Feeding Frosted Forages to Dairy Cattle

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During the fall of the year, the risk for frost poses some concerns for forages fed to dairy cattle. The damage from the frost can certainly affect the levels of dry matter (DM) and nutrients in the plants, but depending on forage type, the risks for prussic acid or nitrate poisonings may exist.

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Sorghum and Sudangrass Forages

Prussic acid poisoning can occur when feeding sudangrass, johnsongrass, 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. As ruminants consume forage containing high concentrations 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 from asphyxiation (lack of oxygen). 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 contained in the feed.

Generally, any stress condition that retards plant growth may increase prussic acid concentrations in plants. Hydrogen cyanide is released when leaves are damaged by frost, drought, bruising, cutting, trampling, crushing, or wilting. Plants growing under high soil nitrogen levels or in soils deficient in phosphorus or potassium tend to have high concentrations of cyanogenic glucosides. Species and varieties differ in prussic acid poisoning potential. Sudangrass varieties are low to intermediate, sorghum-sudangrass hybrids intermediate, and forage sorghums high risk for cyanide potential. Piper sudangrass has low prussic acid poisoning potential, and pearl millet is virtually free of cyanogenic glucosides. The management practices described below may reduce the risk of prussic acid poisoning from forage sorghum, sudangrass, and sorghum-sudangrass hybrids:

  1. Graze or greenchop only when the grass is greater than 18 inches tall.
  2. Do not allow animals to graze wilted plants or plants with young tillers.
  3. Do not allow animals to graze plants during or shortly after a drought when growth has been reduced. Wait 4 to 5 days after rainfall for growth to resume.
  4. Do not allow animals to graze on nights when frost is likely. High levels of the toxic compounds are produced within hours after a frost occurs.
  5. Do not allow animals to graze after a killing frost (temperatures less than 28 degrees F) until the plants are dry. Wait 5 to 7 days to allow the released cyanide to dissipate.
  6. Do not allow animals to graze for two weeks after a non-killing frost (greater than 28 degrees F).
  7. Delay feeding of silage for 6 to 8 weeks after ensiling. Fresh forage is generally higher in cyanide than silage or hay because cyanide is volatile and dissipates as the forage dries. However, silage that likely contained high cyanide concentrations at harvest should be analyzed for HCN content before feeding.
  8. Split applications of nitrogen decrease the risk of prussic acid toxicity, and proper levels of phosphorus and potassium in the soil may also help to reduce the risks. Soil tests are recommended so proper amounts of nutrients for crops may be applied and risks of prussic acid toxicity may be reduced.
  9. Don’t allow hungry or stressed animals to graze young sorghum grass growth.

Nitrate poisoning can occur under conditions of high nitrogen fertilization, heavy manure applications, drought, overcast weather, prolonged low temperatures (not necessarily a frost), or other stress conditions that retard plant growth. Under these stressful conditions, high nitrate concentrations accumulate in the crop. Once forage is fed, nitrate is converted to nitrite in the animal. When nitrite concentrations are high, the animal cannot metabolize it quickly enough, and nitrite inhibits oxygen transport in the blood. Symptoms include rapid breathing, a fast and weak heartbeat, muscle tremors, staggering, and ultimately death if corrective steps are not taken. A veterinarian should be consulted if nitrate poisoning is suspected.

The same management precautions for prussic acid poisoning will help prevent nitrate poisoning. Although pearl millet does not create a potential problem with prussic acid poisoning, it can accumulate high nitrate concentrations leading to nitrate poisoning. Also, corn for silage should be monitored for nitrate concentrations under conditions described above. High nitrate levels will persist when forages are cut for hay, but ensiling the crop can reduce nitrates by approximately 50%. If the forage is suspected of high nitrate concentrations, have it tested before feeding.

Corn for Silage

Maturity of corn and risk of frost damage are often major factors for consideration during the early fall. Both of these factors will affect DM level of the corn. Frosted corn harvested for silage should be no higher than 40% DM (not less than 60% moisture) when stored in upright concrete silos, no higher than 50% DM (not less than 50% moisture) for oxygen-limiting silos, and no higher than 36% DM (not less than 64% moisture) for horizontal silos. These DM benchmarks are based on frosted immature corn rather than the high DM caused by maturity of the plant. Although a preservative (bacterial inoculants are not considered preservatives) is not usually recommended for corn silage, except on the last 2 to 3 wagon loads for upright silos, 20 lb of propionic acid per ton of silage (as-fed basis) should be applied when DM levels are at or near the upper suggested levels which will help to drop the pH and provide for a more stable silage in storage. Urea (8 to 10 lb per ton) and anhydrous ammonia (7 to 9 lb per ton) should not be added to corn silage above 40% DM (less than 60% moisture). These additives will increase silage pH, which in conjunction with inadequate oxygen elimination in high DM silage, can reduce fermentation potential.

Generally, the quality of frosted corn is good until after several frosts or a major killing frost. In such cases, the plant will begin to die, causing leaf loss, and the cell rupturing caused by the frost will allow leaching of nutrients during rainfall. The major factor to be concerned about with frosted corn is not to allow it to become too dry before ensiling as the rate of drying will be accelerated after frosting occurs. The following data on frosted corn silage were collected by researchers in Canada:

Stage Dates of
Dates of
% DM at
% CP % NDF % ADF
Milk 8/30   20.8 7.1 59.0 31.5
Dough 9/7   23.2 6.3 58.8 28.5
1st Frost 9/18 9/14 25.1 6.6 59.4 26.0
2nd Frost 9/26 9/14, 9/26 35.5 7.2 61.6 26.2
5th Frost 10/17 9/14, 9/26,
9/29, 10/11,
45.9 7.0 65.9 28.1

Performance by lactating cows in the study was not negatively affected by the frosted corn until it had been frosted on five times, but high-producing cows based on today’s production levels may be more adversely affected.

The major problem with high DM corn silage is inadequate packing for elimination of oxygen. Therefore, particle size of such silage is very important. Particle size of high DM silage should be smaller than for silage with adequate moisture. Cutter bars should be set at about 0.25 inches and particle size monitored (e.g. 3 to 8, 45 to 65, 20 to 30, and less than 10% of particles on the top sieve, middle sieve, lower sieve, and pan of the Penn State Particle Size Separator based on recommendations from Penn State University) since sharpness of knives, throughput, etc. also affect particle size.

Water can be added, usually at the blower, to increase moisture level for improved packing. The main problem is being able to apply enough water – a high pressure system should be used. Unloading rate (tons/min) and water delivery rate (gal/min) should be monitored to achieve desired results. The table below can be used to determine how much water to add per ton:

Initial% DM Desired % DM
  44 42 40 38 36 34
Gallons of water per ton
46 11 23 36 51 67 85
44   11 24 38 53 71
42     12 25 40 57
40       13 27 42
38         13 28
36           14


Author Information

Maurice Eastridge
Department of Animal Sciences, The Ohio State University

Mark Sulc
Department of Horticulture and crop Science, The Ohio State University