11 Oct 2022
Emma Fishbourne
Job Title
Image: © nordroden / Adobe Stock
Approximately 73 trace elements are found in the body. Some (cobalt, copper, iodine, selenium, zinc and others) are essential for normal cattle health and production, even though they comprise less than 0.01% of the total mass of an organism; some have no known roles; and some can also be toxic (Rabiee et al, 2010).
They form part of numerous enzymes, coordinating a huge number of biological processes and must be provided to cattle in optimal amounts. Requirements, however, are hard to establish, and change during the growth and development of the animal, as well as the production cycle. For example, some trace elements act as antioxidants and are required for the safe removal of potentially harmful by-products from metabolic reactions during periods of acute oxidative stress.
Demand for antioxidants increases with increased milk yield due to an increase in oxygen consumption by the liver and other organs. Demand for trace elements is also higher in cattle that rely on the mobilisation of fat to meet energy deficits; for example, newly calved dairy cows and out-wintered suckler cows due to low levels of trace elements in adipose tissue. This contrasts with cattle close to finishing where growth is occurring primarily in adipose tissue (Suttle, 2004).
The amount of trace elements also varies in different feeds, with soil composition affecting the elemental composition of pasture. The presence of antagonists also affects availability; for example, soluble copper can interact with sulphur and molybdenum in the rumen to form products that are insoluble and unavailable, inducing a copper deficiency, with copper being lost in the faeces.
Due to the variety of roles trace elements play, deficiencies of trace elements can lead to reduced growth, production and immunity. Specific syndromes also exist.
In cattle, copper deficiency is associated with changes in coat colour; selenium deficiency with white muscle disease and cardiomyopathy; cobalt deficiency reduces growth; iodine deficiency leads to the birth of weak neonates; and iodine and selenium causes stillbirth/weak calf syndrome.
A meta-analysis of the effect of trace elements on milk production and reproductive performance by Rabiee et al (2010) showed supplementation could improve production and reproduction in lactating cows. Twenty papers were included in the analysis and supplementation increased milk production by 0.93kg, milk fat by 0.04kg and protein by 0.03kg per day respectively.
Days open was reduced by 13.5 days and the number of services per conception by 0.27. The risk of pregnancy at 150 days in milk was also greater in supplemented cows. Supplementation had no effect on the calving to first service interval or the 21-day pregnancy rate. Supplementary trace minerals have also been shown to have beneficial effects on sperm motility.
However, not all studies have found consistent effects on supplementation. Stokes et al (2018) investigated the effects of repeated trace element injections on beef heifer development and reproductive performance in commercial Angus heifers, and concluded additional trace element supplementation increased copper and selenium, but did not improve heifer performance or reproductive success.
Bodyweight and body condition score did not differ, but the authors highlighted that this has been inconsistent in the literature, with some studies reporting an increase in average daily gain, and suggested this is probably due to differences in timing of treatment, differences in breed and nutrient availability.
Ovarian morphology and follicular development were also not significantly different; however, heifers that received the injection did have decreased attainment of cyclicity, but this had no effect on AI or pregnancy rates.
Vanegas et al (2004) investigated the effects of an injectable trace element supplement on first service conception rate in intensively managed dairy cows. A single injection before breeding had no beneficial effect on first service conception rate, and cows that received two doses, one before calving and one before breeding, had reduced conception at first service. Willmore et al (2021) looked at the effects of a trace element injection containing copper, manganese, selenium and zinc on the reproductive performance of beef cows, and the growth of their calves.
Control cows that weren’t injected had access to free choice minerals and had good mineral status, which may explain why no improvement was observed in cows that received the injection in their reproductive performance or that of their pre-weaned calves, and may also explain some of the results in other studies.
In a review on the influence of trace elements in the peri-parturient period on fertility in dairy cattle, Wilde (2006) highlighted the vital role trace elements play in preventing common problems seen during this period, including retained fetal membranes (RFMs), mastitis and lameness – all of which can affect fertility. The incidence of RFMs in dairy cattle is around 4% and supplementation with selenium has been shown in numerous studies to reduce the incidence, with low levels of selenium-dependent glutathione peroxidase (GSHPx) in plasma associated with higher RFMs. RFMs affect fertility, with increased risk of metritis, extended calving interval, with longer days to first service and more services per conception.
Mastitis incidence in UK dairy herds varies hugely. Studies have shown clinical mastitis before first service increases the days to first service and days open; others that clinical or subclinical mastitis increases days to first service, days open and services per conception. Keratin lines the teat, and acts as a natural chemical and physical barrier to bacteria entering the teat, and zinc is involved in teat canal keratinisation. Supplementary zinc has been shown to reduce the incidence of mammary infections and reduce somatic cell counts.
Zinc has also been implicated in reducing lameness through improving keratinisation on the hoof. Again, lameness can affect fertility with increased calving to first service and conception intervals, with delays in ovarian activity early postpartum.
Internationally, perinatal mortality in dairy calves ranges from 3% to 9%, with mortality after this period and up to weaning ranging between 5% to 11%, with the most common reasons in this age group due to enteritis and pneumonia (Compton et al, 2017).
Failure of passive transfer is a key driver of postnatal mortality, but it has been shown that the neonatal immune system can be affected by the availability of essential trace elements, and that marginal concentrations of zinc, copper and selenium are associated with an increased likelihood of morbidity.
Bates et al (2020) demonstrated that on a spring calving grazing dairy farm, calves injected with a trace mineral supplement containing zinc, manganese, selenium and copper, and challenged with a killed vaccine containing Salmonella species antigens, saw increased phagocytic ability of their white blood cells, with an increased percentage of cells phagocytosing and increased number of bacteria ingested per cell compared to calves that hadn’t received the mineral supplement.
Iodine and cobalt are also important for fertility and youngstock. Iodine plays a vital role in thyroid function, and supplementation has traditionally been used to prevent stillbirths, weak calf syndrome and aid fertility in areas considered marginal by soil analysis.
Cobalt is used by ruminal microbes for the synthesis of vitamin B12, which is involved in glucose production; therefore, a lack of cobalt and B12 leads to a reduction in energy supply with associated clinical signs. Initial signs of cobalt deficiency include poor appetite, weight loss and reduced growth, but with severe deficiency cattle show rapid weight loss, fatty liver and signs of anaemia. Deficiency of cobalt is much less common in cattle compared to sheep and, like iodine, is restricted to certain geographical areas of cobalt deficient soil.
As vets, we should be assisting our clients with nutritional advice to optimise animal health and production, while ensuring human health and food safety. We should remember that cattle most at risk from trace element deficiencies are usually animals at pasture as concentrates can be correctly formulated to meet their needs.
Due to the difficulties in working out requirements for trace elements, it is beneficial to look at levels in the animal through blood analysis and liver biopsies, or liver samples from abattoirs or from on-farm culls, which is where vets can play a vital role.
The timing and delivery of supplements, if required, should concentrate on vulnerable phases of the production cycle. Many different ways exist in which we can supplement trace elements. Which method is selected should be targeted to suit the farm, facilities and budget. Supplements are categorised depending on the speed of absorption as short-acting (includes oral drenches, short acting injections, addition to water of feed) or long-acting (includes intraruminal boluses, slow release injections and trace element amended fertilisers). Intraruminal boluses are often more expensive, but require less handling. Free choice mineral licks are cheap, but don’t ensure all animals receive adequate amounts due to other factors, which affect their intake, as reviewed by Grace and Knowles (2012).
Failure to look to the animal for evidence that its needs are not being met can lead to clinical and subclinical signs of deficiency, but also of toxicity. In recent years, an increasing trend in poisoning from copper, iron and zinc has been identified, raising concerns about over-supplementation, leading to food safety concerns, and adverse effects on cattle health and productivity (Counotte et al, 2019).
In the UK, more than 50% of bovine liver samples collected at abattoirs had copper concentrations above normal; female dairy cattle accounted for the most with almost 40% having copper concentrations in their livers above the APHA reference range, highlighting that a significant proportion of the UK herd is at risk of copper toxicity (Kendall et al, 2015).
Cooper toxicity is characterised as hepatic necrosis resulting in a haemolytic crisis, and can either be due to a large acute dose of copper or the release of elevated copper from the liver.
A belief exists among some vets and farmers that as production has increased, cattle have become more susceptible to trace element deficiencies; however, the increasing trend in poisoning from trace elements highlights that, as vets, we need to look for evidence that an animal’s needs are not being met. When looking at supply, we should also take into consideration all sources before applying appropriate intervention.
