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Robinson, D. E., McNaughton, K. E. and Bilyea, D. 2013. Comparison of sequential preemergence-postemergence and postemergence-alone weed management strategies during critical period of red beet ( Beta vulgaris L.). Can. J. Plant Sci. 93: 863–870. Weed management during the first 4 wk after red beet emergence is critical to prevent yield loss. The purpose of this research was to compare weed control and red beet tolerance of postemergence (POST) treatments alone (full, split- and micro-rates of pyrazon plus triflusulfuron) with sequential preemergence (PRE) applications of s-metolachlor followed by these POST herbicide treatments. The experiment was carried out for 4 yr as a randomized complete block with four replications. Of the POST alone treatments examined, control of velvetleaf (Abutilon theophrasti Medik.), redroot pigweed (Amaranthus retroflexus L.), common lambsquarters (Chenopodium album L.) and green foxtail [Setaria viridis L. (Beauv.)] was 98, 86, 97 and 62%, respectively. Much more consistent control of all species (i.e., greater than 96%) was measured where the sequential treatment of s-metolachlor was applied PRE followed by POST split- or micro-rates in all study years. Red beet density and soluble solid levels were not different than the weed-free check in any of the POST or sequential treatments. However, red beet yield in the POST single and split-rate treatments alone ranged from 9500 to 12100 kg ha−1, and was significantly less than in the weed-free check (19400 kg ha−1). Yields in the sequential treatments were comparable to the weed-free check. The results of this study confirm that a sequential treatment of PRE s-metolachlor followed by POST split- and micro-rate applications of pyrazon plus triflusulfuron can be safely applied to red beet and control annual weed species.

Developmental rates of soybean [Glycine max (L.) Merr. 'Braxton'], johnsongrass [Sorghum halepense (L.) Pers.], quackgrass [Elytrigia repens (L.) Nevski], redroot pigweed (Amaranthus retroflexus L.), sicklepod (Cassia obtusifolia L.), and velvetleaf (Abutilon theophrasti Medic.) were compared among plants grown in all combinations of two temperature levels (avg. day/night of 26/19 °C and 30/23 °C) and two CO2 levels (350 and 700 ppm). Neither temperature nor CO2 affected johnsongrass tillering rate, but plants began tillering earlier at higher temperatures. Adverse effects of higher temperatures on quackgrass development were alleviated by elevated CO2 conditions. Plastochron rate was higher at higher temperatures in all dicot species (soybean, redroot pigweed, sicklepod, and velvetleaf), and was higher at elevated CO2 in all dicots except velvetleaf. Calculating plastochron rates on a degree day basis removed differences between temperature treatments, but did not affect responses to CO2. Responses of dicot branch and branch leaf production to treatments varied among species. Branch production per day increased with higher temperatures in redroot pigweed, decreased with higher temperatures in sicklepod, and was unaffected by temperature in soybean. The relationship between main axis and branch developmental rates was altered by temperature in soybean, and by both temperature and CO2 in sicklepod, but was unaffected by either treatment in redroot pigweed. These results indicate that developmental responses to temperature and CO2 depend on both the species and the aspect of development being considered. Key words: Plastochron index, CO2 by temperature interaction, johnsongrass, quackgrass, redroot pigweed, sicklepod, velvetleaf

Rising atmospheric CO2 levels could affect plant growth both directly, through effects on physiology, and indirectly, through the effects of possible CO2-induced temperature increases. In this study we examined the interacting effects of CO2 enrichment and temperature on the growth and allocation of soybean and five weeds. Individual plants of soybean [Glycine max (L.) Merr. ’Braxton’], johnsongrass [Sorghum halepense (L.) Pers.], quackgrass [Elytrigia repens (L.) Nevski], redroot pigweed (Amaranthus retroflexus L.), sicklepod (Cassia obtusifolia L.), and velvetleaf (Abutilon theophrasti Medic.) were grown in growth chambers in all combinations of two temperatures (avg. day/night of 26/19 °C and 30/23 °C) and two CO2 concentrations (350 and 700 ppm) for 35 d. Leaf area and plant biomass were greater at higher temperatures, regardless of CO2 level, in all species except quackgrass. Quackgrass (C3) produced its greatest leaf area and biomass at elevated CO2, whereas johnsongrass (C4) showed little response. Redroot pigweed (C4) and the C3 dicotyledenous species (soybean, sicklepod, velvetleaf) produced their greatest biomass at high CO2, though effects on leaf area were less consistent or absent. In general, when significant CO2 by temperature interactions were found, CO2 responses were smallest at higher temperatures. These differential responses to elevated CO2 concentrations may cause changes in the relative importance of competitive pressure from these weeds. Key words: CO2 by temperature interaction, soybean, johnsongrass, quackgrass, redroot pigweed, sicklepod, velvetleaf

Field and laboratory studies have demonstrated that bentazon (3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide) can effectively control velvetleaf (Abutilon theophrasti Medic) before the weed emerges from the soil. Laboratory experiments showed that the effectiveness of the bentazon depended upon its concentration in the soil and on the weed seeds being in or above the soil layer containing the herbicide. Experiments in the field relating velvetleaf control to bentazon residues in soil showed that velvetleaf can be controlled in southwestern Ontario when bentazon residues in the surface layers of soil exceed 0.2 kg/ha. Bentazon was rapidly degraded in two soils in southwestern Ontario with residues of less than 0.02 kg/ha 40 days after application. Degradation was closely approximated by first order rate law. Thus, an application of 1.0 kg/ha of bentazon should have residual preemergence activity against velvetleaf for 3–12 days after application.Key words: Degradation, residues, herbicide

Abutilon theophrasti Medic. (velvetleaf) forms extensive weed infestations in all major maize and soybean growing areas of Ontario and Quebec. A review of the literature on the biology of the species is presented. Velvetleaf causes crop losses through competition, allelopathic effects and by hosting insect pests and pathogens of crops. Velvetleaf has a number of features which contribute to its success as a weed, including: the production of a large number of seeds that have high viability with prolonged dormancy and sporadic, continuous germination patterns; robust seedling vigor; and the ability to produce seed under competition. Because of sporadic germination patterns, control is difficult. Effective control measures include the application of pre-emergence and/or postemergence herbicides followed by cultivation and additional herbicide applications to control escapes and later flushes of germination. A triazine-resistant bio-type of velvetleaf has recently been reported from the northeastern United States.Key words: Velvetleaf, Abutilon theophrasti, weed biology