Field and greenhouse studies were conducted in Belle Glade, FL, in 2010 and 2011 to evaluate saflufenacil and glyphosate efficacy on POST burndown of ragweed parthenium. Log-logistic models were used to determine the herbicide dose required to produce 90% control (ED90). The ED90 for rosette ragweed parthenium control in the greenhouse was saflufenacil at 8.0 g ai ha−1 at 14 d after treatment. The rate required to cause 90% growth reduction of rosette ragweed parthenium at 14 d after treatment was 8.9 g ha−1 of saflufenacil. The probability of rosette ragweed parthenium survival decreased with increasing rates of saflufenacil. The ED90 value for bolted ragweed parthenium control in the field was 5.7 g ha−1 of saflufenacil at 21 d after treatment. Ragweed parthenium had no response to glyphosate either in the field or in the greenhouse studies. This demonstrates that saflufenacil can be used as a POST burndown of ragweed parthenium populations that have no response to glyphosate.

Growers have observed lack of control of ragweed parthenium with glyphosate at 0.84 kg ae ha−1 used for weed control in noncrop areas and fallow fields in the Everglades Agricultural Area (EAA) of South Florida. Therefore, studies were conducted to (1) confirm and characterize the level of glyphosate resistance in ragweed parthenium from the EAA, (2) determine if reduced absorption or translocation is the mechanism of resistance of ragweed parthenium to glyphosate, and (3) evaluate the efficacy of POST herbicides commonly used in cultivated and noncrop areas for broadleaf weed control on flowering ragweed parthenium at the full and half labeled rate. A dose-response bioassay was used to determine the response of the rosette ragweed parthenium biotype from the EAA (resistant) and a susceptible biotype from Stoneville, MS, to glyphosate. The bioassay showed that the resistant biotype was 40- to 43-fold less sensitive to glyphosate when compared to the susceptible biotype. There was no significant difference in glyphosate absorption or translocation from the treated leaf to the rest of the plant 168 h after treatment between resistant and susceptible biotypes. This shows that absorption or translocation is not a mechanism of glyphosate resistance by ragweed parthenium. Saflufenacil + dimethenamid-P and hexazinone provided rapid and complete control of flowering ragweed parthenium 3 wk after treatment (WAT). Aminocyclopyrachlor + chlorsulfuron and aminopyralid at the full and the half label rates provided 100% control of ragweed parthenium by 9 WAT. Clopyralid, 2,4-D amine, and glufosinate at the full label rate provided 89 to 98% control of ragweed parthenium 9 WAT. Control of ragweed parthenium 9 WAT was < 75% with flumioxazin, fomesafen, glyphosate, imazapic, imazethapyr, mesotrione, oxyfluorfen, and paraquat, irrespective of use rate.

Studies under Florida conditions for susceptibility of pasture weed species to the new herbicide, aminocyclopyrachlor (ACP), are essential. Sensitivity of weed species was dependent upon the species and environment. Whitehead broom was not susceptible to ACP. Dogfennel control was consistently above 85% 1 yr after treatment with ACP at 140 g ha−1 and all ACP premixes. Milkpea was initially sensitive to all ACP treatments, but regrowth was evident in all treatments after 1 yr at one location. All ACP treatments resulted in 100% control of tropical soda apple 1 yr after treatment at one location, but less than 50% control at a second location. Herbicide applications with ACP, when labeled, will likely provide good to excellent control (80 to 100%) of several weed species.