Drought and Feed Poisoning in Dairy Cattle

DS-16-97, Reviewed April 2008
J. W. Schroeder, Dairy Cattle Specialist

The incidence of prussic acid, nitrate, mycotoxin and other problems may be increased when crops are grown or harvested under extreme weather conditions. These may be kept to a minimum if good judgment is used. Several general procedures may be used to minimize risk:

1. Introduce suspected forages or feeds gradually over a period of one to two weeks.
2. Don't feed suspected items to hungry animals. Make certain that other forages and concentrates are fed prior to the suspected material.
3. Test suspected items for nitrate or myco-toxins, if appropriate. Often materials may be used as part of the ration, depending on levels found.
4. Feed a well-balanced ration with proper nutrient content, proportion of forage dry matter and particle size.
5. Discontinue or severely restrict intake of suspected materials when possibly related problems are encountered.

Some of the more common problems:

Nitrate Poisoning. Recent research indicates that many problems previously ascribed to nitrates may have resulted from other factors. Extremely dry or cool, wet growing conditions may prevent plants from converting nitrate to true protein, so nitrate may accumulate in stressed forages, particularly whole-plant corn silage, sorghum, sundangrass and sorghum-sudan crosses. Heavy or excessive nitrogen fertilization may aggravate the problem, especially if phosphorus and potassium needs are not met. Some weeds, including pigweed and lambsquarter, may accumulate nitrate.

Nitrate levels generally decrease somewhat during ensiling, as dangerous nitrogen oxide gas is formed. However, nitrate levels may increase in hay if it undergoes heating and molding in the bale. Under normal conditions hays and haylages may contain higher levels of nitrate than corn silage.

Risk of nitrate poisoning may be reduced by the following:

• Do not harvest suspected crops for three to five days after an appreciable rain or long cloudy spell.
• Harvest as close to usual maturities as possible.
• Cut the crop somewhat higher above the ground than usual as nitrate often accumulates in stems.
• Gradually introduce suspected forage into the ration over a period of one to two weeks and don't feed it to hungry animals.
• Utilize suspected material for silage rather than green-crop.
• Test all forages and water in the ration for nitrates if one forage contains over 1.0 percent nitrate on a dry matter basis.
• Feed at least 3-5 pounds of concentrate per head per day when suspected forages are fed.

Nitrate toxicity may result when animals suddenly consume large amounts of forage con-taining 2-3 percent or more nitrate ion on a dry matter basis. Even forage with lower levels may adversely affect reproduction or become toxic if animals are nutritionally stressed or metabolically abnormal and suddenly eat a large amount of such forage. Cattle may develop blue mucous membranes from lack of oxygen in the blood. Rumen paralysis may occur. Labored or difficult breathing may be observed. Animals may go down and die rather suddenly.

Subacute or chronic nitrate poisoning may result in more of the usual reproductive problems, including abortions. Milk production and appetite generally are not affected by subacute nitrate intake. Reproductive problems generally may be prevented if feeds are gradually introduced and the nitrate level in the total ration dry matter is kept below 0.40 percent. Because of differences in rate of dry matter intake, grazed forage is about 50 percent less toxic than stored forages.

Recommended uses for forages containing various levels of nitrate are found in Table 1. A high level of nitrate or nitrite in the water may make it necessary to further reduce intakes of nitrate-containing forage. A total intake of 30-45 grams of nitrate ion per 100 pounds of bodyweight is considered acutely toxic in normal animals. Intakes of 8-22 grams per 100 lbs may be toxic when animals are in abnormal condition or are undergoing an abrupt change in ration.

Use of natural or supplemental non-protein nitrogen (NPN) does not aggravate a nitrate problem or increase the incidence of nitrate poisoning. Intravenous treatment of acute cases with 4 percent methylene blue in 5 percent dextrose is often effective.

Silo Gas Poisoning. Silo gas poisoning stems largely from elevated nitrate levels in ensiled forages. Under some conditions large amounts of nitrogen oxide gases may be released during ensiling. These are very toxic to animals, birds and humans. Abnormal or incomplete fermentation appears to produce more of these dangerous gases. In some cases it is present only for a few weeks after ensiling, but it may continue indefinitely in others.

Inhalation of even small amounts of silo gas may cause choking, asthmatic symptoms and burning sensations in the nostrils, throat and chest. Deaths may occur quickly with greater amounts of gas. Severe lung damage may be present in survivors. A sensitivity to the gas may develop. Cows subjected to appreciable levels of nitrogen oxide gases may cough and choke.

Sometimes silo gas has a yellowish-brown color. This does not always occur. Frequently it has an odor similar to laundry bleach. It is heavier than air. This means it may accumulate on the silage surface or flow down silo chutes and accumulate in low places. Dead birds and small animals near the silo may be evidence of this problem.

Treatment of silo gas cases includes drugs to increase aeration and remove fluids which may accumulate in the lungs. Absolute bed rest and general supportive treatment are helpful.

Silo gas formation may be kept to a minimum by ensiling at moisture levels of 60-70 percent to encourage normal fermentation. Also, 8 pounds of sodium metabisulfite may be added per ton of material ensiled. Most important, ventilate silos, silo rooms and barns if suspected forage is ensiled. Don't enter silos for at 7-21 days after filling with suspected forage. Run blowers for 10-15 minutes before entering. Consult your medical doctor and/or veterinarian, depending on the problem encountered.

Prussic Acid Poisoning. This problem is restricted to the use of sorghum, sudangrass and their crosses. It may result when these crops are used at a too immature stage or are severely stressed by weather, such as drought or frost.

Symptoms of prussic acid poisoning are very similar to those of nitrate poisoning. It is very important to differentially diagnose the form of poisoning. Blood drawn from animals suffering from prussic acid or hydrocyanic acid poisoning may turn brilliant or cherry red upon exposure to air. In contrast, blood from nitrate poisoning cases may be normal or chocolate brown in color as drawn.

Acute toxicity from prussic acid may result in lack of coordination and convulsions followed by quick death. Treatment with sodium thiosulfate-sodium nitrite solution is recommended. Repeat treatments with sodium thiosulfate if necessary.

Ensiling does not always alleviate the danger of prussic acid poisoning, but levels generally decline, after about four weeks of ensiling. Drought stricken forage at any height may be more apt to result in prussic acid poisoning. Use as pasture, green-chop or hay is more risky than ensiling. Use suspected material as only part of the ration and heed the other general recommendations on reducing risk of poisoning. Do not feed it to hungry animals.

Mycotoxin or Mold Poisoning. Some feeds or forages may contain highly toxic substances produced by mold. Material with relatively small amounts of mold may contain these poisons. They are more likely to be present in corn and small grains than forage. However, they have been found in some haylages, silages, ensiled grains and hays. Mycotoxins may be produced on some feeds before they are even harvested. Others may be formed during storage.

One of the most common types of mold poison is aflatoxin. Cattle afflicted with aflatoxin may have a black or bloody diarrhea from digestive tract bleeding. Some are uncoordinated and may go down with a posterior paralysis at any stage of lactation. This problem may not yield to milk fever treatment. Liver damage frequently occurs. Subclinical mycotoxicity may result in lower milk production. Another mycotoxin, zearalenone (DON), may result in swollen vulvas and abortions.

A mycotyoxin screening test is available from the NDSU Veterinary Diagnostic Laboratory for $97 (May 1, 2008). Feeds with over 10 ppb should not be sold. Zearalenone levels of 1 to 5 ppm in feeds may result in reproductive problems. Reduced appetite may be noted with only 0.25 to 0.75 ppm zearalenone present. Little or no mycotoxins may be formed in silage during storage if pH falls below 5.0 soon after ensiling.

Other Health Problems. Enterotoxemia, botulism and listerellosis are more apt to occur when crops are flooded or silages fail to develop a pH of under 4.8-5.0. The latter also is true of mycotoxins. The most frequent causes of elevated pH in ensiled materials are excessive addition of urea, anhydrous ammonia or other NPN sources at ensiling and harvesting too dry. Leaking silos and faulty ensiling procedures also may contribute to fermentation failure.

Hypomagnesemia or tetany may occur when cattle are eating small grain pasture in the fall or winter and grasses in spring. Animals may suddenly be found dead or seen in convulsions or tetany. Make certain that cattle on such forage are force fed at least 1-2 ounces of magnesium oxide per head per day.

More problems with infections, reproduction and metabolic disorders may result if cows are fed limited forage, high concentrate rations for an extended period. Feeding corn silage with little or no other forage may aggravate the problem. Avoid limited forage, high grain rations for dry cows and springing heifers.

Poisonous weed problems may also be more prevalent in drought seasons. Ensiling often does not destroy the toxic substances in many poisonous weeds. At lower concentrations they may merely reduce appetite, while at high levels they may result in acute symptoms and death.

An increased incidence of displaced abomasum or twisted stomach may occur when either an acidosis or alkalosis occurs in the rumen. An acidosis is more likely to happen on a high concentrate feeding program, especially when starchy grains are the major energy source and corn silage is fed heavily. Highly acid water with a pH under 5.5 may contribute to the problem. An alkalosis is more apt to occur when excessive amounts of protein and calcium are fed. This is most likely to happen on a ration which contains a lot of good alfalfa as hay or haylage. Highly alkaline water with a pH over 8.5 may also be a contributing factor. Lack of particle size in the forage portion of the ration may also result in more DA's. The pH of rumen contents in most cows should be in the range of 6.7-7.3 when samples are tested immediately at cow side. Sometimes rumen pH may be as low as 6.3-6.4 in normal high producing cows fed considerable amounts of concentrate.

A milkfat test that runs more than 0.3-0.4 percent from breed average often indicates that cows are abnormal metabolically. Steps should be taken to alleviate the problem to prevent possible adverse effects on health and reproduction.