Safflower (Carthamus tinctorius L. ‘Hartman’) in the rosette growth stage or early bolting stage tolerated thiameturon {3-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl] amino] sulfonyl]-2-thiophenecarboxylic acid} at 5, 10, and 15 g ai/ha. Safflower also tolerated chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl] benzenesulfonamide} at 18 g/ha if safflower was taller than 15 cm at time of application. The fresh weight of above-ground biomass of common sunflower (Helianthus annuus L. #3 HELAN), treated at two plant sizes with thiameturon, was reduced more than 88%. Soil activity of thiameturon also reduced the seedling vigor of common sunflower, but thiameturon in soil was not as lethal to common sunflower as were foliar applications.

Intensive surveys were conducted in 2 fields in eastern Nebraska to determine the spatial stability of common sunflower, velvetleaf, green and yellow foxtail, and hemp dogbane over 4 yr (1992 to 1995). The 1st field was planted to soybean in 1992 and corn in 1993, 1994, and 1995. The 2nd field was planted to corn in 1992 and 1994 and soybean in 1993 and 1995. Weed density was sampled prior to postemergence herbicide application at approximately 800 locations per year in each field on a regular 7 m grid. The same locations were sampled every year. Weed density at locations between the sample sites was determined by linear triangulation interpolation. Weed seedling distribution was significantly aggregated, with large weed-free areas in both fields. Common sunflower, velvetleaf, and hemp dogbane patches were very persistent in diameter in the east-west and north-south directions and in location and area over 4 yr in the 1st field. Foxtail distribution and density continuously increased in each of the 4 yr in the first field and decreased in the 2nd field. A geographic information system was used to overlay maps from each year for a species. This showed that 36% of the sampled area was continuously free of common sunflower, 62.5% was free of hemp dogbane, and 11.5% was free of velvetleaf in the 1st field, but only 1% was free of velvetleaf in the 2nd field. The persistence of broadleaf weed patches suggests that weed seedling distributions mapped in one year are good predictors of future seedling distributions. Improved and more efficient sampling methods are needed.

Long term weed management programs for annual weeds should take into account seed production as well as yield losses from weeds. In this research economic optimum thresholds, determined with a simulation model, were compared to the economic thresholds derived from field studies with common sunflower and velvetleaf in soybean. An economic optimum threshold model, including a biological submodel and a submodel evaluating the financial benefit from controlling each weed species in soybean, was constructed to estimate the biological events and processes and economic costs of each weed. Long-term weed management programs were simulated in continuous soybean over a 15-yr period. The model optimized a POST treatment of bentazon by computing costs associated with weed interference and changes in estimated seedbank size. The economic optimum threshold populations were estimated to be 0.3 to 0.4 velvetleaf plants per 10 m2 and 0.4 common sunflower plants per 10 m2. The economic thresholds calculated to be 2.6 velvetleaf plants per 10 m2 and 1.5 common sunflower plants per 10 m2. The economic optimum threshold was calculated to be 7.5-fold and 3.6-fold lower than the economic thresholds for velvetleaf and common sunflower, respectively. The larger ratio for velvetleaf reflected the large quantity and longevity of seeds produced. The smaller ratio for common sunflower reflected the shorter persistence of the seeds and higher seed predation rates.

Scanning electron micrographs of spray droplet residual on wheat, sunflower, and kochia indicated that salts and various adjuvants applied with glyphosate affected deposit crystal content, thickness, and contact with the leaf surface. Spray deposits of glyphosate applied with diammonium sulfate contained distinct crystals, which related to enhanced toxicity of glyphosate applied alone, or to overcoming antagonism of glyphosate toxicity by calcium chloride, when applied to wheat. In general, glyphosate applied with antagonistic calcium chloride salt formed spray deposits that were amorphous, thick, and without crystals. Glyphosate sprayed with nonantagonistic diammonium sulfate and ammonium chloride salts had deposits with crystals and an evenly spread residue beneath the crystals. The various micrographs indicate that the antagonism of glyphosate phytotoxicity by salts may be in part from physical entrapment of glyphosate in the spray deposit. Glyphosate spray droplet residue contact with wheat, sunflower, and kochia surfaces related to observed differences in glyphosate toxicity to these species.

Barnyardgrass and many broadleaf weeds were severely stunted by POST applications of isoxaben. Most grasses were only moderately affected by isoxaben at rates up to 1 kg/ha. POST activity was enhanced by soil interception, but evident with foliar interception alone. Cell swelling in stems and in interveinal leaf lamina was observed after foliar application. A radiotracer study in redroot pigweed showed that only a small amount (3.3% of applied) of isoxaben entered the leaf following foliar application, and that little (0.08% of applied) isoxaben migrated beyond the point of application. The potential of isoxaben as a POST herbicide may be limited, in part, by poor absorption and translocation.

Spring-applied herbicides were evaluated in the field in 1982 through 1984 in western Nebraska for selective weed control in irrigated seedling alfalfa (Medicago sativa L. ‘Appollo’). Weed densities were least in plots treated preplant with benefin [N-butyl-N-ethyl-2,6-dinitro-4-(trifluoromethyl)benzenamine] in combination with postemergence applications of 2,4-DB [4-(2,4-dichlorophenoxy) butyric acid]. Combinations of sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} with bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) and fluazifop-butyl {(±)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl] oxy] phenoxy] propanoic acid} with bromoxynil provided less grass control than sethoxydim or fluazifop-butyl applied alone. Annual weeds did not reduce alfalfa stand density.

Field studies were conducted in 1986, 1987, 1988, and 1989 to determine the stability of crop loss functions across site by year environments. Environment was a significant source of variation for the soybean crop loss function as influenced by velvetleaf, but not as influenced by tall waterhemp and common sunflower. Weed density was a highly significant source of variation for all weed species studied. Regressions between percent soybean seed yield reductions and weed populations were linear. The velvetleaf interference regression equations were divided into two groups, those with high soybean-yielding intercepts and those with low-yielding intercepts, to explain the variance observed across environments. The regression equation for the high-yielding intercept group was Ŷ = 4.24X while the low-yielding group was Ŷ = 2.14X, where Y is percent soybean yield reduction and X is weed density per 10.7 m of soybean row. Tall waterhemp and common sunflower regression equations were determined to be Ŷ = 1.37X and Ŷ = 6.52X, respectively. Confidence intervals were used to account for the variance associated with the mean regression equation for each model and to develop economic threshold models that include risk aversion principles.

Norflurazon [4-chloro-5-(methylamino)-2-(α,α,α-trifluoro-m-tolyl)-3(2H)-pyridazinone] decreased the total chlorophyll content and the chlorophyll a/b ratio of nine plant species. Grain sorghum [Sorghum bicolor (L.) Moench.], wheat (Triticum aestivum L.), and sicklepod (Cassia obtusifolia L. ♯3 CASOB) were the most susceptible plants, and cotton (Gossypium hirsutum L.) was the most tolerant. The soil properties most closely correlated with norflurazon activity were organic matter content and clay component, not clay content. A high treatment rate was necessary for effective control of plant growth in soil high in organic matter content and high in montmorillonite or vermiculite. Approximately twice as much norflurazon was required to reduce chlorophyll content 50% when applied to the soil preplant incorporated as was required when applied preemergence.

Management systems for direct-seeded and transplanted sugarbeets (Beta vulgaris L. ‘Mono Hy D2′) were compared for weed control and sugarbeet selectivity from 1983 through 1985 in western Nebraska. Broadleaf weed density was similar, but yellow foxtail [Setaria glauca (L.) Beauv. # SETLU] density was lower in transplanted compared to direct-seeded sugarbeets. Preplant soil-incorporated applications of cycloate (S-ethyl cyclohexylethylcarbamothioate) plus trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine] at 3.3 plus 0.6 kg ai/ha or ethofumesate [(±)-2-ethoxy-2,3-dihydro-3,3-dimethyl-5-benzofuranyl methanesulfonate] plus trifluralin at 2.2 plus 0.6 kg/ha was noninjurious to transplanted sugarbeets but caused severe injury to direct-seeded sugarbeets. The combination of cycloate or ethofumesate with trifluralin improved weed control over that obtained when cycloate or ethofumesate was used alone. By combining the improved weed control obtained from cycloate plus trifluralin or ethofumesate plus trifluralin with the transplanting crop establishment technique, a superior sugarbeet weed control program was developed.

The seed composition in the upper 15-cm soil horizon was determined and correlated with weed seedlings growing with fieldbeans (Phaseolus vulgaris L. ‘Valley’). The total seed reservoir averaged 250 seed/kg of soil, and 19 species were represented. Seed occurring with the most frequency were redroot pigweed (Amaranthus retroflexus L. ♯ AMARE), common lambsquarters (Chenopodium album L. ♯ CHEAL), and common purslane (Portulaca oleracea L. ♯ POROL). Seed from these plants accounted for over 85% of the seed found. The number of barnyardgrass [Echinochloa crus-galli (L.) Beauv. ♯ECHCG], buffalobur (Solanum rostratum Dunal ♯ SOLCU), common lambsquarters, common purslane, and common sunflower (Helianthus annuus L. ♯ HELAN) seed in the soil was correlated with the number of plants growing in the field with fieldbeans. A correlation occurred between redroot pigweed, yellow foxtail [Setaria lutescens (Weigel.) Hubb. ♯ SETLU], and barnyardgrass growing in corn (Zea mays L.) fields in the fall of the year and plants growing in the field with fieldbeans the following year.

From 1976 to 1981, emergence date, height, and dry weight per plant were taken for eight weed species grown at Bushland, TX, at two stand densities. Height and dry weight per plant were related to precipitation and degree-days for air and soil temperatures using base temperatures of 10, 15.5, or 21 C. Kochia [Kochia scoparia (L.) Schrad. ♯ KCHSC] emerged early in March and was followed by poison suckleya [Suckleya suckleyana (Torr.) Rydb. ♯ SUKSU], sunflower (Helianthus annuus L. ♯ HELAN), and common cocklebur (Xanthium pensylvanicum Wallr. ♯ XANPE) in early April. Buffalobu(Solanum rostratum Dunal. ♯ SOLCU), puncturevine (Tribulus terrestris L. ♯ TRBTE), lanceleaf sage (Salvia reflexa Hornem. ♯SALRE), and barnyardgrass (Echinochloa crus-galli L. Beauv. ♯ECHCG) emerged in late April. The weeds began to increase in height during May but plants did not start increasing in weight until June. Kochia, common cocklebur, and sunflower grew tallest, produced most dry matter, made most efficient use of water, and weighed most per plant. Heat unit accumulations were strongly correlated with plant height.

Resistance to imazethapyr was identified in a population of common sunflower that had been treated with imazethapyr for seven consecutive years. The imazethapyr-resistant biotype of common sunflower was approximately 170 times more resistant to imazethapyr than the susceptible biotype based on the rate required for 25% control. Resistance was due to altered acetolactate synthase (ALS) that is less sensitive to imazethapyr. The imazethapyr concentration required to inhibit in vitro ALS activity by 25% was 210–fold higher in the resistant biotype than in the susceptible biotype. Differences in absorption, translocation, and metabolism of imazethapyr in common sunflower biotypes were not sufficient to explain the resistance to imazethapyr.

Several crop and weed species were field grown to maturity and their residues incorporated with soil to determine the effects of the plant residues on the growth of subsequent plantings in a spring-fall planting sequence in 1980 to 83. Growth of Palmer amaranth (Amaranthus palmeri S. Wats. # AMAPA) was studied in 1984 and 1985, and the effects of soil-incorporated Palmer amaranth on the growth of cabbage (Brassica oleracea var. Capitata L.) and carrot (Daucus carota L. var. sativa) seedlings were studied in 1985. Under warm, moist, saline soil conditions and intense sunlight following the spring plantings of the 1980 to 83 study, soil-incorporated residues of Palmer amaranth inhibited the growth of carrot 49% and onion (Allium cepa L.) 68%. Johnsongrass (Sorghum halepense L. Pers. # SORHA) residues inhibited the growth of onion 67%, and in one of two years inhibited the growth of cabbage 26% and common sunflower (Helianthus annuus L. #HELAN) 10%. In 1984 and 1985, Palmer amaranth grown at population densities of 300 plants/m2 attained heights of nearly 2 m and biomass of 2 to 3 kg/m2 within 10 weeks after planting. Phytotoxicity with these biomasses persisted 11 weeks in cabbage and carrot plantings. In 1985, phytotoxicity was dissipated 16 weeks after soil incorporation of the weed.

Eight field studies were conducted in soybeans at seven locations in Kansas over a 3-yr period to examine the efficacy of using reduced rates of the herbicides acifluorfen, bentazon, chlorimuron, and tank mixes of acifluorfen and bentazon. POST applications of these herbicides at 1/2X rates at 2 wk after planting (WAP) resulted in broadleaf weed control similar to that obtained from standard treatments of 1X rates applied at 4 WAP at six of seven studies with acifluorfen, bentazon, and acifluorfen plus bentazon and at five of seven studies with chlorimuron. One-quarter rates applied 2 WAP were equivalent to standard treatments for broadleaf weed control in four of seven studies with acifluorfen and chlorimuron, five of seven studies with bentazon, and six of seven studies with acifluorfen plus bentazon. One cultivation at 4 WAP, increased the broadleaf weed control with all herbicide treatments.

Interactions between initial weed seedling density and postemergence herbicide and mechanical weed control were studied in two field experiments conducted between 1994 and 1996. Increasing seedbank densities of velvetleaf (0 to 500 seed m–2) in soybean or common sunflower (250 to 2,500 seed 1.3 m–2) in corn or soybean were established at Lincoln and Mead, NE, respectively. Emerged seedlings were treated with increasing intensities of weed control from none to bentazon alone or with interrow cultivation. A positive linear relationship between initial seedling density and density of surviving seedlings was consistently observed. As initial seedling density increased, more survivors were present after treatment. As intensity of weed control increased, the number of seedling survivors decreased. Resulting reproductive fitness decreased with increasing management intensity but remained positive when regressed against surviving seedling densities. Weed management outcomes were dependent on initial seedling density, such that the absolute number of survivors increased, while proportion of survivors appeared constant within the density ranges studied. These research findings emphasize the need to account for weed infestation level when assessing efficacy of weed management systems and provide evidence that patchy weed distributions may persist in part because of the need for considerably higher management intensities in high density patch centers.

The question of when to control weeds traditionally has been approached with the calculation of critical periods (CP) based on crop yields. The concept of economic critical period (ECP) and early (EEPT) and late (LEFT) economic period thresholds are presented as a comprehensive approach to answer the same question based on economic losses and costs of control. ECP is defined as the period when the benefit of controlling weeds is greater than its cost. EEPT and LEFT are the limits of the ECP and can be used to determine when first and last weed control measures should be performed. Calculation of EEPT accounts for the economic losses due to weed competition that occur between planting and postemergence weed control. In this way it is possible to better evaluate the economic feasibility of using preplant or preemergence control tactics. The EEPT for DCPA application is analyzed in the context of onion production in Colorado. The EEPT for DCPA application was calculated from an empirical regression model that assessed the impact of weed load and time of weed removal on onion yields. The EEPT was affected by control efficacy, weed-free yield, DCPA cost, and onion price. DCPA application was economically advisable in only one of 20 fields analyzed because of the tow DCPA efficacy (60%).

A four-year experiment was conducted near Scottsbluff, NE to evaluate the effect of different methods of preplant tillage, post-plant cultivation, and herbicides on weed density and grain yield. Ridge-till enhanced the development of kochia and reduced the density of wild proso millet and common lambsquarters. Tandem disking increased longspine sandbur and redroot pigweed density whereas moldboard plowing increased common sunflower density. Cultivation reduced weed density 86% but weeds that remained after cultivation reduced corn grain yield by 40% as compared with a hand-weeded control. Weed control treatments that integrated cultivation and herbicides controlled a broader spectrum of weed species than cultivation or herbicide treatments alone.

2,4-D diethanolamine control of kochia was influenced by the source of water used as the spray carrier. Sodium bicarbonate in one water source related to the reduced kochia control. Sodium bicarbonate in a water spray carrier antagonized 2,4-D control of all plant species evaluated. Antagonism increased with the tolerance of a species to 2,4-D, concentration of sodium bicarbonate, and spray volume. Sodium bicarbonate antagonism of 2,4-D phytotoxicity was greatest with low temperature and relative humidity.

Chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]benzenesulfonamide} was nontoxic to safflower (Carthamus tinctorius L.) when applied postemergence in 1983 and 1984 at 0.018 and 0.035 kg ai/ha. Trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine] at 1.1 and 1.7 kg ai/ha and pronamide [3,5-dichloro(N-1,1-dimethyl-2-propynyl)benzamide] at 0.8 and 1.1 kg ai/ha were applied previously as preplant soil-incorporated treatments. In both years, safflower was relatively free of weeds where trifluralin was applied alone or in sequential treatments with chlorsulfuron. Pronamide, with or without chlorsulfuron, failed to completely control witchgrass (Panicum capillare L. ♯ PANCA) in 1 yr; thus safflower grain yields were reduced 21 to 35% when compared to weed-free safflower. Chlorsulfuron controlled redroot pigweed (Amaranthus retroflexus L. ♯ AMARE), puncturevine (Tribulus terrestris L. ♯ TRBTE), and common sunflower (Helianthus annuus L. ♯ HELAN).

A common sunflower isolate of Alternaria helianthi collected near Stoneville, MS, was pathogenic to cultivated sunflower and common cocklebur. Soybean, cotton, rice, grain sorghum, corn, wheat, and 22 other weed and crop species/varieties in nine botanical families showed no response or only minor symptoms. Therefore, further research is warranted to determine the potential of A. helianthi as a biocontrol agent against common cocklebur in geographic areas where sunflower is not a crop.