Fungi and mycotoxins in South African forage crops and silage

Several countries have enacted regulations on tolerance limits for common mycotoxins because of the hazardous nature and widespread occurrence of these fungal secondary metabolites in agricultural commodities. Screening of agricultural commodities destined for animal consumption for the presence of mycotoxins is now becoming a prerequisite in several countries as a means of minimizing ingestion of these toxins. Silage samples were analyzed for pH, % dry matter (DM) content, and the presence of total fungi, yeasts and the types of lactic acid bacteria present. The samples were also analyzed for mycotoxins that have been reported to commonly occur in silage. The pH of the samples was found to be acidic ranging from pH 3.4 to 4.7, with few samples having pH values above 6. There was a significant difference in the % DM content amongst the sampling regions. There was no significant difference in the extent of fungal contamination amongst the different regions. Aspergillus fumigatus was the predominant species from all the samples. Most of the yeast species were isolated from the Bergville region. The yeast species isolated from all samples were Trichosporon, Cryptococcus and Candida species, which are all regarded as nonlactate fermenters. Lactobacillus plantarum and Lactobacillus buchneri were the only two lactic acid producers isolated from the samples. Aflatoxins, citrinin and patulin were the most predominant toxins in the samples. Ochratoxin A and deoxynivalenol was not detected in all samples using thin layer chromatography, while the latter two toxins were only detected in two samples using VICAM fluorometry. The level of fumonisins that was found in the forage crops used for silage production was fairly low with the highest level being 9.36 ppb. Most of the mycotoxin extracts were found to reduce the % cell viability of human lymphocytes after 18 hours of incubation as determined by the MTT assay.Professor Mike Dutton Mr. F. E. Van Zy