The probability of maize biomass contamination with weed seeds

Anaerobic digestion in biogas reactors generates a semi-solid leftover, the digestate, which is frequently used as a crop fertiliser. If weed seeds in the maize biomass that is used for feedstock survive the process of anaerobic digestion, the use of digestate could contribute to the spread of weeds. We investigated here the probability that weeds enter the biogas production chain via maize biomass. Five organic and four conventional maize fields were sampled to determine the identity, density and biomass of the weeds present, and the number and viability of seeds produced. This was done separately for plant parts below and above the anticipated cutting height of the maize crop, close to the anticipated harvest date. Weed density varied from 56 to 568 weeds m⁻² and weed seed production from 886 to 229 256 seeds m⁻². Conventional fields tended to have lower weed biomass, lower seed production, and lower proportion of the seeds above the cutting height of maize, with lower seed viability. The main weeds were Chenopodium album and Echinochloa crus-galli. In general, either weed plants were small and produced few seeds that were largely found below the cutting height of maize, or they were tall and produced numerous seeds that were largely found well above the cutting height. Given the numbers of weed seeds produced per hectare in this study, it is likely that some will survive anaerobic digestion in biogas plants, although in low numbers, as seed survival during anaerobic digestion tends to be low.