Why pasture conditions matter
The nutritional composition of pasture is constantly changing. Grass does not provide exactly the same levels of minerals and nutrients throughout the year, and environmental conditions can have a significant influence on what is available to grazing horses.
Periods of cool, wet weather, rapid pasture growth, drought and drought-breaking rain can all change the nutritional characteristics of pasture. Fertiliser use, soil mineral levels, grass species and the stage of plant growth can also influence pasture composition.
This is particularly relevant when considering minerals such as potassium and magnesium, as well as naturally occurring nitrates.
Understanding these changes can help horse owners make more informed decisions about pasture management and the overall diet of their horses.
The relationship between potassium and magnesium
Magnesium is an essential mineral for horses and plays an important role in normal muscle and nerve function.
One of the concerns with rapidly growing pasture is that the grass may contain relatively high levels of potassium. In grazing livestock, high potassium levels in pasture can reduce the absorption of magnesium, increasing the risk of magnesium deficiency under certain conditions.
This relationship is particularly well recognised in grass tetany, also known as hypomagnesemic tetany or grass staggers. Grass tetany is primarily recognised as a serious condition of cattle and sheep grazing lush, rapidly growing pasture, particularly where pasture magnesium is low and potassium levels are high.
Although grass tetany is not considered a common condition in horses in the same way it is in cattle, the relationship between pasture minerals and magnesium availability is still worth understanding when assessing the nutritional quality of pasture.
When can pasture mineral levels change?
Pasture mineral levels can vary considerably depending on growing conditions.
Some situations that can alter pasture composition include:
- rapid growth following rainfall
- cool, cloudy or wet weather
- frost and other environmental stresses
- drought conditions
- drought-breaking rainfall
- nitrogen or potassium fertiliser application
- the type of grass or other plants growing in the paddock
- the maturity and stage of growth of the pasture
- the mineral composition of the soil
Rapidly growing, grass-dominant pastures can be particularly important to consider because they may have a different mineral balance from more mature pasture. High potassium and nitrogen levels in pasture are recognised risk factors for grass tetany in susceptible grazing livestock.
What happens after drought-breaking rain?
Drought-breaking rain can cause significant changes in pasture.
After a prolonged dry period, nitrate levels in the soil can increase because of reduced plant uptake, limited leaching and decomposition of organic matter. When rain arrives, plants may rapidly take up the available nitrate. This can result in elevated nitrate levels in some pasture plants, particularly during the period following drought.
This doesn't mean that every flush of grass following rain is dangerous. The actual nitrate concentration depends on factors including plant species, soil nitrogen, fertiliser use, weather conditions and the stage of plant growth.
Where there is a particular concern about nitrate levels, testing the pasture or forage is a much more reliable approach than assuming it is high based on appearance alone.
Nitrate in horse pasture
Nitrate accumulation is a recognised issue in some pasture plants and forage.
Factors that can contribute to nitrate accumulation include:
- drought
- cold or cloudy conditions
- wilting
- high soil nitrogen
- nitrogen fertiliser
- rapid growth following rainfall
- certain plant species
Ryegrass and a number of other pasture and weed species can accumulate nitrate under suitable conditions.
Horses are generally considered less susceptible to nitrate poisoning than ruminant livestock, but high-nitrate forage can still be a concern. Equine research and extension guidance recommend considering forage testing where nitrate risk is suspected.
This is one reason it can be useful to look beyond simply asking whether a pasture is "green" or "lush". The nutritional composition of that pasture may be quite different depending on how it has grown.
Does grass tetany occur in horses?
Grass tetany is a form of acute magnesium deficiency that is well documented in cattle and sheep. It typically occurs when susceptible animals graze rapidly growing pasture where magnesium intake and absorption are inadequate. High potassium pasture can further reduce magnesium absorption.
There is much less evidence for grass tetany as a common condition in horses.
For this reason, it is more appropriate to think about pasture mineral balance and magnesium availability in horses rather than assuming that changes in pasture are causing grass tetany.
This distinction is important because many different factors can influence a horse's health, behaviour and nutritional status.
Why pasture analysis can be useful
Looking at the grass alone doesn't tell you exactly what minerals your horse is receiving.
Pasture can look healthy while having a mineral profile that is quite different from what you might expect. Soil type, fertiliser history, rainfall, plant species and stage of growth can all influence the nutrient content.
Where there is a particular concern about pasture quality or mineral balance, forage or pasture testing can provide much more useful information than making assumptions based on the appearance of the grass.
Depending on the situation, testing may include minerals such as:
- magnesium
- potassium
- calcium
- phosphorus
- sodium
Nitrate testing may also be appropriate when pasture conditions suggest an increased risk.
What can horse owners do?
There isn't one pasture management approach that is suitable for every horse.
Instead, consider the whole picture:
Look at recent weather conditions
Take note of prolonged drought, drought-breaking rain, frost, unusually cool or cloudy weather and periods of rapid pasture growth.
Consider the pasture itself
Think about the grass species present, how mature the pasture is, whether it has been heavily fertilised and whether there are significant amounts of weeds or other plants.
Consider the whole diet
Pasture is only one part of a horse's nutritional intake. Hay, hard feeds, supplements and access to other forage all contribute to the overall mineral balance.
Avoid making assumptions based on symptoms alone
Changes in behaviour, condition or general health can have many possible causes. A horse's diet and pasture should be considered as part of the overall picture rather than assuming that one mineral imbalance is responsible.
Test when there is a genuine concern
If you are concerned about pasture mineral balance or nitrate levels, testing the pasture or forage can provide considerably more useful information than guessing.
A changing pasture means a changing diet
One of the most useful things horse owners can remember is that pasture is not nutritionally static.
The grass your horse is eating today may have a different mineral and nutrient profile from the same paddock several weeks later. Weather, rainfall, drought, fertiliser, plant maturity and soil conditions can all contribute to these changes.
Understanding the relationship between potassium, magnesium, nitrate and pasture conditions can therefore be a useful part of managing your horse's overall nutrition.
Rather than viewing grass as simply "good" or "bad", looking at the conditions under which it has grown can give you a much better understanding of what your horse is actually consuming.
If you're concerned about your horse's pasture or nutritional requirements, assessing the complete diet and, where appropriate, testing the pasture or forage can help provide a clearer picture.
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