Leverage the fermentation process for feed

By Michelle Chang-Der Bedrosian, Ph.D., and Owen Mickley, D.V.M.

An efficient upfront fermentation is important to the nutrient profile of an ensiled crop.  When forages are ensiled, bacteria ferment water-soluble carbohydrates into lactic acid.  This lowers the pH of the forage mass, thus preventing the growth of undesirable microorganisms during storage.  These lactic acid-producing bacteria originate from the field and also may be applied via a microbial inoculant.  A more efficient fermentation will retain more digestible nutrients for use in the rumen.

Active fermentation ceases after two to six weeks of ensiling if air does not penetrate the silage during that time.  The length of this fermentation process will vary depending on various conditions such as available sugars at ensiling, dry matter of the crop, or the nature of the crop itself.

Once the crop has fermented, it enters the stable phase of ensiling.  It’s historically been assumed the nutritional composition of the silage does not change after the initial fermentation.  However, recent research suggests this isn’t true.  The fermentation acid profile and the starch, protein, and fiber fractions change as time passes.

The biggest change that occurs in the silage mass during storage is in the starch fraction of the plant, which is more pertinent in corn silages than alfalfa silages.  As corn silage sits in storage, enzymes from epiphytic bacteria and from the plant itself digest the waterproof prolamin matrix that surrounds the corn starch, liberating it for digestion in the rumen.

Additionally, the corn silage processing score (CSPS) improves with storage time.  A general rule of thumb from Luiz Ferraretto, Ph.D., University of Wisconsin-Madison, is that CSPS increases 5 to 10 percentage points during storage.

Overall, the total crude protein content remains constant throughout ensiling, but the fraction made of amino acids (true protein) is degraded.  In corn silage, as enzymes digest the prolamin protein matrix, the concentration of soluble protein climbs.

The fiber content of silages typically remains constant during storage.  A decline in neutral detergent fiber digestibility (NDFD) is associated with the initial pH drop of silages due to solubilization of the hemicellulose fraction of the fiber.  However, once this is done, NDFD and concentrations of NDF and acid detergent fiber (ADF) remain steady during ensiling.

Parlay the process

The price of starch has inflated in the last few years and may continue to increase.  Planning for longer carryover of corn silage inventory — by increasing pad space, renting more acres for crop growth, or purchasing crops off the field — can pay for itself in lower purchased feed costs.

Periodic testing for fecal starch content can help farmers prevent losses due to poor utilization by the cow.  Fecal starch scores above 3% have been correlated with reduced pH in the colon (signaling hindgut acidosis) and are indicative of poor fermentation time, poor processing, or a combination of both factors.  One study by the University of Pennsylvania found that each percentage point increase in fecal starch was equivalent to 0.72 pounds of lost milk production.

When we use more homegrown starch and minimize fecal starch content, we maximize nutrient utilization per acre.  This is why we recommend corn silage carryover of at least three to four months.

What about long storage?

Many people ask if silages have an “expiration date” or if they can be stored for too long.  Two- and three-year old silages have been fed to cows with success.  Shrink is the biggest challenge.  If the integrity of the forage mass can be maintained, then additional storage time carries minimal risk.

A corn silage’s starch availability obviously would need to be monitored to mitigate acidosis concerns.  The volatile fatty acid (VFA) profile also would shift toward favoring acetic acid.  But, if the silage is ensiled at the proper dry matter content, treated with a good inoculant to prevent spoilage, and stored under quality oxygen-barrier plastic that does not have holes, it can last for a long time.  This strategy can be used in the battle against escalating starch prices.

This article was originally written for the September 2023 issue of Hoard’s Dairyman.