Potato is the third most important staple food crop globally following rice and wheat. In the United States, potato is grown on approximately 410,000 ha with a farm-gate value of US$1,032 million. In Canada, potato is grown on approximately 134,000 ha with a farm-gate value of US$235 million. The objective of this manuscript, compiled by the Weed Science Society of America Weed Loss Committee, was to estimate potato yield loss caused by weed interference. Potato yield data from weedy and weed-free plots (or plots with >95% weed control) was obtained from researchers working on weed management in potato in the United States and Canada or from published manuscripts from 2000 to 2018. Potato yield loss from weed interference was 12% to 61% when no weed management tactics were implemented. The average yield loss for all states/provinces (where data was obtained) due to weed interference was 44%. Weed interference would cause a farm-gate loss of approximately US$465 million and US$61 in the United States and Canada, respectively, if weeds are not controlled. These results indicate that weed management is critical for successful potato production, and that an ongoing need for research exists on weed management in this crop.

Quackgrass is a problematic agricultural weed in the temperate zones of the world and is difficult to control without herbicides or intensive tillage. However, it may be possible to control quackgrass with less environmental impact by combining multiple low-intensity control methods. A pot experiment was conducted in July to October 2012 and repeated in June to September 2013 to investigate the effect of rhizome fragmentation, competition from white clover, shoot-cutting frequency, and cutting height on quackgrass. Rhizome fragmentation was expected to result in more, but weaker, quackgrass shoots that would be more vulnerable to shoot cutting and competition. However, by 20 d past planting, rhizome fragmentation did not change the total number of quackgrass shoots per pot, because an increase in main shoots was offset by a decrease in tiller numbers. Rhizome fragmentation did not reduce quackgrass biomass acquisition during the experimental period. Although rhizome fragmentation did reduce total fructan content, it did not enhance the effect of clover competition, shoot-cutting frequency, or shoot-cutting height. Clover competition by itself reduced quackgrass shoot numbers by 72%, rhizome biomass by 81%, and belowground fructan concentration by 10 percentage points, compared with no competition. The more frequently quackgrass shoots were cut, the less biomass quackgrass acquired, and a high shoot-cutting frequency (each time quackgrass reached 2 leaves) resulted in a lower belowground fructan concentration than a low shoot-cutting frequency (at 8 leaves). However, in pots without competition, a higher shoot-cutting frequency resulted in more quackgrass shoots. A lower shoot-cutting height (25 mm) had more impact when shoot cutting was more frequent. In conclusion, rhizome fragmentation did not reduce the number of quackgrass shoots or rhizome biomass, but competition from white clover, a high shoot-cutting frequency, and a low shoot-cutting height strongly suppressed quackgrass biomass and fructan acquisition.

The influence of adjuvants, inorganic salts, and herbicide formulations on the antagonistic effect of Na-bentazon on sethoxydim absorption was studied in quackgrass. Adjuvant BCH 815 00, at 2.4 L ha−1 alone or at 1.2 L ha−1 combined with 1.1 kg ha−1 of ammonium sulfate, added to the sethoxydim and Na-bentazon spray mixture overcame the antagonism on 14C-sethoxydim absorption in quackgrass. Crop oil concentrate at rates up to 9.6 L ha−1 did not overcome the antagonism. Adding ammonium phosphate or ammonium nitrate to the spray solution was as effective as adding ammonium sulfate in overcoming the antagonism caused by reduced absorption of sethoxydim even from the combination of Na-bentazon plus Na-acifluorfen. Ammonium salts and NH4-bentazon overcame the antagonism of Na-bentazon by forming readily absorbed NH4-sethoxydim.

Absorption and translocation of 14C-fluazifop-butyl {butyl ester of (±)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl] oxy] phenoxy] propanoate} was compared in susceptible quackgrass [Agropyron repens (L.) Beauv. ♯3 AGRRE] and tolerant soybean [Glycine max (L.) Merr.]. Foliar absorption was more rapid in soybeans with 75% of the recovered 14C absorbed 6 h after treatment compared to 36% in quackgrass. Translocation of the radiolabel occurred in both soybean and quackgrass with no significant difference between the species. Differential absorption and translocation did not contribute to the selectivity of fluazifop-butyl. Herbicidal activity was lower on quackgrass plants at the five- to six-leaf stage when compared to the two- to three-leaf stage. Greater quackgrass control was observed at 30 than at 20 C. Foliar absorption of 14C-fluazifop-butyl was significantly greater at 30 than at 20 C. Translocation of the radiolabel was greater in plants exposed to full light as compared to shade. Moisture stress significantly reduced quackgrass control with fluazifop-butyl.

Surgical experiments were conducted on cultured five-node apical rhizome segments of quackgrass. Removal of scale leaves promoted an initial burst of growth within the axillary buds but did not support the continued growth of buds as effectively as removal of the rhizome apex. Replacement of detached scale leaves over denuded buds temporarily repressed the promotive effect of scale leaf removal. Aqueous extracts of scale leaf material inhibited apical growth in rhizome segments but did not inhibit bud growth. Anatomical sections revealed that removal of scale leaves promoted development of buds: cells enlarged, vascular tissues differentiated, and new nodes began to form within 4 days of the removal of scale leaves. It is suggested that scale leaves contribute to apical dominance by inhibiting the initial development of axillary buds.