Simply feeding diets containing a high level of essential trace minerals to dairy cattle does not necessarily guarantee their mineral requirements are met in today’s high-performance environment. Rather, trace minerals with greater bioavailability might be fed on the milkline, drylot or even replacement heifer pens, so the benefits of their greater nutrient value are received by all animals.
“Bioavailability” of trace minerals often means an ingested trace mineral is absorbed by the cow in a form that can be metabolized and used to meet its specific trace mineral requirement. There are many factors that either promote or reduce trace mineral bioavailability such as: mineral intake, specific mineral source, digestion and absorption in the cow as well as several other existing feed conditions.
Once the cow or replacement heifer eats feed with necessary trace minerals, the importance of actual mineral bioavailability really starts to take shape.
The absorption sites for many trace minerals such as iron, zinc, copper, manganese and selenium are present in duodenum of her small intestine. First, the trace mineral is absorbed by intestinal mucosa cells by either an active transport mechanism and by passive diffusion. Within these mucosal cells, trace minerals are bound to specialized proteins that help them cross this barrier and into the bloodstream. From here, the trace minerals are then bound to other specialized blood proteins, which brings them to all parts of the body as required.
Despite feeding a diet with a good level of essential trace minerals (often based on NRC requirements), there are many dietary factors that will adversely reduce trace mineral bioavailability and thus affect how much essential mineral the animal finally receives or uses. Some dietary factors bind ingested trace minerals in the gut and make them unavailable for absorption in the small intestine. For instance, high levels of molybdenum in the diet combine with copper digested from the feed in the rumen and render it unavailable for absorption.
Even if such antagonistic elements are not present in the diet, other associated dietary minerals, if fed in excess amounts, can adversely affect the uptake of another required trace mineral.
For instance, excessive dietary iron can impede absorption of copper, manganese and zinc, because these four elements all compete for the same receptor proteins, necessary for absorption in the surface intestinal mucosal cells. Researchers found as little as 250 mg/kg of dietary iron can cause a respective copper deficiency in cattle. Their consensus is that high dietary iron impacts copper absorption the most when the cattle are already low or marginal in copper status.
Similarly, there should be a favourable environment in the small intestine for trace minerals to be efficiently absorbed. Normally the pH of this part of the intestine is about a neutral 7.0, which is a necessary condition for many essential trace elements to be absorbed by the mucosa. Acidic conditions caused by common digestive upsets such as sub-clinical acidosis in cows might prevent the release of many trace minerals from the feed digesta and thereby reduce pending bioavailability for intestinal absorption.
In response to these obstacles to good trace mineral absorption and utilization in cattle, the use of “chelated or organic” trace minerals in dairy diets has become very popular.
Chelated-mineral compounds are positively charged trace minerals chemically bound to organic amino acids (re: the building blocks of proteins). Examples include: zinc methionine, copper lysine, and manganese methionine. It should be noted that the trace mineral such as selenium cannot be truly chelated. Rather, organic selenium is produced by feeding inorganic selenium to yeast, which incorporates it into their body proteins.
Just what makes these chelated minerals nutritionally special for dairy cows?
The answer is simple. Unlike their inorganic counterparts, the chelated minerals’ chemical structure allows them to easily cross the cow’s intestinal mucosa and go directly into the bloodstream without much problem. Furthermore, chelated minerals take advantage of their own amino acids’ absorptive properties and do not directly complete with other dietary minerals for binding proteins or mucosal sites in order to be absorbed.
Some scientists think the chelates’ superior bioavailablity has more to do with a reduced electrical charge that makes it less reactive with other molecules that may bind it and thus make it unavailable for absorption. Organic mineral compounds are also believed to be less susceptible to possible changes in pH, which as mentioned above tends to also affect inorganic mineral absorption.
Studies to demonstrate the actual benefits of feeding chelated minerals versus inorganic minerals to cattle have been variable. In one experiment, researchers at North Carolina State University fed marginal copper deficient heifer replacement calves either chelated copper, inorganic copper sulphate, or inorganic copper carbonate. The researchers concluded organic copper may have the same bioavailability as inorganic copper sulphate under normal dietary conditions.
However in a second NCSU experiment, they fed the same treatments, but in the presence of high dietary molybdenum, known to be antagonistic toward dietary copper. As a result, the organic copper was then shown to have greater intestinal absorption and liver retention in the animals than the inorganic copper sources. Such evidence pointed to the advantage of using chelated minerals under special conditions in dairy nutrition.
Such field trials are helpful in demonstrating the source of essential trace minerals for dairy cattle must have a relative high degree of bioavailablity. As a consequence, the dairy animal should not only consume enough of a particular trace mineral, but can effectively absorb, metabolize and retain it in its body; thereby match its respective requirement of that mineral.
PeterVittiisanindependentlivestocknutritionist andconsultantbasedinWinnipeg.Toreachhim call204-254-7497orbyemailat [email protected]