Responsible trace mineral management of dairy cattle

Feeding the correct level of minerals to dairy cows is important for sustainable dairy farming because it is critical to overall health and performance of dairy cows. Correct mineral nutrition of dairy cows is also essential to minimise the environmental footprint of dairy farming. Modern dairy cows have a higher genetic merit, increased levels of production and higher levels of metabolic stress. Next go that, soil balances, crop varieties and fertilisers are changing. Lastly, over use of trace minerals raises environmental concerns. Dairy cows need at least 15 different minerals for good health and productivity (see Figure 1). For this reason, most discussions surrounding mineral use in dairy diets are centred on the consequences of under-supply and the potential impact of deficiencies on production, health and reproductive performance of dairy cows. As a result, the conversation will soon move to additional supplements.

The requirements for a specific cow will depend on factors including genetic merit and production potential, milk yield, pregnancy status and stage of lactation as well as the diet being fed.

Feeding levels of minerals in excess of dairy cow requirements pushes up costs, introduces a risk of toxicity, reduces dairy performance and increases the environmental footprint of dairy farming as excess minerals are excreted into the environment. These risks are increased by an approach that ‘feeding more is better’. By working to balance diets closely, it will be possible to reduce mineral usage and costs on many dairy farms.

To maximise dairy performance, the aim must be to keep dairy cows in a position of optimal supply, meeting the cow’s requirements (see Figure 2). Sub-optimal supply will result in an increasing degree of deficiency while feeding above the optimum will push dairy cows into a risk of toxicity and certainly increase costs with no return on investment.

In a survey carried out in the UK[1], liver samples were recovered from 510 cull cattle and liver copper concentrations were determined. Of the dairy breeds included in the study, 38.3% had copper levels above the Animal Health and Veterinary Laboratories Agency (AHVLA) reference range (see Figure 3), indicating that a significant proportion of the UK cattle herd is at risk of chronic copper toxicity.

Figure 3: Percentage of liver samples from cull cattle with copper values below normal, within the normal range, above normal and above the reference values of the UK Animal Health and Veterinary Laboratories Agency (AHVLA).


The problem of over-supply of trace minerals in dairy herds is widespread. An on-farm study involving over 50 commercial dairy herds in the UK[2] included analysis of sodium, potassium, calcium, magnesium, phosphorus, copper, zinc iron, manganese and molybdenum of the dairy ration. The study involved detailed analysis of the minerals supplied by every component of the diet including the water, as well as any supplements used. Most dairy herds in the study were feeding minerals in excess of dairy requirements for trace minerals. Over-supply of minerals was prevalent in both fresh calved and later lactation dairy cows with commonly calcium, sodium, potassium, magnesium, zinc, iron and manganese being in excess. Particularly in transition cows, calcium and potassium significantly exceeded recommendations, increasing the risk for dairy cows to suffer from hypocalcaemia and milk fever.

Copper levels in the dairy ration will have an impact on dairy cow fertility (see Figure 4).

Figure 4: Fertility parameters of dairy cows fed high levels of copper (27 mg/kg DM) compared to a control group of dairy cows fed normal levels of copper (15 mg/kg DM). High levels of copper resulted in an earlier onset of oestrous, but conception rates were higher in the control group with normal levels of copper fed.

Zinc that is not being absorbed from the gastro-intestinal tract will not be available to the animal. Excess zinc however will not be stored, it will be excreted into the environment (see Figure 5), where it has a strong negative impact on soil microbes that are essential for soil quality.

The starting point for accurate and responsible mineral nutrition of dairy cows is a mineral analysis of all the components in the diet and particularly forages. Information is available on the mineral content of blends, compounds and most straights but very few farmers know the mineral status of their forages. With forages being the principal ingredients in a dairy diet, a forage mineral analysis should be the starting point for assessing the mineral supply in the dairy ration.

The mineral content in grass and forages for dairy cows will be influenced by the soil, the weather, age of the crop at cutting and other factors. As a result, the mineral content in silages varies from year on year, sometimes quite markedly. So average figures for trace mineral levels are no substitute for knowing the nutritional value of the forages in a silage clamp. A mineral assay for silages can be carried out using the sample taken for routine analysis. The values resulting from such an essay can be used to build a diet balanced to meet the mineral requirements of a dairy herd.

While averages can be used for straights, it is important to also understand whether the blend or compound is mineralised, and if so, what the premix levels are. Make sure mineral levels in compounds are known and include these when assessing the mineral supply in the total diet for a dairy herd. Powdered farm minerals, buckets, blocks and boluses should also be included in the calculations.

Comparing mineral supply to mineral requirements of dairy cows will allow nutritionists to assess if any additional supplementation is required. Any supplement, in whatever form, should be seen as the way to top up the mineral supply of the other ingredients in the dairy diet. If the total diet does not meet the trace mineral requirements of the dairy cow, then look for a specific supplement which will supply precisely what is needed.

If, on the other hand, the diet is providing minerals in excess of the dairy cows’ requirement for tarce minerals, then reduce supplementary sources. On many dairy farms, there will be an opportunity to remove supplementary sources of some minerals for dairy cows, saving costs, possibly streamlining tasks and certainly reducing the environmental impact of dairy farming by reducing the amount of mineral excreted into the environment.