Nutrition Update
Volume 15 No.2, November 2004

Potassium (K) levels in some forages, e.g. cereals and legumes are on the rise.  They are often over 2% and in some cases, over 3%.  This has created difficulties in balancing rations for cattle.  Information on K and dairy cows can be found in previous Nutrition Update articles:  Potassium in Dairy Cow Diets, May 2001; Preventing Udder Edemas, May 2002. This article will deal with beef cattle and K.

Background

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), K is one of the three major electrolytes in the body and functions to maintain cation-anion balance (blood pH).  K plays an important role in the transportation of oxygen and carbon dioxide through the blood.  Along with sodium, calcium (Ca) and magnesium (Mg), K acts to maintain nerve and muscle activity.  Tetany ratios and dietary cation : anion balance (DCAB) are both used to describe electrolyte balance.

Tetany Ratios

In 1957, research showed that when the ratio of K to the sum of Ca and Mg (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 above 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 (i.e. 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.   

Source: Oetzel, 1993.

Dietary Cation – Anion Balance (DCAB)

The DCAB refers to the relative amounts of cations (the positively charged ions - Na, K) to anions (the negatively charged ions – Cl, sulphur (S) ). It is an indicator of acid-base status as the body strives to maintain equibilibrium between acidic and basic components i.e. 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 large 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 - +200 may be sufficient to prevent milk fever (Alberta Agriculture, 1999).

What is Tetany?

Grass tetany is a complex disorder characterized by hypomagnesemia (low blood magnesium). 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 calcium).

Saskatchewan’s Experience

During the winter of 2001/2002, over 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 (2-3 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. Supplemental limestone and magnesium oxide were added to diets at the rate of 3 oz and 1-1.5 oz per head per day, respectively. This appeared to be effective as there no more problems were observed in the herds afterwards.
Cereal greenfeeds comprised the bulk of most cows’ winter rations. Potassium levels in the greenfeeds from 5 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 1-7:1. The DCAB ratios were high, ranging from 316-518.

Blood work for the 5 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 K 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?

Cowbytes 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. Alberta Agriculture, Food and Rural Development has indicated there are no imminent plans to change the way tetany ratios are calculated in the Cowbytes software. Attempting to meet the Cowbyte tetany ratio, however, can require some feeding practices which are difficult to implement.

Ration Formulation

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

• Be particularly concerned when forage K levels exceed 2.5%.

• Consider alternative forages (ie low K) for the critical period 6 – 8 weeks before calving.

• Ensure calcium to phosphorus ratios are between 2:1 and 7:1. 2 -3 oz of limestone may be required.

• There appears to be some leeway with the tetany ratio, as calculated in Cowbytes. Aim for a Cowbytes tetany ratio of less than 3.5:1.

• The DCAB may be more important in preventing metabolic problems around calving than the tetany ratio but there are no specific recommendations for a suitable DCAB in beef cow diets. Keep the DCAB as low as possible especially in the last 6-8 weeks of pregnancy. The University of Saskatchewan noted tetany and/or milk fever problems with DCABs exceeding 300 mEq/kg (316-518 mEq/kg – 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 S and 0.5% for chloride if analyses are not available. Use 0.2% for both chloride and sulphur when entering grain values. These numbers are needed for Cowbytes to calculate a DCAB ratio.

• Add 1-1.5 oz of MgO to the diets of downer cows.
  
  Sources:
  

• Cowbytes Beef Ration Balancer. Alberta Agriculture, Food and Rural Development.

• Doig, Bryan. 2004. Personal communication. Saskatchewan Agriculture and Food.

• McDowell, Lee. 1992. Minerals in Animal and Human Nutrition. Academic Press, Inc.

• McKinnon, John. 2004 Personal communication. University of Saskatchewan.

• Oetzel, Garrett. 1993. Use of Anionic Salts for Prevention of Milk Fever in Dairy Cattle. Compendium on Continuing Education for Practicing Veterinarians 15:1138.

• Walker, Amanda. 2003. A Case Study of Factors Influencing Tetany-Like Symptoms in Beef Cows. University of Saskatchewan.