Rhizomes of johnsongrass [Sorghum halepense (L.) Pers.], cut into 2.5-cm or 15-cm lengths, were planted in the field within and below a 7.5-cm layer of soil mixed with trifluralin (a,a,a-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) at 0, 1.7, or 3.4 kg/ha. When planted 3.8- and 10-cm deep in nontreated soil, survival was 90% and 69%, respectively, for the longer rhizomes and only 39% and 10%, respectively, for the shorter ones. When planted within trifluralin-treated soil, 95% and 100% of the shorter rhizomes and 54% and 81% of the longer rhizomes were killed at 1.7 and 3.4 kg/ha, respectively. When planted below treated soil, only 11% or 19% of the longer rhizomes were killed. The shorter rhizomes planted below the treated soil survived at less than 10%.

The influence of fall (late September or late October)-applied dicamba (3,6-dichloro-2-methoxybenzoic acid) on spring-planted soybean [Glycine max (L.) Merr.] and sunflower (Helianthus annuus L.) was studied at five sites in Minnesota during 1983 and 1984. At the two southern Minnesota locations, only late-October applications of dicamba at the highest rate (4.5 kg ae/ha) reduced yields of spring-planted soybean and sunflower. At the central and the two northern Minnesota locations, both late-September and late-October applications of dicamba at 1.1 kg/ha and higher caused injury and yield reductions to soybean and sunflower planted the following year. The two southern Minnesota locations had generally higher levels of precipitation, higher temperatures, higher organic matter soils, and lower pH levels compared to the central and northern Minnesota locations. These climatic and soil characteristics reduced dicamba persistence at the two southern Minnesota locations.

The translocation of foliar-applied 14C-labelled 2,4-D [(2,4-dichlorophenoxy)acetic acid] in seedlings of leafy spurge (Euphorbia esula L.) was investigated. Approximately 90% of the tracer was extracted from the tissues with 80% ethanol. Seven days after treatment 48% of the 14C in the shoot extract and 75% of that extracted from the root were identified chromatographically as 2,4-D. Translocation out of the treated leaves and into the shoot, root, and root buds was significantly increased both by decapitation of the shoot and by increasing the nitrogen supply. This effect is attributed to the marked promotion of bud growth produced by both of these treatments.

Gas exchange of jointed goatgrass leaves was affected by temperature, irradiance level, and soil matric potential. Net photosynthesis of leaves under saturating irradiance (PPFD3 = 1850 (μE·m–2·s−1) was optimum at about 20 C. At 25 C, net photosynthesis was nearly 90% of maximum at a PPFD of 800 μE·m–2·−1. Transpiration, and presumably water use, increased steadily with temperature from 10 to 40 C. Dark respiration rate and compensation points for light and for CO2 increased exponentially, or nearly so, from 10 to 40 C. Soil moisture deficits of −130 kPa reduced net photosynthesis and transpiration by about 30 and 55%, respectively, compared to well-watered plants.

Absorption, translocation, and metabolism of AC 263,222 by leafy spurge were studied over 8 d. Based on the amount of herbicide applied and recovered from the leaf surfaces, 40% of applied AC 263,222 was absorbed by leafy spurge 2 d after treatment (DAT), with no further absorption observed by 8 DAT. Eight DAT, 19% of applied [14C]-AC 263,222 had translocated to below-ground plant parts while 4% was exuded from the roots into the sand media. AC 263,222 was not metabolized 2 DAT in the crown, root, and root buds, but 42% was metabolized in the treated leaves. Only 17% of recovered [14C] was AC 263,222 in treated leaves 8 DAT (83% metabolized), while AC 263,222 accounted for 70% of recovered [14C] in the root and root buds. HPLC analysis indicated that the balance of [14C] was associated with a single, polar metabolite. Total recovery of [14C] was 88% at 8 DAT.

In greenhouse studies, atrazine (2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine killed Canada thistle [Cirsium arvense (L.) Scop.] when applied as preemergence or postemergence treatments. Postemergence atrazine applications caused more rapid kill when allowed to cover the ground as well as the foliage compared to only foliar atrazine applications. In three field experiments conducted in 1967, 1968, and 1969, Canada thistle was controlled with atrazine and amitrole (3-amino-s-triazole) + NH4SCN. Control resulting from atrazine was as good or superior to amitrole + NH4SCN when both were applied to foliage prior to plowing. Control from 2.2 kg/ha of atrazine applied prior to plowing was 78% for the 3-year average. Atrazine applied preemergence at 2.2 kg/ha resulted in inferior control, whereas postemergence atrazine applications resulted in thistle control similar to treatments applied prior to plowing.

Corn and giant foxtail response to foliar-applied sethoxydim at 67, 134, and 200 g ai/ha was evaluated in field studies. Sethoxydim applied over the top of corn (60 cm tall) caused greater whorl damage and reduced corn grain yield more than postdirected sethoxydim. Sethoxydim controlled giant foxtail best when used in conjunction with a preemergence application of metolachlor (2.2 kg/ha) and atrazine (1.7 kg/ha).

A sequential extraction procedure was developed to study the fate of pendimethalin [N- (1 -ethylpropyl) -3,4- dimethyl - 2,6 - dinitrobenzenamine], trifluralin (α,α,α -trifluoro - 2,6 - dinitro - N,N - dipropyl - p - toluidine), and oryzalin (3,5 -dinitro-N4,N4 -dipropylsulfanilamide) in soil over a 6 - month laboratory incubation period. The distinction of non - bound residues was based on an acidic - methanol extraction followed by an extraction with an acetone :H2O : HCl (95:4:1, v/v/v) solution and then extraction with 0.5 N NaOH with further fractionation of the NaOH extract by partial precipitation. After 6 months, soil - bound 14C-residues of pendimethalin, trifluralin, and oryzalin contained 15, 14, and 23%, respectively, of the radioactivity applied to non - sterilized soil. The proportion of bound radioactivity associated with each organic fraction was nearly identical for each herbicide; 46, 12, 12, and 30% was associated with the acetone:H2O:HCl-soluble, fulvic acid, humic acid, and humin organic fractions, respectively. Soil sterilization reduced herbicide degradation by 31% and radioactivity in the fulvic acid, humic acid, and humin organic fractions by approximately 50% during the 6-month period. Radioactivity in the acetone:H2O:HC1-soluble fraction was reduced less than 10% by soil sterilization.

Large Crabgrass (Digitaria sanguinalis (L.) Scop.) and barnyardgrass (Echinochloa crusgalli (L.) Beauv.) plants were grown from seedlings to maturity under five light qualities of equal energy levels. For crabgrass, both fresh and dry weight yields of plant tops grown under red light and under cool white light were significantly greater than those under green, yellow, or blue light. Length of stem followed this same order. Effects of light quality on reproduction were not as consistent; green and blue light caused the largest numbers of seed heads to form, while yellow light delayed flowering and resulted in the least number and weight of seed heads. Somewhat similar responses to light were found with barnyardgrass plants, except that yields under yellow light were close to those under red light for this species. Blue light caused smallest plants in both kinds of weeds.

Volunteer corn (VC) in hybrid corn has become more prevalent in recent years and can reduce grain yield. Nitrogen (N) management can influence VC interference in corn. Field experiments were established to determine the effects of N fertilizer management and VC interference on hybrid corn growth and grain yield. Treatments consisted of three VC densities (control, 0 plants m−2; low density, 1 plant m−2; high density, 4 plants m−2) and six N fertilizer treatments (0 kg N ha−1, 67 kg N ha−1 at planting, 67 kg N ha−1 at planting + 133 kg N ha−1 at V5 corn growth stage, 67 kg N ha−1 at planting + 133 kg N ha−1 at V10 corn growth stage, 200 kg N ha−1 at V5 corn growth stage, and 200 kg N ha−1 at V10 corn growth stage). The effect of VC on hybrid corn was dependent on N rate. When 200 kg N ha−1 was applied, regardless of application timing, hybrid corn dry weight, hybrid corn N content, and hybrid corn grain yield were reduced by the high VC density. However, when VC grain yield was added to hybrid corn grain yield, VC density did not affect total grain yield. When 0 and 67 kg N ha−1 were applied, neither hybrid corn dry weight nor hybrid corn N content was affected by either VC density, but the high VC density reduced hybrid corn grain yield for both N rates by 19% and total grain yield by 9 and 10%, respectively. Application timing of N fertilizer had no effect on hybrid corn dry weight, N content, or grain yield. However, late N fertilizer applications (200 kg N ha−1 at V10 and 67 kg N ha−1 at planting +133 kg N ha−1 at V10) resulted in greater VC N content, VC grain yield, and total yield. Assuming the harvestability of VC, the ability of a late N treatment (V10) to maximize total grain yield allows growers to use a late N application to reduce the competitive effects of VC in hybrid corn.

In many cases, the logical presentation of scientific results masks the actual unfolding of experimental progress. Our work to identify the mechanisms by which chemicals break red rice seed dormancy is presented as an example. Doing the simplest experiments first, allowing the data to drive subsequent work, frequent and aggressive study of published literature, and examination of potentially key findings using multiple, interlocking strategies have been the keys to defining the relationship between the structure and action of dormancy-breaking chemicals. Our experiments show that chemical activity is dependent upon the lipophilicity as well as the nature and position of functional groups. However, the chemical that is applied to a dormant seed is not necessarily the chemical that actually breaks dormancy. A combination of approaches using structure–activity studies, metabolic inhibitors, and physiological measurements indicate that alcohols must first be metabolized to carboxylic acids to elicit germination of red rice. This report summarizes these findings as a personal account, illustrating the importance of logic, luck, and imagination in scientific discovery, as well as to highlight some important lessons about conducting research.

Quinoclamine is an herbicide under development for control of liverwort, a weed common in nursery crops. With respect to liverwort control, quinoclamine has been considered to primarily have POST activity. However, some PRE activity has been reported. Growth media sorption studies with 14C-quinoclamine indicate that only 0.64% of the quinoclamine amount that enters the media remains unadsorbed and thus available to be taken up by established plants or propagules. Computer modeling revealed that a large portion of the surface of the quinoclamine molecule is positively charged, which likely is the reason for its high adsorptivity. In a simulation of PRE activity, hydroponically grown liverwort and germinating gemmae were exposed to increasing quinoclamine concentrations. Phytotoxicity to both plants and gemmae was obtained with a minimal concentration of 4 to 6 mg L−1. Based upon the projected use rate, and assuming minimal vertical infiltration depth, the theoretical concentration of quinoclamine within the aqueous phase of a pine bark substrate would be approximately 8 mg L−1. In toto, results indicate that the projected use rate will result in sufficient quinoclamine in the aqueous phase of a pine bark substrate to provide PRE control of gemmae propagules as well as to contribute to the efficacy of POST applications to established liverwort.

Previous evidence indicates that changes in well-defined phases of dormancy in underground adventitious buds of leafy spurge in late summer and autumn are regulated by complex sensing and signaling pathways involving aboveground sugar signals. However, little information exists concerning seasonal photosynthesis and carbohydrate partitioning of leafy spurge, although such information would help to elucidate the involvement of sugar in controlling bud dormancy. An outdoor study was conducted over two growing seasons to determine and model seasonal patterns of photosynthesis and aboveground carbohydrate partitioning and their relationship to underground adventitious bud carbohydrate status. Photosynthesis and total nonstructural carbohydrate (TNC) content of aboveground tissues was greatest during vegetative growth. Photosynthesis gradually declined over the growing season, whereas TNC decreased sharply during flowering, followed by a gradual decline between midsummer and autumn. Leaf starch increased dramatically to midsummer before declining sharply throughout late summer and early autumn, whereas sucrose content responded inversely, indicating a mobilization of starch reserves and export of sugars to overwintering belowground sink tissues. Because newly formed underground adventitious buds showed a continuous increase in TNC from midsummer through autumn, export of sugars from aboveground tissues likely contributed to the increase in TNC. These results may facilitate new strategies for biological control of leafy spurge.

Aminocyclopyrachlor has provided excellent control of many perennial weed species including leafy spurge, but control of yellow toadflax has been inconsistent. 14C-aminocyclopyrachlor absorption was rapid in both leafy spurge and yellow toadflax and averaged 72% 48 h after treatment (HAT). However, translocation within the plant differed by species. More 14C translocated to the aboveground portion of yellow toadflax (28% of applied) compared to leafy spurge (16.5% of applied). There was rapid translocation of 14C-label to the roots of both species but more reached the belowground portion of leafy spurge than yellow toadflax. Over 12% of applied 14C translocated into leafy spurge roots within 24 HAT but declined to 2% by 192 HAT. In comparison, only 2% of applied 14C was found in yellow toadflax roots 24 HAT, and just 0.15% remained in belowground plant parts by 192 HAT. The inconsistent long-term control of yellow toadflax with aminocyclopyrachlor is likely due to poor translocation to the root system, which would allow for rapid regrowth in this hard to control perennial species.

Buthidazole {3-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-4-hydroxy-1-methyl-2-imidazolidinone} may be useful for selective weed control in corn (Zea mays L.) but injures corn seedlings at high rates. Greenhouse studies were initiated to test potential antidotes for buthidazole injury to corn seedlings. NA (1,8-naphthalic anhydride) and CDAA (N,N-diallyl-2-chloroacetamide) were the most effective of six chemicals evaluated. A ratio of 1:3 (buthidazole: CDAA) appeared to be optimal for the protection effect. CDAA appeared to be more effective than NA. Since both CDAA and NA offered limited protection but R-25788 did not, this action appears to be through a different mechanism than the one proposed for the thiocarbamate or acetanilide herbicides. The other chemicals tested were R-25788 (2,2-dichloro-N,N-diallylacetamide), R-29148 (2,2-dimethyl-5-methyl-dichloroace-tyloxazolidine), carboxin (2,3-dihydro-5-carboxanilido-6-methyl-1,4-oxanthiin) and gibberellin (GA3). Seven herbicides, members of the acetanilide, thiocarbamate, and dinitroaniline groups, were also tested for their antagonistic interactions but none offered protection.

Absorption and translocation of buthidazole [3-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-4-hydroxy-1-methyl-2-imidazolidinone] was determined in several plant tissues. Buthidazole caused slight effects on seed germination. Germination of soybean (Glycine max L. Merr. ‘Williams’) seeds was inhibited up to 15% and velvetleaf (Abutilon theophrasti Medic.) was stimulated up to 13%. Buthidazole was absorbed by germinating soybeans, velvetleaf, and corn (Zea mays L.) with rate of absorption increasing when roots were capable of absorption. Buthidazole concentrations of 0.5 μM or greater inhibited growth of soybean in the early second-trifoliolate growth stage when supplied to the roots in nutrient solution. Within 96 h, 29% of the buthidazole available to soybeans was absorbed from nutrient solution and 89% of that absorbed was found in the shoots. The expanded leaves of soybean accumulated the majority of the radioactivity. Radioactivity in roots of soybean approached a steady state condition within 96 h, whereas 14C continued to increase in shoots. Foliar absorption of buthidazole in soybean greatly increased when any one of several surfactants were used. Soybean dry weight reduction nearly doubled by adding surfactants. Over 60% of the 14C-buthidazole applied to soybean leaves was absorbed within 1.5 h, and increased to 73% by 96 h when the nonionic surfactant, AL-411-F3 [Phytobland Spray Oil (83%) plus ATPLUS 300F (17%)] was used compared to only 7% in 96 h without the surfactant. There was little or no movement of buthidazole from the treated leaves. Translocation was typical of apoplastic movement.

Intercropping is an eco-friendly approach for reducing weed problems through nonchemical methods. Intercrop effects on weed community structure have rarely been studied. A 6-yr study was initiated in 1999 and the response of aboveground weed flora (1999–2002 and 2005) and seed bank (2005) to the intercropping of saffron and black zira, two perennial crops was investigated. Mixtures consisted of 0/100, 25/75, 50/50, 75/25, and 100/0 saffron/black zira ratios, each planted at three densities: 30, 50, and 70 plant m−2. The effect of planting density on weed populations was variable and in most cases not significant. However, mixture ratios caused drastic species compositional changes in the weed community for which univariate and multivariate analyses explored four major associations: (1) weeds that favored a higher ratio of saffron in mixtures (e.g., grasses, field bindweed, pigweeds), (2) weeds that preferred a higher ratio of black zira in mixtures (e.g., Persian speedwell, Brassicaceae complex, Polygoaceae complex, and earthsmoke), (3) weeds that were more abundant in 50/50 mixtures (e.g., Caryophyllaceae complex), and (4) weeds that showed no specific pattern (e.g., common lambsquarterss). Pigweeds, prostrate knotweed, and common lambsquarters dominated the viable seed bank with relative densities of 48, 28, and 8%, respectively. The seed bank of most weed species responded to mixture ratios in a similar manner to those of their corresponding aboveground flora. Seed density decreased as soil depth increased, leading to the accumulation of 66, 22, and 12% of viable seeds in soil layers of 0–5, 5–15 and 15–25 cm, respectively. Greater weed and seed densities were found in more pure stands of black zira. These findings contribute to improving current understanding of crop–weed community structures and may help in developing weed management practices.

Defoliation can affect plant competition in at least two ways: negative effects on the performance of defoliated plants and positive effects on nondefoliated plants due to neighbor defoliation. It has been suggested that these effects can alter competitive relations between plants, and this study was designed to test this in an experiment using Crofton weed as the invasive category, and its two neighbors Japanese brome and Dichrocephala as the native category. The plants were grown from seed and potted either singly or in pairs. The pairs consisted either of two conspecific plants (same pairs) or of one plant of each category (mixed pairs). Randomly preselected plants were defoliated twice in the growing season, at 9 wk and again at 5 wk before final biomass sampling. Competition reduced growth by 51 to 78%, with the plants of the native category consistently more affected than the invasive category. When grown singly, Crofton weed grew larger (average 67%) than the native species. As compared with plants grown singly, Crofton weed plants grown in mixed pairs were 40% smaller. The corresponding data for the native species were 78% smaller for Japanese brome and 73% smaller for Dichrocephala. The effects of neighbor defoliation differed among categories and competition. All the nondefoliated plants grown singly were larger than plants grown with a defoliated conspecific neighbor. For the corresponding relationship in mixed pairs, the native plants grown singly were significantly larger than the same native species grown together with a defoliated, invasive neighbor. Defoliation of Crofton weed reduced the growth of native neighbors and increased negative competitive effects on native plants. For the invasive species, however, Crofton weed plants grown in mixed pairs with neighbor defoliation compensated fully for the competitive effects, i.e., they were not significantly different from nondefoliated plants grown singly. Consequently, the effects of defoliation on competition between invasive Crofton weed and its native neighbors may depend on the ability to undertake compensatory growth, and probably on the allelopathic exudation of Crofton weed to herbivory. Our data suggest that herbivory by biocontrols can make an already superior competitor even stronger, especially if the biocontrol agent does not effectively damage or kill the target plant. Owing to complex interactions between competition and defoliation, the indirect effects of herbivory may be more complicated than currently conceived, and understanding the indirect effects of biocontrol agents before their release is crucial.

Wild oat (Avena fatua L.) seedlings were exposed to vapors arising from soil treated with triallate [S-(2,3,3-trichloroallyl)diisopropylthiocarbamate]. Vapor from the liquid triallate formulation inhibited seedling growth more than vapor from the granular formulation, and the degree of inhibition was least in Fargo clay, intermediate in Hecla sandy loam and greatest in washed sand. Growth inhibition from vapors of soil-applied liquid or granular triallate increased as soil moisture content and soil temperature increased. The site of effective vapor action on wild oat seedlings was below the soil surface.

Chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]benzenesulfonamide} was nontoxic to safflower (Carthamus tinctorius L.) when applied postemergence in 1983 and 1984 at 0.018 and 0.035 kg ai/ha. Trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine] at 1.1 and 1.7 kg ai/ha and pronamide [3,5-dichloro(N-1,1-dimethyl-2-propynyl)benzamide] at 0.8 and 1.1 kg ai/ha were applied previously as preplant soil-incorporated treatments. In both years, safflower was relatively free of weeds where trifluralin was applied alone or in sequential treatments with chlorsulfuron. Pronamide, with or without chlorsulfuron, failed to completely control witchgrass (Panicum capillare L. ♯ PANCA) in 1 yr; thus safflower grain yields were reduced 21 to 35% when compared to weed-free safflower. Chlorsulfuron controlled redroot pigweed (Amaranthus retroflexus L. ♯ AMARE), puncturevine (Tribulus terrestris L. ♯ TRBTE), and common sunflower (Helianthus annuus L. ♯ HELAN).