How Potassium Affects Dairy Cattle

For lactating dairy cattle, potassium is an essential nutrient that plays a vital role in milk production, feed efficiency, and overall health. While potassium is often abundant in forage-based diets, its levels need to be carefully managed depending on the cow’s life stage, particularly around calving.

Why Potassium Matters for Dairy Cows

Potassium is involved in a range of critical biological functions:

• Muscle and nerve activity
• Cardiac health
• Fluid balance and kidney function
• Cellular metabolism

For dairy cows, potassium is particularly important due to the large amounts lost daily through milk production. If this nutrient isn’t replenished, cows can experience decreased milk yield, reduced feed intake, and in severe cases, health complications such as displaced abomasums.

According to the National Research Council, extreme potassium deficiency (0.06% to 0.15% of dry matter) can significantly reduce plasma and milk potassium, weaken appetite, and slow weight gain. To avoid this, dairy diets must be carefully balanced, especially in high-producing herds.

Potassium Requirements: Dry Cows vs. Lactating Cows

The amount of potassium needed in a cow’s diet changes depending on her stage of production:

• Dry cows: Ideal potassium levels range from 2–2.5% in hay.
• Lactating cows: Potassium levels should increase to between 2.5–4% to support high milk output.

Early lactation (<75 days post-calving) is a period when cows often fall short in potassium. This can negatively affect milk production and milk fat content. Providing high-potassium feed during this phase can improve outcomes significantly.

When Less Potassium Is Better: Pre-Calving Period

While most of a dairy cow’s life benefits from a potassium-rich diet, the 3–6 weeks before calving is the exception.

During this time, the cow shifts large amounts of calcium into the mammary glands to produce colostrum. High dietary potassium can raise blood alkalinity, which interferes with calcium mobilisation. This can lead to hypocalcemia (milk fever), a major concern in fresh cows.

To reduce this risk, it’s important to feed a low-potassium (anionic) diet during the dry period just before calving. Once the cow has calved, potassium levels should be brought back up quickly to support milk production.

Managing Potassium in Forage: Tips for Farmers

Producing the right hay for each stage of the dairy cycle takes planning. Here are some tips to help manage potassium levels in your feed:

• Soil testing: Regular testing helps you monitor potassium levels in fields, particularly high-producing ones.
• Plant testing: Identify which paddocks yield high or low potassium forage to match feed to cow lifecycle.
• Maturity at cutting: Young plants contain more potassium. Later cuts typically have lower levels.
• Cutting height: In Lucerne hay, potassium concentrates in the stems. Higher cuts tend to contain more potassium.
• Rain-damaged hay: Rain can leach potassium out of hay, reducing its mineral content.
• Fertiliser use: Potassium-based fertilisers (e.g., potash) boost forage yield and quality, but must be balanced to avoid excess.

Summary: Get the Timing Right

Potassium is vital for dairy cattle, but timing matters:

• High-potassium diets support milk production, feed intake, and overall health, especially in early lactation.
• Low-potassium diets are essential in the lead-up to calving to reduce the risk of milk fever.

By adjusting potassium levels in line with your cows’ lifecycle, you can improve animal welfare and productivity. If you’re unsure of your hay’s potassium content, consider feed testing or consulting a nutritionist to get the balance right.

Disclaimer: LocalAg provides this article as general guidance based on industry knowledge. Always consult with a vet or qualified nutritionist before making significant changes to your feeding strategy.