Mineral Requirements in Out-Wintered and Grazed Cattle

There are several areas and questions that we need to consider when we look at fluctuations in animal demand.  

• Age – a calf does not have the same mineral demand as an adult cow. 

• Mineral status – is the animal’s current status high, low, or optimum? 

• Milk yield – how much milk are we asking the cow to produce? 

• Growth rate – what daily liveweight gain are we expecting from the cattle? 

• Pregnancy – the cow will partition some of the mineral to the foetus - is her mineral status optimum to do that and provide the calf with optimum mineral status at birth? 

• Immune status – minerals are vital in the immune system. 

• Weather – need to consider mineral status during cold and hot spells. 

• Interactions – this can cover interactions between the above which change the demand for the animal. 

Antagonism and minerals in the diet 

Antagonism in the diet is when minerals can block or prevent the absorption of other minerals. Similarly, goitrogens are chemical compounds which are present in certain crops (brassicas) that act like an antagonist in the bloodstream and impact the absorption of iodine. In these situations, the supplementation of iodine is particularly important to consider when grazing certain forage crops.  

In grazing systems, particularly during winter months, soil intake must be considered as minerals within the soil are normally unavailable for the cow, but can act as antagonists. Soil intake can increase the amount of iron, aluminium and lead ingested by the cow. Strip grazing and bale grazing can offer a way of reducing the risk of potential soil intake in cattle.  

Weather can influence the requirements of the cattle - during frosty, cold weather, the cow needs to maintain her body temperature which she does by increasing her metabolic rate and this increases her demand for minerals.  

Three minerals to consider in these systems are:  

• Copper – critical for fertility, growth and the immune system. Antagonists of copper are sulphur, molybdenum and iron - all three of these can make copper unavailable to the cow and these may be present in high levels in the soil.  

• Iodine – critical for energy metabolism as it is vital in the production of thyroid hormone. Goitrogens are antagonists of iodine and therefore supplementation must happen when animals are being grazed on brassicas. 

• Selenium – critical for growth and muscle development, correct liver function and the immune system. Fluctuations can be seen in selenium in grazing systems, particularly, across the country.  

A final mineral that needs to be discussed in a grazing system is magnesium. The skeleton contains between 60 and 70% of the total magnesium in the body, and it is also important in muscle contractions and the rumen microbiome. Magnesium works as a catalyst for many enzymes in the body. For grazing, grass staggers becomes an issue when magnesium in the blood drops rapidly due to low levels in the grass and high potassium levels. Grass staggers is typically seen in the spring months when grass growth is rapid, but can occur at other times of the year so supplementation is key.  

In grazed cattle, it is vital to get their mineral status to the optimum level to ensure the health and performance of the animal. If you require assistance with mineral supplementation for your cattle discuss any changes with your nutritionist.