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By Mikaela Baker, B. Ag Sci, TRAC Ruminant Productivity Consultant

A topic of interest this season has been magnesium, specifically its unavailability or lack of in our pastures this growing season, that has resulted in stock losses, production losses and challenges on farm. This year particularly, there seems to be higher stock losses, relating to and around magnesium and nitrate levels in pastures. The notes below will go through a few important factors around Magnesium, and some key points taken from Owen Rees’ webinar on magnesium which is available on the TRAC Members Portal for more in-depth detail.

Magnesium is a macro mineral essential to life and can be a critical limiting factor. The challenge with magnesium is that it is needed in relatively high amounts, and intake is required daily as ruminants don’t have the capacity to store magnesium for later use or mobilise magnesium from skeletal resources. They can, however, easily excrete excesses within reason. The specific regulatory mechanisms are still unknown, while the importance is highly recognized. Magnesium is required for calcium, phosphorus and vitamin D absorption, glucose production, bone formation, enzyme reactions and helps in the circuit board of messages for the central nervous system; often why animals can behave quite erratically when they are low, or die when they are very low. Other common impacts from low magnesium are an increase in ewe mortality during pregnancy and abnormal fetal development in addition to lamb losses through impaired thermogenesis, and most commonly – grass tetany.

Magnesium is absorbed in the rumen and has very minimal ability to be absorbed further down the system such as the small and large intestine. This is important to acknowledge as not all magnesium is created equally. Firstly, there are different forms such as the most common feed additives: magnesium sulphate and magnesium oxide. Secondly, the quality can vary hugely. Poor quality sources will have low availability which can mean it is going straight through the animal, or the granule size is too large, and it is still dissolving in the intestine which can cause negative health impacts. Magnesium sulphate and magnesium oxide are sometimes used at different times depending on the diet.

Magnesium oxide has a larger amount of magnesium, but availability can be slower, so it is often used in summer and TMR diets, or a smaller amount in grass diets. Magnesium sulphate has lower magnesium, it is very rapidly available and comes with the sulphate component which in high amounts, is not too friendly on gut microbes. Magnesium sulphate is often a great source of magnesium in diets where it moves through the animal faster, such as full grass. Magnesium does exist in forages, unfortunately the antagonistic effects of Potassium negate this and often the balance of minerals is not favourable to animal production.

Challenges that arise from low magnesium in the animal can stem from low amounts in the feed or having excess potassium (e.g. excessive potash applications on pastures), sudden increases of ammonia (urea application and grazing management) and rapid pH changes.

Ways in which absorption of magnesium can be improved can be:

1. Slowing down the passage of feed. This includes putting animals on a pasture or crop with a good bite length or adding fibre, allowing the minerals to sit in the rumen longer, offering more chance of absorption. Where there is loose manure and dirty behinds, the feed is moving through too quickly.

2. Feeding a good quality mineral. Preventative management includes having a magnesium supplement available for daily intake beginning when the animals first go onto green grass, through to the end of the end of the growing season.

3. Feeding salt, which can help balance the high potassium levels.

4. Managing pasture fertilizers and avoiding excessive using of potassium heavy fertilizers and managing grazing times post urea application.

The most common times issues around Magnesium deficiency are seen, are through Winter & Spring pastures and diets that are high in vetch or lucerne.

Shawn McGrath has produced some excellent research through Charles Sturt University focused on magnesium and lamb production. The graphs show the imbalance of minerals in a wheat crop and highlight the importance of daily supplementation through the statistically significant difference in daily weight gain for lambs (grams per day) when supplemented on grazing wheat. It has been found that there is no clear benefit to magnesium supplementation when they are not deficient, however animals will underperform, or at worst die, when there are low levels of Magnesium.

To discuss how we can apply this information on your farm,

please speak with your local TRAC consultant today,

your Experts in Ruminant Productivity or

phone the TRAC Office on 08 8733 1888


Our Consultants


Tom Thorn

0427 243 319

Owen Rees

0429 437 823

Mikaela Baker

0457 243 319


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A Note on Magnesium
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