Contamination of hay and haylage with enteric bacteria and selected antibiotic resistance genes following fertilization with dairy manure or biosolids.

The present study evaluated if enteric bacteria or antibiotic resistance genes carried in fecal amendments contaminate the hay at harvest, representing a potential route of exposure to ruminants that consume the hay. In the field experiments, dairy manure was applied to a hay field for three successive growing seasons, and biosolids were applied to a hay field for one growing season. Various enteric bacteria in the amendments were enumerated by viable plate count, and selected gene targets were quantified by qPCR. Key findings include the following: at harvest, hay receiving dairy manure or biosolids did not carry more viable enteric bacteria than hay from unamended control plots. The fermentation of hay did not result in a detectable increase in viable enteric bacteria. The application of dairy manure or biosolids resulted in a few gene targets being more abundant in hay during the first harvest. Fermentation of hay resulted in an increase in the abundance of gene targets, but this occurred with hay from both the amended and control plots. Overall, the application of fecal amendments resulted in an increase in the abundance of some gene targets associated with antibiotic resistance in the first cut hay.