Colony Forming Units (CFUs): Counting the Microbial Army

During ensiling there is an unseen battle between armies of microbes. High-quality forage inoculants supply armies of billions of elite, scientifically selected beneficial bacteria to help win this battle in your silage crops.

Using the right amount of bacteria can drive a rapid, efficient ensiling fermentation. These "fermentation aids" speed the pH drop and are applied at a minimum of 100,000 CFUs per gram of forage. But what is a "CFU"?

Microorganisms are very small and difficult to count directly. Instead, we use a laboratory technique to count CFUs. This involves making dilutions of a microbial suspension and spreading them onto thin layers of jelly (agar) containing growth nutrients in petri dishes (referred to as "agar plates"). After incubation in ideal conditions, some plates are completely covered with microbial growth; some have little to nothing growing; and some are covered with isolated individual "spots," which are microbial colonies. Since a colony may be formed by one single organism or a cluster of microbes, the spots represent "colony-forming units" (CFU). Count the number of colonies, multiply by the dilution and you have the "plate count" in CFU.

100,000 CFUs per gram of forage is the standard for driving the pH drop. Yet, for aerobic stability, a high dose rate of Lactobacillus buchneri 40788 has been proven in independent trials to be the most consistently effective option. L. buchneri 40788 applied at a rate of 400,000 CFU per gram of forage (600,000 for HMC) has been reviewed by the FDA and allowed to claim improved aerobic stability.

The number of CFUs per gram of product should be clearly stated on the product label.

In addition to checking the level of CFUs, producers should ask to see independent data to support claims made for any inoculant product. It's important to check that both the level and the strains of bacteria used in these studies match what is being sold. Strains are usually identified by a unique number (like L. buchneri 40788). Different bacterial strains are unique and will give different results.

Then, be sure to apply inoculants at the correct rate using proper handling practices, especially keeping the product cool during application. These steps will help ensure your inoculant investment pays off with stable, high quality silage.

For additional tips on producing stable, high-quality silage, visit www.qualitysilage.com or Ask the Silage Dr. on Twitter or Facebook.