Microbiological Status of Grass Silages in Relation to Agricultural Practices: A Four-Year Study

The microbiological quality of silage, a fundamental component of cattle feed, is critical to both animal health and human safety. This long-term study evaluated the microbiological status of grass silages collected from Polish cattle farms, aiming to assess the prevalence of zoonotic agents and hygiene indicators in relation to pre-ensiling and ensiling agricultural practices. A total of 160 grass silage samples were randomly collected nationwide by veterinary officers across various municipalities. The researchers measured pH values and conducted analyses to determine the presence and loads of pathogens, including Listeria spp., Clostridium spp., Salmonella spp., Escherichia coli, and Bacillus cereus. Only 17.5% of grass silages fell within the optimal target pH range. Conversely, 80.6% exceeded the upper reference threshold, indicating widespread poor fermentation and low silage quality. Listeria species were detected in 5.6% of the forages, occurring exclusively in silages with an incorrect pH. Clostridium spp. were highly prevalent, testing positive in 82.5% of all samples, while C. perfringens (predominantly toxotype A) was identified in 26.2%. Both Salmonella spp. and botulinum neurotoxin-producing Clostridia were detected in separate, single samples, both characterized by high pH levels. Additionally, fungal counts reached high, non-feedable levels exceeding 6 log₁₀ cfu/g in 10.6% of the tested silages. Silage inoculation, practiced by 26.2% of farmers, significantly lowered pH values and reduced the counts of spoilage microorganisms. Additionally, inoculated silages had a more than three-fold greater likelihood of achieving the correct pH. In contrast, organic fertilization was associated with an increase in the mean counts of Clostridium spp. and B. cereus. In conclusion, grass silage is highly vulnerable to poor fermentation and microbial contamination.