Using capture-recapture analysis we estimate the effective size of the active Amazon Mechanical Turk (MTurk) population that a typical laboratory can access to be about 7,300 workers. We also estimate that the time taken for half of the workers to leave the MTurk pool and be replaced is about 7 months. Each laboratory has its own population pool which overlaps, often extensively, with the hundreds of other laboratories using MTurk. Our estimate is based on a sample of 114,460 completed sessions from 33,408 unique participants and 689 sessions across seven laboratories in the US, Europe, and Australia from January 2012 to March 2015.

The effectiveness of activated charcoal in protecting seedling cotton (Gossypium hirsutum L. ‘Stoneville 213’) from diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] at 1.78 and 3.55 kg/ha was investigated under field conditions over a 5-yr period. Charcoal did not provide adequate protection when applied through the seed hopper box or sprayed in the seed furrow over each hill. Charcoal at 83 and 167 kg/ha applied as a spot over each hill of cotton gave adequate protection. Shallow incorporation of the charcoal significantly reduced the degree of protection. The optimum combination, providing maximum weed control and minimum crop injury, was diuron at 3.55 kg/ha applied over non-incorporated charcoal at 83 kg/ha.

Greenhouse experiments in central Texas assessed the relative importance of above- and belowground interactions of semidwarf Mit wheat and Marshall ryegrass during vegetative growth. One experiment used partitions to compare the effect of no (controls), aboveground only, belowground only, and full interaction for 75 d after planting (DAP) one wheat and nine ryegrass plants in soil volumes of 90, 950, and 3,800 ml. The results with the different soil volumes were similar. Wheat growth in the aboveground interaction only did not differ from controls. However, the full or belowground only interaction of wheat with ryegrass reduced wheat height, leaf number, tillering, leaf area, percent total nonstructural carbohydrates in shoot, and dry weights of leaves, stems, and roots 45 and 75 DAP compared to controls. Wheat in full and belowground interaction only did not differ from one another in growth. A replacement series experiment of 56 d also showed that the competitive advantage of ryegrass was relatively greater in root than in shoot growth. No allelopathic response of wheat to ryegrass occurred. While the tallness of the semidwarf wheat minimized aboveground interference by ryegrass, the root growth of the thinner and more fibrous roots of ryegrass greatly enhanced its belowground competitiveness.

Research was conducted from 1983 through 1985 in east central Texas to determine the effect of grass-specific herbicides, herbicide rates, and weed height on johnsongrass [Sorghum halepense (L.) Pers. #3 SORHA] control and cotton (Gossypium hirsutum L. 'Stoneville 825’) yield. Seven herbicides were applied to johnsongrass less than 30 cm tall with 2 to 5 leaves approximately 4 weeks after cotton emergence. Good-to-excellent johnsongrass control was observed for each herbicide except cloproxydim {(E,E)-2-[l-[[(3-chloro-2-propenyl)oxy]imino] butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} at 0.56 kg ai/ha and sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} at 0.28 kg ai/ha. Few significant differences were found in seed cotton yields even though johnsongrass control ranged from 43 to 99% 26 days after treatment (DAT). These data suggest that johnsongrass regrowth and/or reinfestation following initial control of 75% or higher during the first 4 to 6 weeks after cotton emergence has a minimum negative effect on cotton yield. Johnsongrass control was highest when either fluazifop-P {(R)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]propanoic acid} or haloxyfop {2-[4-[[3-chloro-5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy] propanoic acid} was applied to 15 to 30 cm tall johnsongrass.

Research was conducted to determine the utility of a single, temperature-independent Weibull function for describing cumulative seed germination under several temperature regimes with 14 sets of weed and crop seed germination data. A modified cumulative Weibull function was derived to distribute germination times for individuals within the population and distributed the occurrence of germination given ample sample size and appropriate sample interval. The descriptive and predictive attributes of the stochastic model component are well suited for incorporation into seed germination models and are likely applicable to models to predict distribution of times for other developmental processes of plants.

In six years of field studies, early-season releases of the native moth Bactra verutana Zeller were evaluated as a biological control for purple nutsedge (Cyperus rotundus L.). Establishment from adult releases required at least 10 pairs of moths per release; establishment from larval release could be achieved with a single release of two or five newly-emerged larvae per shoot. When shoots averaged 4 or 7.5 cm high at the time of such releases, aboveground growth was reduced 32 to 62% after 30 days. Three to eight weekly releases of larvae reduced the topgrowth of nutsedge planted in rows of cotton (Gossypium hirsutum L. ‘Stoneville 213’) 30 to 68% 6 to 8 weeks after the last release. Weekly releases of about five larvae per shoot repeated three, four, or five times in 1976 and 1977 reduced nutsedge growth so that seed cotton yield equalled that of cotton in plots without nutsedge. MSMA (monosodium methanearsonate) applied at the rate of 2.2 kg/ha 5 and 7 weeks after cotton emergence gave greater control of purple nutsedge topgrowth and a yield of seed cotton equal to that resulting from three to five larval releases of about five larvae per shoot beginning when purple nutsedge averaged 7.5 cm high. When MSMA was applied at 7.5 and 8.5 weeks after cotton emergence, the addition of two early-season releases of larvae were also necessary to achieve nutsedge control and seed cotton yield equal to that produced by three to five releases of larvae.

Greenhouse experiments were conducted to elucidate the effects of water stress on photosynthetic parameters of soybean [Glycine max (L.) Merr. ‘Hutton′] and velvetleaf (Abutilon theophrasti Medik. # ABUTH). Stomatal conductance of both species responded curvilinearly to reductions in leaf water potential. At leaf water potentials less negative than −2.5 MPa, stomatal conductance, net photosynthetic rate, and transpiration rate were greater in velvetleaf than in soybean. Soybean photosynthetic rate was linearly related to stomatal conductance. Velvetleaf photosynthetic rate increased linearly with stomatal conductances up to 1.5 cm s–1; however, no increase in photosynthetic rate was observed at stomatal conductances greater than 1.5 cm s–1, indicating nonstomatal limitations to photosynthesis. As water stress intensified, stomatal conductance, photosynthetic rate, and transpiration of velvetleaf declined more rapidly than in soybean.

In a 2-yr field study conducted on a Weswood silt loam soil (Fluventic Ustochrepts), interspecific competition between soybeans [Glycine max (L.) Merr. ‘Hutton′] and velvetleaf (Abutilon theophrasti Medik. # ABUTH) resulted in greater than 40 and 50% reductions in soybean and velvetleaf seed yield, respectively. Leaf area index, number of mainstem nodes, total number of leaves, and plant dry weight of monocultured and intercropped velvetleaf differed significantly as early as 4 weeks after emergence. Interspecific competition had litttle or no effect on soybean morphology before 8 weeks after emergence. Soil water extraction occurred to 1-m depths in a monoculture of velvetleaf (five plants/m2) in 1984 and 1985. Monocultured soybeans (32.5 plants/m2) extracted water from a 1.5-m or greater depth of the soil profile during the same years. Soil water extraction in the intercropped plots resembled that of the monocultured velvetleaf treatment until soybeans attained R6, when soil water was extracted to a 1.5-m depth. The potential for interspecific competition for water existed early in the season before late-season soybean root development. Relative water content and leaf water potential (Ψw1) did not differ (0.05) between monocultured and intercropped soybeans in 1984 or 1985. In 1985, Ψw1 differed between monocultured and intercropped velvetleaf during anthesis. Leaf water potential values in the youngest, fully expanded leaves were approximately 0.3 and 0.4 MPa lower during midmorning and midday hours, respectively, in intercropped and monocultured velvetleaf. Transpiration and stomatal conductance did not differ between monocultured and intercropped soybeans or velvetleaf at any time during 1984. Photosynthetic and transpiration rates, stomatal conductance, and Ψw1 were lower in intercropped than in monocultured velvetleaf during anthesis in 1985, suggesting interspecific competition for soil water. Soybean water relations were not affected in either year. The data suggest that soybean yield reductions in soybean-velvetleaf interspecific competition are attributable to resource limitations other than water in south-central Texas.

Seeds of 20 weed species buried at depths of 8, 23, and 38 cm in 1972 and 1973 were exhumed at regular intervals and tested for viability and germination. Burial depth had little to no significant influence on longevity. After 5.5 yr 48% of johnsongrass [Sorghum halepense (L.) Pers.], 36% of velvetleaf (Abutilon theophrasti Medic.), 33% of purple moonflower (Ipomoea turbinata Lag.), 30% of spurred anoda [Anoda cristata (L.) Schlecht], 18% of hemp sesbania [Sesbania exaltata (Raf.) Cory], and 13% of pitted morningglory (Ipomoea lacunosa L.) seeds were still viable. No more than 6% of the original population for any other species was viable at 5.5 yr. Hard seeds comprised a high percentage of the longer-lived seeds. Seeds of longest-lived species were losing viability at an average of 26% of the existing population each year from 3.5 to 5.5 yr.

A greenhouse experiment compared the vegetative growth in pure cultures and mixtures of winter Triticum aestivum cultivar ‘Mit’ and Lolium multiflorum cultivar ‘Marshall’ in continuously watered controls and drought treatments. Control L. multiflorum in pure culture 14 wk after planting produced more leaf area, tillers, and dry weights of stem and root than control T. aestivum in pure culture. The greater seed size, larger initial leaf area, and height allowed T. aestivum to produce greater final leaf area and dry stem weight in control mixtures than L. multiflorum. Watering following drought shifted the relative performance of the two species in pure cultures and mixtures compared to controls. The ability of T. aestivum to maintain a greater leaf expansion rate during drought and a greater leaf area afterward than L, multiflorum allowed T. aestivum to attain greater growth than L. multiflorum in pure cultures exposed to temporary drought followed by watering. Conversely, drought and its relief enhanced the relative competitiveness of L. multiflorum compared to controls in mixtures with T. aestivum. During 4 wk of watering following the drought, L. multiflorum in mixtures grew vigorously and was similar to T. aestivum in all measures except in height and dry stem weight. Thus, L. multiflorum was similar in root growth with T. aestivum in control and drought mixtures and had its aboveground competitiveness amplified by the cycle of drought and watering in this study. There was no evidence of an allelopathic interaction between the two species.

Preliminary greenhouse studies indicated the presence of a synergistic interaction between low rates of fluometuron [1,1-dimethyl-3-(α,α,α-trifluoro-m-tolyl)urea] or diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] in combination with linuron [3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea] or chlorbromuron [3-(4-bromo-3-chlorophenyl)-1-methoxy-1-methylurea]. In a 3-yr field study, moderate injury of cotton (Gossypium hirsutum L. ‘Stoneville 213’) with no yield reduction was observed with fluometuron or diuron at 1.68 and 1.12 kg/ha, respectively, but higher rates (5.04 and 3.36 kg/ha, respectively) caused injury and yield reductions. Soybean [Glycine max (L.) Merr. ‘Hill’] injury ratings during the second year indicated a synergistic interaction between residue from fluometuron at 5.04 kg/ha combined with linuron at 2.48 kg/ha or chlorbromuron at 3.85 kg/ha, and from diuron at 3.36 kg/ha combined with linuron at 2.48 kg/ha. These interactions were not apparent in soybean yield data. Soybean injury the third year ranged from 30 to 60% with single or repeated applications of linuron or chlorbromuron, although yields were not affected. Residual fluometuron and diuron from 2 yr of application at 5.04 and 3.36 kg/ha, respectively, injured soybeans, but did not reduce yields. Significant synergistic interactions were observed through injury and yield measurements after two annual applications of fluometuron at 5.04 kg/ha combined with linuron or chlorbromuron. A synergistic interaction was detected in soybean yields with chlorbromuron at 3.85 kg/ha preceded by two annual applications of diuron at 3.36 kg/ha. Such interactions may occur under field conditions, but are unlikely to be economically significant. Chemical analysis indicated little evidence of accumulation, even with repeated application of fluometuron or diuron at 5.04 and 3.36 kg/ha, respectively.

In 1972, a 50-yr study of seed longevity was initiated at Stoneville, Mississippi. Weed seeds were collected from 20 locallygrown species and mixed with soil. Replicate samples were placed in polypropylene screen envelopes and buried at soil depths of 8, 23, and 38 cm. Seed germination and viability were determined by germination tests and 2,3,5-triphenyl-tetrazolium chloride (TTC) treatments with recovered seeds. In general, the depth of burial had little effect on seed longevity. Based on the averages of the means at the three depths, the percentages of seeds still viable after burial for 2.5 yr were: spurred anoda [Anoda cristata (L.) Schlecht.] 71%; purple moonflower (Ipomoea turbinata Lagasca y Segura) 71%; johnsongrass [Sorghum halepense (L.) Pers.] 62%; velvetleaf (Abutilon theophrasti Medic.) 58%; goosegrass [Eleusine indica (L.) Gaertn.] 33%; hemp sesbania [Sesbania exaltata (Raf.) Cory] 29%; Texas panicum (Panicum taxanum Buckl.) 24%; common cocklebur (Xanthium pensylvanicum Wallr.) 18%; common eveningprimrose (Oenothera biennis L.) 14%; large crabgrass [Digitaria sanguinalis (L.) Scop.] 12%; sicklepod (Cassia obtusifolia L.) 10%; common purslane (Portulaca oleracea L.) 10%; white morningglory (Ipomoea lacunosa L.) 8%; redroot pigweed (Amaranthus retroflexus L.) 7%; prostrate spurge (Euphorbia supina Raf.) 6%; prickly sida (Sida spinosa L.) 5%; redvine (Brunnichia cirrhosa Gaertn.) 3%; Florida beggarweed [Desmodium tortuosum (Sw.) DC.] 3%; barnyardgrass [Echinochloa crus-galli (L.) Beauv.] 1%; and chickweed [Stellaria media (L.) Cyrillo] 0%.

An experimental directed sprayer has been improved for use in conservation-tillage and no-tillage field conditions and variable row widths. The directed sprayer consists of a rear-mounted toolbar with individually flotational spraying units on lightweight runners which move over field surfaces without the collection of loose residues. Improvements allow convenient adjustments which combined lateral, vertical, angular, and fore-and-aft repositionings to correctly aim the hydraulic nozzles for desired pesticide targeting. Such new designs are needed to deliver pesticides to targets at minimum application rates and with minimum environmental impacts.

The influence of seed age, planting depth, and temperature on the germination and emergence of ivyleaf morningglory [Ipomoea hederacea (L.) Jacq. var. hederacea], white morningglory (Ipomoea lacunosa L.), and entireleaf morningglory (Ipomoea hederacea var. integriuscula Gray) was studied by using pure seed of each taxa that were collected in 1972, 1973, and 1974. Germination of acid-scarified white and entireleaf morningglory seed ranged from 64 to 98% at continuous 20, 25, 30, 35, and 40 C and at an alternating 20/30 C cycle. The ivyleaf morningglory germination range was 50 to 98% at these temperatures, except at 40 C germination was <25%. Germination of nonscarified seed was generally 30 to 60% lower than scarified seed for the three taxa. Germination of seed collected in 1972 for all three taxa was less than 10% with 6 days of exposure to 45 C and 100% relative humidity, and germination of seed collected in 1973 and 1974 ceased with 12 days of exposure. Maximum emergence for all taxa occurred at planting depths of 1.3 and 2.5 cm. Seed age influenced seedling emergence of white morningglory and ivyleaf morningglory but not entireleaf morningglory. Total numbers of seed produced per plant for white, entireleaf, and ivyleaf morningglory were 15,200, 14,600, and 5,800, respectively.

The feasibility of herbicide and crop rotation for the control of johnsongrass [Sorghum halepense (L.) Pers.] in corn (Zea mays L.) was studied in field experiments. Light infestations of johnsongrass were initially present, but it became the predominant weed after 4 yr of continuous corn treated with atrazine [2-chloro-4-(ethylamino)-6-(isopropylamine)-s-triazine], cyanazine {2-[[4-chloro-6-(ethylamino)-s-triazin-2-yl]amino]-2-methylpropionitrile}, and linuron [3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea], alone and in combinations at rates of 2.24, 2.24, and 0.84 kg/ha, respectively. The infestation of johnsongrass was effectively controlled by growing corn in rotation with cotton (Gossypium hirsutum L.) in a cropping sequence of corn-cotton-cotton-corn, in which trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine), fluometuron [1,1-dimethyl-3-(α,α,α-trifluoro-m-tolyl)urea] and MSMA (monosodium methanearsonate) at 0.56, 2.24, and 2.24 kg/ha respectively, were used for weed control in cotton. In the corn-cotton-cotton-corn cropping sequence, the herbicide treatments also prevented increases in the populations of other indigeneous weeds including prickly sida (Sida spinosa L.), prostrate spurge (Euphorbia supina Raf.), spotted spurge (Euphorbia maculata L.), common purslane (Portulaca oleracea L.), tall morningglory [Ipomoea purpurea (L.) Roth], common cocklebur (Xanthium pensylvanicum Wallr.), spurred anoda [Anoda cristata (L.) Schlecht.], hemp sesbania [Sesbania exaltata (Raf.) Cory], redroot pigweed (Amaranthus retroflexus L.), goosegrass [Eleusine indica (L.) Gaertn.], junglerice [Echinochloa colonum (L.) Link], large crabgrass [Digitaria sanguinalis (L.) Scop.], and purple nutsedge (Cyperus rotundus L.).

Alligatorweed is a perennial, invasive weed in southern United States rice production, but knowledge on effective management of this weed is limited, especially in conventional (non-imidazolinone-resistant) rice fields. Field studies were conducted in multiple environments in southeastern Texas to evaluate different herbicide options involving penoxsulam, propanil, triclopyr, halosulfuron, bispyribac-sodium, bensulfuron, and quinclorac for alligatorweed control in conventional drill-seeded rice when applied at early POST (EPOST), late POST (LPOST), or both. Among the herbicide options evaluated, penoxsulam alone (up to 83%), penoxsulam plus triclopyr (up to 87%), or bispyribac-sodium plus triclopyr (92%) provided superior alligatorweed control. Plots treated with penoxsulam plus triclopyr EPOST produced the highest yields (9,550 kg ha−1), which were comparable to plots receiving penoxsulam plus triclopyr LPOST (9,320 kg ha−1), penoxsulam alone EPOST (9,280 kg ha−1), and penoxsulam plus halosulfuron LPOST (9,180 kg ha−1). Considering both weed control and rice grain yields, penoxsulam plus triclopyr applied EPOST was found to be the best option among the treatments tested. The treatments bensulfuron alone, bensulfuron plus propanil, penoxsulam plus propanil, triclopyr plus propanil, and bispyribac-sodium plus propanil provided poor (≤ 65%) alligatorweed control. Results also suggest the likelihood for antagonistic interactions when tank-mix combinations tested in this study included propanil.