The facts on potassium in the horse’s diet

Equine nutritionist Dr Lucy Waldron PhD delves into the myths and misinformation on the role of potassium in horse diets

There has been much discussion of late on various social media websites about the role of potassium from grass in equine nutrition, with some strange ‘advice’ being bandied about as to how to combat it.

This seems to have taken off during the lush grass growth seen in many parts of New Zealand recently. Hence, it is timely for me to clarify and report what is really going on with potassium, from a purely scientific point of view!

Potassium (chemical symbol K) is essential for the early, vigorous growth of various pasture species, especially the clover and ryegrass which predominate in New Zealand due to the widespread sowing of dairy pastures. 

Research into the best way of fertilising pastures has been strong since the 1940s, when there was a focus on boosting agricultural yields after the end of the austere rationing of World War II. It was determined that fertilising soil with potassium was necessary for establishing early and prolonged pasture growth. 

While in those early years, potassium-rich inorganic fertiliser was widely used; with the testing now available it’s much preferable that soils are fertilised only after laboratory analysis – we determine what’s already in the soil and apply only what’s needed to balance the required minerals.

However, earlier fertilising practices lingers on in various soils.

The expression of nutrients, including minerals, changes in growing pasture from hour to hour, as the activity in plants is dependent on temperature, light exposure and water availability. So, all data should be taken as broad averages. 

In addition, the time that forage is consumed or cut has a major impact on the levels of potassium consumed. The table below shows the typical, published levels of potassium in various types of pasture, and how this equates to the daily requirements of horses, as published by NRC (2007). It’s plain to see that all forages, at whatever stage of development and growth, exceed the recommended intake levels. 

Below: The levels of potassium (K) in common forages harvested at different growth periods and what this means in terms of daily intakes (based on pasture contributing half the 1.5% of body weight feed intake per day for a 500 kg horse being provided as forage = 3.75 kg on a dry matter basis). 

DM basisK levels %Levels g of K per kg DMLevel of K intake gLow  work intake% of needHard  work intake% of need
(NRC 2007)

Mature grass hay1.9719.773.928.5259%53.3139%
Immature grass hay2.5725.796.428.5338%53.3181%
(Ashland 1994)

Alfalfa immature3.8238.2143.328.5503%53.3269%
Alfalfa mature2.3623.688.528.5311%53.3166%
Alfalfa plus rye grass immature3.4534.5129.428.5454%53.3243%
Alfalfa plus rye grass mature2.2422.484.028.5295%53.3158%
Red clover immature3.535.0131.328.5461%53.3246%
Red clover mature2.3823.889.328.5313%53.3167%
Red clover plus rye grass immature4.1441.4155.328.5545%53.3291%
Red clover plus rye grass mature2.5625.696.028.5337%53.3180%
Birdsfoot trefoil immature3.1431.4117.828.5413%53.3221%
Birdsfoot trefoil mature2.4224.290.828.5318%53.3170%
BFT plus ryegrass immature4.3343.3162.428.5570%53.3305%
BFT plus ryegrass mature2.8628.6107.328.5376%53.3201%
(Pritchard et al 1964)

Timothy immature2.424.090.028.5316%53.3169%
Timothy mid-mature1.9819.874.328.5261%53.3139%
Timothy mature1.3413.450.328.5176%53.394%
White clover3.333.0123.828.5434%53.3232%
Rye grass3.535.0131.328.5461%53.3246%

Published authors are in brackets at the head of each column

Work by Joe Pagan in 1998 compiling research done in the USA examined various nutrient levels, and reported 75.4% digestibility for potassium. They used maximum levels of 3.3% (or 269g/kg) K in their trial diets. So, the figures above are, in reality, about three -quarters of the actual amount in terms of uptakes from the gut. 

The main issue – if it is an issue – is the high level seen when ryegrass is consumed. 

Firstly, why do animals need potassium? Well, this mineral is important in cells, where it acts as a cation (that is, a positive charged ion) in maintaining the water content and correct mineral balance in cells and in tissues; its movements in and out of cells is important to ensure the survival of cells, and for muscle contraction. It is essential for the correct function of neurone cells and in membranes. 

In mammals, it has been shown that potassium supplementation can reduce hyperactivity in the nervous system; it appears that the central nervous system is involved in controlling the levels of circulating potassium. Specific research in horses has shown that deficiencies are rare – mostly this mineral is consumed in excess, due to its high levels in forages. In addition, the chemically organic form of potassium in forage is better digested and utilised compared to inorganic forms, such as potassium chloride. 

When excess potassium is consumed by horses, urine excretion is increased, followed by faecal excretion. Mineral balance studies have shown that 95% or more of ingested potassium is excreted in urine and faeces, as well as in sweat (Groenendyk et al., 1988). 

The horse’s kidneys have evolved to be very efficient at removing excess potassium from the body – no doubt an adaptation to their dependency on forage and pasture in the wild. Indeed, when potassium intakes are lower, the horse is at great risk of showing deficiency syndromes and fails to conserve this essential mineral in its body (Johnson, 1995). 

Other research has shown that hard-working horses have a higher requirement for potassium, as it is lost in sweat, and can adjust digestibility to take up more of this mineral, in a deliberate response to increased needs. 

The nutritional guidelines released by the National Research Council in 2005 declared a maximum level for potassium of 1% of daily dietary intake (ie. feed intake range for a 500kg horse 7.5-12.5 kg dry matter basis per day, equivalent to 75-125 g potassium per day), however, they reported that the higher levels seen in forages caused no problems in horses, as it is easily regulated and excreted from the body. 

The NRC is a committee made up of senior scientists, who compile all available peer-reviewed research papers to develop global feeding guidelines. 

Other research has reported that horses are able to respond to high potassium forage by reducing consumption, and will drink more to assist in its removal from the body (Lewis, 1995). 

More recent data has reported that maximum levels in forage should not exceed 1.1% for grazing animals – even though all the forage these researchers tested were very much in excess of this figure (Juknevicius and Sabiene, 2007).

So, how should horse owners feed their horses to help keep potassium intakes in balance? Is it actually a worry? Or is it a ‘manufactured problem’ to sell more supplements?

Extremely high potassium intake can result in heart irregularities and muscle tremors, which occur mainly in animals with kidney impairment or genetic disease. Care must be taken, as potassium toxicity (hyperkalaemia) is notoriously difficult to diagnose, and is only normally seen by vets in foals with ruptured bladders (Kablack et al., 2000). Treatment in the first instance typically is to increase calcium levels to balance out the excess potassium. 

Sometimes potassium is blamed for diarrhoea in horses on lush grass – but there isn’t evidence of this (and other research in monogastric animals have hinted at a link – but only at very high intakes), and this has shown currently to be much more likely to be due to high levels of sugar in the pasture.

High levels of sugar, typically found in short pasture, is more likely to cause problems than potassium levels

There is an important relationship between calcium and potassium intakes. If potassium is too high and calcium is too low, this is when problems occur. Primary vet treatment of hyperkalaemia (potassium excess), as mentioned above, is infusion with calcium.

If you are worried about potassium, the following should be done (and not just for this mineral, but to ensure best balance of all minerals expressed in pasture):

  1. Get your paddock soil tested – and only apply the mineral mix to balance the soil. Don’t just dump anything on. A soil test is cheap and easy to do. In addition, high lime applications without a soil test has been shown to increase potassium expression in plants, especially clover.
  2. Try to rotate your paddocks to give the horses access to older pasture – as this will contain lower amounts of potassium. Short, actively-growing plants will always be higher in potassium, as the base of the plant and its top leaves contain the highest levels. 
  3. When cutting for baleage or hay, try to use longer, stalkier plants that have ‘gone to head’.
  4. Avoid rye-clover pastures – if possible resow with higher fibre, animal-friendly combinations. 
  5. Never use single (especially inorganic) mineral supplements in feed, unless a qualified professional has identified a need for this. Mineral imbalances cause a massive amount of problems that can be difficult to diagnose and rectify, and may take years to show symptoms and organ damage.