Abdel O. Mesbah

Common Sunflower (Helianthus annuus) and Green Foxtail (Setaria viridis) Interference in Dry Bean1

Field experiments were conducted in 1994 and 1995 under sprinkler irrigation at the University of Wyoming Research and Extension Center at Torrington to evaluate the effects of season-long interference and the effects of duration of interference of several common sunflower and green foxtail densities, alone or in combination, on pinto bean yield. Green foxtail densities did not significantly affect pinto bean yield in 1994 and reduced yield only at the highest density in 1995. In contrast, sunflower densities reduced pinto bean yield, except at the lowest density in 1994. Pinto bean yield was reduced as the combined density of green foxtail and sunflower increased. Compared with yield losses from each weed species alone, yield reductions from mixed species were additive in 1994 and at low weed densities in 1995 and less than additive at higher weed densities in 1995. The minimum number of weeds per m of row that will economically reduce pinto bean yield was estimated to be 1.6 to 2.9 for green foxtail and 0.12 to 0.2 for sunflower. Pinto bean yield reduction increased as the duration of green foxtail and sunflower interference increased, whether grown alone or in combination. The maximum duration that green foxtail, sunflower, and green foxtail plus sunflower can interfere with pinto bean before causing economical losses was estimated to be 4.5, 3.2, and 2.5 wk, respectively. Nomenclature: Common sunflower, Helianthus annuus L. #3 HELAN; green foxtail, Setaria viridis (L.) Beauv. # SETVI; pinto bean, Phaseolus vulgaris L. ‘Bill Z’. Additional index words: Competition, time of removal.

Canada Thistle (Cirsium arvense) Control in Established Alfalfa (Medicago sativa) Grown for Seed Production1

Canada thistle is one of the most troublesome and difficult weed species to control in established alfalfa grown for seed production. Current tools available for control are limited because of cultural management strategies associated with seed production. Alfalfa seed losses due to Canada thistle interference include both reduced yields from competition and increased seed loss during seed cleaning operations. Additional tools are needed to alleviate these losses. Field experiments were conducted in 1998, 1999, and 2000 at two locations in Park County, WY, to evaluate Canada thistle control and alfalfa tolerance to several postemergence herbicides. Bentazon, imazamox, imazethapyr, and MCPB were applied, alone or in combination, at different Canada thistle growth stages. Methylated seed oil (MSO) was added at 1.5% v/v to the treatments containing imazamox or imazethapyr. MCPB applied alone when Canada thistle was 7.5- or 15-cm tall caused severe alfalfa injury (28 to 40%) and resulted in less Canada thistle control (23 to 27%). Imazamox or imazethapyr applied alone when Canada thistle was 15-cm tall did not cause any significant alfalfa injury but resulted in unsatisfactory Canada thistle control (29 to 35%). Bentazon was the only treatment containing a single herbicide that provided more than 50% Canada thistle control. The treatments providing the best balance between Canada thistle control (>80%) and alfalfa injury (<13%) were a single application of bentazon combined with either imazamox or imazethapyr. These two treatments also yielded the highest, more than 800 kg/ha. Split applications of bentazon combined with imazamox or imazethapyr were similar to single applications. Nomenclature: Bentazon; imazamox; imazethapyr; MCPB; Canada thistle, Cirsium arvense (L.) Scop. #3 CIRAR; Alfalfa, Medicago sativa L. Additional index words: Application timing, herbicide mixtures, methylated seed oil, MSO, postemergence, weed management, CIRAR. Abbreviations: IMI, imidazolinone-resistant; MSO, methylated seed oil.

Wild Buckwheat (Polygonum convolvulus) Interference in Sugarbeet

Abstract Field studies were conducted in Powell, WY in 2006 and 2007 to determine the influence of season-long interference of various wild buckwheat densities and duration of interference on sugarbeet. Percent sucrose content was not affected by wild buckwheat interference. Root and sucrose yield loss per hectare increased as wild buckwheat density increased. The estimated percent yield loss as wild buckwheat density approaches infinity was 64 and 61% for root and sucrose yield loss, respectively. The estimated percent yield loss per unit weed density at low weed densities was 6% for both root and sucrose yield loss. Greater durations of wild buckwheat interference had a negative effect on sugarbeet root yield. The critical timing of weed removal (CTWR) to avoid 5 and 10% root yield loss was 32 and 48 d after sugarbeet emergence (DAE), respectively. These results show that wild buckwheat is competitive with sugarbeet and should be managed appropriately to forestall any negative effects on sugarbeet root and sucrose yield. Nomenclature: Wild buckwheat, Polygonum convolvulus L. POLCO; sugarbeet, Beta vulgaris L

Lanceleaf Sage (Salvia reflexa) Interference in Sugarbeet

Abstract Field studies were conducted in Powell, WY, in 2006 and 2007 to determine the influence of season-long interference of various lanceleaf sage densities and durations of interference on sugarbeet. The rectangular hyperbola model with the asymptote (A) constrained to 100% maximum yield loss characterized the relationship between lanceleaf sage density and sugarbeet yield loss. The estimated parameter I (yield loss per unit weed density as density approaches zero) was 3% for both root and sucrose yield loss. Increasing duration of lanceleaf sage interference had a negative effect on sugarbeet root yield. The critical timing of weed removal to avoid 5 and 10% root yield loss was 37 and 52 d after sugarbeet emergence, respectively. Lanceleaf sage interference did not affect percentage of sucrose content. These results indicate that lanceleaf sage is not as competitive as other weeds but that appropriate control measures should be undertaken to minimize sugarbeet yield loss from interference. Nomenclature: Lanceleaf sage, Salvia reflexa Hornem. SALRE; sugarbeet, Beta vulgaris L.

Venice Mallow (Hibiscus trionum) Interference in Sugarbeet

Abstract Field studies were conducted in Powell, WY, in 2006 and 2007 to determine the influence of season-long interference of various Venice mallow densities and duration of interference on sugarbeet. Sucrose concentration was not affected by Venice mallow interference. The effect of Venice mallow density on sugarbeet root and sucrose yield loss was described by the rectangular hyperbola model. Root and sucrose yield loss increased as Venice mallow density increased. The estimated asymptote, A (percent yield loss as density approaches infinity) was 61% for both root and sucrose yield loss, and the estimated parameter, I (percent yield loss per unit weed density as density approaches zero) was 6% for both root and sucrose yield loss. Sugarbeet root yield decreased as the duration of Venice mallow interference increased. The critical timing of weed removal to avoid 5 and 10% root yield loss was 30 and 43 d after sugarbeet emergence, respectively. Results show that Venice mallow is competitive with sugarbeet implying that it should be managed appropriately to reduce negative effects on yield and prevent seed bank replenishment and re-infestation in subsequent years. Nomenclature: Venice mallow, Hibiscus trionum L. HIBTR; sugarbeet, Beta vulgaris L.

Redstem Filaree Control in Sugarbeets with Micro-rate Herbicide Treatments

Field experiments were conducted at the University of Wyoming Powell Research and Extension Center to evaluate redstem filaree control and sugarbeet response to several herbicide treatments. Preplant herbicides used were ethofumesate and pyrazon applied alone or in combination. Postemergence herbicides included desmedipham-phenmedipham-ethofumesate plus triflusulfuron plus clopyralid at various application rates and timings. A standard postemergence treatment was compared with a micro-rate system, which included 1.5% methylated seed oil. Redstem filaree control was influenced by number and time of applications. Treatments applied at cotyledon stage provided better control than those applied at 2-leaf sugarbeet stage. Redstem filaree control increased as the number of applications increased. With or without preplant herbicides, four sequential applications using micro-rate provided more than 90% redstem filaree control. Micro-rate treatments with methylated seed oil adjuvant were significantly superior to standard treatments in controlling redstem filaree. There was no significant difference in redstem filaree control between treatments containing preplant herbicides and those without, suggesting that preplant herbicides may not be necessary. Preplant herbicides followed by standard rate postemergence treatments showed more injury than those followed by micro-rate treatments. Sugarbeet root yield was higher in treated plots than in the untreated check and was closely related to both redstem filaree control and sugarbeet injury. Sugar content was not affected by any of the herbicide treatments. With or without preplant herbicides, four sequential micro-rate applications provided significantly higher yields than standard postemergence treatments.

Flixweed (Descurainia sophia) Shade Tolerance and Possibilities for Flixweed Management Using Rapeseed Seeding Rate

Brassicaceae weeds can be problematic in canola varieties that have not been modified to resist specific broad-spectrum herbicides. The overall objective of this study was to evaluate the potential for increased rapeseed seeding rate as a management strategy for flixweed. To accomplish this objective, a field study was conducted to determine crop seeding rate effects on canopy light transmission and rapeseed yield characteristics, as well as a greenhouse study to determine morphological and photosynthetic responses of flixweed to decreasing irradiance levels. Results from the field study indicated that light transmittance through the canopy decreased linearly as crop seeding rate increased from 1.8 to 9.0 kg ha-1. Increasing crop seeding rate did not influence rapeseed aboveground biomass, seed yield, and harvest index, but negatively affected rapeseed seed oil content in one of two siteyears. Greenhouse study results indicated that declining irradiance levels caused reductions in flixweed biomass, root allocation, and photosynthetic light compensation point. Flixweed leaf allocation, foliage area ratio, and specific foliage area increased in response to decreasing irradiance levels. Combined results of field and greenhouse studies suggest that increasing rapeseed seeding rate can suppress flixweed growth while not causing yield penalties from increased intraspecific competition. However, increased rapeseed seeding rate might not be an adequate control strategy on its own because flixweed displays characteristics of a shade-tolerant species. Nomenclature: Flixweed, Descurainia sophia (L.) Webb. ex. Prantl; rapeseed, Brassica napus L.