Intraspecific interference of populations of sunflower (Helianthus annuus L.) and velvetleaf (Abutilon theophrasti Medic), as well as interspecific populations of these two broadleaf weeds, in sugarbeets (Beta vulgaris L. ‘Mono Hy D2′) was determined in 2-yr field experiments. Sunflower was more competitive in sugarbeets than was velvetleaf. At densities of 6, 12, 18, and 24 sunflower plants/30 m of row, root yields were reduced 40, 52, 67, and 73%, respectively. At the same densities of velvetleaf, root yields were reduced only 14, 17, 25, and 30%, respectively. Interspecific interference of these two broadleaf weeds at the same densities reduced root yields 19, 36, 43, and 56%, respectively. The minimum number of weeds required/30 m of row to reduce sugarbeet root yields was predicted to be 1 for sunflower, 9 to 12 for velvetleaf, and 2 to 7 for an equal population of sunflower and velvetleaf.

Interference within a mixture of equal densities of common lambsquarters (Chenopodium album L.), kochia [Kochia scoparia (L.) Schrad.], and redroot pigweed (Amaranthus retroflexus L.) in sugarbeets (Beta vulgaris L. ‘Mono Hy D2’) was determined in a 3-yr field study. Yield of sugarbeet roots and sucrose per hectare decreased as intensity of competition from equal populations of these three weeds increased. At densities of 6, 12, 18, and 24 broadleaf weeds per 30 m of row, root yields were reduced 13, 24, 33, and 39%, respectively. Sucrose yields were reduced similarly. Fewer than three weeds per 30 m of row did not significantly reduce root yield. Reduction in root yield (Y) of sugarbeets caused by specific densities (X) of the three broadleaf weeds was predicted by using the linear equation Y = 1.64 + 1.88 X. The actual reductions in yield were always less than the predicted reductions when this equation was tested against 36 weed control systems because the competitive ability of broadleaf weeds that were treated with herbicides, but not killed, was suppressed during the growing season. By harvest, broadleaf weeds present in weed-control-system plots weighed an average of 75 to 85% less than broadleaf weeds present in nontreated plots.

The effect of common lambsquarters (Chenopodium album L. #3 CHEAL) density on spring barley (Hordeum vulgare L. ‘Lidal’) was studied in a 2-yr field experiment near Fairbanks, AK. The data were fit to a rectangular hyperbola model. Common lambsquarters densities explained 74 to 75% of the variability in barley yield. The maximum yield loss attributable to common lambsquarters was 23% in 1982 and 36% in 1983.

Interference of Powell amaranth (Amaranthus powellii S. Wats. ♯ AMAPO) in sugarbeet (Beta vulgaris L. ‘Mono Hy A4’) was determined in field experiments. Yield of sugarbeet roots and recoverable sucrose/ha decreased as the density of Powell amaranth increased. At densities of 6, 12, 18, and 24 Powell amaranth plants/30 m of row, root yields were reduced 8, 14, 24, and 25%, respectively, and recoverable sucrose yields were reduced 7, 13, 23, and 24%, respectively. The minimum number of Powell amaranth plants required per 30 m of row to reduce sugarbeet root yields was estimated to be nine in 1982 and 11 in 1983.

Multiple regression and response surface plots were used to analyze the effects of common lambsquarters population density and interference duration on weed growth and soybean seed yield. Under favorable growing conditions in 1986, weed biomass production at all population densities and interference durations was four to five times that produced in 1987, under less favorable conditions. However, there was no significant treatment by year interaction for soybean seed yield reduction by common lambsquarters, and production of each kg/ha weed biomass resulted in an average soybean yield reduction of 0.26 kg/ha. Utilizing 5% yield loss as an arbitrary threshold level, the regression equation predicted a common lambsquarters density threshold of 2 plants/m of row for 5 weeks of interference after crop emergence and 1 plant/m of row for 7 weeks. Seed production by individual common lambsquarters plants was highly correlated (r=0.92) with weed dry weight, and seed production ranged from 30 000 to 176 000 seeds/plant.

Field experiments were conducted to study the feasibility of applying glyphosate [N-(phosphonomethyl) glycine] postemergence with a recirculating sprayer and a vertical roller for the control of common lambsquarters (Chenopodium album L.), common sunflower (Helianthus annuus L.), kochia [Kochia scoparia (L.) Schrad.], redroot pigweed (Amaranthus retroflexus L.), and velvetleaf (Abutilon theophrasti Medic.) in sugarbeets (Beta vulgaris L.). Glyphosate was applied twice each year at 1.7 kg/ha with a recirculating sprayer in 1977 and 1978, or twice as a 20% (v/v) solution with a vertical-roller applicator in 1979. By harvest, 70 to 74% of the total population of treated common sunflower, kochia, and redroot pigweed, 61% of the common lambsquarters, and 30% of the velvetleaf was dead. Root yields in glyphosate-treated plots, when averaged over 3 yr, were increased 5800, 8500, 12 500, and 13400 kg/ha at densities of 6, 12, 18, and 24 broadleaf weeds (equal densities of common lambsquarters, kochia, and redroot pigweed)/30m of row, respectively. Where equal densities of common sunflower and velvetleaf competed with sugarbeets, root yields in glyphosate-treated plots, when averaged over 2 yr, were increased 4400, 11900, 11700, and 10700 kg/ha, respectively, at these same densities.

Interference of common lambsquarters (Chenopodium album L.) in sugarbeets (Beta vulgaris L. ‘Mono Hy D2′) was determined in a 2-yr field experiment. Yield of sugarbeet roots and recoverable sucrose/ha decreased as the density of common lambsquarters increased. At densities of 6, 12, 18, and 24 common lambsquarters plants/30 m of row, root yields were reduced 13, 29, 38, and 48%, respectively, and recoverable sucrose yields were reduced 11, 27, 37, and 46%, respectively. The minimum number of common lambsquarters plants required per 30 m of row to reduce sugarbeet root yields was estimated to be six in 1980 and four in 1981.

Interference studies utilizing manipulated and natural infestations of giant foxtail (Setaria faberi Herrm. ♯ SETFA) in soybeans [Glycine max (L.) Merr. ‘Williams’] were conducted at Urbana, IL, in 1982 and 1983. Giant foxtail clumps, each consisting of 3 to 6 plants per dm2, were used to determine the effect of clump density on giant foxtail dry weight and soybean seed yield. In both years, regression equations predicted a soybean seed yield loss of 0.8% for each giant foxtail clump per 9 m of row. The highest density of 30 clumps per 9 m of row reduced soybean yield 26%. In a second study, fluazifop {(±)-2-[4-[(5-(trifluoromethyl)-2-pyridinyl)oxy]phenoxy]propanoic acid} at 0.28 kg ai/ha or fluazifop + alachlor [2-chloro-2’,6’-diethyl-N-(methoxymethyl)acetanilide] at 0.28 + 2.24 kg/ha applied 10 or 15 days after emergence gave maximum giant foxtail control with little or no reduction in soybean seed yield. Interference from uncontrolled giant foxtail began to reduce soybean dry-matter accumulation 15 days after emergence under favorable growing conditions in 1982. Under less favorable conditions in 1983, similar effects were not observed until 25 days after emergence.

Inputs, including herbicides, used in crop production may create negative environmental impacts. One solution to minimize these adverse effects is the adoption of integrated weed management (IWM) with the intention of reducing herbicide use. This study, conducted in 2010, estimates the willingness of farmers to pay for the adoption of more effective weed management methods. Results suggest that the willingness to pay (WTP) for IWM is greater than the WTP for other weed management methods, including chemical weed management and chemical and mechanical weed management. This study also identified a number of factors that influence the adoption of IWM on wheat farms in Iran using a multinomial logit model. Total annual income, area under irrigated wheat, wheat yield loss due to weeds, perennial nature of the weeds, and having awareness of weed resistance to herbicides had a positive effect on the adoption of IWM practices. However, having rain-fed (dryland) wheat cultivation and a larger number of plots on the farm had a negative influence on the choice of IWM.