I recently saw a very odd thing when I observed a farm adviser balance a ration for some replacement heifers. Based on the analyses of the forage and the trace mineral mixture and his estimate of feed consumption, his spreadsheet showed that magnesium intake would only be 6.7 grams per day.
His reference books listed the magnesium requirement at 7.8 grams per day. So, logically, he recommended increasing the magnesium level in the mineral mixture. The odd thing is that he assumed the magnesium shortfall of 1.1 grams is a real number.
On a forage diet, mineral availability in the digestive tract is really not well understood, certainly not enough to put decimal places on availability numbers. Forages contain fiber, and fiber can interact with minerals in ways that should make us seriously question our assumptions about mineral availability.
One main reason for this is a concept called “cation exchange capacity” (CEC). I’ll explain this with a discussion that involves glass tubes, clay, fruit and heat-damaged hay.
In chemistry, there is a well-known laboratory procedure called “column chromatography,” which is used to assay and separate chemical compounds. Column chromatography involves a long, vertical glass tube (the column) packed with some kind of gelatin-like substance.
A solution containing the compounds is poured into the top of the tube. As the solution flows slowly downward through the gel, some of the dissolved molecules “stick” onto the gel particles and thus come out of solution. This sticking – called adsorption (spelled with a “d”) – occurs when molecules electrostatically attach themselves to the outside surface of the gel particles, kind of like when you rub balloons on a carpet and then they tend to “stick” to that carpet.
In any case, after the first solution clears the column, then a different solution is poured into the column, which causes some of those compounds to detach from the gel and go back into solution. By strategically using different solutions and different gels, laboratories can routinely separate and analyze a vast array of compounds.
The important concepts here are that small molecules or even atoms will come out of solution when they are adsorbed onto solid particles, and they go back into solution only when something comes along and removes them from those particles.
Let’s switch our discussion to soils. Soils are composed of four main types of particles: sand, silt, clay and organic matter (humus). (We’ll ignore roots and boulders). The surfaces of these particles tend to have a negative electric charge. The smallest particles – clay and humus – have the largest amounts of charge because they have a very high surface area compared to their weight.
The negative charges on these soil particles attract atoms that have positive electric charges (cations) – primarily potassium, calcium, magnesium, sodium and hydrogen. In the soil, these cations readily adsorb onto soil particles, and they are exchangeable – meaning hydrogen atoms can replace the calcium or magnesium atoms.
The capability of soil particles to hold on to these cations is the soil’s cation exchange capacity. Each soil has its own CEC value, which depends primarily on the percentages and types of soil particles. Soils with a high CEC can potentially hold more nutrients than soils with a low CEC.
Laboratories can measure the proportions of each cation in a soil’s CEC, and agronomists use these numbers to help guide their crop and fertilizer recommendations. In practice, the CEC gives us an estimate on a soil’s potential to act as a reservoir of plant nutrients. Again, the important concept is that atoms can be adsorbed onto soil particles.
Now let’s focus on mineral nutrition for our animals. Let’s start with a very hypothetical scenario: If we have a completely empty digestive tract and pour a solution of minerals into the animal’s mouth, those minerals would flow down the tract unimpeded until they reach the small intestine, where a certain percentage of each mineral would be absorbed (absorbed with a “b”) across the gut wall into the blood.
The actual percentage that crosses the gut wall would depend on the solubility of each mineral’s compound (oxide, sulfate, carbonate, etc.) and the specific biochemical mechanisms used to transport that mineral across the gut wall.
For example, if a heifer’s daily magnesium requirement was 8 grams, and we knew that the efficiency of absorption across the gut wall was 50 percent, then we could feed 16 grams of magnesium and be confident we had met the magnesium requirements. So far, so good.
But we live in a real world where things are more complicated. On a real farm or ranch, we feed complete rations to our animals rather than simple mineral solutions, and these rations include forages. Here’s the rub: forages contain fiber, which is composed of a variety of large molecules like cellulose, hemicellulose, lignin, pectin, beta-glucans, and also other compounds like the Maillard products in heat-damaged hay.
When animals consume forages, some fiber is fermented in the rumen and some fiber proceeds down the digestive tract into the small intestine. In addition to the fiber in feedstuffs, the rumen bacteria contain cell walls – which are a type of fiber – and when these bacteria die, their remnants wash out of the rumen and also move down the digestive tract.
Let’s add one more thing to the mix: the cation exchange capacity of the fiber. This is the same concept as for soils. Over the past 25 years of fiber research, nutritionists have determined that plant fibers exhibit a CEC, similar to the CEC of soil particles (interestingly, measuring fiber CEC with the rare earth element neodymium, the same element used in the super magnets you can buy at home improvement stores).
And like the variety of soil particles, each type of fiber molecule has its own CEC level. Some fiber molecules have only a moderate CEC; some show very high levels of CEC. Two types of fiber with very high CEC values are pectin, found commonly in fruits and feedstuffs such as soyhulls and beet pulp, and the Maillard products found in heat-damaged hay.
Since every forage contains a complex mixture of many types of fiber molecules, every forage has a different CEC depending on the percentages and types of fiber molecules in that forage.
Now the big picture: We can think of the digestive tract as a flexible, living example of column chromatography. The digestive tract is the column; the fiber inside the digestive tract is the gel that packs that column. The gel consists of the fiber molecules that pass through the rumen into the lower tract.
Every ration will have its own unique mix of fiber molecules. And the liquid that moves down through the column contains the minerals that our animals require.
As this liquid moves through the digestive tract, some of its minerals will be adsorbed onto the fiber because of the fiber’s CEC, similar to what happens in the glass tube in a laboratory. Once mineral atoms are adsorbed onto the fiber, they can’t be transported into the blood until they are released back into solution. If they remain stuck on the fiber all the way through the digestive tract into the manure, they are completely unavailable to the animal.
In practice, this situation can get even more complex. Let’s say that we feed a balanced ration of hay, corn and a good free-choice trace mineral mixture. Each mineral in this ration will have a certain availability associated with it. A few weeks after we begin feeding this ration, we replace the first batch of hay with a different batch of hay that has been heat-damaged in the barn.
Heat-damaged hay contains Maillard products (the brown goo that resembles caramel). Maillard products are large molecules that are completely indigestible but they have high CEC values, so more mineral atoms will be adsorbed onto them, which may reduce the mineral availabilities to the animal even though the total mineral levels of the ration remain unchanged.
From a nutritionist’s point of view, fiber CEC adds a very big question mark to mineral nutrition. For any ration, what percentage of minerals will be adsorbed onto the fiber? Which minerals will be affected most? Under what conditions will these minerals be released back into solution?
Each mineral is affected differently, each ration contains its own combination of fiber molecules, and each combination of fiber molecules has a different impact on mineral availability. You might say there are a few unknowns here.
So how do we cope? Well, the first thing is to ask serious questions about the numbers. When our ration-balancing calculations show small differences between mineral requirements and intake, we should take these numbers with a grain of salt.
We certainly should avoid using decimal places or making recommendations based on decimal places. And secondly … well, perhaps we should step back, look at the bigger picture, and don’t cut things too closely. Those mineral numbers may not be what they seem. 
ILLUSTRATION: Illustration by Getty Images.
Woody Lane, Ph.D., is a livestock nutritionist and forage specialist in Roseburg, Oregon. He operates an independent consulting business and teaches workshops across the U.S. and Canada. His book, From The Feed Trough: Essays and Insights on Livestock Nutrition in a Complex World, is available through Woody Lane.
