Nutrition in cattle – essential trace elements

“You are what you eat” sums up the need for a balanced diet. When reviewing the diet for cattle under our care, consideration must be given to the potential for suboptimal performance or disease when a failure to deliver occurs. This article focuses on the trace elements essential for a balanced diet in cattle, whether adult or youngstock.

A variety of feedstuffs lessens risk, and cattle rations containing compound feeds, straights, blends, mixed forages or simply conserved forages tend to carry a lower risk of deficiency.

The grazed animal, especially when not supplemented, is often at greatest risk of trace element deficiencies. Overlaid on this, local geology, soil type pH and grassland management – including fertiliser application – plus seasonal factors will all impact on trace element levels.

Local knowledge is clearly helpful, and farmers may well build in a safety net through the use of external supplements to ensure adequate supply. These can most simply include free access mineral buckets, but still risk variable intakes between individuals.

Better still is to ensure all individuals have a known, adequate level through balanced nutrition – or, failing this, the use of boluses or long-acting mineral injections.

Measure

Prior to supplementation, it is essential to assess the trace element status and risk. Very often, assessment follows investigation of a clinical presentation, but far better would be a regular review.

This should indicate planned feed changes, available information such as soil or forage analysis, and prior risks and issues.

A discussion can then be taken on whether further information is needed, such as a blood profile, or other diagnostic approaches. In some cases, information may be sufficient to proceed with diet modification or with supplementation.

For example, in late pregnancy, a natural decline occurs in trace element status due to the increasing demand from the fetus. Liver copper stores will decline during this period.

In addition, oxidative stress can be high after calving, and given protective effects of selenium and vitamin E, we need to ensure adequacy to avoid ill health, such as retained fetal membranes and metritis.

Manage

When clinical cases due to deficiency occur, a more rapid response is usually deployed when parenteral treatment is used. The only single trace element product available is in the form of copper as copper methionate. Note, use in cattle is off licence. No licensed single selenium or iodine product is available.

Broad-spectrum treatment is available, however, and is licensed for use in cattle. This would appear to be the treatment of choice in any clinical presentation involving copper or selenium, once regard is given to any potential toxicity given multiple trace elements present.

Monitor

We continue to move away from an approach based on reaction to a clinical presentation, towards a proactive preventive approach. Herd metabolic profiles, and reviews of forage and mineral levels can help establish whether a need for supplementation exists.

This long-term supplementation can then be used for prevention in the future and maintaining efficient intakes. A wide range of oral boluses covering the spectrum of trace elements is required. Choice is likely to depend on specific requirements, duration, availability and personal preference.

Bulk milk sampling for trace element status offers an interesting route to trace element testing as it gives a simple opportunity to assess whole herd status as a snapshot, and could be, therefore, used at key times in the season.

While it is offered by one laboratory in the Netherlands, it is not commercially available in the UK, and Brexit has ruled out export of milk samples to European laboratories. We will need to see if this approach is taken up by any UK laboratories. Until then, the standard approach is to screen six animals in any profile to give us reasonable reliability in interpretation.

Iodine

A school of thought exists that dry cows are turned out for a “rest”. This can be a “rest” from supplementation, including trace elements, to a “more natural” diet of grass.

However, if this grass is deficient, and fetal demand for trace elements increases, then the farmer is running a considerable risk – as has been noted previously.

Iodine deficiency is a common presentation in grazed late gestation cattle. Iodine is an essential trace element, incorporated into the thyroid hormones – thyroxine and triiodothyronine – responsible for energy metabolism, metabolic rate and protein synthesis.

Clinical presentation is seen as late abortions, stillbirths or weak calves. Confirmation is through fetal postmortem and identifying an enlarged thyroid gland. A pooled blood sample can be tested for plasma inorganic iodine. Deficiency is not confirmatory, but can be easier to establish as fetuses may not be available due to loss and scavenging by wildlife.

An Irish study of 44 dairy farms found a pasture-only diet would have provided only 50% of iodine requirements, with 87% of forages containing less than 0.5mg iodine per kg dry matter (DM). This would justify iodine supplementation for dry cows eating only grazed grass.

Iodine supplementation can be achieved through bolus delivery, which will give 10mg to 20mg of iodine at the recommended dose rate for adult cattle. As the daily requirements are 12mg daily for a lactating cow, this requirement will be met by the bolus. While excess may occur, fortunately the safety margins in cattle for iodine are large – perhaps as high as 100 times (National Research Council guidelines).

However, a responsible approach must be taken with the end consumer in mind, and so care needs to be taken with milking animals as cows’ dietary iodine has a direct relationship with milk iodine. Excess supplementation can result in excess levels in milk, potentially used to manufacture infant milk formula (IMF). The Irish feed industry has reduced iodine inclusion rates to reflect this risk.

Iodine concentration in bulk milk must be maintained between 20μg/kg and 150μg/kg to produce milk safe for IMF.

Potassium iodide can be used as a nutritional supplement and has the advantage of simply supplementing iodine. This can be useful when no need exists for other trace elements. As potassium iodide is 75% iodine, addition of 1g per cow per fortnight into the water trough will deliver 50mg per cow, or four times requirement.

This is probably acceptable for the short duration a cow is dry and allows for variation in individual intakes. The author prefers water trough supplementation over the application of iodine products to the skin of cattle, where uptake may be unpredictable depending on grooming and rainfall.

Selenium

Youngstock, whether growing beef or dairy replacements, offer a further opportunity for trace element assessment and, when necessary, supplementation. Rearing can often be on more distant fields with reliance on grazing.

The use of long-acting anthelmintics has probably reduced the frequency of bringing animals into worm and this may mean clinical presentations have become more commonplace.

Over the past few years, we have seen both beef and dairy yearlings presenting with muscle stiffness and lameness due to white muscle disease, as a result of selenium deficiency.

Diagnosis has been confirmed using the glutathione peroxidase test, when blood results showed levels below the normal range and confirmed with results below 30; most supplemented animals show levels around 100.

A review of recent practice profiles where glutathione peroxidase tests were run showed one third of animals tested were found to be deficient. While this does not reflect the normal population, simply when the vet thought it was worthwhile looking for selenium deficiency, it does demonstrate deficiency is commonplace.

Copper

Copper deficiency has been a hugely controversial condition throughout the past 40 years, and clinically relevant given local herds on the “teart” pastures of the Somerset Levels, where exceptionally high levels of molybdenum – a copper antagonist – are found.

In the past this has resulted in very high levels of copper supplementation.

Legal limits were introduced to cattle feeding levels, and a prescription required once levels exceed 40mg/kg DM. The normal total copper in the ration is typically 20mg/kg DM from all sources, and this supplies 200% of requirements, so it builds in a large margin for safety.

Toxicity occurs when, over time, the surplus feed results in accumulation of copper in the liver. While this can classically manifest as sudden death with severe jaundice and pathognomic gun-metal grey kidneys at postmortem, more likely is a subclinical impact on general health.

Copper supplementation was seen by some as a panacea, with some farmers encouraged to increase levels to address poor health and fertility. We have seen this approach impacting on health due to oversupply. Liver biopsy screens from two dairy farms in the recent past established liver levels were high and allowed a reduction in copper inclusion in dairy cake.

Improvements in health and fertility have been seen subsequently. While improvements in health were multifactorial, it shows the importance of taking a holistic approach, and considering both copper deficiency and the potential for toxicity.

A high mortality rate was seen on another farm where dairy heifers at around four months of age were dying due to respiratory syncytial virus pneumonia despite vaccination, and good housing and husbandry. Further investigation established calves were receiving cow cake, rather than calf cake.

Cow cake contains higher levels of copper than that designed for the calf, as the absorption of copper is much greater in the pre‑ruminant calf, resulting in accumulation in the liver. Elevated copper was suspected to be causing an immunosuppression.

Screening for copper consists of blood sampling a cohort of animals for plasma copper levels. This approach gives some information with gross deficiency on intakes, but may not be wholly reliable when antagonists are present.

A review of practice results where copper status was assessed still showed 50% of samples indicated deficiency. Blood samples are not likely to tell us when the animals are verging on toxicity.

Liver biopsies are a more accurate approach, and the technique is easily learned. Alternatively, liver samples can be obtained from the abattoir for testing.

Zinc, cobalt and manganese

Other trace elements to be considered include zinc, manganese and cobalt.

Again, these may be an issue due to the geology and soil type of the farm.

Zinc deficiency is associated with lameness and poor skin health. Cobalt deficiency is a recognised condition in sheep, causing ill thrift, but the evidence for clinical disease in cattle is limited. Manganese is also associated with poor performance and surveys indicate widespread deficiencies, but linking this to a clinical presentation is inconclusive, as is a response to supplementation.

Summary

Trace element deficiencies continue to cause clinical disease – particularly in grazing cattle.

Trace element status and assessment should, therefore, form part of the herd health approach to animals under our care.

Prevention is possible using the range of boluses or parenteral products available.

Treatment usually requires parenteral treatment. Toxicities are less common, but copper excess can cause mortality, and have significant health and production outcomes.