Cotton, spurred anoda, and velvetleaf were grown in controlled-environment chambers at day/night temperatures of 32/23 or 26/17 C and CO2 concentrations of 350 or 700 ppm. After 5 weeks, CO2 enrichment to 700 ppm increased dry matter accumulation by 38, 26, and 29% in cotton, spurred anoda, and velvetleaf, respectively, at 26/17 C and by 61, 41, and 29% at 32/23 C. Increases in leaf weight accounted for over 80% of the increase in total plant weight in cotton and spurred anoda in both temperature regimes. Leaf area was not increased by CO2 enrichment. The observed increases in dry matter production with CO2 enrichment were caused by increased net assimilation rate. In a second experiment, plants were grown at 350 ppm CO2 and 29/23 C day/night for 17 days before exposure to 700 ppm CO2 at 26/17 C for 1 week. Short-term exposure to high CO2 significantly increased net assimilation rate, dry matter production, total dry weight, leaf dry weight, and specific leaf weight in comparison with plants maintained at 350 ppm CO2 at 26/17 C. Increases in leaf weight in response to short-term CO2 enrichment accounted for 100, 87, and 68% of the observed increase in total plant dry weight of cotton, spurred anoda, and velvetleaf, respectively. Comparisons among the species showed that CO2 enrichment decreased the weed/crop ratio for total dry weight, possibly indicating a potential competitive advantage for cotton under elevated CO2, even at suboptimum temperatures.

Resistance in waterhemp to herbicides that inhibit protoporphyrinogen oxidase (PPO) previously was shown to result from the deletion of a glycine codon at position 210 (ΔG210) in the PPO-encoding gene, PPX2. Research was conducted to determine if this same mechanism accounted for resistance in geographically separated populations—from Illinois, Kansas, and Missouri—and, if so, to determine if the mutation conferring resistance was independently selected. A dose–response study with lactofen indicated that the resistant populations had different levels of resistance. These differences, however, could be accounted for by different frequencies of resistant individuals within populations and, therefore, the dose–response data were consistent with the hypothesis that the populations contained the same resistance mechanism. Direct evidence in support of this hypothesis was provided by DNA sequencing, which showed that nearly all resistant plants evaluated contained the ΔG210 mutation. A variable region of the PPX2 gene was sequenced and resulting sequences were aligned and organized into a phylogenetic tree. The phylogenetic tree did not reveal clear clustering by either geography or phenotype (resistant vs. sensitive). Possibly recombination within the PPX2 gene has masked its evolutionary history.

The herbicide, 3–(p–chlorophenyl)–1,1–dimethylurea (CMU), first reported by Bucha and Todd (1), is finding increasing uses in the field of weed control. The mechanism of action of this compound, however, is as yet unknown. One of the early symptoms of CMU toxicity is a chlorosis (2) which develops in both monocotyledonous and dicotyledonous species. Muzik et al. (5) have shown that while CMU will inhibit root growth, the primary toxic action of this compound is on the aerial portion of the plant. Cooke (3) showed that CMU brought about a large decrease in the sugar content of plants and suggested that the mode of action was through an interference with photosynthesis.

The effects of bristly starbur interference with peanuts were studied from 1980 through 1983 on a Dothan sandy loam (Plinthic Paleudults). Peanut seed yields, which included kernels plus hulls, were reduced as bristly starbur densities increased from 2 to 64 plants/7.5 m of row. Full-season interference from 8, 16, 32, and 64 bristly starbur plants/7.5 m of row reduced peanut seed yields 14, 26, 43, and 50%, respectively. The seed yield of peanuts maintained weed free for 6 weeks after crop emergence was reduced by no more than 3% compared to peanuts maintained weed free for the entire season. Weed interference for 2 weeks after crop emergence reduced seed yield by an average of 4% over the 3-yr period, based on prediction equations. Bristly starbur interference for 13 weeks after crop emergence reduced peanut forage dry weight yield by 54%. Similarly, peanut interference for 13 weeks after emergence reduced bristly starbur forage dry weight yield by 32%. Bristly starbur density in the forage dry weight studies averaged 35/7.5 m of row. These bristly starbur were confined to a 35-cm band over the drill. Peanut density averaged 72/7.5 m of drill.

Field experiments were conducted at Stuttgart, AR, from 1986 to 1988 to determine effects of season-long interference of red rice densities of 1, 2, 5, 10, 20, and 40 plants m–2 on ‘Lemont’ and ‘Newbonnet’ rice cultivars. This interference reduced straw dry weights of Newbonnet and Lemont by 100 and 130 kg ha–1 per red rice plant, and grain yields by 178 and 272 kg ha–1 per red rice plant, respectively. Grain yield reductions were due to decreases in panicle number and length, and in number of grains per panicle. Ten plants m–2 or more reduced height of Lemont, while 40 plants m−2 were required to reduce height of Newbonnet. Red rice at 10 plants m−2 or more reduced total milled and head rice yields of Lemont, but red rice did not affect these components for Newbonnet. Red rice interfered with rice, even at densities as low as two plants m−2, and interference was greater in Lemont, a semidwarf cultivar with mature plants 92 cm tall, than in Newbonnet, a conventional cultivar with mature plants 115 cm tall. Red rice produced more panicles m−2 and 31 to 64% greater straw dry weight when grown with Lemont than when grown with Newbonnet because red rice shaded Lemont more than Newbonnet. Red rice plants grew taller as red rice densities increased and also were taller when grown in Newbonnet than when grown in Lemont.

The use of 1,8-naphthalic anhydride, at 0.5% by corn (Zea mays L. ‘501D’) seed weight, eliminated most of the corn yield loss caused by S-ethyl dipropylthiocarbamate (EPTC) at 3.4, 6.7, or 10.0 kg/ha. The seed protectant partially reduced the moderate corn yield loss from S-ethyl diisobutylthiocarbamate (butylate) at 9.0 kg/ha. This seed protectant provides a new approach to increased weed control while maintaining selectivity in the production of corn.

Absorption and translocation of 14C-glyphosate [N-(phosphonomethyl)glycine] by moisture-stressed common milkweed (Asclepias syriaca L. ♯ ASCSY) was studied in greenhouse and growth chamber experiments. Water-stressed [13% (w/w) soil moisture] common milkweed plants treated with glyphosate at 1.1 kg ae/ha produced shoot regrowth equal to untreated plants, whereas shoot regrowth of nonstressed [25% (w/w) soil moisture] glyphosate-treated plants was only 6% of untreated plants. All shoot regrowth originated from buds on the proximal half of roots. Common milkweed plants, maintained at 25% soil moisture, absorbed 44% of the 14C-glyphosate applied and translocated 20% from the treated leaf, whereas plants at 13% soil moisture absorbed 29% and translocated 7%. Wiping the leaf with tissue paper wetted with distilled water or chloroform prior to 14C-glyphosate application increased absorption from 35 (unwiped leaves) to 62 and 77%, respectively, for plants at 25% soil moisture, and increased absorption from 14 to 42 and 46%, respectively, for plants at 13% soil moisture. Wiping failed to increase translocation out of the treated leaf for plants at either soil moisture regime. Latex samples taken from the abaxial leaf surface opposite 14C-glyphosate-treated leaves and from petioles of treated leaves did not contain 14C, indicating that glyphosate did not enter laticifers. Proximal root buds accumulated less 14C-radioactivity than distal root buds and had lower respiratory rates, suggesting that proximal root buds are more dormant than distal root buds and thus accumulate less glyphosate.

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.

Intensive field surveys were conducted in eastern Nebraska to determine the frequency distribution model and associated parameters of broadleaf and grass weed seedling populations. The negative binomial distribution consistently fit the data over time (1992 to 1993) and space (fields) for both the inter and intrarow broadleaf and grass weed seedling populations. The other distributions tested (Poisson with zeros, Neyman type A, logarithmic with zeros, and Poisson-binomial) did not fit the data as consistently as the negative binomial distribution. Associated with the negative binomial distribution is a k parameter. k is a nonspatial aggregation parameter related to the variance at a given mean value. The k parameter of the negative binomial distribution was consistent across weed density for individual weed species in a given field except for foxtail spp. populations. Stability of the k parameter across field sites was assessed using the likelihood ratio test There was no stable or common k value across field sites and years for all weed species populations. The lack of stability in k across field sites is of concern, because this parameter is used extensively in the development of parametric sequential sampling procedures. Because k is not stable across field sites, k must be estimated at the time of sampling. Understanding the variability in it is critical to the development of parametric sequential sampling strategies and understanding the dynamics of weed species in the field.

Laboratory experiments were conducted to determine the mechanism of resistance to diclofop in two wild oat biotypes (designated ‘B’ and ‘C’ biotypes) from the Willamette Valley of Oregon. Resistance could not be attributed to differential absorption, translocation, or metabolism of diclofop. Resistance was not correlated with membrane plasmalemma repolarization following diclofop acid treatment. Compared to a susceptible (’S') wild oat biotype, acetyl CoA carboxylase from the B and C biotypes showed a 10.3 and 4.5 fold increase in the level of resistance, respectively, to diclofop acid. Cross-resistance to fenoxaprop acid was 5.5 and 7.3 times higher in the B and C biotypes, respectively than the S biotype. Correlation between resistance at the whole plant level and at the ACCase level was good for diclofop and fenoxaprop in the B biotype. For the C biotype, this correlation was not as good. Possible reasons for the discrepancy are given.

The effects of 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine), N,N-dimethyl-2,2-diphenyl-acetamide (diphenamid), and N-(p-bromophenyl)-N'-methyl-N'-methoxyurea (metobromuron) upon autotrophic growth of four unicellular green algae were studied. Growth of Chlamydomonas reinhardi Dangeard was completely inhibited by 1 μg/ml metobromuron or 0.5 μg/ml atrazine and was stimulated by diphenamid. Five μg/ml metobromuron were toxic to Chlamydomonas eugametos Moewus, while atrazine and diphenamid had little effect on growth. Growth of Chlorella vulgaris Beijerinck was partially inhibited by atrazine and stimulated by diphenamid and metobromuron. At high concentrations, atrazine or metobromuron inhibited Chlorella pyrenoidosa Chick while growth was stimulated by diphenamid. The results obtained by treating with a combination of atrazine and metobromuron varied greatly with the organism. The effect of these three herbicides upon heterotrophic growth of Chlamydomonas reinhardi also was investigated. Both metobromuron and diphenamid were toxic in the dark. Atrazine had no effect upon heterotrophic growth.

The transfer of the imazethapyr-resistant gene from Clearfield™ (CL) rice to red rice is an ecological risk. Flowering synchronization and genetic compatibility between cultivated rice and red rice could influence gene transfer. We examined the (1) variability in maximum outcrossing rate between 12 red rice biotypes and ‘CL161’ rice during their peak flowering overlap in the field and (2) genetic compatibility of red rice biotypes with CL161 rice. Experiments were conducted at Stuttgart, AR, and Fayetteville, AR, from 2005 to 2007. To evaluate the flowering synchrony of red rice and CL161 rice as well as its impact on outcrossing rate, field experiments were conducted at four planting times from early April to late May. The red rice biotypes were planted in the middle row of nine-row CL161 plots and flowering was monitored. Outcrosses were evaluated in subsequent years by herbicide response and simple-sequence-repeat marker assays. To determine compatibility, manual crosses were performed between 12 red rice biotypes and CL161 rice in the greenhouse. The flowering duration of all red rice types ranged from 5 to 16 d after the onset of flowering in contrast to 6 d in CL161 rice. Ten of the twelve types of red rice had ≥ 70% overlap in flowering time with CL161 rice in at least one planting date. The maximum field outcrossing rate between red rice biotypes and CL161 ranged from 0.03 to 0.25%. The field outcrossing rate between red rice biotypes differed (P < 0.01), but flowering synchronization was not directly related to outcrossing rate. Manual crosses resulted in seed sets of 49 to 94%. The majority of red rice biotypes had similar compatibility with CL161 rice. Thus, other factors must contribute to hybridization rates in the field. Follow-up experiments should investigate other plant factors and environmental influence on hybridization rate.

The concept of integrated pest management (IPM) was first introduced in the 1960s. Since then many definitions of IPM have appeared in the literature. According to the 1988 U.S. National IPM Coordinating Committee, the primary goals of IPM programs are to reduce pesticide use and the subsequent environmental impact and to rely more on alternative strategies to control pests. Integrated weed management (IWM) comes under the umbrella of IPM with similar objectives of using multiple management tactics and incorporating the knowledge of weed biology and crop physiology into the weed management system. The goals of IWM range from maximizing profit margins to safeguarding natural resources and minimizing the negative impact of weed control practices on the environment. The acceptance of IWM by farmers will depend on their perceived risk to management, individual management capability, and environmental interactions that will influence the economic viability of the cropping system. We have been in this process of developing, integrating, and adopting potential IWM practices for the last 25 to 30 yr. However, strategic directions for research in the future must continue to discover opportunities for enhanced profitability and sustainability.

Dose-response studies estimating GR40 values indicated different levels of propanil resistance in junglerice populations from fields previously treated with propanil, compared to a check population collected where this herbicide had never been used. The GR40 for susceptible populations ranged from 0.36 to 0.50 kg ai ha−1 and for resistant populations ranged from 1.10 to 3.10 kg ai ha−1. Considerable variability in growth and morphology existed among populations. Variability in cumulative leaf area, aboveground biomass, mean relative growth rate, mean net assimilation rate, and mean leaf area ratio could not be related to propanil resistance. Competitiveness was not related to propanil resistance either. of several vegetative and reproductive parameters measured at maturity, only grain weight per plant and number of grains per plant were correlated with GR40 (r = −0.73, P = 0.06). This trend towards lower reproductive fitness in propanil-resistant junglerice plants may reduce its ecological success when growing with propanil-susceptible plants in the absence of this herbicide.

Laboratory, greenhouse, and field studies were conducted to determine the allelopathic potential of lantana and Siam weed on the germination and growth of five crops. Emergence and dry weight of bioassay species were affected when lantana or Siam weed debris was present on the soil surface or incorporated into the soil. However, presence of Siam weed on the soil surface did not affect dry weight of rape. Emergence and dry weight of crops (except emergence of spinach) were not affected when crops were grown in soil samples collected from beneath field-grown lantana. Dry weight of Chinese cabbage and chilli was reduced when these crops were grown in Siam weed-contaminated soil. Germination of Chinese cabbage, chilli, and rape decreased progressively when exposed to increasing concentrations of aqueous lantana extract. However, the lantana extract at full strength (66.7 g L−1) did not reduce germination of spinach and cucumber seed. Siam weed extract, when applied at full strength to seed of spinach, Chinese cabbage, rape, and chilli, reduced germination by 10, 12, 21, and 19% of control, respectively. Full-strength extract of lantana and Siam weed decreased seedling length and fresh weight of all crops.

The effect of the organophosphate insecticide malathion on the response of resistant rigid ryegrass population SLR 31 to the herbicides trifluralin, pendimethalin, clomazone, and triallate was investigated. The insecticide and herbicides were soil-applied prior to emergence of plants grown under controlled conditions. In the absence of malathion, the resistant population exhibited significant resistance to the four herbicides compared with a susceptible population. Levels of resistance, as determined by comparison of herbicide rates required to inhibit growth by 50%, were 35, 11, 2.4, and 2.4 for pendimethalin, trifluralin, triallate, and clomazone, respectively. Malathion had a synergistic effect on pendimethalin in the resistant population but not in the susceptible population. Malathion had no synergistic effect on trifluralin, triallate and clomazone. Resistance to triallate and clomazone was found despite the fact that the resistant population had never before been selected with these herbicides. This resistance, selected by other herbicides, further indicates that the use of alternative herbicides to control multiple resistant weeds is unlikely to be a successful resistance management approach.

Field studies were conducted at Florence and Clemson, South Carolina to measure the influence of soybean [Glycine max (L.) Merr.] planting dates on the length of early-season weed control needed to prevent yield reductions, the rate of shade development, and suppression of annual weeds by soybeans. The rate of shade development was similar for both planting dates during the 9- to 11-week period after planting for Florence and Clemson, respectively. The period of weed-free maintenance required to prevent soybean yield reductions was not affected by planting dates. With cultivation between rows, early- and late-planted soybeans required 3 weeks of weed-free maintenance to achieve maximum yields. Lower weed weights resulted from late than early soybean plantings. At Clemson, 3 weeks of weed-free maintenance for early and late plantings reduced weed weights 97 and 91%, respectively. Weed weights at Florence were reduced 85% with 3 weeks of weed-free maintenance for the late plantings, whereas 5 weeks were required to reduce weed weights 88% for early plantings.

In field studies, bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-(4)3H-one 2,2-dioxide] was applied as postemergence sprays over the top of weeds and soybeans [Glycine max (L.) Merr.]. Bentazon at 0.84 to 1.68 kg/ha applied as an early postemergence treatment controlled wild mustard [Brassica kaber (DC.) L.C. Wheeler var. pinnatifida (Stokes) L.C. Wheeler], common ragweed (Ambrosia artemisiifolia L.), velvetleaf (Abutilon theophrasti Medic.), Pennsylvania smartweed, (Polygonum pensylvanicum L.), common cocklebur (Xanthium pensylvanicum Wallr.), and wild common sunflower (Helianthus annuus L.). Pigweeds (Amaranthus sp.) were controlled by applications in the three true-leaf stage but became more resistant at later stages. Control of common lambsquarters (Chenopodium album L.) was erratic. The optimum time for controlling weeds with bentazon was around the first trifoliolate stage of soybeans. Rainfall within several hours after treatment reduced weed control. Eight yield studies, two of which included eight cultivars, were conducted on weed-free soybeans. In none were yields reduced significantly by bentazon at 3.36 kg/ha (the highest rate studied). Eight yield studies were conducted on soybeans infested with common cocklebur or velvetleaf. Weed control was generally excellent with 0.84 kg/ha of bentazon. Where infestations were sufficient to reduce yields, bentazon treatments increased the yields to levels generally comparable with those of the handweeded checks. One exception was an application of bentazon to soybeans growing in a low area that was periodically flooded by heavy rains. In that experiment the benefit of controlling common cocklebur was offset by bentazon injury to the soybeans, and yields from the treated plots were about the same as those of the weedy check.

Greenhouse studies were conducted to determine the response of common lambsquarters and velvetleaf to glyphosate applied alone or with 20 g L−1 of ammonium sulfate (AMS). Minimal response of common lambsquarters to glyphosate plus AMS was observed. The GR50 values for velvetleaf decreased dramatically from 451 to 92 g ha−1 for glyphosate applied alone and glyphosate plus AMS, respectively. The addition of AMS did not affect foliar absorption of 14C-glyphosate in common lambsquarters but increased absorption in velvetleaf. A twofold increase in translocation, as a percentage of total 14C-glyphosate absorbed, occurred in velvetleaf with the addition of AMS. Increased control of velvetleaf with glyphosate plus AMS may be partially explained by greater glyphosate absorption and translocation. Increased translocation of glyphosate applied with AMS in velvetleaf was an indirect effect of greater foliar uptake as well as greater partitioning of glyphosate out of the treated leaf.

In three replacement series experiments, winter wheat (Triticum aestivum L.), jointed goatgrass (Aegilops cylindrica Host. #3 AEGCY), and downy brome (Bromus tectorum L. # BROTE) were paired in all possible combinations to determine competitive relationships during vegetative growth. Under growth chamber conditions of ample fertility and soil moisture and day/night temperatures of 18/10 C, relative yield totals for the three species were similar, indicating that they compete for the same resources. Both winter wheat and jointed goatgrass had greater plant growth and higher relative crowding coefficients than downy brome, which indicated a hierarchy of relative competitiveness of winter wheat > jointed goatgrass >> downy brome. In other growth chamber studies, winter wheat was slightly more competitive than jointed goatgrass regardless of fertility levels. Winter wheat was the superior competitor at 18/10 C and −33 kPa (soil moisture), whereas jointed goatgrass was superior at 27/10 C and −300 kPa, conditions that are frequently encountered in the Pacific Northwest.