2 to 3 weeks before feeding will give you an added safety factor, says Fischer. Certainly, do not feed the material as green-chop.

• Grazing alfalfa, clover, and other perennial forages is not a concern following a frost because they do not produce toxins and can be grazed or baled and fed to livestock even after a frost. The only caution will perhaps be a slight chance for ruminants to bloat if grazing pure legume pastures. They should be offered other feedstuffs to avoid overeating the succulent pasture.

• Harvesting alfalfa after a killing frost is acceptable only when the plant is going into dormancy. This killing/dormant frost occurs when temperatures reach 28 degrees Fahrenheit, or lower, overnight (4 hours or more). If the plants are harvested predormancy and regrowth occurs 1 to 2 weeks before going dormant, significant plant damage can occur during overwintering due to reduced food reserves in the plant to survive the cold temperatures, notes the Illinois Extension specialist. If the forage is not needed, it is advisable to leave the crop uncut for the winter.

• Immature soybeans that are caught by the frost due to late plantings can be harvested for forage if the crop is free from applied pesticides. The soybean forage should be cut, wilted, and harvested at similar moisture levels as would be used for alfalfa or other legumes. It is best to begin harvest no later than when the bottom leaves turn yellow and begin to drop off. Since late fall drying conditions are very marginal, chopping as silage is preferred over baling.