Q. What is a colony forming unit and how many should a forage inoculant contain?
A. CFUs are determined by taking a microbial suspension, making a series of 10-fold dilutions of this suspension and spreading a small amount of each dilution on petri dishes with a nutrient agar (“jelly”). These samples are then incubated for 24 to 48 hours at a temperature ideal for the microbe(s) under test (generally around body temperature). After incubation, the agar plates are covered in spots, which are microbial colonies. Since we do not know if the colony was formed by one single organism or a cluster of bacteria, the spots are referred to as “colony-forming units.”
Using the right amount of inoculant bacteria can help drive a rapid, efficient ensiling fermentation. It is generally accepted that fermentation aids — which are designed to dominate the initial fermentation and increase the speed of pH drop — should be applied at a minimum of 100,000 CFUs per gram of fresh forage. The number of CFUs should be clearly listed on the silage inoculant product label, though this may only be given on a per gram of product basis, requiring some calculation to determine the rate that will be applied to the crop.
To reliably produce a silage with good aerobic stability, inoculation with a significantly higher dose rate of Lactobacillus buchneri 40788 has been proven in independent trials to be the most 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 of silages and HMC.
In addition to ensuring a product contains the target level of CFUs, producers should always ask to see independent data to support claims made for any inoculant. It’s important to check the level the product was used at in these studies and ensure it matches what is being sold.
Then, be sure to apply inoculants at the correct viable level using proper handling practices. These steps will make sure your inoculant works as expected — and result in stable, high quality silage.