How and why should we determine mineral status in ruminants? - Veterinary Practice
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How and why should we determine mineral status in ruminants?

It is important to ensure that we meet each animal’s nutrient and mineral requirements while protecting against the effects of over-supply

Nutritional management in ruminants: 1 of 2

Ruminants have an essential requirement for minerals. This includes the macrominerals calcium, magnesium, phosphorus, sodium, potassium, chloride and sulphur, and microminerals or trace elements, such as iron, zinc, copper, manganese, iodine, selenium and cobalt. Each of these minerals fits onto a dose-response curve shown in Figure 1. Thus, it is important for us to be able to determine an animal’s status to ensure it remains optimal.

FIGURE (1) A dose-response curve illustrates the importance of optimal intake without under- or over-supply, which can lead to subclinical, often non-specific, signs (eg poor growth), or clinical signs, such as pica in cobalt deficiency or a haemolytic crisis in copper toxicity

There are also other minerals, which are not required, that have the potential to interact with essential minerals or to cause toxicity (lead, arsenic, cadmium, molybdenum). 

Understanding nutrient requirements

The National Research Council (now the USA-led National Academies of Science, Engineering, and Medicine Committee) reviews published work to derive an animal’s requirements (defined as the amount considered to meet the animal’s functional needs plus a safety margin) (NASEM, 2021).

In addition to considering an animal’s mineral requirements, there are also other published figures to consider when formulating a diet or examining the total intake of a ruminant. These are the maximum permissible level (MPL), an EU-derived number outlining the maximum concentrations of some elements allowed in the total diet, and the maximum tolerable level (MTL), which is the maximum intake considered to not cause any detrimental issues to the animal (Table 1). 

Magnesium (Mg)g/kg DM6 6 
Calcium (Ca)g/kg DM15 15 
Phosphorus (P)g/kg DM70 60 
Sodium (Na)g/kg DM30, 45* 40 
Potassium (K)g/kg DM20 20 
Chloride (Cl)g/kg DM30, 45* 40 
Sulphur (S)g/kg DM3, 5† 4, 5† 
Iron (Fe)mg/kg DM500852.3500568.2
Zinc (Zn)mg/kg DM500136.4300136.4
Copper (Cu)mg/kg DM4034.11517
Manganese (Mn)mg/kg DM2,000170.52,000170.5
Selenium (Se)mg/kg DM50.5750.57
Cobalt (Co)mg/kg DM251.1251.1
Iodine (I)mg/kg DM505.86, 11.365011.36
Molybdenum (Mo)mg/kg DM52.852.8
Lead (Pb)mg/kg DM100 100 
Cadmium (Cd)mg/kg DM10 10 
Boron (B)mg/kg DM150 150 
Aluminium (Al)mg/kg DM1,000 1,000 
TABLE (1) The maximum tolerable levels (MTL) and maximum permissible levels (MPL)(converted to dry matter based on original source, 88 percent dry matter basis) expressed as g/kg DM or mg/kg DM for cattle and sheep. *30 dairy, 45 growing; † 3/4 lower value concentrate, 5 forage; 5.86 dairy, 11.36 beef

Determining an optimal diet

To determine whether an animal’s diet falls within these boundaries we must consider the formulated diet, how this differs from the diet prepared and the diet consumed by the animal.

There are many and varied reasons why a formulated diet may differ from the one prepared. For example, if a diet is formulated for a silage analysis of a given dry matter, the actual silage dry matter will vary through the clamp or due to weather conditions. Another example is if a component (eg mineral premix) is formulated at 19.5kg but comes in 20kg bags. Here, the weight might be rounded up inadvertently when using the whole bag for ease.

Even if inconsistencies such as these are mitigated, there will be issues with feed selection and sorting by the animals as they eat. For grazing animals, there is increased inherent variation (eg season, weather, sward selection), and intakes are substantially harder to monitor.

Analysing feed

Ideally, forages, grazing, and large inclusion rate, home-produced straights will be analysed not only for mineral composition, but also for the standard energy and protein analysis, as they often form the bulk of the diet. It is important that these are true representative samples (Figure 2). Bought-in feed should have a “feed bag” label; however, this usually only contains the basic legally required information. (Companies are legally obliged to provide the full specifications, even if they are reluctant to do so.)

FIGURE (2) Methods to effectively collect pasture and conserved forage samples for analysis

Water is an important dietary component and source of minerals, some of which may be detrimental, especially if it is not from the mains. The mains supply can be checked for mineral content by postcode through the supplier’s website, while boreholes, wells and other private sources can be sampled and analysed from mid-flow at the source. Soil is commonly sampled, but although this is easy and cost-effective, it does not translate into pasture. Due to this poor correlation, it is recommended that pasture samples over soil are used in animal nutrition.

Forage mineral analysis is analytically determined in a laboratory, and its techniques can give total elemental concentration down to impressively low limits. Forage reports often include indicator bars, but it is important to note these display plant requirements or forage averages and are not related to animal requirements. Also, the results will not indicate whether the mineral is available to the animal. For most minerals in forage, it is estimated that between 40 and 90 percent is bioavailable; as such, it is important that animal status is also considered.

Determining animal status

To determine animal status, firstly on-farm records can be used to detect if a clinical condition is present (eg neonatal ataxia) or if there is a lack of performance (for example, a drop in milk yield, reduction in growth rates, lower conception/scan rate, etc). Next, animal samples can be collected – primarily blood, liver and urine – which can provide different information over different timescales.

It is important to bear in mind that for each animal, these parameters can be influenced by other factors, such as stress and disease

However, it is important to bear in mind that for each animal, these parameters can be influenced by other factors, such as stress and disease, which can limit their use.

Blood samples

Blood may be used to show deficiencies and short-term supply using simple concentration in serum or plasma. Whole blood parameters tend to have a six- to eight-week half-life, reflecting the lifespan of red blood cells. Blood also contains functional indicators which often play an active role (enzymes, vitamins), such as glutathione peroxidase activity for selenium status and vitamin B12 concentration for cobalt status.

Urine samples

Urine is useful to indicate the recent supply of macrominerals and iodine as these are regulated in the blood, so they remain in the normal range unless the animal is actually suffering a clinical condition. However, dilution corrected (creatinine) urine concentration is able to show whether the mineral is currently being excreted as it is in abundant supply, or being retained and conserved as supply is low.

Liver samples

The liver is very useful for monitoring mineral accumulation, and is especially useful for minerals which have toxicity issues (eg copper accumulation in the liver predisposes copper toxicity).

Other considerations

One of the hardest aspects of determining mineral status can be gaining an accurate history […] often mineral supplements are overlooked, forgotten or hidden like a guilty secret

Surprisingly, one of the hardest aspects of determining mineral status can be gaining an accurate history of mineral supplementation from the farmer. Often mineral supplements are overlooked, forgotten or hidden like a guilty secret. Conducting a farm walk and looking in bins and on shelving can be useful to trigger reminders and discussion.

Final thoughts

Together, all these aspects allow us to determine an animal’s mineral status. It is important to ensure that we meet animal requirements while protecting against the effects of over-supply. To do this, we must analyse the input to determine the total supply before assessing how the animal responds to the diet. Then we can make any supplemental decisions based on the data, and lastly, but most importantly, we must check whether the action we have taken is producing the desired effect.

Minerals are easy to blame for a lack of performance when more often than not, the cause is a lack of dry matter intake, a lack or imbalance of energy and protein or an animal health issue

Remember that minerals are easy to blame for a lack of performance when more often than not, the cause is a lack of dry matter intake (not enough food), a lack or imbalance of energy and protein (food quality) or an animal health issue, such as parasitism or disease. These should be investigated ahead of or parallel to investigating a mineral “problem”.

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