A number of the substituted symmetrical triazines have been shown to possess considerable herbicidal activity. Their high biological effectiveness against a wide spectrum of plants has aroused much interest in their use. While they have been found useful as soil sterilants, they are particularly valuable as selective herbicides on a number of crops. Two of the more promising triazines are simazine (2–chloro–4,6–bis(ethylamino)–s–triazine) and atrazine (2–chloro–4–ethylamino–6–isopropylamino–s–triazine). Both of these compounds have found extensive use in the control of weeds in corn. For this reason, it was of interest to determine the absorption, translocation and metabolism of these compounds by corn plants. The studies of this nature reported here were undertaken using C14 ring-labeled triazines. The labeling of the triazine was accomplished using C14 tagged cyanogen chloride in the synthesis, thus giving a random labeled ring.

The addition of 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine) to nutrient solutions of corn (Zea mays L.) increased the percent of N and total mg of N in one test regardless of the N level in the solution. Under the low N level, the magnitude of this increase was 90 percent with 0.5 ppm simazine. However, in a second experiment, under environmental conditions more favorable for corn growth, the total mg of N was not increased, although the percent nitrogen in the shoots was increased at the lower nitrogen level. Corn plants treated with hydroxy simazine responded similarly to the controls.

Root sections of Canada thistles (Cirsium arvense (L.) Scop.) from 10 locations in Montana, Idaho, Washington, and Wyoming were planted at Bozeman, Montana. Leaf margins varied from deeply ruffled and lobed with numerous spines, to entire margins essentially without spines. Flowers varied in color from shades of purple to pale blue and white. Seed size ranged from 300,000 to 673,000 seeds per pound. The earliest eco-type emerged about 2 weeks before the latest and shoot and flower development and height of plants were varied.

Multiyear field data from Spain were used to model seedling emergence for three bedstraw species (Galium) that can coexist in winter cereal fields. The relationships between cumulative emergence and both growing degree days (GDD) and hydrothermal time (HTT) in soil were analyzed as sigmoid growth functions (Weibull). Iterations of base temperature and base water potential were used to optimize the HTT scale. All species were well described with Weibull functions. Both GDD and HTT models provided good descriptions of catchweed bedstraw emergence, as its seedlings have less dependence on soil water potential than false cleavers and threehorn bedstraw, which were described best with HTT. The HTT model for catchweed bedstraw was validated successfully with independent data from the United Kingdom. The models may be useful for predicting bedstraw emergence in semiarid Mediterranean regions and elsewhere.

Multiple-herbicide resistance represents an added weed management challenge to growers as it can considerably reduce their options for weed control. The widespread nature of triazine resistance in Ontario coupled with the more recent appearance of resistance to ALS inhibitors in Amaranthus species warranted documenting biotypes with multiple resistance. A collection of Powell amaranth and redroot pigweed biotypes that had previously been characterized for resistance to ALS inhibitors was therefore screened with atrazine. Dose–response analysis with atrazine and imazethapyr was also conducted. High-level resistance to imazethapyr and atrazine was determined in a Powell amaranth biotype from Perth County, Ontario. This biotype had a > 1,860-fold and 109-fold resistance to atrazine and imazethapyr, respectively. Sequence analysis was conducted for the psbA and ALS genes that code for the target sites of the triazines and imidazolinones, respectively. A mutation in the psbA gene was identified that coded for an amino acid substitution of glycine for serine at residue 264 of the D1 protein. This mutation is the most likely cause for triazine resistance in this biotype. Similarly, a nucleotide substitution was identified that codes for threonine in place of serine at position 652 of the ALS protein. This mutation in the ALS gene has only been observed previously in laboratory-selected mutants of arabidopsis and tobacco and is known to endow resistance to imidazolinones in plants. It is concluded that multiple resistance in this Powell amaranth biotype is due to the presence of altered target sites for triazine and imidazolinone herbicides.

The degradation of 14C ring-labeled 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine) was studied in the laboratory with four Hawaiian soils selected for their wide range of organic matter contents (4 to 29%), pH values (4.4 to 6.3), and different mineralogies. Atrazine concentrations of 1, 5, 10, 50 and 100 ppmw in soils incubated at 30 C gave similar 14C-recovery curves. Degradation was rapid in all four soils; atrazine recovery at 34 days (30 C) ranged from 15 to 30% of the quantity applied. Degradation was accelerated by a temperature increase from 30 to 50 C, suggesting a chemical rather than a biological process. The loss of atrazine from soils approached a first order reaction rate at 30 C, and deviated from this at 50 C. Hydroxyatrazine was the main degradation product with only a slight amount of an unidentified “Product B”. It was concluded that chemical degradation (hydrolysis) was the major pathway of atrazine loss in these soils. This process was more closely related to pH, i.e. faster on low pH soils, than with organic matter content or adsorption. The fraction of atrazine adsorbed on soil increased with the decrease in total atrazine caused by degradation. Extraction of soils with both water and methanol and subsequent analysis of extracts by thin-layer chromatography showed that some of the methanol-extractable atrazine was not readily desorbed by water. These results indicate that some of the residual atrazine extracted chemically may not be available for uptake by plants and would be relatively immobile in soil water.

Decomposition of 11 dinitroaniline herbicides was determined following 7 days exposure to unfiltered solar radiation in July and compared with that in the dark. Radiolabeled herbicides were applied at 1 kg/ha to dry soil thin layer plates. Decomposition of all dinitroanilines was higher when exposed to sunlight than if held in the dark under otherwise similar conditions. Thermal decomposition was noted for all dinitroanilines except profluralin [N-(cyclopropylmethyl)-α,α,α-trifluoro-2,6-dinitro-N-propyl-p-toluidine] and dinitramine (N4,N4-diethyl-α,α,α-trifluoro-3,5-dinitrotoluene-2,4-diamine). Photodecomposition for each herbicide was: 7.8% AC 92390 [N-(1-ethylpropyl)-2,6-dinitro-3,4-dinitro-3,4-xylidine], 8.2% isopropalin (2,6-dinitro-N,N-dipropylcumidine), 9.0% butralin [4-(1,1-dimethylethyl)-N-(1-methylpropyl)-2,6-dinitrobenzenamine], 9.9% pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine], 17.1% benefin (N-butyl-N-ethyl-α,α,α-trifluoro-2,6-dinitro-p-toluidine), 18.4% trifluralin (α,α,α-trifluoro-2,6-dinitro-N-N-dipropyl-p-toluidine), 26.6% oryzalin (3,5-dinitro-N4,N4-dipropylsulfanilamide), 30.4% fluchoralin [N-(2-chloroethyl)-2,6-dinitro-N-propyl-4-(trifluoromethyl)aniline], 40.6% nitralin [4-(methylsulfonyl)-2,6-dinitro-N,N-dipropylaniline], 47.6% profluralin, and 72.3% dinitramine.

Soil profile distribution of a single simulated seed dispersal event after tillage followed a nonlinear model. Approximately 40% of recovered simulated seed were found within 4 cm of the surface following chisel plowing and disking, and density declined steadily with depth to 20 cm. Moldboard plowing placed 50 to 60% of simulated seed to a depth of 11 to 16 cm. Simulated seed were evenly distributed to 8 cm depth by disking and did not differ regardless of whether plots were chisel or moldboard plowed prior to seed dispersal. More than 90% of seed remained within 2 cm of the surface with no-tillage. Simulated seed distribution was the same after a tillage rotation of chisel plowing, disking, moldboard plowing, and disking regardless if the dispersal event occurred prior to chisel or moldboard plowing. Two years of the tillage rotation and two dispersal events resulted in a distribution pattern greatly influenced by the most recent major tillage of chisel or moldboard plowing. Seedling emergence of common milkweed averaged 80% for seed buried 0.5 to 4 cm deep and emergence was negligible for seed on the surface or buried 7 cm in soil in pots in a growth chamber.

The use of eucalyptus leaves for weed control in maize-based cropping systems is proposed. Aqueous extracts of eucalyptus are known to exert phytotoxicity on many weeds and crops, but there is also experimental evidence of the relative tolerance of maize. Based on in vitro dose-response bioassays of leaf aqueous extracts, we conducted greenhouse pot experiments testing incorporated eucalyptus leaves as green manure. The phytotoxic effects were tested on the germination, establishment and growth of maize and some representative accompanying weeds, in comparison to the PRE herbicide metolachlor. Eucalyptus fresh leaves incorporated into the soil as green manure at 1 and 2% w/w reduced the emergence of the dicot weed species redroot pigweed and black nightshade. After one month of incorporation, both doses reduced aerial biomass >94% two monocot weed species (barnyardgrass + large crabgrass) with respect to the eucalyptus-free pots, and around 80% for the small seeded dicots. Although the aerial biomass of maize was reduced by 33%, the final relative yield of maize biomass with respect to the untreated control increased by 37%. On the assessment of the temporal phytotoxic effects, the reduction of aerial biomass in maize could be overcome by adopting a relay-planting of maize after 12 to 15 days from eucalyptus incorporation. Our results constitute evidence that the incorporation of E. globulus residues to soil could be a feasible practice to reduce the reliance on synthetic herbicides in maize-based cropping systems.

A strain of Fusarium oxysporum was isolated from infected sicklepod stems in Stoneville, MS, in 1989. When formulated as granules either in a fungus-infested rice preparation or encapsulated in a wheat-gluten matrix called ‘Pesta’, biocontrol of sicklepod, coffee senna, and hemp sesbania was achieved with preemergence or preplant-incorporated treatments. Liquid conidial applications were less effective. Plants were killed by a preemergence damping-off. Applications made postemergence were significantly less effective. Results of these tests suggest that this fungus has potential as a mycoherbicide for controlling sicklepod, coffee senna, and hemp sesbania.

In greenhouse replacement series experiments, ‘Atlantic’ and ‘Russet Burbank’ potatoes and barnyardgrass were more competitive than redroot pigweed as measured by relative competitive ability. In additive design field studies, four redroot pigweed or barnyardgrass per meter of row did not reduce Atlantic tuber yield when planted between the row following hilling (6 to 7 weeks after planting). However, as few as one barnyardgrass or redroot pigweed per meter of row reduced marketable tuber yield 19 to 33% when seeded in the row at the time of potato planting. Redroot pigweed reduced tuber yield more than barnyardgrass did in 1 yr of research. The correlation coefficients were similar for both weed density and weed biomass regressed on tuber yield. Aboveground potato biomass was not consistent in predicting total tuber yield. Furthermore, variability in aboveground potato biomass was not due to increasing weed weight or density. Neither specific gravity nor tuber quality was altered by the presence of any density of either weed species.

Laboratory studies were conducted to examine the leaf surface, epicuticular wax content, and spread area of primisulfuron spray droplet with and without surfactant on leaf surface of barnyardgrass and green foxtail. Adaxial and abaxial leaf surfaces were examined using scanning electron microscopy and leaf wax was extracted and quantified. The spread of 1-μl droplets of distilled water, primisulfuron solution (without surfactant), primisulfuron solution with a nonionic low foam wetter/spreader adjuvant (0.25% v/v), and with an organosilicone wetting agent (0.1% v/v) was determined on the adaxial leaf surfaces of each of the weed species. Stomata and trichomes were present on adaxial and abaxial leaf surfaces in both species. Green foxtail had more stomata per unit area on the adaxial as compared to the abaxial leaf surface. Barnyardgrass had more stomata on the abaxial than on the adaxial leaf surface. There was no significant variation in the number of trichomes per unit leaf area of green foxtail, and the number of prickles per unit area of leaf was significantly higher in adaxial than the abaxial leaf surface, in both young and old leaves. In barnyardgrass, there were more trichomes on abaxial than adaxial leaf surface. The mean value of the wax content per unit of leaf area in barnyardgrass and green foxtail was 35.9 μg cm−2 and 19.1 μg cm−2, respectively. On both species primisulfuron with a nonionic surfactant had more spread area than that without a surfactant, and the spread was even greater with organosilicone wetting agent. The spread area of primisulfuron droplet was higher on the leaf surface of barnyardgrass than on green foxtail when surfactant was added.

Field trials indicated that addition of ammonium sulfate to imazethapyr plus nonionic surfactant increased quackgrass control, especially at low imazethapyr rates. In greenhouse experiments, approximately twice as much imazethapyr was absorbed by quackgrass leaves when the herbicide was applied in combination with nonionic surfactant plus ammonium sulfate than when the herbicide was applied with nonionic surfactant alone. Black Mexican Sweet maize (BMS) suspension-cultured cells were used to evaluate the effects of ammonium sulfate and nonionic surfactant on cellular absorption of imazethapyr in the absence of a cuticular barrier. Imazethapyr absorption by BMS cells was diffusion-mediated, energy-dependent, and exhibited a pH optimum of approximately 3. Over the concentration range of 0.1 to 10.0 μM, the equilibrium concentration of imazethapyr in BMS cells was a linear function of the external concentration. Addition of ammonium sulfate to the external medium of BMS cells enhanced both the rate of imazethapyr uptake and medium acidification. There was a linear correlation between the ability of ammonium sulfate (0.5 to 10 mM) to promote medium acidification and imazethapyr uptake by BMS cells. The ammonium sulfate-induced stimulation of imazethapyr absorption in BMS cells was sensitive to plasma membrane adenosine triphosphatase inhibitors (sodium vanadate, diethylstilbestrol), the uncoupler carbonyl cyanide m-chlorophenylhydrazone, and energy metabolism inhibitors (sodium azide, nitrogen gas), demonstrating that this effect was dependent on ATP production and the functioning of the plasma membrane ATPase. It is hypothesized that cytoplasmic acidification in BMS cells due to ammonium assimilation stimulates the plasma membrane ATPase to pump protons across the plasma membrane which in turn acidifies the cell wall promoting cellular accumulation of imazethapyr by ion-trapping. Cell wall acidification due to ammonium assimilation may contribute to the ability of ammonium sulfate to enhance the efficacy of imazethapyr and other foliar-applied herbicides.

In 1996 a common sunflower population near Howard, SD, was suspected to be cross-resistant to imazethapyr and chlorimuron. Whole-plant acetolactate synthase (ALS) assays confirmed ALS-inhibitor resistance in the Howard biotype. The I 50 values (inhibition of 50% of the enzyme activity) indicated that the resistant population required 39 and 9 times more imazethapyr and chlorimuron, respectively, to obtain the same level of enzyme inhibition compared with the sensitive biotype. Herbicide dose response data supported the whole-plant enzyme assay data; control (> 90%) was not achieved with less than a four-times application rate of chlorimuron. Control with imazethapyr was not achieved even with a 16-times rate. Chlorimuron and imazethapyr controlled 70 and 95% of the population, respectively, when a four-times rate of each herbicide was applied separately. Differences in 14C-herbicide absorption were observed, suggesting that there may be physical or chemical differences in leaf surface composition between the resistant and sensitive biotypes. Although translocation of 14C-herbicide was less in the resistant biotype than in the sensitive biotype, the differences were not enough to explain chlorimuron and imazethapyr selectivity between the two biotypes. Overall results suggested that the differences in the common sunflower populations were attributed to an altered site of action on the ALS enzyme.

We studied germination of seeds of 44 morphologically distinct, vegetative ecotypes of johnsongrass (Sorghum halepense (L.) Pers.). Seeds of most ecotypes were highly dormant. Without low temperature pretreatment, best germination occurred with continuous fluorescent light, a KNO3 substratum, and an alternating 20 to 35 C temperature regime. Pretreatment of the seeds for 2 weeks at 10 C, followed by a temperature shift to 40 C for 2 hr was effective in breaking seed dormancy. Germination of each of 12 ecotypes, from harvests in each of 2 years, was markedly similar. Interpretation of the evidence suggests that seed dormancy is largely imposed by the relative degree of mechanical restriction afforded by the seed coat.

Seeding date and the duration of weed emergence influenced the duration of the critical weed-free period in carrot. The critical weed-free period extended up to 930 growing degree days (GDD), when carrot was seeded in late April. In contrast, the critical weed-free period was short and lasted 414 to 444 GDD, when seeded in mid to late May and weed biomass was less than 650 g m−2. It is important for growers to scout fields for weeds until 930 GDD to protect the yield potential of the carrot crop in earlier planted crops; however, for carrot planted in mid to late May, weeds emerging after 444 GDD did not reduce yield. A useful strategy to reduce reliance on herbicide application would be to delay planting until late in May.

Experiments were established in 1982 to study the loss of viability of spotted knapweed seeds (achenes) in soil. Greater than 50 and 25% of buried seeds remained viable but dormant after 5 and 8 yr at two locations, respectively. Decline in seed viability was also measured in two natural seedbanks. Although the soil seed reserve decreased by 95% over a 7-yr period, approximately 400 000 viable seeds per ha remained, indicating that spotted knapweed seeds will last for many years in soil.

Field experiments were conducted to study the phenology and growth of broadbean (Vicia faba L. ‘Alameda’) and crenate broomrape (Orobanche crenata Forsk. # ORACR) in relation to planting date. The establishment of crenate broomrape on broadbean planted between mid-December and mid-January coincided with the flower initiation on the crop. Establishment occurred during the vegetative phase, several weeks before flowering from a mid-November and mid-October planting. In general, no consistent relationships were observed between growth of the host and attachment or subsequent growth stage of the parasite. The maximum number of crenate broomrape plants successfully attached to the host as well as the duration of the underground stage of crenate broomrape decreased as broadbean planting was delayed. Broadbean dry weight decreased as infection rate increased. This effect was more intense in early than late plantings.

Most nonnative plants that have become naturalized and even invasive were deliberately introduced into their new range—prompting the quip that we have often “invited trouble” by enthusiastically importing species as putative sources of food, fiber, or fuel without assessing their potential damage. Gant reed is a case in point: a large, rapidly growing grass introduced long ago in the United States that has already become a riparian invader in California and Texas. Concern about the ability of giant reed to wreak environmental damage has taken on new urgency as it is now being touted as a potential biofuel feedstock. Ambitious proposals call for creation of huge plantations devoted to the grass's cultivation. Needed is neutral, comprehensive, transparent accounting of the pros and cons (literally, the credits and debits) of the widespread planting of nonnative biofuel candidates, especially giant reed. Otherwise, the United States could embark on plans to introduce giant reed throughout a much larger new range, while simultaneously paying for its removal—obviously conflicted policies.

Tubers from purple nutsedge (Cyperus rotundus L.) plants grown in the greenhouse for 2 to 10 weeks did not germinate after foliar applications of 4 kg/ha glyphosate (N-(phosphonomethyl)glycine). Some tubers from 12- and 24-week-old purple nutsedge plants survived glyphosate application. Translocation of 14C-glyphosate from treated purple nutsedge leaves to other plant parts increased from 5% of the amount applied at 1 day to 19% at 4 days after application. Specific activity of 14C in tubers was greater than in leaves of plants 2- to 6-week-old. With increasing plant age, specific activity decreased in both tubers and leaves. As purple nutsedge grew older, total 14C translocated increased in tubers, and decreased slightly in leaves. Thin layer chromatography showed no evidence of glyphosate metabolism in purple nutsedge.