Staggers and the Risks from High Potassium Fertiliser

Hypomagnesemia is a serious metabolic disorder that affects cattle, particularly during the spring and autumn months. It occurs when there is a deficiency of magnesium in the bloodstream, leading to muscle tremors, convulsions, and even death. While this condition can be triggered by a variety of factors, one of the primary contributors is the use of high potassium (K) fertilisers on pastures. High levels of potassium can interfere with magnesium absorption in cattle, making them more susceptible to hypomagnesemia. Understanding the link between potassium fertilisation and magnesium deficiency is crucial for farmers to protect their herds and maintain a healthy, productive farm. 

What is Staggers? 

Hypomagnesaemia often referred to as "staggers” or “grass tetany," is a metabolic condition which most commonly affects recently calved cows. The condition occurs when there is a deficiency of magnesium in the blood. Magnesium is an essential electrolyte which plays a vital role in muscle function, nerve transmission and heart function. Staggers is mainly associated with lush grass growth when magnesium and fibre content are low. Low fibre increases gut transition time which reduces overall ruminal absorption time. This is most prevalent in spring and autumn. Lactating cattle are at the highest risk, due to a large concentration of magnesium being secreted into the milk. Bad weather or metabolic stress will also play a role.  

There are a range of clinical signs which can be exhibited, from mild to severe. In the initial stages, cows can be seen to go off their feed and display twitching of muscles. This progresses to incoordination and being separate from the herd. In severe cases, animals will collapse, exhibit paddling of limbs and potentially death. Diagnosis is based off clinical signs and magnesium blood concentration; however, measurably low blood magnesium only occurs in severe cases. Treatment of hypomagnesemia is a veterinary emergency and consists of replacing circulating magnesium levels. 

Adult animals store a high concentration of magnesium in their bones. Unfortunately, cows are unable to access this store. This results in animals being entirely reliant on dietary intake and ruminal absorption. The total diet should contain 25g per day of magnesium to meet requirements of lactating suckler cows at pasture. There are numerous methods to supplement this requirement, for example, high magnesium rolls, salt licks, top dressing, magnesium salt supplementation in water. During high-risk periods such as during lush grass growth in spring, consider supplementing the diet.  

What Role Does High Potassium Fertiliser Play? 

One of the major risk factors for hypomagnesemia in cattle is the overuse of high-potassium fertilisers on pasture. Potassium is a vital macronutrient for grassland. It is essential for water and nutrient movement within the plant, and deficiency has an impact on overall yield. While potassium is beneficial for pasture productivity, an excess amount in the soil can interfere with the magnesium uptake by plants, leading to reduced magnesium levels in the pasture. 

In the soil, potassium competes with magnesium for absorption into the plant roots. When potassium levels are too high, magnesium uptake is inhibited, resulting in lower magnesium concentrations in the pasture grasses. Consequently, when cattle graze on these pastures, they consume less magnesium, increasing the risk of hypomagnesemia. Potassium also has an inhibitory effect within the rumen. High potassium concentrations will reduce magnesium uptake from the rumen into the blood stream, again, putting them at risk of hypomagnesemia. 

This issue is particularly prominent in the spring and autumn months when cattle are grazing lush, rapidly growing pastures. Luxury uptake of potassium occurs in rapidly growing spring grass which has the ability to take up a lot more potassium than it needs for normal plant functioning. These conditions, coupled with the higher demand for magnesium in lactating cows, make the animals more vulnerable to magnesium deficiency.  

Grassland should be analysed whenever cultivated or every four to five years to check macronutrient concentrations are appropriate. Grassland which is being grazed should receive only a light dose of potassium fertiliser for two main reasons. Firstly, heavy dressings of potassium in spring can reduce magnesium uptake in cows. Secondly, pasture being coated with muck from grazing livestock usually contains an adequate potassium concentration. Consequently, applications to grazed grass should be made mid-season onwards.  

Here are some key tips to help reduce the risk of staggers this spring: 

Good cow management 

Animals under stress due to factors such as negative energy balance, milk production, sudden dietary changes, or harsh weather conditions are more susceptible to developing hypomagnesemia. Ensuring cows calve down at a good body condition score and are receiving an appropriate ration will help to mitigate this risk. 

Magnesium supplementation

Provide magnesium supplementation to cattle during high-risk periods, especially in early lactation and during lush pasture growth. 

Recognising signs of hypomagnesemia 

Recognising the signs of staggers early will help to prevent severe cases and loss of stock. Cattle off their feed, straying from the herd and displaying muscle twitches may be in the early stages of hypomagnesemia. 

Application rates and soil analysis 

Limit application rates to 90kg/ha of potassium for first cut. Any surplus should be applied later in the season to prevent luxury uptake. Soil analysis can be extremely useful at identifying when potassium levels are high, farmers should adjust their fertiliser applications accordingly. Using balanced fertilisers that provide adequate levels of magnesium in addition to potassium can help mitigate the risk of magnesium deficiency. 

Rotate grazing pastures 

Rotating pastures can help reduce the overuse of any one area. This practice also allows pastures to recover and regenerate, promoting better forage quality and reduced risk of nutrient imbalances. 

Captiva Mills, Consultant, captiva.mills@sac.co.uk

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Posted by Unearthed News on 15/04/2025