Viable Weed Seed Transfer in Feedlot Manure

Summary

This project examined feedlot manure as a possible source of viable weed seed transfer to cultivation land. The project consisted of winter and summer phases utilising manure with various background treatments in weed seed germination observations in glasshouse pots, field plots and in nylon bags within manure stockpiles. Results show that viable weed seeds of non-resident species were introduced to the observation site in manure from all feedlots. Seed viability was destroyed when it was left in porous nylon bags placed in small observation stockpiles of manure. In the winter glasshouse pot observations, germinations were observed in only one of six loose stockpile replicates. No germinations were seen in other treatments. In the summer phase of the glasshouse pot observation, germinations were observed in some replicates of the loose and composted piles at all three feedlots but not in the grain co-product or compacted treatments. Germinations were recorded in all manure treatments in the winter and summer field plot observations.

When control plot weed species were excluded, all bar the compacted manure pile treatment recorded some germinations in winter and all bar the grain co-product treatment recorded germinations in summer. Temperatures in most of the background treatment manure piles exceeded those required for pathogen removal. However, the composting process was not considered complete in most treatments. Fusarium spores were not detected in any tests, which included freshly harvested pen manure and post storage summer treatments. Irrespective of manure background treatment and stockpile temperatures recorded in this observation, viable weed seeds were introduced to cultivation land in feedlot manure. Incomplete heating of all manure in the storage, inadequate temperatures and re-contamination of manure stockpiles or windrows are thought to contribute to viable weed seed transfer. Lower weed counts are expected in commercial operations as in small observation piles thorough heating is less assured and there is more surface area to re-contaminate.

Some of the analytical data collected and information from previously reviewed unpublished observations relates to feedlot manure nutrient availability. Laboratory analyses of manure, from all treatments in winter and summer showed 60% of total phosphorus was present as Colwell phosphorus. A review of five field trials showed 72% of total manure phosphorus was either removed by the harvested crop or remained in the top 10 cm of soil. There was some variation in duplicated laboratory tests. Most but not all of the difference was due to test methodology. This is an important point for licensing authorities, when specific laboratory methodology is not specified.