| | Grass tetany refers to animals that exhibit tetany-like symptoms frequently associated with cattle being turned out on lush pasture. Grass tetany is a disease caused by low levels of magnesium in blood of cattle. It can affect different types of cattle, but the incidence is higher in older beef cows and on those post-partum that are in the high peak of lactation (6-8 weeks) during winter or early spring. These cows need a constant source of magnesium to replace the large amount lost from the body in milk. Even with low feed levels of magnesium, the loss of magnesium in the milk remains the same. Also, poorly doing and over-conditioned cows may be at risk.
Contributing Factors and Causes
Low magnesium levels in blood of an animal can be caused by low magnesium levels in feed and/or reduced magnesium absorption.
- Absorption of magnesium is influenced by the amount of calcium, phosphorus and potassium in the diet. High dietary levels of potassium, sulphates, nitrogen or phosphorous and low levels of calcium in the diet and low rumen sodium, reduces absorption of magnesium. Cows depend on a continuous supply of magnesium from the digestive tract to maintain normal blood magnesium concentrations.
- Normally, levels of potassium in forages ranges from 1.5% to 1.9% (100% dry basis). Drought or dry growing conditions and regions where acidic soils exist (low pH) contribute to the accumulation of potassium in plants, especially cereal crops.
- Alfalfa can also accumulate high levels of potassium. High levels of potassium in feeds can also be caused by fertilizing crops with high levels of nitrogen and potassium, and by repeated or high levels of manure application to soils.
- Winter tetany can also occur when cattle are fed poor quality hay, cereal greenfeed or straw, which contain low levels of magnesium.
- Winter tetany in Alberta is often associated with feeding grain and straw, or greenfeed based rations. Cereal forages are typically low or borderline in magnesium compared to the cow's requirements.
- Low energy intake, fasting or sudden changes in feed.
- Stress during transportation, treatments/vaccinations, inclement weather.
- Low roughage intake (young grasses have low roughage and often poor palatability).
Signs
Tetany and milk fever (caused by low levels of blood calcium) are two metabolic diseases that produce tetany-like symptoms.
Symptoms include:
- Depressed appetite, reduced weight gain, nervousness, staggering, stiff gait, convulsions and paralysis usually in mature cattle.
- There may be marks or signs on the ground beside the animal indicating they were leg paddling on the ground before death (lying on their side with stiff outstretched legs that thrash backwards and forwards or paddling on the ground around head and legs).
Often the first signs of a problem in the beef herd is a dead cow. It is often difficult to tell whether the cow died from milk fever or tetany.
- When paddling marks are present, it suggests that the animal died from tetany. Animals that have died from milk fever usually don’t show any signs of struggling or convulsions. Cows that are still alive and are suffering from low magnesium levels or tetany are often excitable, or "flighty". They appear uncoordinated, have a stiff gait, tremble, stagger or may be down. Farmers sometimes call these cows "downer cows".
- Producers should contact their veterinarian immediately if their cattle show any of these symptoms.
- Other diseases that may be causing similar neurological signs are BSE and listeriosis.
- If the cow is down, it is recommended to draw a blood sample before treatment is given. Have the blood sample checked for Calcium and Magnesium levels in order to determine if the cow that was down was due to low magnesium (tetany) or low calcium (milk fever) levels.
Treatment
The plan of action in this case is to restore blood magnesium levels as soon as possible. In severe or acute cases, magnesium and calcium (if required) solutions ca be given under the skin.
- Treatment includes slow intravenous or subcutaneous administration of solutions containing magnesium and/or calcium salts.
- If one cow is magnesium deficient, it is assumed that the whole herd could be borderline deficient.
- The results of the blood test can help your livestock nutritionist develop a preventative mineral mix for the rest of the herd. Cattle suffering from tetany usually have low blood magnesium (Mg) and adequate blood calcium (Ca) levels. Cows suffering from milk fever will usually have low blood Mg and Ca levels.
Treatment initiated before the animal become a downer, or just as they go down, is often successful. The longer the animal remains as a downer, the least success in the therapy.
Prevention
Prevention should be aimed to:
As a result farmers should:
- Provide good quality hay and silage, which ensures that high energy is provided and roughage intake is reached. Older animals may be moved to pastures having good proportions of legumes.
- Planting clovers
- Grains can be supplemented provided that there is a period of adaptation to this new feed
- Mineralized salts containing good levels of magnesium and calcium could be supplemented
- Handle animals carefully to reduce stress and provide shelter
- Correct acidity in soils with limestone or dolomite salts and reduce application of fertilizers containing high levels of potassium, sulphates, or nitrogen (urea) until acidity is corrected.
One of the easiest approach to prevent this disease is by supplementation the ration with magnesium oxide and limestone. Limestone is a source of calcium, which is often deficient in cereal-based rations. It is important to ensure that the cows are receiving adequate and balanced levels of calcium and magnesium. A feed analysis will indicate the levels of available nutrients and minerals in the greenfeed or silage. Based on the results of the feed & blood tests, proper mineral/vitamin supplementation can then be formulated.
- Producers should work with a nutritionist from their feed company to design a program that fits with their management system. Supplementation programs should be designed to provide approximately 40 grams of magnesium oxide and 80 grams of limestone per cow per day. Greater amounts may be required if the potassium level in the ration is extremely high. Magnesium oxide is very unpalatable and should be mixed with grain or screenings based supplements, or with silage, to achieve this level of intake.
- Cereal based rations also require supplementation of salt, trace minerals and vitamins and, if a lot of straw is used, protein and energy.
Table 1 provides a general guideline for creating mixes that provide supplemental magnesium and calcium for 590 kg (1300 lb) beef cows on a greenfeed-based ration that contains 2.0% to 2.5% potassium. For higher amounts of potassium in the ration the amount of minerals may have to be adjusted.
Table 1: Supplemental Mix for Calcium & Magnesium for 590 Kg (1300 Lb) beef cow.
| Feeding rate of Grain Based mix (per cow /Day) |
Kg | Lbs | Kg | Lbs | Kg | Lbs |
1 | 2 | 2 | 4 | 4 | 9 |
| Kg (Lbs) of Mineral supplement to add to 1,000 kg (2205 Lbs) of Grain Mix |
| Limestone | 70 | 155 | 50 | 110 | 29 | 62 |
| Magnesium Oxide | 36 | 80 | 19 | 42 | 12 | 26 |
If you have questions or require further assistance on this topic, please call the Ag-Info Centre at 310-FARM (3276) |
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