Effect of invasive slug populations (Arion vulgaris) on grass silage. II: Microbiological quality and feed safety

Abstract

This study aimed to explore how invasive slug populations of Arion vulgaris influenced the microbiological quality and animal feed safety of grass silage, and the efficiency of silage additives and wilting to control the microbiology of slug contaminated crops. The effect of four slug contamination levels, including control, of a grass crop harvested for silage production, was evaluated in laboratory scale. The crop was wilted to two dry matter (DM) levels: low (253g DM/kg) and high (372g DM/kg). Adult slugs were applied to the low DM crop corresponding to 5 (low level), 10 (medium) and 20 (high level) seven-gram sized Arion vulgaris per m2 in an assumed harvested regrowth yield of 2.5ton DM/ha. For the high DM crop, slug weights corresponding to 6 (low level), 12 (medium) and 24 (high level) slugs per m2 were applied. At low DM level, the effect of four additive treatments; control (C), inoculation with Lactobacillus plantarum (LP), a formic, propionic and benzoic acid mixture (ACID) and a chemical additive containing benzoic acid, NaNO2, hexamethylenetetramine and propionic acid (CHEM) were tested. Slugs, slug feces, grass, soil and silages were analyzed for lactic acid bacteria (LAB), Enterobacteriaceae, Listeria monocytogenes, Clostridium tyrobutyricum, molds and yeasts by cultivation methods and Clostridium botulinum type C by real-time PCR analysis.Increasing slug contamination reduced the microbial quality of silages by increasing C. tyrobutyricum levels at both silage DM levels. Only silages without slugs and silages treated with the nitrite containing additive CHEM had non-detectable mean levels of C. tyrobutyricum. Increasing slug contamination increased LAB enumerations in silages. No microbes of risk to human or animal health were detected in anaerobic silages even at the highest slug contamination.