Potassium and Tetany Ratios
Potassium (K) levels in many forages—particularly cereals and legumes—are increasing, sometimes exceeding 2–3%, creating challenges in balancing beef cattle rations. High potassium influences mineral absorption and can contribute to disorders such as grass tetany and milk fever.
 
1. Background
Potassium is the third most abundant body mineral, concentrated in muscle and skin. It plays key roles in:
  • Osmotic balance within cells
  • Electrolyte regulation with Na and Cl
  • Blood pH control (cation–anion balance)
  • Oxygen and carbon dioxide transport
  • Nerve and muscle function (along with Ca and Mg)
Two concepts describe electrolyte interactions in cattle diets:
  • Tetany Ratio: K / (Ca + Mg)
  • DCAB: Dietary Cation–Anion Balance
 
2. Tetany Ratios
Research indicates:
  • Tetany ratio < 2.2: Very low incidence of tetany
  • Tetany ratio > 2.2: Marked increase in tetany cases
Tetany ratio uses milliequivalents (mEq/kg DM), accounting for both molecular weight and electrical charge. High K, low Ca, or low Mg all increase the ratio and raise susceptibility.
mEq conversion factors (per % DM):
  • Na: 434.98
  • K: 255.74
  • Cl: 282.06
  • S: 623.75
  • Ca: 499.00
  • Mg: 822.64
 
3. Dietary Cation–Anion Balance (DCAB)
DCAB reflects the balance of positively charged ions (Na, K) and negatively charged ions (Cl, S).
  • Helps predict blood pH and metabolic risk.
  • Negative DCAB = improved calcium mobilization prior to calving and reduced incidence of milk fever
  • For beef cattle, limited research exists; some sources suggest DCAB < +150 to +200 mEq/kg may help reduce milk fever risk.
 
4. Grass Tetany
Grass tetany is a metabolic disorder caused by low blood magnesium (hypomagnesemia).
High K can reduce Mg absorption, increasing risk.
Signs include:
  • Reduced appetite, lethargy
  • Poor coordination, stiffness
  • Nervousness, tremors
  • Collapse and potential death (mortality >30% if untreated)
Higher-risk groups:
  • Older cows (reduced ability to mobilize Mg)
  • Late gestation and early lactation cows
Hypomagnesemia often occurs with hypocalcemia, complicating diagnosis.
 
5. Case Study
Over 50 herds experienced “downer cow” outbreaks. Key findings:
  • Cows were close to calving or newly calved.
  • Symptoms suggested both milk fever and tetany.
  • Supplementation with limestone (3 oz/head/day) and MgO (1–1.5 oz/head/day) resolved issues.
  • Forages (mostly cereal greenfeed) had K levels 1.5–3.26%, but tetany ratios were below 2.2.
  • DCAB levels were high (316–518 mEq/kg).
  • Blood tests: Mg normal/high; Ca and P low → primarily milk fever.
Conclusion: High K likely contributed via increased DCAB, not tetany ratio.
 
6. Importance of Tetany Ratios
  • Cowbytes software calculates tetany ratios using percentages, not mEq, which can overestimate risk.
  • A ration with a %‑based tetany ratio of 2.1 may have an mEq ratio of 1.0.
  • The recommended ratio of 2.2:1 originates from mEq‑based research.
  • Meeting Cowbytes ratios can require impractical ration adjustments.
 
7. Ration Formulation Recommendations
When working with high-potassium forages:
For pre‑calving cows (6–8 weeks before calving):
  • Pay close attention when forage K exceeds 2.5%.
  • Consider alternative, lower‑K forages.
  • Maintain Ca:P ratio between 2:1 and 7:1 (may require 2–3 oz limestone).
  • Cowbytes tetany ratio target: < 3.5:1.
DCAB Management:
  • Aim to keep DCAB as low as possible; avoid values > 300 mEq/kg.
  • High DCAB may be more influential in metabolic issues than tetany ratio.
Software Inputs:
  • If no analysis available:
    • For forages: use 0.2% S and 0.5% Cl
    • For grains: use 0.2% S and 0.2% Cl
  • Necessary for accurate DCAB calculation in Cowbytes.
Treatment/Prevention:
  • Add 1–1.5 oz MgO for downer cows or tetany‑risk groups.