Many farmers routinely do this by stockpiling late-summer grass, especially tall fescue, but this option doesn’t work with earlier growth or corn plants.
If we store the forage, we must overcome two major enemies, mold and bacteria, which will grow on the forage and use up nutrients before our animals can get to it. We can prevent both if we dry the forage enough so nothing will grow on it. That’s hay or one of its derivatives, like cubes or stacks.
Another technique is: We can enclose wet forage in airtight containers to prevent mold growth and let it rot so the low pH prevents bacterial growth. That’s silage or one of its derivatives, like baleage, haylage or hay crop silage. We are all familiar with hay, but silage is a horse of a different color. Let’s talk about it.
Making silage is really a process of fermentation, roughly analogous to the fermentation in a rumen, only different. The silage process occurs in five phases. (I know, I know, some publications list four phases or even six phases. But I’m not fazed – these categories overlap, and authors make arbitrary distinctions, so five is a perfectly good number).
In the first phase (which scientists predictably call “Phase I”), we cut green forage and stuff it into an airtight container. The plants still contain 30 to 70 percent moisture, depending on the type of vegetation and the type of silage. Initially, there may still be a bit of residual oxygen in the container.
Because the forage was green when we cut it, many plant cells are still alive and carrying out respiration. Which means that, for a short while, those cells are still using some sugar. Also, some oxygen-loving (aerobic) bacteria on the surfaces of the cut forage continue to live and metabolize during this period.
But within a few hours, all the oxygen has been used up, and all the plant cells and aerobic bacteria die, signaling the end of Phase I. The pH of this forage is 6.0 to 6.5.
In “Phase II,” the forage begins to rot or, should I say, ferment. Since the container now contains little or no oxygen, the only life that can survive are bacteria that can live without oxygen. These are called anaerobic bacteria. In fact, oxygen may be toxic to some of them.
A valuable characteristic of silage is: All molds require oxygen, so by eliminating oxygen, we eliminate molds. Meanwhile, the anaerobic bacteria begin to ferment the soluble carbohydrates in the forage and produce acetic acid. Since a silo is a closed container, the acetic acid has no place to go, and it begins to build up in the forage. This increase in acidity is reflected in the silage pH, which begins to drop below 6.0.
We should remember: A silo, while it is a fermentation chamber, is not the same as a rumen, which is also a fermentation chamber. A rumen is actually a flow-through system. Things enter through the mouth and esophagus, ferment in the rumen and exit through the tube of the gastrointestinal tract to the true stomach (abomasum). In contrast, a silo is a tightly closed static system.
Once things are put into the silo, they stay there, and any fermentation products also stay there and build up over time.
This is an important concept because within three days, the buildup of acetic acid causes the silage pH to drop to around 5.0, which initiates “Phase III” of the ensiling process. The acetic acid-producing bacteria die off and are replaced by different bacteria that thrive at this lower pH, like lactobacillus and leuconostoc.
These anaerobic bacteria produce lactic acid. Lactic acid is a stronger acid than acetic and, as the lactic acid accumulates, the forage pH drops further. In approximately two weeks, the lactic acid accumulates to more than 60 percent of the organic acids in the silage, and the silage pH drops to below 4.5 and even below 4.2.
Which moves us to “Phase IV” – stability. The silage pH is now so low even the lactic acid-producing bacteria cannot survive. In effect, they produced so much acid they pickled themselves. But here’s the bottom line: Nothing else can live in that environment. We have reached our goal. The forage is no longer losing nutrients to fermentation. The forage is stable and safe.
Unless things don’t go quite right. Then the silage can slide into another phase: “Phase V.” This occurs when the silage pH doesn’t drop low enough. The silage may have contained too much water or not enough readily fermentable carbohydrates for the lactic acid bacteria. In any case, in Phase V, the fermentation shifts to favor a different type of bacteria: clostridia.
These bacteria are not our friends. Instead of fermenting carbohydrates like the well-behaved lactic-acid producers, these clostridial bacteria thrive by fermenting protein. This, of course, reduces the amount of protein in the forage. And as they break down protein, these bacteria produce a number of distinctive aromatic compounds. I say distinctive because here are some names: cadaverine and putrescine.
The term “stench” would be an understatement. Phase V indicates a poor silage – an unpalatable forage with significantly reduced nutritional value.
But these phases (I, II, III, IV and sometimes V) pretty much sum up the process of ensiling forage. Prevent the growth of molds and bacteria by making the container airtight, and reduce the pH to create an inhospitable, stable environment.
Which opens up lots of possible variations to exclude oxygen and drop the pH. A silo can be any airtight container, like the classic, tall Midwestern barn silo, which takes advantage of the forage weight to force out the air. Or those big blue metal silos, which can be made more airtight than traditional silos.
Or we can chop the forage into small pieces and stack it in a three-sided concrete bunker (a bunker silo) or a three-sided open trench (a trench silo). Then we run tractors back and forth over it to press it down and then cover it with plastic and old tires. Or we can blow the forage pieces into a long white plastic sleeve like a monstrous sausage.
Or we can cut the hay, let it wilt slightly and make large bales containing 50 percent moisture – far wetter than anyone would ever put into a barn – and wrap those bales in layers of white plastic, mummy-like. This is baleage – a good technique, particularly if we keep a supply of sturdy repair tape for holes in the plastic caused by moles and rocks and falling meteors.
We can also manipulate the silage pH. We can make sure the forage contains lots of rapidly fermentable carbohydrates for the bacteria. We can include extra grain or dried whey or anything else with sugars and starch. We can also add commercial additives during loading to accelerate the fermentation process, like lots of extra bacteria, particularly lactobacillus.
Or enzymes to break down the large starch molecules into smaller soluble sugars these bacteria prefer. Or mold inhibitors to prevent, well, mold, in case a dry forage doesn’t pack well or contains extra air pockets.
But here’s another idea: If we want to make forage more acidic, why not just add acid? Like strong acid. Well, in the 1920s, someone developed a system to do just that. A Finnish scientist named Artturi Virtanen devised a way to drop the forage pH to under 4.0 very quickly and thus completely avoid the fermentation process and its associated nutrient losses.
He simply sprinkled mixtures of hydrochloric acid and sulfuric acid directly onto the forage as it was put into the silo. The system genuinely worked, and for a time it was used on farms throughout Scandinavia and elsewhere.
It was, however, rather hard on equipment – and I’m sure farm crews were not thrilled about handling jars of concentrated acid – but nonetheless, in its day, this method was widely known. I suspect, however, in today’s world of seatbelts and roll bars, pouring concentrated acids into open silos would not be favored by most regulations.
Oh yes, Dr. Virtanen’s middle name was Ilmari (Dr. Artturi Ilmari Virtanen) and he patented his process as the “A.I.V. Method” for making silage. You might not want to try it at home, but you can safely look it up on Google. 
PHOTO: The silage pH is now so low even the lactic acid-producing bacteria cannot survive. In effect, they produced so much acid they pickled themselves. But here’s the bottom line: Nothing else can live in that environment. We have reached our goal. The forage is no longer losing nutrients to fermentation. The forage is stable and safe. Staff photo.
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.
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Woody Lane
- Lane Livestock Services
- Roseburg, Oregon
