Contributed by Lynn Sosnoskie – Horticulture Section, School of Integrative Plant Science, Cornell University
Weeds can interfere with crop growth and development, directly, via competition for water, nutrients, and light. Indirectly, weeds can physically interfere with crop production operations, including harvest. Weeds that escape in-season control often produce significant quantities of seed, which are threats to future yields. While most seed will be deposited in the same field in which it was produced, some may be transported between sites on combines. Throughout the US, university personnel have reported harvesters as being important mechanisms of seed dispersal for some economically important weed species, such as herbicide resistant biotypes of Palmer amaranth (Amaranthus palmeri) and waterhemp (Amaranthus tuberculatus).
While it may not always be feasible to thoroughly clean equipment between every field, removing as much plant debris as possible before transferring combines between sites is a valuable strategy for controlling weed seed spread. Newly purchased, but previously owned harvesters should be inspected thoroughly to prevent new weed species or weedy biotypes from being introduced. Here in NY, seeds recovered from a combine recently acquired from an out-of-state grower were identified as waterhemp. Results from subsequent herbicide resistance screening efforts found that the developing seedlings could not be controlled by glyphosate (WSSA Group 9) and some of the ALS-inhibiting (WSSA Group 2) herbicides, with possible resistance to at least two additional chemical classes. Ultimately, the careful examination of this harvester may have prevented the establishment of a difficult-to-control weed with a novel resistance profile.
Before engaging in combine clean-out efforts, make sure all personnel are equipped with PPE such as safety glasses, gloves, dust masks, and ear protection and that everyone involved is familiar with the safe operation of any equipment. Although the combine head and feeder house are likely areas for weed residue to accumulate, tissue and seeds can accrue in all parts of the harvester including the rotor, rock trap, grain tank, and unloading auger. Be strategic with site selection when conducting end-of-season equipment clean-outs; choose a location where dislodged or removed seed cannot be easily blown, picked up by footwear, washed away or otherwise transported back to fields. Vacuuming up all residue and disposing of it securely will help prevent unintentional seed return. The removal of debris may have additional economic benefits (beyond its impacts on weed control efforts) if it prevents unnecessary wear-and-tear and helps to preserve equipment functionality over time.
While harvester clean-out is important for limiting weed seed movement, other strategies can also minimize weed dispersal. This includes avoiding overly dense patches of weeds in a field (especially if you suspect herbicide resistance) and arranging harvest operations to ensure that the weediest fields are harvested last. Remember that unwanted seed can also be picked up and spread on tires and on tillage and planting equipment. Remove clumped soil from implements and tractors to avoid spreading weed seeds, as well as devastating soil-borne pathogens.
See: the following links for more details: end-of-season-combine-clean-out-fact-sheet.pdf (ndsu.edu) and Weed Seed Movement via Combines.pdf (wiscweeds.info) and Weed Seed Management at Crop Harvest.pdf (wiscweeds.info).
