Colony-forming units key to quality silage

Making sure forage inoculants contain enough CFUs of the right strains is critical to producing quality silage.

When producers use a research-proven forage inoculant, they are loading up their silage with billions of scientifically selected bacteria.

"We are looking at a war at a microscopic level between 'good' bacteria that can ensure maximum quality silage and 'bad' bacteria that cause nutrient and energy losses and may cause severe spoilage," said Dr. Bob Charley, forage products manager at Lallemand Animal Nutrition. "To make sure we have enough 'good' bacteria to prevail, we need to look at the colony-forming units (CFUs) on the inoculant label."

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/g of fresh forage.

CFUs are determined by making a microbial suspension, then making a series of 10-fold dilutions of this suspension and spreading a small amount of each dilution on petri dishes containing a layer of nutrient agar. These are then incubated for 24-48 hours at a temperature ideal for the microbe(s) under test. After incubation, the agar plates are covered in individual microbial colonies, which are counted.

While 100,000 CFUs of selected bacteria per gram of fresh forage is the accepted minimum standard for driving a rapid pH drop, in order to reliably produce silage with good aerobic stability, independent trials have shown that inoculation with a significantly higher dose rate of Lactobacillus buchneri 40788 is the most effective option, Charley said. The Food & Drug Administration has reviewed the application of L. buchneri 40788 at a rate of 400,000 CFUs/g of forage (600,000 CFUs for high-moisture corn) and has allowed the claim of improved aerobic stability of silages and high-moisture corn.

The number of CFUs should be clearly listed on the silage inoculant product label.

In addition to ensuring a minimum level of CFUs, producers should always ask to see independent data to support claims made for any inoculant. Charley recommended that producers check what level of the product was used in these studies and ensure it matches what they're being sold.

"CFUs alone will not guarantee the desired results," he said. "Be sure to look for specific strains that are proven in independent research to meet specific silage goals. Then, make sure you apply inoculants at the correct viable level with proper handling practices to help ensure your inoculant works as expected — and results in stable, high-quality silage."

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