Pesticide Persistence in Turfgrass Clippings
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Turfgrass clippings are collected during mowing and may be composted or used as garden mulch, where they may provide cultural weed control and essential plant nutrients as they decompose. Unfortunately, if the area has been treated with pesticides, they may release as well as the clippings decompose. Previous research has shown clopyralid, a broadleaf herbicide, can cycle within a turfgrass system and be detected in clippings for over a year after application. Further, certain pesticides can release from clippings at levels that are injurious to aquatic and terrestrial organisms. While these are relatively unique occurrences, it is fair to say foliar-applied pesticides used in turfgrass may be detected in turfgrass clippings in the days and weeks after application due to foliar spray interception in established turfgrasses coupled with pesticide properties. For this reason, our research program at NC State University has conducted numerous research trials investigating persistence of common pesticides in established turfgrass species throughout the mid-Atlantic United States.
Research trials were initiated at the Lake Wheeler Turfgrass Field Laboratory (Raleigh, NC) in 2013 and 2014 to quantify persistence of 2,4-D, a common broadleaf herbicide, and azoxystrobin, a broad spectrum fungicide (Heritage®), in clippings collected from hybrid bermudagrass, tall fescue or zoysiagrass maintained in accordance with home lawn recommendations. In short, pesticides were applied at their currently registered single maximum application rate and clippings were collected 32, 16, 8, 4, 2, 1 or 0 days after treatment (DAT) from unique plots. Pesticide residues were quantified in clipping vegetation via laboratory analysis. An additional trial was also completed to measure pesticide release from clippings into water, where clippings collected from field plots were applied to the surface of containers filled with pond water and water samples were taken over time to quantify 2,4-D or azoxystrobin residue concentrations.
Results from field trials suggest pesticide persistence varied across turfgrass species from clippings collected 0 to 2 DAT, with a greater proportion of the applied pesticide in hybrid bermudagrass compared to tall fescue or zoysiagrass. This may be due in part to a greater proportion of the applied pesticide being retained in the upper portion of the hybrid bermudagrass canopy, which is removed during a mowing event, than tall fescue or zoysiagrass. Across turfgrasses from 0 to 2 DAT clipping collections, 17 to 42 and 12 to 30% of the applied 2,4-D and azoxystrobin, respectively, were removed via clipping collection. These pesticide residues is of concern both from ecological and pest control perspectives. Overall, from clipping collections 4 to 32 DAT 2,4-D and azoxystrobin persisted similarly across turfgrasses, with detection of both pesticides at 32 DAT. This suggests clippings should be managed and properly handled following pesticide applications to prevent off-target injury. This is highlighted, as most pesticide labels currently do not provide information regarding mowing scheduling or clipping management greater than one week prior to, or following application.
Results from greenhouse trials suggest pesticides release similarly across turfgrass species; however, a greater proportion of the 2,4-D released from clippings than azoxystrobin. Water samples collected 2 days after clippings were applied determined 29 to 60 % of 2,4-D originally in/on turfgrass clippings, released into water; whereas only 2 to 15 % of azoxystrobin released.
In conclusion, pesticides can persist in turfgrass clippings after application. Further, appreciable amounts of pesticide within clippings can release as they decompose, which may cause off-target injury. However, with the implementation of sound turfgrass management practices, potential off-target pesticide movement via clipping collection can be mitigated. Practices specific to mowing include scheduling pesticide applications such that the first mowing event is at least two days following application, and subsequent mowing events are routinely performed with clippings returned to the canopy. Pest management practices include using granular products when possible (examples: for soil-dwelling insects or root-sorbed herbicides), coordinating foliar-applications with irrigation or light rainfall to wash off the foliage (while balancing time required on the foliage for effective control) and selecting pesticides with properties that do not favor accumulation in clippings. Finally, create buffers or zones near ecologically sensitive areas that call for additional measures regarding pesticide applications and mowing practices.
This research is funded by the The Centere for Turfgrass Environmental Research and Education at NC State University.