Trace elements are essential dietary components for organisms. They must be ingested in very small amounts, unlike macroelements. Concentrations of specific trace elements within the body are usually <50 mg/kg body weight, with the exception of iron (National Research Council, 2005).
Current recommendations in animal feeding in regards to trace minerals, include total dietary concentrations. This is due to the highly variable concentration levels and bioavailability of plant-based foods which many times don’t meet animal needs(Suttle, 2010).
In ruminants, there are special recommendations regarding Cobalt in order to address specific requirements of rumen microbiota and promote cobalamin synthesis.
Special forms have also been commercialized in recent years. From which the most significant are probabbly nanoparticles (e.g. nano ZnO).
Within the European Union, feed additives require authorisation in accordance with Regulation (EC) No 1831/2003. The authorisation procedure must ensure that a feed additive is safe for: the animals, personnel handling the product, consumers and the environment. This implies that trace element supplements are used in sufficient quantities in order to minimize total dietary concentrations needed. Avoiding excess accumulation in animals, edible products and the environment.
Defining “bioavailability”
The term bioavailability is often used as a synonym for absorption or retention capacity. However, this is misleading as it implies that trace elements are 100% bioavailable as long as they are present as an available chemical compound within the gastrointestinal (GI) tract.
The content of essential trace elements in the body is actively regulated by the intestinal barrier through specific absorption mechanisms, as well as various absorption and excretion mechanisms according to their metabolic state (Windisch, 2002).
Therefore, the bioavailability of a trace element source can be defined as its potential to supply the physiological functions dependent on that specific element, within the animal’s metabolism(O’Dell, 1984).
These physiological functions are closely related to the organism’s total requirements and the kinetics of that element within the body. In regards to animal nutrition,the bioavailability estimate is related to the estimate of gross requirements, which is expressed as:
“the minimum concentrations of total feed required to provide sufficient bioavailable quantities under ad libitum feeding conditions.”
Basic concepts of trace element metabolism
[register]Trace elements tend to circulate in their ionic form bound to chemical ligands. The role that these ions play within biochemical reactions as cofactors of the structure and activity of peptides is considered to be irreplaceable by other substances. This highlights the base of their essentiality for animals (Maret, 2016).
In intensive or semi-intensive farm animal production systems, clinical presentations due to trace element deficiencies are rare and most deficiency presentations are subclinical.
A subclinical deficiency compared to a clinical one is defined by the total absence of visible symptoms of pathophysiological adaptations. Therefore, it only becomes evident due to changes in metabolic levels.
Total Zn body content is regulated at the GI tract absorption level and through fecal excretion.
Parallel to the upward regulation of the GI tract absorption capacity, the excretion of endogenous Zn is reduced to a minimum under conditions of Zn deficiency in the diet. Excretion increases when additional Zn supply surpasses the requirements’ threshold (Weigand and Kirchgessner, 1980).
- Cereal phytate may affect Zn status in growing young pigs without sufficient dietary supplementation and in the absence of endogenous and/or exogenous phytase activity (Schlegel et al. , 2013).
Brugger et al., 2020 suggest that the Zn status of laying hens may be affected by phytic acid under conditions comparable to those previously applied in weaned piglets (Brugger et al., 2014).
- This contrasts sharply with previous studies done on broilers, where these seemed to be more efficient than pigs in regards to the use of Zn (Schlegel et al. , 2013). This was presumably due to microbial phytase activity.
Ruminants harbor significant microbial phytase activity in their rumen ecosystem (Humer and Zebeli, 2015). Therefore, their susceptibility to phytate-induced Zn deficiency should be lower than in non-ruminants.
- However, reports of Zn deficiency in high-yield dairy cows indicate that certain dietary scenarios may have the potential to affect Zn absorption in ruminants(Cope et al. , 2009).
Microbial breakdown of phytic acid in the rumen is generally not complete and can be significantly reduced under conditions where ruminal feed passage increases. For example under conditions of high DM consumption (Humer and Zebeli, 2015).
In addition, forage contamination with soil (especially silage) can produce high concentrations of Fe (up to 1500 mg/kg DM) which have been shown to impair the use of Zn in feed (Standish et al., 1969).
Hepatic Cu represents the body’s primary Cu storage site(Suttle, 2010).It is stored bound to the enzyme Metallothionein (MT) or undergoes the synthesis of Cu protein (especially ceruloplasmin (CP)), biliary excretion or it is directly transferred to other tissues for Cu peptide synthesis.
The particular purpose of Cu transport depends on animal’s Cu state and its requirements(Bremner, 1993):
- Under conditions of excess Cu in the diet, hepatic Cu accumulates and becomes increasingly subjected to biliary excretion. The speed at which the liver is able to transfer accumulated amounts into bile secretion in order to prevent poisoning seems to depend on the animal species (Suttle, 2010).
Dietetic phytate can chelate Cu and block its absorption. This is especially true in non-ruminant farm animals.
In fact, phytate has a much higher affinity for other ions such as Ca2+ and Zn2+. Therefore, the association of Cu with phytate is less likely when there are considerable amounts of such ions(Humer et al., 2015).
- In addition, the total daily Cu requirements of pigs are considerably low (National Research Council, 2012).
Therefore, dietary Cu deficiencies in pigs fed with common ingredients are usually rare(Suttle, 2010).
There appear to be no practical observations of Cu deficiency in poultry. Cu availability indifferent ration ingredients has been thoroughly assessed. Although Cu availability for chickens is reduced in the presence of high phytate concentrations, the apparent absorption does not drop to zero.
Ruminants tend to be the most susceptible to Cu deficiency. Increased intake of sulfur (S) and molybdenum (Mo), mainly from fiber, leads to the formation of thiomolybdates in the rumen. These quelate Cu, which extremely reduces the apparent absorption of Cu to <1%, e.g. in sheep (Suttle, 1983, Allen and Gawthorne, 1987).
Spears et al. (2004) demonstrated that Cu supplemented as tribasic Cu chloride for growing cattle has superior bioavailability compared to Cu sulphate under conditions of high levels of Mo and S in the diet. This appears to be due to lower solubility in the rumen of tribasic Cu Chloride and therefore reduced susceptibility to thiomolybdate antagonism.
- In contrast, Hanauer (2017) suggested that this superiority can be reversed when the Mo and S in the diet are moderate, because the lower solubility can become a disadvantage in terms of bioavailability, as demonstrated in non-lactating cows.
In turn, excess Mn in the diet may further promote molybdenum-associated hypocupremia, as demonstrated in growing cattle (Hansen et al., 2009).
More in “Nutritional aspects of Copper for animal production”
Manganese is one of the least abundant essential transition metals in animal tissues (Suttle, 2010). Given the narrow range of concentrations of Mn that fulfills its biochemical purpose, its body reserves must be strictly regulated.
- However, data obtained in farm animal models suggest that absorption relative to intake cannot always be regulated as accurately by state (Pallauf et al., 2012).
- Therefore, their gross needs under practical conditions are higher and therefore there is a greater risk of suboptimal dietary supply.
The high concentrations of phytate and fiber are the most relevant antagonisms in this regard. There is another antagonism between excessive levels of P in the diet and gastrointestinal absorption of Mn (Baker and Odohu, 1994).
In addition, excessive supply of Fe in the diet can cause Mn deficiency in all livestock species (Suttle, 2010).
The content of Mn in forage is quite variable depending on its geographical origin. In addition, it also depends on other factors, such as:
- botanical composition
- the growth stage
- the harvest number
- conservation (Schlegel et al., 2018)
In alkaline soils, the absorption of Mn by plants can be affected (Suttle, 2000). Conversely, acidic soils can harbor so much Mn available to plants that animal poisoning could become a problem (Grace and Lee, 1990).
Therefore, the determination of Mn in food is especially important for feeding ruminants.
Conclusions
Flows of essential trace elements are actively regulated in response to dietary intake levels that vary below or above metabolic requirements.
- For example, when animals are poorly supplied, their gastrointestinal absorption capacity increases and/or endogenous losses are reduced as far as possible.
There are relative differences in bioavailability with different sources of trace elements in the diet, as well as between different animal species
Source: Abstract taken from ” Review: Bioavailability of trace elements in farm animals: definition and practical considerations for improved assessment of efficacy and safety”Brugger et al., 2022
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