Hypomagnesaemia is the term used to describe grass staggers, a metabolic disease associated with the deficiency of dietary magnesium in cattle and sheep. It occurs when the level of magnesium in the fluid surrounding the brain and spinal cord, as well as in the blood, drops below a critical level. The clinical signs seen in stock as a result include nervousness, muscle tremors and a stereotypical ‘staggering’ movement when walking forwards.
Statistical estimations suggest that 1% of UK cattle will experience clinical grass staggers each year, with up to 30% of these cases resulting in death. However, the likelihood is that many more animals are suffering sub-clinical symptoms of staggers, and this will have a significant impact on their overall health and performance, and subsequently diminish a farm’s production and profitability.
Magnesium deficiency can develop all year round, but spring is a particular period of risk due to the sudden seasonal growth of grass. Flush, spring grass is a great source of protein, however, it has a very low magnesium content (0.1% to 0.2% in dry matter) so does not provide the animal with sufficient to avoid deficiency. To add to this, repeated fertiliser application can increase potassium levels in grass and upon ingestion this disrupts the potassium: sodium ratio in the digestive tract and interferes with magnesium absorption. Similarly, increased nitrogen levels typical of spring grass can affect rumen pH, which affects magnesium solubility. This, combined with its high-water content, means spring grass transits through the rumen very rapidly, which decreases the amount of magnesium available for the animal to absorb and further exacerbates the risk of staggers.
Magnesium absorption can also be compromised due to other factors aside from just the low magnesium content in spring grass. Fluctuations in seasonal weather may increase the risk of grass staggers amongst cattle and sheep, as it disrupts grazing routine and therefore grass intake. This is typical following very wet weather as minerals are leached from grazing. Periods of metabolic stress can also cause an imbalance of minerals, locking up any available magnesium.
As a macro-nutrient essential for bone growth and maintenance, cattle and sheep store approximately 70% of bodily magnesium in their bones. This means it is not readily available when dietary supply is compromised, such as during the spring risk period.
Daily magnesium supplementation is therefore necessary to help maintain optimal blood magnesium levels and give animals the best possible chances to avoid grass staggers.
Following a reasonably mild winter and good grass availability so far in 2022, stock may be turned out earlier than usual this year. Farmers are therefore encouraged to get ahead of the game by considering the risks and solutions to avoid magnesium deficiency as spring grass starts to grow.
A decision tree can help producers determine the risk of grass staggers occurring in their herd. If the decision tree suggests a “medium” or “high” risk, there are a number of management practices that can be implemented to help alleviate the risk of clinical grass staggers. It is important to ensure animals receive a source of long fibre in their diet, in the form of hay or silage to help slow the transition of lush spring grass through the rumen. Avoiding overuse of potassium fertilisers on land and minimising disruption to grazing are also likely to help increase magnesium availability, however perhaps the most reliable way to combat staggers is to help prevent deficiency through ensuring stock have daily access to a good quality magnesium supplement.