Introduction

In years of drought and also in those of excess rain feed quality can be compromised for the wintering beef herd. Problems have been observed throughout Western Canada with cows going down prior or after calving. While the single cause is often not pinpointed, it is suspected that often a multitude of factors play a role in causing cows to go down. A prolonged negative energy balance, hypomagnesimia, an inadequate calcium to phosphorus ratio or an inadequate balance of dietary cation to anion. For example, in years of drought, cows are often fed more greenfeed and straw. Both of these being low in calcium may create some predisposition to milk fever. In other years moldy feeds may have to be used. These again are compromised in quality. Perhaps we are seeing reduced levels of vitamins, but also a reduction of calcium or maybe higher levels of potassium.

The following summary will familiarize you with different conditions and underlying causes. Some causes for down cows are the following.

Milk Fever

Milk fever, or acute hypocalcemia, occurs when calcium in blood falls below 1.6mmol/L. Symptoms include muscular weakness, subnormal temperature, increased heart rate, sternal recumbancy and loss of consciousness. The primary cause lies in the reduced ability of the animal to mobilize calcium from the bones. Treatment with intravenous or subcutaneous calcium gluconate will usually resolve the problem.

Hypomagnesimic Tetany

Hypomagnesimic tetany, also called grass tetany or winter tetany, is caused by low levels of magnesium (<0.65mmol/L plasma). Symptoms can be nervousness, stiff gait, staggering, convulsions and paralysis. In mature lactating beef cows magnesium should be at <0.2%. Tetany can be treated by injection with calcium magnesium solutions. Subcutaneous injection of magnesium sulfate is also a solution. Magnesium oxide can be included in the diet.

Pregnancy Toxaemia

Pregnancy toxaemia in cattle is the result of fetal carbohydrate or energy demand in excess of maternal supply in the third trimester of pregnancy. It is precipitated by large or multiple fetuses, feed that is low in energy or protein, and health conditions that increase energy demand or decrease ability to take in nourishment. It occurs in cows that are rapidly losing weight or in cows that are due to calve and are in poor body condition. Diagnosis in one or two individuals may indicate a dietary energy deficiency being experienced by the whole herd. Affected cows need to be injected or drenched with high energy supplements. These cows often require calving induction before they rise and have a relatively poor prognosis for successful recovery.

Key Minerals Involved in Tetany and Milk Fever

Calcium: Structural component of bones and teeth; important in muscle contraction nerve impulse transmission, blood clotting and enzyme activation

Phosphorus: Structural component of bones and teeth; important in energy metabolism

Potassium: Osmotic pressure, acid-base/water balance; important in muscle contraction, nerve impulse transmission and membrane function

Magnesium: Nerve impulse transmission; enzyme cofactor and important in energy production

Background

Potassium (K) is the third most abundant mineral in the body. It is found primarily in skin and muscle and is important for maintaining proper osmotic balance within cells. Along with sodium (Na) and chlorine (Cl), potassium is one of the three major electrolytes in the body and functions to maintain cation-anion balance (blood pH). Potassium plays an important role in the transportation of oxygen and carbon dioxide through the blood. Along with sodium, calcium (Ca) and magnesium (Mg), potassium acts to maintain nerve and muscle activity. Tetany ratios and dietary cation-anion balance (DCAB) are both used to describe electrolyte balance.

Repeated or high levels of manure application to soil can cause high accumulation of potassium in feeds. Dry growing conditions and regions where acidic soils exist (low soil pH) contribute to the accumulation of potassium in plants. Normally the levels of potassium in feeds range from 1.5 to 1.9% on dry matter basis. Feed tests in Saskatchewan in cereals grown in 2001 indicate potassium levels as high as 4.2%. Similar results were reported in Alberta and Manitoba. Gestating cows need 0.6% of dry matter intake as potassium, and lactating cows need 0.7% potassium. Indeed, the maximum tolerable level is 3% of dry matter intake.

Tetany Ratios

In 1957, research showed that when the ratio of potassiumto the sum of calcium and magnesium (the tetany ratio) was less than 2.2, there were very few cases of tetany (0.77% of 4658 animals). The incidence of tetany increased to 6.66% of 1908 animals when the tetany ratio was greater than 2.2. The tetany ratio is expressed as K/(Ca+Mg) in milliequivalents (mEq) per kg of dry matter. Milliequivalents take into account both molecular weight and valence (ie, charge) because acid-base balance is affected by electrical charge rather than mass. Table 1 shows how to calculate mEq/kg from dietary percentages. High potassium, lowered calcium and lowered magnesium can all cause the tetany ratio to increase and predispose animals to tetany.

Table 1. How to calculate mEq/kg from dietary percentages

Element	Factor to multiply by for converting from % to mEq per kg
Sodium	434.98
Potassium	255.74
Sulfur	623.75
Calcium	499.00
Magnesium	822.64

Dietary Cation:Anion Balance (DCAB)

The DCAB refers to the relative amounts of cations (the positively charged ions Na and K) to anions (the negatively charged ions Cl and S). It is an indicator of acid-base status as the body strives to maintain equilibrium between acidic and basic components (to ensure the sum of the positive charges is equal to the sum of the negative charges). Calculation of the DCAB also requires the use of "equivalent weights."

A great number of research studies show that a negative DCAB prior to calving reduces the incidence of milk fever in dairy cows. A negative DCAB (acidic) will help to increase the mobilization of bone calcium in an attempt to buffer the blood and return it to a neutral state. DCAB values for beef cattle are not well studied but some sources suggest that a DCAB value of below +150 to +200 may be sufficient to prevent milk fever (Alberta Agriculture, Food and Rural Initiatives, 1999).

What is Tetany?

Grass tetany is a complex disorder characterized by hypomagnesemia (low blood Mg). A high K intake can decrease the absorption of Mg from the gut inducing hypomagnesemia. Grass tetany may also be referred to as lactation tetany, winter tetany and milk tetany in calves. Animals initially exhibit a depressed appetite and a dull lethargic appearance. As the disease progresses, stiffness, a staggering gait, nervousness, excitability, muscular tremors, collapse, thrashing about and finally death may follow. Mortality among untreated clinical cases can be greater than 30%. Older animals (third or more pregnancy) are more susceptible to grass tetany because of their decreased ability to mobilize skeletal Mg. Cows are most susceptible to tetany immediately prior to calving and when they are nursing a calf. Often hypomagnesemia occurs along with hypocalcemia (low blood Ca).

Saskatchewan's Experience

During the winter of 2001/2002, more than 50 herds in the North Battleford area experienced problems with "downer cows" - significantly more than observed in previous years. Cows were in the later stages of pregnancy (two to three weeks prior to calving) or very early stages of lactation. The cows showed clinical signs of both milk fever (lethargy) and/or grass tetany (head held back, twitching). One of the major differences between these cases and those from previous years was the very rapid onset of symptoms and a more rapid rate of deterioration. Larger than normal doses of treatment solutions were also required to cure the affected cows.

Cereal greenfeeds comprised the bulk of most cows’ winter rations. K levels in the greenfeeds from five selected herds ranged from 1.5 to 3.26% but the tetany ratios were well below the recommended 2.2:1. calcium to phosphorus ratios were within the recommended range of 1:1 to 7:1. The DCAB ratios were high, ranging from 316 to 518.

Blood work for the five selected herds showed several interesting things. Magnesium was within the normal to high range for all affected cows indicating that they were likely not suffering from hypomagnesemia. Affected cows showed low blood levels of calcium and phosphorus indicating the occurrence of milk fever. Clinical symptoms, however, were not always consistent with milk fever suggesting that some other complications may have been occurring at the same time.

It was concluded that if potassium played a role in the outbreak, it was likely not through an increased tetany ratio as originally hypothesized, but by its contribution to an increased DCAB in dry cow diets.

How Important are the Tetany Ratios?

For Cowbytes (see References), Alberta Agriculture, Food and Rural Initiatives calculates tetany ratios using percentages. The research behind the recommended ratio of 2.2:1 was based on milliequivalents (mEq) which take into account electrical charge, not just weight. When tetany ratios are calculated using mEqs, the results are lower than if the calculations are based simply on percentages. For example, the tetany ratio of a ration may be 2.1 when calculated using dietary percentages but drops to 1.0 when calculated using mEq.

Prevention of this condition is possible through supplementation of the diet with magnesium oxide and limestone. Limestone is a source of calcium. 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 diet is very high. Magnesium oxide is very unpalatable and needs to be mixed with grain or screening based supplements to achieve this level of intake. Producers should work with a nutritionist from a feed company to design a program that fits with their management program. The goal should be to achieve a tetany ratio of lower than 2.2 using percentages, however bearing in mind that in some cased this is impossible. Remember that the actual calculation should be done using milliequivalents, which allows some flexibility on the ratio of 2.2:1. The nutritional program should also aim to get the calcium/phosphorus ratio to greater than 2 in the total diet. Cereal-based rations also require supplementation of salt, trace minerals and vitamins and, if a lot of straw is used, protein. The minerals taken in by animals in their drinking water can also affect the situation, positively or negatively. As has been said, the calcium/magnesium imbalance has the greatest likelihood of occurring just before or during calving in the form of tetany or milk fever. Cases may be visible by finding a dead cow that was calving or laying on her side in a slight hollow. These could be misdiagnosed as calving difficulty or weak cow syndrome.

If your feed samples were analyzed, ask yourself, "Were they properly collected and are you comfortable that it is a representative sample of what you are feeding?" Check to see if you have analyzed for calcium, phosphorus, potassium and magnesium.

Do you have a water analysis you can provide to your nutritionist? Check to see if your feed analysis was done by the more costly, wet chemistry or the cheaper Near Infrared Reflectance Spectroscopy (NIRS) method. In the NIRS method the mineral analysis is less accurate than a wet chemistry mineral analysis and should be used only with great caution.

Finally, if you are now providing the necessary mineral package to the animals' free choice, are their intakes at the recommended levels? Calcium is not palatable and magnesium is very unpalatable. If feeding a free choice mineral mix, add flavoring agents and a 58% granulated magnesium oxide for best results. Granulated gets better intake than other more powdery magnesium forms. Monitor the intakes to see that they are meeting recommendations. Manage the location and freshness of the mineral.

If intakes are still low, you are concerned and have not had a feed analysis done, you may want to force feed 1 to 2 ounces of magnesium and some additional calcium in a grain ration. This is a much more preferred method for feeding minerals.

Force feeding 1 ounce of magnesium oxide if the problem is just a magnesium deficiency works well or, up to, but no more than 2 ounces if the ration also has an excess of potassium. To compensate for the excess of potassium affecting calcium and magnesium absorption (called the DCAB ratio), the feeding of calcium and magnesium needs to be higher than recommended. Multiple factors can play a role: underlying energy deficiencies, twin fetuses, moldy feeds and overall cow health and condition can amplify the problem.

Ration Formulation

Keep in mind all of the following points when working with high-potassium feeds:

• Be particularly concerned when forage potassium levels exceed 2.5%.
  
• Consider alternative forages (ie, low-potassium) for the critical period six to eight weeks before calving.
  
• Ensure calcium to phosphorus ratios are between 2:1 and 7:1 - 2 to 3oz of limestone may be required.
  
• There appears to be some leeway with the tetany ratio, as calculated by Cowbytes. Aim for a Cowbytes tetany ratio of less than or equal to.
  
• The DCAB may be more important in preventing metabolic problems around calving than the tetany ratio. Recommendations for a suitable DCAB in beef cow diets lie at +150 to +200 mEq/kg DM. Keep the DCAB as low as possible especially in the last six to eight weeks of pregnancy. The University of Saskatchewan noted tetany and/or milk fever problems with DCABs exceeding 300 mEq/kg (316 to 518 mEq/kg in a limited number of cows). A different forage may be required.
  
• When entering forage feed values into Cowbytes, use an average value of 0.2% for sulfur and 0.5% for chloride if analyses are not available. Use 0.2% for both chloride and sulfur when entering grain values. These numbers are needed for Cowbytes to calculate a DCAB ratio.
  
• Add 1 to 1.5oz of magnesium oxide to the diets of downer cows .

June 2006