As atmospheric carbon dioxide concentration ([CO2]) increases, it is anticipated that the competitive ability of C3 crops could be enhanced relative to C4 weeds in agricultural systems. However, given the different nitrogen use efficiencies of C3 and C4 plants, it is unclear whether any effect of increasing [CO2] on C3/C4 competition is nitrogen dependent. To determine the interaction of [CO2] and N availability on species growth and competitive outcomes, the growth of rice (C3 photosynthetic pathway) was examined in both monoculture and in competition with a common weed, barnyardgrass (C4 photosynthetic pathway) at two levels of N supply (0.357 and 1.071 mmol N L−1) and two levels of [CO2] (ambient and ambient + 200 µmol mol−1) under field conditions in eastern China. In monoculture, the biomass response of rice to elevated [CO2] depended on N supply, whereas the response of barnyardgrass to elevated [CO2] was less dependent on nitrogen. Consequently, when grown in mixture, the proportion of rice biomass increased relative to that of barnyardgrass under elevated [CO2] if the supply of nitrogen was adequate. However, if N was low, elevated [CO2] significantly reduced the proportion of leaf area and root biomass relative to barnyardgrass biomass. Although data from this experiment confirm that competitiveness of rice could be enhanced relative to C4 weeds in response to rising [CO2] in situ, the data also indicate that such a response could be contingent on the supply of nitrogen. This suggests that, for rice cropping systems where N is in limited supply, rising atmospheric CO2 could still exacerbate competitive losses, even from C4 weeds.

We studied the inactivation of herbicides applied to the soil by activated carbon in the greenhouse and field. Degree of inactivation depended upon herbicide type, dosage, plant species, and quantity of activated carbon applied. In greenhouse experiments with Toa silty clay, 2-chloro-4,6-bis(isopropylamino)-s-triazine (propazine) was inactivated at activated carbon: herbicide ratios of 66:1, whereas 4-amino-3,5,6-trichloropicolinic acid (picloram) required ratios of 3600:1. Beans (Phaseolus vulgaris L., var. Black Valentine) were used as a bioassay. In the field, activated carbon rates up to 600 lb/A did not completely prevent injury to oats (Avena sativa L., var. Markton), cucumbers (Cucumis sativus L., var. Ashly), and Black Valentine beans by propazine at 2.5 lb/A in Toa silty clay 0, 37, 72, and 93 days after herbicide application. Activated carbon protected oats from picloram at 0.5 lb/A but did not completely protect cucumbers or beans at rates up to 600 lb/A. Activated carbon in untreated herbicide soils was not harmful to growth of beans, oats, and cucumbers in Toa silty clay.

Herbicides applied shortly after wheat (Triticum aestivum L.) harvest combined with no tillage or with several minimum tillage treatments were compared with conventional tillage for weed control from wheat harvest 1 year through sorghum (Sorghum bicolor (L.) Moench) production the next year. Three lb/A of 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine) with no tillage did not completely control weeds, but over 4 years of sorghum grain yields on the atrazine-no tillage plots and on conventional tillage plots were not significantly different. One tillage either immediately before or immediately after application of 3 lb/A atrazine controlled essentially all weeds from wheat harvest until fall, but in some years grass weeds emerged in the spring. One pre-planting tillage or one cultivation after sorghum emergence reduced weed competition so that no additional weed control measures were needed. Sorghum yields from those plots were substantially above yields resulting from conventional tillage.

Effectiveness of bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-(4)3H-one 2,2-dioxide] for control of common cocklebur (Xanthium pensylvanicum Wallr.) in soybeans [Glycine max (L.) Merr.] was evaluated in field research over a 3-yr period. Bentazon, applied over-the-top at 0.6 to 1.1 kg/ha, controlled common cocklebur more effectively than over-the-top or directed applications of dinoseb (2-sec-butyl-4,6-dinitrophenol) at 0.8 to 1.7 kg/ha or 2,4-DB [4-(2,4-dichlorophenoxy)butyric acid] at 0.2 kg/ha. Single applications of bentazon were highly effective for control of common cocklebur, but multiple applications usually provided the best control and highest soybean yields. A preemergence treatment of metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazine-5(4H)one] at 1.1 kg/ha and a postemergence application of bentazon at 1.1 kg/ha provided 92 to 99% control of common cocklebur. Control of common cocklebur with bentazon reduced discounts in harvested soybean seed caused by excessive moisture content and foreign material. The reduction in discounts improved soybean seed grades.

Agronomic research on the effects of nitrogen fertilizer and weed control in corn has focused primarily on maintaining or increasing yield. Few studies have examined the effect of nitrogen (N) fertilizer rate or weed competition (or both) on whole plant growth and development. The objectives of this research were to determine how N influences the growth and development of corn and to explore how green foxtail density affects this relationship. Field experiments were conducted on a sandy low organic matter soil from 1999 to 2001. The experiment was designed as a factorial with N rate ranging from 0 to 200 kg N ha−1 and targeted green foxtail density ranging from 0 to 300 plants m−2. Under weed-free conditions, a higher rate of N fertilizer increased corn leaf and grain N content, leaf area index (LAI), plant height, and aboveground dry matter (DM) production, including kernel weight. However, in the presence of green foxtail, corn leaf N content, LAI, growth rate, plant height, and aboveground DM were reduced at each N level. Despite having significant main effects, there was no interaction between N rate and green foxtail density. Results indicate that in corn grown on a coarse-textured soil with low organic matter, the additional stress brought about by the presence of green foxtail exacerbated the effect of low N rates on corn growth and development. More intensive weed management may be required in corn if N fertilizer rates are reduced.

The primary objectives of this research were (1) to characterize intraspecific variation in Powell amaranth seed germination and emergence response to nitrogen fertilization, and (2) to evaluate whether germination and emergence characteristics varied between seeds from populations originating on organic vs. conventional vegetable farms. We hypothesized that nonherbicide–based weed management and use of slower-releasing forms of N on organic farms may have selected for seeds with lower dormancy and lower germination sensitivity to N fertilization than seeds from conventional farms. Seeds were collected from five conventional and five organic vegetable farms in central New York State. A second generation of seeds, produced under common greenhouse conditions and stored for at least 3 mo at 5 C was tested for both germination in petri dishes and emergence in the field under multiple rates of ammonium nitrate (NH4NO3). Both seed germination and emergence were greater for seeds originating from organic compared with conventional vegetable farms. However, seed responsiveness to fertilization did not vary significantly by habitat of origin. Reduced rates or split applications of NH4NO3 significantly reduced emergence in the field in 2003 but had no significant effect on emergence in 2004. Large interpopulation variation in germination and emergence patterns suggests that for Powell amaranth and similar weed species, (1) species-level models of emergence may not be very robust across different farms, and (2) the effectiveness of manipulating emergence through soil fertility practices is likely to vary substantially according to farm and year.

Plants of dormant and nondormant wild oat (Avena fatua L. # AVEFA) lines were grown under temperatures of 15, 20, and 25 C. A number of physiological and morphological characters in the plants and seed of both lines were influenced by temperature. Duration of dormancy in the progeny seed increased in both lines that had experienced low temperatures (15 C) during development, and decreased in seed of both lines that had experienced high temperatures (25 C) during development. High, compared to low, temperatures of development decreased plant height, vegetative and seed development time, seed numbers per plant, seed dry weight, and seed water content.

The growth and development of field horsetail (Equisetum arvense L. # EQUAR) was measured outdoors using potted plants grown from one-node rhizome fragments. Shoot growth rate increased slowly after planting in March, accelerating to a maximum during July with maximas for shoot height during August and shoot number in September. Rhizome system growth rates initially followed a similar pattern to that of the shoots achieving a maximum during July. Dry matter accumulated in the rhizomes until October and declined thereafter throughout the winter. Tubers initiated during August grew in size and number until November, well beyond the period of active shoot growth and onset of senescence. The characteristics of the growth and development of field horsetail are discussed in relation to achieving weed control via cultural and herbicide treatments.

Experiments were conducted to evaluate the effects of preemergence herbicides on the growth characteristics and competitiveness of sicklepod and common cocklebur to soybean. Alachlor was applied at 1.1 or 2.2 kg ai ha–1 and metribuzin at 0.2 or 0.4 kg ai ha–1 in glasshouse and field experiments. Alachlor at 2.2 kg ha–1 reduced sicklepod dry weight and leaf area 4 weeks after planting. Sicklepod water use was initially lowered by both rates of alachlor while soybean and common cocklebur water use was initially lowered by metribuzin at 0.4 kg ha–1. Sicklepod and common cocklebur fresh weights were reduced at soybean harvest by the high rates of alachlor and metribuzin. Soybean competing with sicklepod injured by 1.1 or 2.2 kg ha–1 of alachlor produced yields greater than soybean competing with untreated sicklepod. Soybean yields were not improved when metribuzin was used on common cocklebur.

Field and greenhouse studies were conducted to determine the influence of soil organic matter on herbicidal activity using four Pocomoke soils with organic matter contents of 4.8, 7.0, 11.9, and 17.0%. In field experiments, control of giant green foxtail [Setaria viridis var. major (Gaud.) Posp.] was greater than 75% from all treatments of 2-chloro-2′,6′-diethyl-N-(methoxymethyl) acetanilide (alachlor), 2-chloro-N-isopropylacetanilide (propachlor), and N,N-diallyl-2-chloroacetamide (CDAA) with no statistical differences between chemicals, rates, or organic matter levels. Control was statistically lower from all 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine) treatments except 2.6 kg/ha at 7% organic matter and 4.4 kg/ha at 7 and 17% organic matter. Under greenhouse conditions, giant foxtail (Setaria faberi Hermm.) was used to establish GR50 values on soils or soil mixtures containing 0.8, 1.9, 3.9, 6.4, 11.0, and 18.7% organic matter for the herbicides used in the field study and for 2-chloro-N-(1-methyl-2-propynyl)acetanilide (prynachlor). Generally as organic matter levels increased, the GR50 values for herbicides increased; the rate of GR50 increase varied with herbicides. A ranking of herbicides from most toxic to least toxic is: propachlor>alachlor>prynachlor>CDAA. Atrazine GR50 values varied from a low similar to propachlor at 0.8% organic matter to 20 times greater than propachlor at 17% organic matter.

In semi-arid areas, good control of field bindweed with 1 lb/A of 2,4–dichlorophenoxyacetic acid (2,4–D) will occur if there is at least 1.5 inches of available soil moisture when runners are 6 to 10 inches long. Under these conditions carbohydrates are high in the leaves and low in the roots. Analysis of rainfall records from Amarillo, Texas indicates that soil moisture conditions during May will be favorable for good bindweed control with 2,4–D 54% of the time.

With the introduction of new chemicals for weed control, a tremendous effort has been expended toward finding a chemical weed control method for soybeans. Some of this research has been reported in the Research Reports and Proceedings of the North Central Weed Control Conference from 1946 to date. Much of the effort has been devoted to pre-emergence treatments on soybeans because of the apparent sensitivity of soybeans to foliage sprays of the present day herbicides.

Field studies indicated that liming an acid Bladen silt loam from pH 5.5 to 7.5 increased the phytotoxicity of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] and prometryn [2,4-bis-(isopropylamino)-6-methyoxy-s-triazine]. Liming greatly increased the persistence of atrazine, but did not affect prometryn dissipation. Liming increased the 14C-concentration present in the shoots of corn (Zea Mays L. ‘Pioneer 3369A’), cotton (Gossypium hirsutum L. ‘Coker 201’), and soybeans [Glycine Max (L.) Merr. ‘Ransom’] from soil treated with 14C-ring labeled atrazine, prometryn, and hydroxyatrazine [2-hydroxy-4-(ethylamino)-6-(isopropylamino)-s-triazine] in greenhouse studies. Decreases in 14C-uptake by the crops were associated with adsorption and degradation of the compounds in the soil. Atrazine was taken up in much greater amounts than hydroxyatrazine. Cotton absorbed less of the s-triazines than soybeans or corn from soil.

Group IV Glycine max cultivars, chlorimuron postemergence, and paraquat + sodium chlorate applied as preharvest desiccants were evaluated for their effect on weed seed production and viability. Early-maturing G. max cultivars reduced seed weight, seed production, and seedling growth in Ipomoea lacunosa in most instances and usually reduced seed weight, germination, emergence, and growth in Sesbania exaltata by allowing harvest prior to physiological maturity of these weeds. Tall, late-maturing G. max decreased weed seed production and seed weight of both species, presumably through increased competitiveness of G. max. The seeds produced by weeds that emerged after chlorimuron application were usually lower in weight, germination, and emergence, and growth of seedlings was reduced because most weeds present at the time of application were controlled. Thus, only late-emerging or stunted plants were present to produce seed. Preharvest desiccation of late-maturing S. exaltata parent plants with paraquat + sodium chlorate reduced height and fresh weight of the resulting S. exaltata seedlings 35 to 50% by reducing the amount of time for the S. exaltata seeds to mature.

Basic factors contributing to the rapid evolution and broad distribution of acetolactate synthase (ALS)-inhibiting herbicide resistance in smallflower umbrella sedge L. have not yet been investigated. The objectives of this study were to examine patterns of cross-resistance to ALS herbicides and genetic diversity within and among smallflower umbrella sedge populations in California rice fields to provide insight into the processes contributing to resistance spread. Twelve different patterns of herbicide cross-resistance were found across the 56 populations sampled. The frequency of populations with at least one resistant individual in the North, Central and South Sacramento Valley, and the San Joaquin Valley were 76, 86, 67, and 50%, respectively. Analysis of the genetic diversity of 29 populations using 73 sequence-related amplified polymorphism molecular markers revealed little genetic diversity within populations, with estimates of Nei's gene diversity index, h, ranging from 0 to 0.049, and Shannon's information index (I) ranging from 0 to 0.079. Hierarchical analyses of molecular variance indicated that the majority of genetic variation was partitioned among populations, rather than within populations or among regional groups. No isolation by distance was evident. Unweighted pair group method with arithmetic averages analysis indicated that population clustering was not region specific. The results suggest that resistance to ALS-inhibiting herbicides in smallflower umbrella sedge populations from California rice fields appears to have evolved independently multiple times rather than spread from a single population where resistance originated. Consequently, prevention and management of smallflower umbrella sedge in California rice fields should emphasize in-field strategies that focus on decreasing the selection pressure caused by ALS-inhibiting herbicides.

Sprouting percentage was estimated for purple nutsedge tubers in the field from daily fluctuating soil temperatures. Tuber sprouting under alternating temperatures ranging from 20 to 45 C for 14 d responded quadratically to alternations of high and low temperature. A response surface regression of the cumulative sprouting percentage accounted for 88% of the variation. The cumulative sprouting percentage curves were sigmoidal, and the Richards function satisfactorily regressed the characteristics of the curves. A simulation model was developed for the cumulative sprouting percentage by estimating sprouting from daily high and low temperatures and accumulating daily increments of tuber sprouting. Five weeks of soil solarization with clear polyethylene film at Waimanalo, Hawaii raised the mean soil temperature at 15-cm depth by 5.8 C in spring and by 7.2 C in summer. Solarization also increased the mean daily temperature difference from 1.5 to 3.7 C in spring and from 2.3 to 3.8 C in summer. Solarization increased the final sprouting percentage in the field from 74 to 97% in spring and from 97 to 100% in summer. The simulation model estimated the final field sprouting of tubers within 95% confidence intervals of the observed means.

The absorption and translocation of radioactive 2,4–D in bean plants has been studied by many workers (2, 4, 5, 7). The translocation of radioactive carbon from applied 2,4–D has been demonstrated to be more rapid in susceptible bean plants than in resistant plants such as corn and wheat seedlings (2, 3, 4). The difference in reaction of 2,4–D with plant constituents was further established between these two types of plants (3, 6). A compound designated as “unknown 1” was the major metabolic product formed in 2,4–D treated bean plants while “unknown 3” was found mainly in 2,4–D treated corn and wheat seedlings.

Field experiments were conducted using entireleaf morningglory seed collected from areas of adaptation throughout the United States to determine whether biotypes or ecotypes exist and whether differences in susceptibility to acifluorfen exist. Initiation of first flower for the entireleaf morningglory populations ranged from 63 to 81 d after emergence. The interval between emergence and initiation of first flower decreased 2.8 d for each increase in degree of latitude from which the seed was collected. Plants originating from southern latitudes remained in the vegetative phase longer and tended to produce more total dry-weight biomass than plants originating from northern latitudes. Thus, ecotypes do exist for entireleaf morningglory because of adaptation to a specific environment. The adaptation allows ecotypes to utilize the length of the growing season associated with the area of origin. Trichome density on the adaxial leaf surface ranged from 147 to 206 cm−2 across the ecotypes and was not correlated with latitude or differences in acifluorfen susceptibility. Ecotypes differed in susceptibility to acifluorfen, but resistance among ecotypes was not evident.

Translocation of alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl) acetanilide] in soybean [Glycine max (L.) Merrill] and wheat (Triticum aestivum L.) was primarily apoplastic with greater uptake and translocation in wheat (susceptible) than in soybean (resistant). Root-applied 14C-alachlor translocated throughout both plants. Root uptake was greater for wheat than for soybean and older leaves accumulated more than did younger actively growing tissue in both species. Foliar applications to the primary leaves of soybean and wheat resulted in some apoplastic translocation within the treated leaf of wheat but only slight translocation in soybean. Absorption of 14C-alachlor reached equilibrium at 4 hr by excised wheat root tissue, while absorption continued after 32 hr in excised coleoptile and leaf tissue. Absorption of alachlor by leaf and coleoptile tissue was greater in light than in the dark.

The gross morphological effects of the herbicide 4-methyl-sulphonyl-2,6-dinitro-N,N-dipropylaniline [nitralin] on roots of corn (Zea mays L., var. U. S. 13) suggested abnormal cytological behavior. Roots of corn plants treated preemergence with nitralin developed digitate to globose swelling in the region of active cell division of the root tip. Cytological examination of the affected area showed that the effects of the herbicide were prevention of cell wall formation, enlargement of cells, and extensive replication of nuclei, and suggests that a primary effect of nitralin is on cell division and that this may be a primary mechanism of action as a herbicide.