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Susceptibility of a system to colonization by a weed is in part a function of environmental resource availability. Doveweed [Murdannia nudiflora(L.) Brenan] can establish in a variety of environments; however, it is found mostly in wet or low-lying areas with reduced interspecies competition. Four studies evaluated the effect of mowing height, interspecies competition, and nitrogen, light, and soil moisture availability onM. nudifloraestablishment and growth. A field study evaluated the effect of mowing height onM. nudifloraestablishment. In comparison with unmowed plots, mowing at 2 and 4 cm reduced spread 46% and 30%, respectively, at 9 wk after planting. Effect of mowing height and nitrogen fertilization on ‘Tifway’ bermudagrass (Cynodon dactylonBurtt-Davy×C. transvaalensisL. Pers.) andM. nudiflorainterspecies competition was evaluated in a greenhouse trial.Murdannia nudifloracoverage was 62% greater in flats maintained at 2.6 cm than flats maintained at 1.3 cm. Supplemental application of 49 kg N ha−1mo−1increasedM. nudifloracoverage 75% in comparison with 24.5 kg N ha−1mo−1. A difference inM. nudifloracoverage could not be detected between flats receiving 0 and 24.5 kg N ha−1mo−1, suggesting moderate nitrogen fertilization does not encourageM. nudifloracolonization. Effect of light availability onM. nudifloragrowth and development was evaluated in a greenhouse study. Growth in a 30%, 50%, or 70% reduced light environment (RLE) did not affect shoot growth on a dry weight basis in comparison with plants grown under full irradiance; however, internode length was 28% longer in a 30% RLE and 39% longer in a 50% and 70% RLE. Effect of soil moisture onM. nudifloragrowth and development was evaluated in a greenhouse study. Plants maintained at 50%, 75%, and 100% field capacity (FC) increased biomass>200% compared with plants maintained at 12.5% or 25% FC.

Glyphosate is used in the transition zone to control annual bluegrass in fully dormant warm-season grasses. A suspected resistant (R) biotype of annual bluegrass was identified on a golf course in South Carolina after at least 10 consecutive years of glyphosate application. Greenhouse bioassays revealed the R biotype was 4.4-fold resistant to glyphosate compared with a standard susceptible (S) biotype. Further studies were conducted to investigate the mechanism conferring glyphosate resistance in the R biotype. Leaf discs of both biotypes accumulated shikimate in response to increasing glyphosate concentration, but the glyphosate concentration resulting in 50% EPSP synthase inhibition as a result of shikimate accumulation (I50) was 4.2-fold higher in the R biotype compared with the S biotype. At the whole plant level, similar levels of shikimate accumulation were observed between biotypes at 6 and 24 h after treatment (HAT) with glyphosate, but greater shikimate accumulation occurred in the S biotype at 72, 120, and 168 HAT. Shikimate levels decreased in the R biotype after 72 HAT. There were no differences in14C-glyphosate absorption between biotypes. However, more14C-glyphosate translocated out of the treated leaf in the R biotype and into root tissues over time compared with the S biotype. Partial sequencing of the EPSP synthase gene revealed a point mutation that resulted in an Ala substitution at Pro106. Although other mechanisms may contribute to glyphosate resistance, these results confirm a Pro106to Ala substitution is associated with resistance to glyphosate in the R annual bluegrass biotype.

Acetolactate synthase–- (ALS-) inhibiting herbicides are frequently used to control annual bluegrass in managed turfgrass systems. Recently, the number of annual bluegrass populations resistant to these herbicides has increased on golf courses. Previous research involving annual bluegrass resistance to ALS inhibitors has included elucidating the mechanism of resistance and in vivo response of ALS to ALS-inhibiting herbicides. The purpose of this research was to generate baseline information on enzymatic parameters of resistant (R) and susceptible (S) annual bluegrass biotypes and further distinguish these biotypes by comparing vegetative growth and reproductive characteristics. The R biotype contained a mutation in the ALS gene resulting in a Trp574to Leu amino acid substitution. Compared to the S biotype, the R biotype exhibited a 27- and 10-fold resistance to trifloxysulfuron at the whole-plant level and under in vitro conditions, respectively. No significant differences were observed in substrate concentration at one-half maximum rate of enzyme activity (pyruvate) or extractable ALS activity between biotypes, but the maximum rate of enzyme activity was higher for the R biotype. The feedback inhibition of ALS activity by the branched-chain amino acids was higher for the R biotype than the S biotype, with leucine, valine, and isoleucine inhibiting ALS activity 20, 6, and 4% more in the R biotype, respectively. The R biotype produced more inflorescences and seeds per plant in comparison with the S biotype, but relative growth rates between biotypes were similar at all harvest intervals. Our research provides baseline information regarding ALS enzyme response, vegetative growth, and reproductive characteristics of annual bluegrass biotypes resistant and susceptible to ALS-inhibiting herbicides.

Annual bluegrass is the most problematic winter annual weed in managed turfgrass. Acetolactate synthase (ALS)-inhibiting herbicides are effective for annual bluegrass control, but reliance on this mode of action can select for herbicide-resistant biotypes. Two annual bluegrass biotypes not controlled with ALS-inhibiting herbicides were reported at golf courses in South Carolina and Georgia. Research was initiated at Clemson University to verify the level of resistance of these biotypes to ALS inhibitors. Two ALS-susceptible (S) and suspected resistant (SCr, GAr) annual bluegrass biotypes were established in a greenhouse. Dose-response experiments were conducted on mature annual bluegrass plants using trifloxysulfuron, foramsulfuron, and bispyribac-sodium, all ALS-inhibiting herbicides. Additionally, a rapid diagnostic ALS activity assay was optimized and conducted using the same herbicides. For dose-response experiments, the rate of herbicide that reduced shoot biomass 50% (I50) values for the S biotypes were 13.6 g ai ha−1for trifloxysulfuron, 7.0 g ai ha−1for foramsulfuron, and 38.3 g ai ha−1for bispyribac-sodium. Fifty percent shoot biomass reduction was not observed in either the SCr or GAr biotypes at eight times the labeled field rate of all ALS-inhibiting herbicides tested. For in vivo tests of ALS activity, the SCr biotype yielded I50(concentration of herbicide that reduced ALS activity 50%) values 3,650, 3,290, and 13 times the S biotypes following treatment with trifloxysulfuron, foramsulfuron, and bispyribac-sodium, respectively. Similarly, I50values for the GAr biotype were 316, 140, and 64 times greater than the S biotypes following the same herbicide treatments. This research indicates high levels of annual bluegrass resistance to multiple ALS-inhibiting herbicides in South Carolina and Georgia. Future research should focus on the mechanisms of ALS resistance in these annual bluegrass biotypes as well as alternative options for control not targeting the ALS enzyme.

Field experiments were conducted from 2004 through 2006 at Pendleton and Clemson, SC, to determine the influence of seasonal emergence of wild radish on phenological development, survival, and seed and biomass production in a noncompetitive environment. The duration of four developmental phases, emergence to bolting, bolting to flowering, flowering to silique production, and silique production to maturity, were recorded following wild radish sowing at monthly intervals from October 2004 through September 2006. Seedling emergence occurred 2 to 4 wk after sowing. Mortality of seedlings that emerged from December through March was greater than that of seedlings that emerged in all other months. Wild radish that emerged from April through August completed its life cycle by summer or early autumn. Wild radish that emerged from September through November was able to survive the winter and complete its life cycle the following spring. The developmental phases most affected by time of emergence were emergence to bolting and bolting to flowering. The duration of emergence to bolting ranged from 249 to 479 growing degree days (GDD), and bolting to flowering from 270 to 373 GDD, depending on the month of emergence. The total life cycle of wild radish varied from a low of 1,267 GDD following June emergence to 1,503 GDD following November emergence. Multiple regression analysis revealed that emergence to bolting and silique production to maturity phases were dependent on accumulated heat units and photoperiod. Seed and biomass production were influenced by month of emergence. An average of 1,470 seeds plant−1was produced when emergence occurred in July and 10,170 seeds plant−1when emergence occurred in November. Plants that emerged in autumn exhibited minimal growth during the winter months, but conditions were conducive for growth in mid-March and April, with biomass production of 809 g plant−1at silique production.

Knowledge of the germination requirements of wild radish will help in determining the favorable conditions for germination and emergence and allow better management of this weed. Experiments were conducted during 2005 to 2006 and 2006 to 2007 to evaluate wild radish temperature and light requirements over a 12-mo period beginning in July on seeds placed on the soil surface and at a 10-cm depth. Germination response was influenced by temperature, light, duration of burial, and burial depth. Freshly harvested seeds (July) had no more than 18% germination whereas seeds allowed to after-ripen in the field for 3 to 6 mo (October to January) had up to 40% germination. The germination of wild radish retrieved from the soil surface was 1.2 to 1.5 times greater at alternating temperatures (2.5/17.5, 7.5/22.5, and 12.5/27.5 C) than at constant temperatures (10, 15, and 20 C) at 0, 3, and 6 mo after maturation. The light requirement for germination varied by time of year with no differences in germination between light and dark conditions for freshly harvested seeds. Far-red light inhibited germination of wild radish, indicating that wild radish may become sensitive to light following an after-ripening period.

Experiments were conducted to investigate the effects of shading on and location of the mother plant on germination and hormone content of Palmer amaranth seed. Increasing the shading from 0 to 87% decreased germination of fresh, viable seeds in dark from 25 to 12%. Abscisic acid (ABA) content of seeds from plants in 0% shade increased from 13.3 ng g−1dry seed to 19.1 ng g−1dry seed with 87% shade. Shading of the mother plant did not influence the 1,000-seed weight of Palmer amaranth. Seeds that matured in the top and middle third of the mother plant had 67 to 78% greater germination than those that matured in the bottom third of the mother plant. Endogenous gibberellic acid (GA) content of seeds did not differ between locations on the mother plant; however, the ABA content of seeds produced on the bottom third of the plant was 46 and 59% higher than the ABA content of seeds produced at the middle and top third of the plant, respectively. Endogenous ABA or GA content of seeds and 1,000-seed weight had no relationship with seed germination over and above the treatment effects. This research suggests that shading and plant location can influence germination of fresh, viable seeds of Palmer amaranth, which will be a dormancy strategy for seed dispersal over time.

Experiments were conducted to investigate the acclimation of Palmer amaranth to shading. Plants were grown in the field beneath black shade cloths providing 47 and 87% shade and in full sunlight (no shading). All photosynthetic measurements were taken 4 wk after initiating the shade treatments. Photosynthetic rates of Palmer amaranth grown under 47% shade increased with increasing photosynthetic active radiation (PAR) similar to 0% shade-grown plants. Light-saturated photosynthetic rates were predicted beyond the highest measured PAR of 1,200 µmol m−2s−1for plants grown under 0 and 47% shade. Plants acclimated to increased shading by decreasing light-saturated photosynthetic rates from 60.5 µmol m−2s−1under full sun conditions to 26.4 µmol m−2s−1under 87% shade. Plants grown under 87% shade lowered their light compensation point. Rate of increase in plant height was similar among shade treatments. Plants responded to increased shading by a 13 to 44% reduction in leaf appearance rate (leaf number growing degree days [GDD]−1) and a 22 to 63% reduction in main-stem branch appearance rate (main-stem branch number GDD−1) compared with full sunlight. Palmer amaranth specific leaf area increased from 68 to 97 cm2g−1as shading increased to 87%. Plants acclimated to 47% shade by increasing total leaf chlorophyll from 22.8 µg cm−2in full sunlight to 31.7 µg cm−2when shaded; however, the increase was not significant at 87% shading. Thus, it is concluded that Palmer amaranth shows photosynthetic and morphological acclimation to 87% or less shading.

Field experiments were conducted near Blackville, SC, and Tifton, GA, in 2004 and 2005, to evaluate the effect of wild radish and rye cover crops on weed control and sweet corn yield when used in conjunction with lower-than-recommended herbicide rates. Cover crop treatments included wild radish, rye, and no cover crop, alone and in conjunction with half and full rates of atrazine (0.84 and 1.68 kg ai ha−1) plusS-metolachlor (0.44 and 0.87 kg ai ha−1) applied before sweet corn emergence. Florida pusley, large crabgrass, spreading dayflower, ivyleaf morningglory, and wild radish infested the test sites. Wild radish and rye cover crops without herbicides reduced total weed density by 35 and 50%, respectively, at 4 wk after planting (WAP). Wild radish in conjunction with the full rate of atrazine plusS-metolachlor controlled Florida pusley, large crabgrass, and ivyleaf morningglory better than rye or no cover crop treated with a full herbicide rate in 2004 at Blackville. In 2005, at Blackville, weed control in sweet corn following wild radish cover crop plots alone was not different from that following rye. Wild radish or rye in conjunction with a half or full rate of atrazine andS-metolachlor controlled > 95% Florida pusley, wild radish, and large crabgrass in sweet corn at Tifton during both years. Ten glucosinolates, potential allelopathic compounds, were identified in wild radish, including glucoiberin, progoitrin, glucoraphanin, glucoraphenin, glucosinalbin, gluconapin, glucotropaeolin, glucoerucin, glucobrassicin, and gluconasturtin. Sweet corn yields at Blackville and Tifton following wild radish or rye cover crops were similar between the half and full rates of atrazine plusS-metolachlor. Sweet corn in wild radish or rye cover crop plots without herbicides produced less-marketable ears than herbicide-treated plots, indicating that a combination of cover crops and herbicides are required to optimize yields and to obtain desirable weed control.

The influence of soybean row width and glyphosate application timing was determined on survival, biomass, and seed production of cohorts from a mixed population of Palmer amaranth and pusley species (Florida and Brazil pusley) along with soybean seed yield. The first Palmer amaranth and pusley cohort comprised plants that emerged from soybean planting through the V3 (3 wk after soybean emergence [WAE]) soybean stage (cohort 1). The second cohort comprised plants that emerged between the V3 to V6 (5 WAE) soybean stages (cohort 2), and the third cohort emerged after the V6 through the R2 soybean stage (cohort 3). Glyphosate at 840 g ae ha−1was applied at V3; V6; V3 and V6; and V3, V6, and R2 in rows either 19 or 97 cm wide. A nontreated control was included for comparison in each row width. Sequential glyphosate applications at V3 and V6 or at V3, V6, and R2 soybean stages resulted in 1 to 3% survival of cohort 1 compared with 23 to 28% survival after a single glyphosate application. Vegetative biomass production by cohort 1 accounted for 71% of the total pusley biomass produced in the nontreated plots. Cohort 1, 2, and 3 contributed 68, 31, and 1%, respectively, of the total 37,900 seeds m−2produced by pusley plants in nontreated plots. Delaying a glyphosate application to the V6 stage resulted in higher biomass and more than twice the seed produced from cohort 1 when compared with cohort 2. Glyphosate applied at V3 and V6 stages prevented pusley seed production from cohort 1, and an additional glyphosate application at the R2 stage prevented seed production from cohorts 2 and 3. No Palmer amaranth emergence occurred after the V6 soybean stage in either row width. A single glyphosate application at the V3 or V6 stage eliminated cohort 1 of Palmer amaranth in narrow rows. Palmer amaranth plants from cohort 1 in wide rows that survived the V3 glyphosate application produced 3.3 g m−2biomass and 600 seeds m−2. Averaged over years and row widths, soybean yields after sequential glyphosate applications were 2,490 to 2,640 kg ha−1compared with 1,850 to 2,020 kg ha−1after a single glyphosate application at the V3 or V6 stage. This research confirms that sequential glyphosate applications are superior to a single application for minimizing pusley and Palmer amaranth survival, biomass, and seed production along with an improvement in soybean yields.

Experiments were conducted to determine the influence of soybean row width and glyphosate application timing on survival, biomass production, and fecundity of three sicklepod cohorts along with soybean seed yield. The first cohort comprised sicklepod plants that emerged from soybean planting through the V3 stage of soybean (two fully developed trifoliate leaves plus the unifoliate leaves; cohort 1). The second cohort comprised plants that emerged between the V3 to V6 stages of soybean (five fully developed trifoliate leaves plus the unifoliate leaves; cohort 2), and the third cohort emerged after the V6 stage through the R2 stage of soybean (full bloom; cohort 3). Glyphosate was applied at V3; V6; V3 and V6; and V3, V6, and R2 in rows 19 and 97 cm wide, and a nontreated control was included for comparison in each row width. Survival of cohort 1 in 2004 in glyphosate-treated plots occurred only after a single glyphosate application at V3 or V6 in wide rows. Narrowing the soybean row width reduced ...

Kyllingaspecies are becoming more prevalent in turfgrass sites throughout North America. The effects of nitrate (50, 200, and 400 mg L−1), temperature (33/24, 25/17, 19/11 C day/night, respectively), and light on seed germination of threeKyllingaspecies were investigated in growth chambers. Nitrate concentrations did not stimulateKyllingaspecies seed germination compared with untreated seeds. AllKyllingaspecies seeds failed to germinate in darkness but resumed germination once they were placed in light. This is an important pest management strategy because a dense, uniform turfgrass stand with its minimum light penetration to the soil would minimizeKyllingaspecies seed germination. Higher temperatures increased seed germination rate and percentage of each species after 8 wk. Maximal (> 90%)K. brevifoliagermination occurred 2 to 4 wk after initiation in every seed study, whereasK. squamulataseeds germinated continuously. Minimal (< 10%)K. pumilaseeds germinated until alternating diurnal temperatures were imposed.

Kyllingaspecies are becoming more common throughout the southeastern United States. Two species,Kyllinga brevifoliaandKyllinga squamulata, in particular are prevalent weeds in turfgrass. To better understand these weeds, growth chamber studies determined the growth ofK. brevifolia, K. squamulata, andCynodon dactylon×Cynodon transvaalensisas influenced by three temperature regimes (33/24, 25/17, 19/11 C day/night, respectively). Temperature influenced almost all aspects ofKyllingaspecies growth. Plant height of bothKyllingaspecies increased nearly twofold after 8 wk at high temperatures. Plants were mowed each week to 2.5 cm; both species produced more than twice as many clippings by 8 wk at high (33/24 C) temperatures than at low (19/11 C) temperatures. Destructive analysis at 8 wk revealed thatK. brevifoliashoot and root weight increased with decreasing temperature, whereasK. squamulatashoot and root weights were not affected by temperature. Shoot weight percentage for bothKyllingaspecies increased from 59% in medium temperatures to 69% in high temperatures.K. brevifoliashoot weight percentage decreased to 53% in low temperatures, whereasK. squamulatashoot weight percentage increased to 72%.K. brevifoliainflorescences formed at 2, 3, and 5 wk in high, medium, and low temperatures, respectively, whereasK. squamulataflowered immediately in all temperatures.C. dactylon×C. transvaalensisandKyllingaspecies growth were similar within each temperature regime throughout the 8-wk study.

Norflurazon adsorption and dissipation under field and laboratory conditions, and distribution within the soil profile were determined in three soils representative of cotton-growing regions of the southeastern U.S. Norflurazon adsorption was greater in soil from 0 to 8 cm in a Lexington silt loam (Tennessee) and a Beulah silt loam (Mississippi) than in a Dothan loamy sand (Georgia). Adsorption was directly related to organic matter. Norflurazon degradation under controlled conditions in soil from 0 to 8 cm from each state was not different among locations, with half-lives ranging from 63 to 167 d. Degradation at 30 C in soil from the 30- to 45- and 60- to 90-cm depths was not different among locations, and was slower at the 60- to 90-cm depth than in surface soil. Norflurazon dissipation was more rapid under field conditions than under laboratory conditions, with half-lives ranging from 7 to 79 d in the 0- to 8-cm soil horizon. Dry field conditions slowed norflurazon dissipation. Norflurazon was not detected below 15 cm in the profile in any soil, and concentrations in the 8- to 15-cm soil zone were < 36 ppbw 112 d after treatment.

Fluometuron adsorption and dissipation under field and laboratory conditions, and distribution within the soil profile was determined in 3 soils from Tennessee, Mississippi, and Georgia that are representative of the cotton-growing regions of the southeastern United States. Fluometuron adsorption was correlated with organic matter, but not with clay content or soil pH. First-order kinetics explained fluometuron dissipation under field and controlled conditions (r2≥ 0.82). Field dissipation of fluometuron was slower under dry conditions. Fluometuron was not detected below 15 cm in the soil profile in any soil, and concentrations in the 8- to 15-cm soil zone were < 15 ppbw 112 d after treatment. Fluometuron dissipation was more rapid in soil from the 0- to 8-cm depth in Tennessee soil than in Mississippi soil under controlled conditions. Dissipation was more rapid under field conditions than under laboratory conditions at 2 of 3 locations. Fluometuron half-lives in soils from the 0- to 8-cm depth ranged from 9 to 28 d under field conditions and from 11 to 43 d in the laboratory. Fluometuron dissipation in soils from 30- to 45- and 60- to 90-cm depths was not different among soils, with half-lives ranging from 58 to 99 d under laboratory conditions. Fluometuron half-life was positively correlated with soil depth and inversely correlated with organic matter. These data indicate that organic matter, soil depth, and environmental conditions affect fluometuron dissipation.

A model of competition for light between peanut and three broadleaf weed species has been developed to run with the PNUTGRO model. The model simulates shading of the peanut canopy by reducing the total daily PAR received by the peanuts in a manner that realistically represents timing and quantity of light capture by the weeds. Data were col- lected in nursery plots of Florida beggarweed, sicklepod, and wild poinsettia in 1989,1990, and 1991. These data provided the values for the critical parameters: maximum attenuation of PAR by the weed, time when the weed overtops the peanut canopy, time when maximum attenuation is reached, and the distance of influence of the weed. Florida beggarweed over- topped the peanut canopy 52 DAP, and reduced PAR reach- ing the peanuts 45 % by 73 DAP. Sicklepod overtopped the peanut canopy 42 DAP and reached an attenuation of 41% 79 DAP. Wild poinsettia overtopped the peanut canopy 44 DAP, and had an attenuation value of 39% 85 DAP. The distances of influence were 162, 150, and 192 cm for Florida beggarweed, sicklepod, and wild poinsettia, respectively. Ob- served yield losses in the distance of influence were 26, 27, and 22%, respectively. The model predictions accounted for at least 90% of the yield losses observed in field studies. The model also proved capable of simulating competitive differ- ences between morphologicaily and phenologically different populations of Florida beggarweed. Simulation models will play an important role in reducing the expenditure of time and resources required to document yield losses due to weeds in peanuts. Nomenclature: Florida beggarweed, Desmodium tortuosum (Sw.) DC. #3 DEDTO; sicklepod, Cassia obtusifolia L. # CASOB (now-Senna obtusifolia); wild poinsettia, Euphorbia heterophylla L. # EPHHL; peanut, Arachis hy

Research was conducted during 1988 and 1989 in Georgia and Florida to determine the densitydependent effects and critical periods of wild poinsettia interference with peanut. Peanut yield loss ranged from 0 to approximately 50% over the wild poinsettia density range from 0 to 32 plants 9 m−1peanut row. Predicted peanut yield losses across 2 yr in Georgia were 0, 4, 8, 12, 15, 26, 40, and 54% for season-long wild poinsettia interference at densities of 0, 1, 2, 3, 4, 8, 16, and 32 plants 9 m−1of row, respectively. Across 2 yr in Florida, predicted peanut yield losses were 0, 9, 14, 22, 30, 37, and 41% for wild poinsettia densities of 0, 1, 2, 4, 8, 16, and 32 plants 9 m−1row, respectively. Peanuts had to be maintained poinsettia free for 10 wk after peanut emergence to prevent yield loss. Wild poinsettia that interfered with peanut for more than 2 wk after peanut emergence reduced peanut yield.

Research was conducted to determine foliar absorption of14C-clethodim and its polar degradation products with five adjuvants and to compare the relative herbicidal activity of clethodim and its polar degradation products. Differential14C-clethodim and14C-polar degradation products absorption occurred among the five adjuvants. Adjuvants that mediated rapid foliar absorption of clethodim did not always mediate rapid foliar uptake of polar degradation products. Herbicidal activity of clethodim exceeded activity of polar degradation products regardless of time of day of application or whether adjuvant was used. Inclusion of adjuvant was required for herbicidal activity of polar degradation products. Clethodim activity declined when applied at 0800 h EDT compared to applications at 2000 h EDT. To maximize herbicidal activity of clethodim, adjuvants should be selected that mediate rapid foliar absorption of both clethodim and the polar degradation products of clethodim and that minimize photodegradation of clethodim.

The dissipation of flurtamone was determined in three Georgia soils in 1987 to 1989. Flurtamone dissipation in soil was initially rapid but gradually slowed throughout the sampling interval. Dissipation was not affected by rate (0.8 or 1.7 kg ai ha-1) or application method (PPI or PRE). Dissipation rates were slightly more rapid in the Greenville and Dothan soils than in the Cecil soil, with calculated initial half-lives for each respective soil being 6 to 7, 8 to 10, and 9 to 23 days. There was no effect of previous flurtamone exposure on the dissipation rate in soil at any location. Nomenclature: Flurtamone, 5-(methylamino)-2-phenyl-4-[3-(trifluoromethy1)phenyl1-3-(2H)-furanone. Additional index words. RE-40885, persistence, enhanced degradation, peanut, Arachis hypogea L., cotton, Gossypium hirsutum L., grain sorghum, Sorghum bicolor L.

Glasshouse experiments were conducted to eval- uate the effects of preemergence applications of metribuz- in, pendimethalin, alachlor, and imazaquin on the competitive relationship between soybean and sicklepod, tall morningglory, or common cocklebur. An equal-ratio series design was employed where the proportion of species in a mixture remained constant and total plant density was varied. The ratio of soybean:sicklepod fresh weights was altered by alachlor and imazaquin while the ratio of soybean:tall morningglory fresh weights was altered only by imazaquin. The ratio of soybean:common cocklebur fresh weights was altered by metribuzin and imazaquin. Increasing herbicide rates resulted in higher soybean:weed fresh weight ratios and higher herbicide response coefficients. Nomenclature: Metribuzin, 4-amino-6- (1,1-dimethylethyl)-3-(methylthio) - 1,2,4-tria- zin-5(4H)-one; pendimethalin, N-(1-ethylpropyl)-3,4-di- methyl-2,6-dinitrobenzenamine; alachlor, 2-chloro-N-(2,6- diethylphenyl)-N-(methoxymethyl)acetamide; imazaquin, 2-(4,5-dihydro - 4 - methyl - 4 - (1-(methylethyl)- 5-oxo-lH-imidazol-2-yl)-3-quinolinecarboxylic acid; soy- bean, Glycine max (L.) Merr. 'Gordon'; sicklepod, Cassia obtusifolia L. #3 CASOB; tall morningglory, Ipomoea purpurea (L.) Roth. # PHBPU; common cocklebur, Xanthium strumarium L. # XANST. Additional index words. Competition, equal-ratio series, herbicide response coefficient, CASOB, PHBPU, XANST.

Research was conducted from 1982 through 1984 on a Shipps clay soil (Udic chromustert) in east-central Texas to determine the density-dependent effects and critical periods of johnsongrass (Sorghum halepense (L.) Pers. #3 SORHA) competition with cotton (Gossypium hirsutum L.). Two experiments were conducted each year, one to assess cotton yield response to increasing johnsongrass density and the second to establish the johnsongrass-free requirement of cotton and the johnsonigrass competition period. A critical minimum density of 2 plants/9.8 m of row was observed. Full-season johnsongrass competition densities exceeding 2 plants/9.8 m of row resulted in seed cotton yield reductions each year. Yield decline was most rapid as density increased from 2 to 8 plants/9.8 m of row. Yield loss averaged 1, 4, 14, 40, 65, and 70% for johnson- grass densities of 1, 2, 4, 8, 16, and 32 plants/9.8 m of row, respectively. Percent yield loss was proportional to yield potential at johnsongrass densities of 4 and 8 plants/9.8 m of row. Three to four weeks of rhizome johnsongrass competi- tion or 6 weeks of seedling johnsongrass competition re- sulted in significant yield reductions. Maintaining cotton johnsongrass free for 4 weeks did not prevent significant yield reductions. Additional index words. Critical minimum density, com- petitive threshold, interspecific competition, intraspecific competition, Gossypium hirsutum, SORHA.

Research was conducted from 1978 through 1980 at Headland, AL, to evaluate the use of herbicides, mechanical cultivation, and hand-hoeing as components of a total weed control system for peanuts (Arachis hypogaeaL.). Herbicides or two cultivations alone failed to provide acceptable weed control, peanut yields, or net returns for the 3-yr period. However, adding two cultivations to the herbicide treatments produced acceptable weed control, peanut yields, and net returns for the 3-yr period. Average peanut yield for herbicide(s) plus two cultivations was 3200 kg/ha with an average net return of $260/ha. Two cultivations plus two hoeings without herbicide(s) produced good results with an average 3-yr yield of 3380 kg/ha and a net return of $280/ha. Herbicide(s) plus two cultivation treatments that had net returns equal to two cultivations plus two hoeing treatments were: dinoseb (2-sec-butyl-4,6-dinitrophenol) at 5.0 kg ai/ha applied at ground-cracking ($300/ha); dinoseb + alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] at 1.7 + 3.4 kg ai/ha applied at ground-cracking ($310/ha); and alachlor at 4.5 kg/ha applied preemergence ($320/ha). Treatments containing herbicide(s) plus two cultivations plus two hoeings did not substantially improve weed control, peanut yields, or net returns over herbicide(s) plus two cultivations.

Research was conducted from 1983 through 1986 to compare the competitiveness of three cotton cultivars of different height with johnsongrass. Cultivars evaluated were: '1209-619-7', 'TAMCOT SP37-H', and 'Stoneville 213', which had an average height at maturity of 66, 122, and 168 cm, respectively. Each cultivar was grown with johnsongrass densities of 0, 4, and 6 plants/6 m of row with a 1-m row spacing. Johnsongrass biomass data confirmed that all three cultivars did compete with johnsongrass, but regression analyses using cultivar as a covariate revealed no difference in the competitiveness of the three cultivars. Nomenclature: Cotton, Gossypium hirsutum L.; johnsongrass, Sorghum halepense (L.) Pers. #3 SORHA. Additional index words. Cotton varieties, height, interspecific competition, intraspecific competition, weed biomass, SORHA.

Research was conducted from 1979 through 1982 in east-central Alabama to determine the effect of various cropping and sicklepod (Cassia obtusifoliaL. # CASOB) management systems on yields and total net returns to land and management. Intense weed management inputs resulted in higher levels of sicklepod control than did standard inputs with spring tillage, but sicklepod control was good regardless of management intensity with no spring tillage. Sicklepod control in soybean [Glycine max(L.) Merr.] – corn (Zea maysL.) rotations was similar to that in continuous soybeans regardless of planting method. Even though intense management inputs often resulted in increased sicklepod control, total net returns from these systems were often less due to increased herbicide and application costs. Total net returns from summer-fallow systems were generally low since only a winter wheat crop was produced each year.

A population level, two-compartment, temperature- dependent model that predicts date of seedling johnsongrass flowering was formulated. The model consisted of a four- parameter poikilotherm rate equation to describe develop- ment rate as a function of temperature and a temperature- independent Weibull function to distribute flowering times for the population. Experiments were conducted to determine the effect of temperature, nitrogen availability, and water availability on development of seedling johnsongrass. De- velopment was most sensitive to temperature while the effect of nitrogen concentration and water availability was minimum and inconsistent. The model was tested against three indepen- dent field data sets and provided accurate prediction of flowering dates for each data set. Nomenclature: Johnson- grass, Sorghum halepense (L.) Pers. #3 SORHA.

Research was conducted from 1979 through 1982 in east-central Alabama to monitor changes in sickle- pod (Cassia obtusifolia L. #3 CASOB) seed numbers in the soil under various cropping and weed management systems in soybeans (Glycine max (L.) Merr.). Significant declines in sicklepod seed numbers were attained only when mechan- ical summer fallow each year prevented replenishment of seed in soil. Repeated disking favored germination and emer- gence of sicklepod seed and thus caused a reduction in seed numbers. Chemical summer fallow, which relied on post- emergence control of emerged sicklepod, did not result in a disturbed seedbed and was less effective in reducing seed numbers. Permitting subcompetitive densities of sicklepod, 0.45 and 0.90 plants/m2, to reach maturity each year re- sulted in increases in seed numbers in the soil. Increases were more dramatic in conventionally planted soybeans than in no-till culture. Sicklepod produced more pods per plant in tilled culture than in no-till. Sicklepod seed dis- tribution in the upper 30 cm of the soil was not affected by the type of primary tillage. Additional index words. Tillage systems, competition, weed seeds, thresholds, Glycine max, CASOB.

Field experiments were conducted to evaluate placement techniques for preemergence applications of pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine] in grain sorghum [Sorghum bicolor(L.) Moench.]. The first technique consisted of row shields mounted behind the planter units. Shields maintained an untreated strip over the crop drill and allowed successful crop establishment with pendimethalin at 1.1 kg ai/ha, despite a simulated, intense rainfall of 3.8 cm within 24 h after planting. A second technique, which consisted of a special nozzle arrangement, was evaluated in no-till grain sorghum. The nozzle arrangement allowed a broadcast herbicide application but maintained an untreated strip over the crop drill. No stand reductions occurred using this technique at pendimethalin rates of 1.1 and 2.2 kg/ha. In a growth chamber experiment, preemergence applications of pendimethalin severely injured grain sorghum when the soil was wet at the time of emergence, but injury was reduced under hot, dry conditions.