Mineral supplementation should match the determined need, whether this is from a comparison of feed intake with requirement or in response to a clinical condition (now or in the previous annual cycle).
Effective mineral supplementation
A frequently overlooked fact of mineral status is that different elements have different effective sites for absorption, to act with a co-factor or to prevent detrimental interactions. Key examples include:
- Cobalt has only one role: vitamin B12 synthesis occurs via rumen microbes. Therefore, cobalt supplementation must supply cobalt to the rumen in a rumen-available form to be effective
- Copper function is inhibited through thiomolybdate formation in the rumen. Therefore, a rumen-available copper source is needed to detoxify and prevent absorption in the rumen. Subsequently, thiomolybdate in the intestine will also bind copper, preventing thiomolybdate and copper absorption
Toxicity and over-supplementation
The use of copper as an example also highlights another crucial consideration for mineral status in ruminants: over-supplementation. Copper toxicity has a greater risk of death than copper deficiency or any other copper interaction effects. Recent work has shown not only substantial liver copper loading in the UK dairy herd (Kendall et al., 2015) but a general overfeeding of copper on many dairy farms (Sinclair and Atkins, 2015).
Selenium can also be over-supplemented and tends to have very similar sub-clinical signs for both over and undersupply (signs of which are antioxidant related). This highlights the issues of supplementing to observed signs which, especially for trace elements, are often non-specific. One of the sub-clinical signs for selenium is the incidence of retained placenta in cattle (Kendall and Bone, 2006) as seen in Figure 1.
There are a variety of supplementation strategies and lots of products available. These can generally be classified under the following approaches.
If the animal is fed a complete-type diet, then in-food mineral supplementation via concentrate or premixes is often the easiest and cheapest option. If it is on a concentrate and forage-type diet with the concentrate fed to yield, it is worth considering macrominerals in the “fed to yield” component and trace elements in the forage component. This ensures the macrominerals meet the yield while controlling intakes of trace elements and preventing oversupply in the higher producing animals.
If the animal is fed a complete-type diet, then in-food mineral supplementation via concentrate or premixes is often the easiest and cheapest option
Water approaches can be an option for mineral supplementation, but only when the drinking resource can be controlled. This often requires fencing off streams/springs and should also include a dosing system to ensure a consistent content in the water. Remember that water intakes can sometimes be fulfilled by in- and on-feed water, especially in non-dairy animals. Rain will reduce trough water intake, but intake will increase with increased temperature.
Direct-to-animal approaches involve the administration of products directly to individual animals and can include drenches, boluses, injections and some topical applications. The effectiveness and best route of these approaches depend on the element at risk and the system itself.
Boluses should provide a long-term sustained release of mineral supplementation across the boluses stated lifespan. However, many seem to dissolve/erode quicker than expected, which means the daily dose is increased.
They are usually formulated to fully meet requirements (rather than the gap between requirement and background basal intake), meaning over-supply and potential toxicity are a worry. However, some boluses do not dissolve/erode at all and can be recovered largely intact years after administration in some cases.
It can be informative to attempt to recover boluses from animals that die or are slaughtered to double-check dissolution rates.
Drenches are often very short-acting, providing a short-term response that can be useful as a starter dose for mineral supplementation.
Drenches are often very short-acting, providing a short-term response that can be useful as a starter dose for mineral supplementation
Responses differ by element: for example, cobalt may be largely ineffective unless drenching is weekly (due to the rumen microbes’ requirement for cobalt and not the animal’s per se), so a regular supply is imperative. Selenium, on the other hand, has a different response, with a drench able to elevate plasma selenium. The elevated plasma selenium is incorporated into additional glutathione peroxidase, most of which is contained in erythrocytes, which, due to their six- to eight-week half-life, results in a prolonged response to a single oral drench of selenium.
This continued effect for selenium is also seen with boluses, where a post-bolus dissolution response is observed, whereas other elements, such as cobalt or iodine, will not respond beyond a few days of bolus dissolution.
Injections and topical application
Injections are only available for a few elements. A normal B12 injection has a similar duration to a vitamin B12 drench (around one week). However, there are some long-acting vitamin B12 injections that are very effective over a month before relying on passive storage (accumulation of vitamin B12) for a longer duration of efficacy.
Topical application of iodine is also used for mineral supplementation; however, it is unclear whether the reason for its efficacy is due to allogrooming or absorption through the skin.
Free access approaches to mineral supplementation include mineral blocks, feeding buckets and molasses licks. The intakes for these devices are variable between animals, especially for mineral-only products. Approaches that also provide feed are less variable in intake and may be useful where supplementary energy/protein is also required.
A method used outside the UK involving a free-access buffet or “smorgasbord” of individual mineral elements relies on animals selecting their own requirement for each element. Unfortunately, animals do not have this level of nutritional wisdom and the free-access feeding of individual elements is illegal in the UK/EU.
There are various pasture fertiliser and field treatment options for mineral supplementation. These, however, rely on either the treatment being a contaminant or plant absorption. Plants have no requirement for many trace elements and therefore have no absorption mechanism, so these options usually rely on the contaminant route.
There are various pasture fertiliser and field treatment options for mineral supplementation. These, however, rely on either the treatment being a contaminant or plant absorption
Clever management options are often low-cost approaches to mineral supplementation and result from knowledge gained over a period of prolonged strategic testing. For example, a farm identifies an area with fields low in cobalt and another area with fields adequate in cobalt. Preferentially, sheep are now grazed on the higher cobalt fields. The low-cobalt fields are used for conserved forage production to be fed as a part of a (mineralised) total mixed ration when the sheep are housed or for groups being creep fed.
Doing nothing can be pragmatic or lazy. Where the identified risk is low, the economic response may be less than the cost of the supplement. In this case, doing nothing is a good option as long as welfare is not compromised.
It is important to regularly check that any mineral supplement strategy you use is working and still needed. If this is done incorrectly, it can be misleading.
Cobalt status may be found to increase after a low status has been identified and a supplement administered in the summer/early autumn. Further testing in the late autumn can show a normal status, and it can be assumed that the supplement has given this response. However, pasture cobalt is lowest in the dry summer/early autumn and will naturally increase as the autumn progresses and it becomes wetter. Therefore, the rise in cobalt status seen is not necessarily the result of the supplement.
Maintaining some unsupplemented animals will enable you to check the efficacy of the supplementation strategy
Maintaining some unsupplemented animals will enable you to check the efficacy of the supplementation strategy. When working with “direct-to-animal” supplementation, this is much simpler. One option is to identify a small subset (6 to 12 animals) from the group to remain unsupplemented and act as sentinels. A fair trial is another approach and can be used to compare supplementation strategies or a strategy against no strategy. However, fair allocation is essential. Do not use the first half against the latter half, as the latter are more likely to include lameness, stragglers and strugglers.