Synthetic auxin herbicides are widely utilized in turfgrass settings for selective broadleaf weed control. Aminocyclopyrachlor (AMCP) is a newly registered pyrimidine carboxylic acid with similar chemical mode-of-action and structure to clopyralid. Off-target injury has been documented following exposure to composted turfgrass clippings previously treated with clopyralid.  To address this issue, the current AMCP label requires all turfgrass clippings be returned indefinitely following an application. However, clipping displacement can occur from mowing equipment, heavy rain or irrigation, wind, and/or other influences, resulting in herbicidal residues being deposited into sensitive aquatic or riparian communities. Research was conducted to determine the response of alligatorweed [Alternanthera philoxeroides (Mart.) Griseb.] and parrotfeather [Myriophyllum aquaticum (Vell.) Verdc.] to tall fescue [Lolium arundinaceum (Schreb.) Darbysh] clippings previously treated with synthetic auxin herbicides.

Experiments were conducted in 2010 at the Weed Science Annex in Raleigh, NC. Herbicide treatments included AMCP (84 g ae ha^-1) and Confront^® [triclopyr (315 g ae ha^-1) plus clopyralid (105 g ae ha^-1); (TRIC+CLPY)] applied to mature tall fescue.  Clipping collection took place 0, 1, 3, 7, and 14 days after application (DAA). Clippings (25 g) were placed into 15 L growth containers containing alligatorweed and parrotfeather (9.5 L pondwater; pH ≈ 7.8). A nontreated mulch and nontreated control were also included for comparison.  Treatments were arranged in a 4 x 5 factorial (four clipping treatments by five clipping collection timings) in a randomized complete block design with four replications and two experimental runs. Visual plant injury (0 to 100% scale) was recorded weekly and plant shoot length (cm) was recorded 69 days after initiation. Data were subject to ANOVA using MIXED model methodology and means separated using Fisher’s Protected LSD (P<0.05).

All herbicide treatments and clipping collection timings caused significant effects to alligatorweed and parrotfeather growth compared to the nontreated mulch and nontreated control.  In general, visual injury increased and shoot length decreased as clipping collection neared 0 DAA.  Alligatorweed visual injury was greater from AMCP clippings collected 1, 3, 7, and 14 DAA than similar TRIC+CLPY clipping collection timings. Alligatorweed shoot length was reduced more with AMCP than TRIC+CLPY clippings collected 0 and 7 DAA.  Similarly, parrotfeather visual injury and shoot length reduction was greater with AMCP clippings collected 3, 7, and 14 DAA than similar TRIC+CLPY clipping collection times.  Based on these data, turfgrass managers must properly return AMCP and TRIC+CLPY treated turfgrass clippings in a manner which avoids potential off-target plant injury.