In cattle breeding, evolution of the current agriculture (monocultures, high productive breeds, deficient soils…) are responsible for a growing gap between the supply in trace elements and the needs of the animals (figure 8). This increases the risk of having deficiencies. Furthermore, because trace elements role in metabolic reactions is better known, it leads to more frequent diagnosis of deficiencies.

In this context, several tools allow the veterinarian to advice to his farmers management methods of trace elements deficiencies or excess within the farm.

Review of the ration as a whole

First, it is important to review the ration as a whole. Indeed, when food-born origin of a disorder is highlighted, major elements of the ration should first be reviewed such as energy, fibre, crude protein supply… (Arthington, 2003; Chantreau, 2017). In this instance, the following elements should be checked :

Last but not least, it has been shown that the vast majority of the rations supplied to ruminants are deficient in minerals. It is even more important in the case of trace elements. Therefore, it is sometimes possible to directly have a look at the supplementation in minerals without analysing forages (Rollin, 2002)

Identification of poor absorption due to hazardous practices

Poor absorption hazardous practices correspond to practices leading to secondary deficiencies.

Secondary deficiencies in Copper are relatively common. Especially throughout competitions between Molybdenum, Sulphur, or Iron. The first two elements inhibit the absorption of Copper per collection and complexation of Copper. In this way they constitute molecules that are very less absorbed in the intestine and with a low bioavailability. When cattle graze on pastures where grass is too short, the ingestion of soil, including Iron that can represent up to 10% of dry matter, can be responsible for a Copper deficiency (Guerin, 2010). An excess in Sulphur, sometimes present in water for instance, also inhibits absorption of Selenium. Thus, usage of a well collecting rainwater, or the utilization of a fertilizer rich in Sulphur on crops are two hazardous practices for Copper and Selenium deficiencies (Arthington, 2003).

Any events responsible of alteration of the digestive tract can be responsible for a secondary deficiency in trace elements by decrease of the digestive absorption. Therefore, as an example, an untreated chronic diarrhoea can be responsible for deficiencies (Graham, 1991).

Identification of poor supplementation hazardous practices

Most of forages are deficient in trace elements. We therefore understand that lack of mineral supplementation represents a risky practice of having important deficiency (Rollin, 2002).

However, in most animal nutrition works, the majority of the studies concerns major elements of the ration such as digestible proteins, energy or nitrogen; then on calcium and phosphorus, and finally, few concern trace elements. Therefore, the recommendations in this area are often limited to one commercial product containing trace elements. However, this product should be affordable while being of good quality, which means being sufficiently bioavailable for cattle (Adams, 1975; Meschy, 2010).

Available galenics

The type and frequency of trace elements supplies highly affect bioavailability. This latter notion corresponds to the proportion indeed active in the organism and metabolized compared to the quantity of elements absorbed. This corresponds to the valued part of the ration for trace elements.

Therefore, trace elements excretion increases when supplies increases. For instance, if we look at minerals distributed in the ration, the quantities supplied are relatively high in order to cover the needs, but frequency is low: only two times per day. Mechanisms of absorption are thus rapidly saturated, and a significant part of supplies are directly eliminated. Even if the quantity distributed seems to be enough to meet the animals needs, the quantity absorbed is too low – because of the type of supplementation. On the opposite, a quantity given during longer period, such as methods with continuous release, will be better absorbed by the animal and thus best used (Spears, 2014).

Solubility of trace elements is another major parameter influencing their bioavailability. The latter varies depending on the biochemical form given. It can also influence desintegration times of the delayed action mechanisms.

Chemical forms of supply

We mentioned the different macroscopic forms trace elements can have. However, even if this form is of great significance for trace elements bioavailability, it is important to take into account their chemical form. Indeed, sources of trace elements cannot be used the same way by the animals: they can be completely assimilated and used by the animal, partially, or completely eliminated, which makes their bioavailability vary (McDowell 2003).

Numerous chemical forms of supply in trace elements exist. They can be split in two groups: inorganic salts and organic salts. Since the late 1990s, a third category exists: the hydroxy trace elements.

Hydroxyanalogue forms

A hydroxy trace element is composed of a mineral element, bonded to a hydroxyl goup and a chloride group by covalent bonds. These are usually elements that are not soluble in water. Therefore, they are non-hygroscopic and less reactive to sulphates. Furthermore, by improving bioavailability of trace elements, the quantity excreted in the environment through faeces are less important. Likewise, their low solubility to pH of the rumen, allows to reduce the impact on the rumen microorganisms. Indeed, cattle needs in trace elements are higher than those from the rumen microorganisms. However, if concentrations that are too high, especially in Zinc and Copper, they can be responsible for a deterioration of the rumen microorganisms ((Intellibond), 2013).

The type of salts used do not only determine trace elements bioavailability. For instance, supply of organic or inorganic forms determines the speed of disintegration of boluses from a galenic point of view. A bolus mainly made of organic salts will have a shorter disintegration time than a bolus manly made of inorganic salts.

Therefore, the idea for the manufacturer is to find the right balance between the disintegration time/desired length of action and the composition.

Assessment of the trace elements status of the animals

To assess cattle’s assimilation of trace elements of their ration makes it possible to assess the effective intake of the ration. The status in trace elements can be obtained through different samples: blood, milk, saliva or animal tissues.

When the objective is to determine a plasma profile of livestock in trace elements, it is relevant to select animals at random in herds, but should represent best the herd. Indeed, it increases the impact of the predictive values of the analyses and the random selection makes it possible to reduce individual variations, especially those related to the physiological stage (Herdt, 2000; Siliart, 2014). However, it is important not to sample them when they are too young, less than 2 months, neither in the month before or after calving, as the variations in minerals during theses periods are very high and not representative of the global status of the herd. Moreover, the sampled animals have to be clinically healthy, as trace elements status are strongly affected by different diseases – for instance, steric concentrations in Copper increases during the inflammatory process (Herdt & Hoff, 2011).

The optimum number of animals to sample corresponds to 10% of the total number of cows of the herd. But it also depends on the objective of the analysis: estimate the average concentrations in trace elements of the herd, assess a deficiency prevalence or highlight it. The minimum number of animals to sample varies from 4 to 12 depending on the authors (Herdt & Hoff, 2011; Kincaid, 1999; Oetzel, 2004). In order to lower the costs, sometimes pooled samples from several animals can be taken. Then, between 5 and 20 animals can be sampled depending on the authors, and group average values can be compared to reference values of this group. However, when dosing glutathione peroxidase for the assessment of Selenium, the number of individuals to sample would be of 5 maximum. Beyond, the obtained value for the mix would statistically be too different from the average value of the individuals (Pitel, Besnier, Reisdorffer, Defontis, & Richard, 2014). However, the use of pooled samples to assess a herd’s status is subject to controversy. Indeed, pooled blood sample may hide heterogeneity of status between animals. Moreover, just one extreme value can significally change the results. Thus, only very high or very low results could be interpreted (Guyot & Rollin, 2007; Siliart, 2014).

Finally, during blood tests, it is relevant to choose the right sample tube. Indeed, tubes made of glass or those with a rubber cork are at risk to contaminate the sample, favouring hemolysis or by artificially increasing Zinc level. It is recommended to sample on EDTA K2 tubes, specifically treated on the cork to make trace elements dosage possible.

These tubes are supplied by Vétalis to all veterinarians who request them.

Sample of tissus

These samplings allow an access to different trace elements stocking organs. Therefore, liver biopsy gives a direct assessment of Copper, Iron and Cobalt stock. Even though this sample is relatively easy and quick to do, it is more expensive and slower than a blood sample. This is why it is not often used in Europe.

During a longer fast, stress, inflammations or infections – even starting – quantities of trace elements stocked change. This happens before apparition of any clinical sign or any variation of plasma concentrations (Suttle, 2010). This is why, when sampling is possible, biopsies can be worth to be carried out – even more in clinical context (Herdt & Hoff, 2011). Finally, chronic exposure to high doses of minerals causes their accumulation in the stocking organs, quite often without increasing their plasma levels. Therefore, biopsies make possible to objectify intoxication before clinical consequences appear (Auza, 1983).

Milk sample

For Iodine and Selenium, excreted amounts in the milk reflect the short-term status of the animal. The analysis can be done either on one individual or on the milk from the tank. However, it is important to make sure that the contaminated milk by erythrocytes hasn’t been put in the mixed milk. This would distort the Selenium value (Guyot, Saegerman, Lebreton, Sandersen, & Rollin, 2009).

However, Iodine cannot be sampled on the colostrum as it contains three to five times more Iodine than the milk. Moreover, when using iodine-based products, this sampling is useless as the products artificially increase the iodine level measured (Meschy, 2010).

Other samples

Urinary and hair profiles could seem interesting at first sight because easy to do for many people. However, they are not the most relevant samples. Indeed, urinary concentrations in trace elements are not correlated to blood concentrations: when feed supplies increase, urine elimination also increases. Furthermore, urine excretion depends on the trace element and on the intake type. For instance, Selenium renal excretion is increased when the latter is supplied in the inorganic form compared to an organic supply. In general, when there is no kidney damage, trace elements in urine come from an excessive supply or a sudden weight loss (Guyot & Rollin, 2007; Siliart, 2014).

Hair concentrations highly vary depending on the colour: black hair is richer in Zinc, red hair is richer in Copper. Moreover, these concentrations are lagging indicators of a possible deficiency. Finally, hair is very often contaminated by the external environment (when friction against the fence for example) and dust. Therefore, hair concentrations in trace elements depend more of the hair colour, the age of the animal and the external environment than of his status (Meschy, 2010; Siliart, 2011, 2014).

Thus, urine or hair samples are not suitable for assessing the trace element status of cattle.