What precautions should I take feeding frosted forages to dairy cattle?

Q. What precautions should I take feeding frosted forages to dairy cattle?
A. 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.

Q. Are there special concerns with sorghum and sudangrass forages?
A. 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.

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. 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.

Q. What conditions can cause nitrate poisoning?
A. 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.

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.

Q. How can frost effect corn for silage?
A. 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:

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.
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.

Authors:  Maurice Eastridge, 
Department of Animal Sciences, The Ohio State University
Mark Sulc,
 Department of Horticulture and crop Science, The Ohio State University

Link to full article: