Redroot pigweed is a troublesome weed in the sorghum-growing regions of North America. In 1994 and 1995, field studies were conducted at two locations near Manhattan, KS, to determine the influence of redroot pigweed density and environmental conditions on physiological determinants of redroot pigweed growth: duration of plant growth, light interception, radiation-use efficiency, and dry matter partitioning. In addition, specific leaf area was determined. Redroot pigweed was seeded at monoculture densities of 2, 4, and 12 plants m−1 of row each year at each location. Duration of redroot pigweed growth was not influenced by plant density. Light interception was defined as a simple exponential function of leaf area index. Specific leaf area did not change over the season and averaged 135 cm2 g−1. Partitioning of redroot pigweed dry matter was not influenced by plant density or environmental conditions but did not change within vegetative and reproductive stages. Radiation-use efficiency was not influenced by redroot pigweed density; the most reliable estimate was 1.74 g dry matter MJ−1 of intercepted photosynthetically active radiation. Physiological determinants described were not affected by redroot pigweed density or environmental conditions and therefore provide a starting point for the development of a redroot pigweed growth module. The module could be coupled with available crop growth models (e.g., the sorghum growth model SORKAM) to simulate redroot pigweed–sorghum competition.

The low-nitrogen status of highly weathered soils may offer a potential alternative for weed suppression in agricultural systems with N2-fixing crops. In this study, we used sicklepod as a model to evaluate weed response that might occur with managed reductions in nitrogen-soil fertility. A field study was conducted with the parental generation supplied 0, 112, 224, or 448 kg N ha−1. Decreased nitrogen fertility led to reduced shoot biomass, seed number, and total seed mass. Individual seed mass was lower, but seed % nitrogen was not affected. Analysis of seed-mass distribution confirmed that low parental fertility was associated with more small seeds as a proportion of total seeds produced. Additional experiments in hydroponics culture revealed slower growth rates of seedlings produced from small seeds when grown under low-nitrogen conditions. Competitiveness of plants from small (low nitrogen) and large (high nitrogen) seed classes was determined in a replacement-series experiment conducted in sand culture in a controlled environment at two densities and two levels of nitrogen nutrition. Plants produced from smaller seeds were less competitive in low-nitrogen fertility conditions, but plants from small and large seeds competed similarly when grown under high-nitrogen fertility. The results support the hypothesis that comprehensive management strategies to reduce nitrogen availability for weed growth in low-fertility conditions could decrease weed interference by decreasing growth and seed production of parental plants and through maternal effects that lower competitiveness of offspring.

Plants of catchweed bedstraw from different Norwegian locations and from three other countries were compared with respect to morphological factors, herbicide sensitivity, and genetic variation. For the morphological comparison of cotyledons, whorls, leaves, and fruits five populations, grown outdoors but sheltered from rain, were used. Plants from Belgium and Sweden showed a high similarity, whereas one Norwegian population differed significantly in nearly all parameters. The same populations were used for a comparison of the sensitivity to the herbicide mecoprop-P. In this study, only slight differences appeared between the five populations. Finally, a deoxyribonucleic acid (DNA) sequence analysis of the internal transcribed spacer (ITS) regions was performed. The entire sequence of the ITS1 and ITS2 and the 5.8S subunit of ribosomal DNA were obtained from 15 populations (12 from Norway and one each from Sweden, Belgium, and Germany). The sequences had a length between 590 and 662 base pairs; intraspecific length variation was observed. Based on six insertions–deletions and 26 nucleotide substitutions, two DNA types could be distinguished. The first type consisted of eight Norwegian populations, whereas the second one contained the other seven populations including all non-Norwegian populations. The sequence alignments were used to build a phylogenetic tree. The results of the morphological comparison mostly corresponded with the results of the ITS sequence analysis. The variation was only to some extent correlated with the geographic distribution of the populations.

Hoary cress is a perennial herbaceous weed that has invaded agricultural and natural areas of western North America. Invasions are often composed of dense patches, and it is unclear whether clonal growth via lateral rhizomes or seedling recruitment is the dominant method of patch expansion. To study the clonal structure of this invasive, six patches from three USA populations (194 ramets) were analyzed with the use of Amplified Fragment Length Polymorphisms (AFLPs). Known siblings and clones were also included to ensure sufficient variation for discrimination between clonal and nonclonal ramets. Patches had low genet/ramet ratios (mean G/N = 0.25) and low diversity levels (mean D = 0.49) compared to similar clonal studies. Single genets represented 55–85% of the ramets sampled in a patch, and the largest genet was 38 m across. Hoary cress exhibits a strong bias toward patch-size increase from clonal reproduction rather than from seedling recruitment. Results indicate that biological control methods that focus on reducing or eliminating seed production would do little to stop expansion of a patch. Despite the domination of a patch by one or a few large genets, other smaller genets are able to persist or are occasionally recruited into dense areas of a patch.

The addition of surfactant (trimethylnonylpolyethoxyethanol) at 0.5% (v/v) of the spray solution caused a twofold increase in the deposit of clopyralid (3,6-dichloro-2-pyridinecarboxylic acid) applied at 0.14 or 0.28 kg ae/ha on greenhouse-grown honey mesquite (Prosopis glandulosa Torr. # PRCJG) and increased transport into the plants compared to clopyralid applied alone. Application of equal rates of benazolin [4-chloro-2-oxo-3(2H)-benzothiazoleacetic acid] with clopyralid at 0.28 + 0.28 kg/ha enhanced transport of clopyralid into honey mesquite. Spray deposit, absorption, and translocation of clopyralid was unaffected by addition of 0.5% (v/v) of d'limonene (4-isopropenyl-1-methyl-cyclohexane). Greater spray deposit of clopyralid on plant surfaces after addition of surfactant was associated with reduced liquid surface tension and greater percentage of spray volume in small droplets (<204-μm diam).

Lipid metabolism and other biochemical responses to alachlor [2-chloro-2’,6’-dimethyl-N-(methoxymethyl) acetanilide] and the protectant flurazole [2-chloro-4-(trifluoromethyl)-5-thiazolecarboxylic acid, (phenylmethyl ester)] in germinated grain sorghum [Sorghum bicolor (L.) Moench. ‘MFA GS 301’] were studied. Treatments included flurazole, applied to grain sorghum seed at 2.5 g ai/kg, alachlor at 8.2 × 10−6 M, and alachlor plus flurazole at these rates. Treatments did not alter total lipid synthesis or fatty acid composition. Seeds that germinated for 48 h in media containing alachlor or alachlor plus flurazole had a greater triglyceride:phospholipid ratio than those treated with flurazole or the control seeds, indicating that alachlor plus flurazole retarded catabolism of storage lipid. Oxygen consumption of seeds was reduced after the herbicide plus protectant treatment. Oxidation of 14C-pyruvate to CO2 was inhibited by alachlor and alachlor plus flurazole treatments, but not by flurazole alone.

Effects of bearded sprangletop interference durations on Lemont and Newbonnet rice cultivars were studied. Interference durations of 63, 70, and 130 d after rice emergence reduced Lemont grain yields 11, 21, and 50%, respectively, and lowered Newbonnet grain yields 11, 13, and 37%, respectively. Interference durations of 21 to 56 d after emergence did not reduce grain yields of either cultivar. Bearded sprangletop grown in Lemont rice produced more biomass than that in Newbonnet. Season-long interference reduced plant height and straw dry weight of Lemont more than that of Newbonnet.

Broadleaf weeds, including spurred anoda, emerge after direct-seeded chile peppers are thinned. Field experiments were conducted in 1989, 1990, and 1991 to determine the effect of spurred anoda density on green and red pepper yield, quality, and ease of hand harvest. Spurred anoda was established immediately after peppers were thinned at initial densities of 0, 3, 6, 12, 24, or 48 plants 9 m−1 row. The 1991 experiment also evaluated the influence of delayed pepper thinning and concurrent spurred anoda establishment on the competitive effect of spurred anoda. Spurred anoda were beginning to flower at green harvest and senescing at red harvest regardless of planting date. Spurred anoda were taller and accumulated more biomass when planted at a pepper thinning stage of 10 cm compared to 20 cm. Spurred anoda that emerged after thinning peppers reduced yield and ease of harvest of green and red peppers but not the quality of green peppers. Yield reduction at the highest spurred anoda density was 31 to 49% and 12 to 27% when peppers were thinned at 10 or 20 cm, respectively. Yield reduction was smaller when peppers were thinned at 20 cm tall than 10 cm tall and appeared to be associated with reduced spurred anoda biomass. Time required to hand harvest 1 kg of green or red peppers increased as spurred anoda density increased when peppers were thinned at 10 cm.

Cowcockle, an introduced summer annual weed of the Northern Great Plains, is being considered for domestication because of its high quality starch, cyclopeptides, and saponins. Loss of seed dormancy is one of the key desirable traits for domestication. To determine the potential for domestication of this species, an understanding of the seed dormancy and germination patterns is required. The objectives of this study were to evaluate seed dormancy in cowcockle ecotypes and determine how temperature and light affect seed dormancy. We evaluated 15 populations of cowcockle for primary dormancy by exposing them to five temperatures (5, 7.5, 10, 15, and 20 C) under two temperature regimes (constant and alternating) in both dark and light conditions. Freshly matured seeds of all the populations showed high levels of primary dormancy except ‘Mongolia’. Lower levels of dormancy at medium temperatures (10 and 15 C) and greater dormancy at low and high temperatures suggest conditional dormancy, a state at which seeds germinate over a narrower range of conditions compared to nondormant seeds. The effects of temperature regime, light, and their interaction was significant only at suboptimal (5 and 7.5 C) and supraoptimal (20 C) temperatures. Under these conditions, alternating temperatures were more effective in breaking the conditional dormancy, followed by light. The variation in optimum temperature, light, and their interactions among the cowcockle populations may be due to the plants evolving to adapt to their local environments. From a domestication perspective, the conditional dormancy in cowcockle can be observed as an evolutionary mechanism that prevents untimely germination following maturity and may not be a major obstacle for its domestication.

The exotic Brazilian peppertree is a serious invader of both disturbed and natural areas in central and south Florida, forming fast-growing, impenetrable thickets that dominate entire ecosystems. Brazilian peppertree has been targeted for biocontrol, and two defoliating insect species may eventually be released. This study was done to consider the possible effectiveness of defoliating biocontrol agents. The research investigated the effects of different frequencies of defoliation on height, crown diameter, and berry production of young Brazilian peppertrees. All the foliage was manually clipped from 36 trees in field plots once or twice per year for ≥ 1 yr. The effect on berry production of clipping 100% of the leaves from scattered individual branches of one large Brazilian peppertree was ALSo examined. Trees that were completely defoliated five times at 6-mo intervals were significantly smaller and produced significantly fewer fruits than undamaged controls. Plants defoliated one time only, two times in 1 yr, and two times in each of 2 yr were comparable to the undamaged controls. From this simulated herbivory study, we infer that multiple defoliations by insect defoliators have the potential to significantly suppress the growth and fruit production of Brazilian peppertree in Florida.

Greenhouse and growth chamber studies were conducted to investigate effects of herbicide placement in soil on toxicity to emerging seedlings. The use of a technique involving a plastic bag over the plants for 3 to 4 weeks to reduce soil moisture loss did not change the apparent site of 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine) uptake by green foxtail (Setaria viridis (L.) Beauv.) or oats (Avena sativa L.). Atrazine was more effective on oats through root exposure and more effective on green foxtail through shoot exposure when an untreated buffer zone around the seed was used. But in subsequent studies without the buffer zone, atrazine was most active when placed immediately below the seed of both oats and green foxtail. Proximity to the seed increased effectiveness of atrazine, S-ethyl dipropylthiocarbamate (EPTC), and 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron). Also, a narrow concentrated layer of these three herbicides adjacent to the seed was more effective than a wider dilute layer above or below the seed. A diurnal fluctuating temperature regime with a low night temperature increased toxicity of EPTC to oats through root exposure, when compared to a constant temperature regime.

Prior to germination, seed of some plant species either adsorb or absorb some herbicides applied to the soil. Equations were developed describing the concentration of the herbicide expected in the seed both when diffusion of the herbicide in the soil is the limiting factor and when diffusion of the herbicide within the seed is the limiting factor. Expected concentrations of the herbicide in the seed were presented for combinations of three seed radii (seed were assumed to be spheres) five diffusion times, and five diffusion coefficients. The calculated concentrations of the herbicide in the seed in ppm increased as seed size or radii decreased. This theoretical result was consistent with experimental data and potentially explains why some large-seeded plant species are difficult to control with the use of herbicides. The equation developed for a perfect “sink” predicted within experimental error the concentration of isopropyl m-chlorocarbanilate (chlorpropham) absorbed by soybean (Glycine max (L.) Merr.) seed from aqueous solutions of chlorpropham.

Because soil characteristics and weed densities vary within agricultural landscapes, determining which subfield areas are most favorable to weed species may aid in their management. Field and greenhouse studies were conducted to determine whether subfield environments characterized by higher soil organic carbon (SOC), or ridge vs. furrow microsites, affect common sunflower seed germination after winter burial, seedling emergence, or the control afforded by a preemergence herbicide in a ridge-tillage corn production system. Among seeds buried in situ during winter months and germinated in the laboratory, no differences in common sunflower seed germination or mortality were detected between high-SOC (1.8% mean) and low-SOC (1.1% mean) locations. However, seeds buried at 5-cm depth had about 40% laboratory germination compared with about 10% for seeds stratified on the soil surface or under crop residues. In field emergence and survival experiments, the SOC main-plot effect indicated 25% greater seedling survival in high- than in low-SOC locations. In the absence of herbicide, both emergence and survival were ≥ 35% greater in the ridge than in the furrow microsite, and seedling survival was 48% greater in high- vs. low-SOC furrow environments. However, common sunflower seedling survival was similar between herbicide-treated high- and low-SOC ridges. Greenhouse studies indicated a 13 to 24% increase in common sunflower seedling biomass per 1% increase in SOC under three atrazine doses. Altered or additional weed management tactics should be considered for common sunflower in high-SOC environments to offset the greater potential for seedling survival and growth.

Under Texas conditions, the potassium salt of 4-amino-3,5,6-trichloropicolinic acid (picloram) usually was more effective than the isooctyl ester formulation for control of live oak (Quercus virginiana Mill.), yaupon (Ilex vomitoria Ait.), winged elm (Ulmus alata Michx.), huisache (Acacia farnesiana (L.) Willd.), and honey mesquite (Prosopis juliflora (Swartz) DC. var. glandulosa (Torr.) Cockerell). Possible reasons for the difference in effectiveness of picloram formulations were studied in the laboratory. Extensive degradation of the ester of picloram (96%) occurred in open Petri dishes after 72 hr exposure to ultraviolet (uv) light, compared to a loss of 26% for the salt of picloram. Forty-five percent of the isooctyl ester of picloram was lost at high temperatures (60 C), whereas only 2% of the potassium salt of picloram was lost after 1 week at 60 C from open Petri dishes in a dark oven. Application of the ester to soils reduced thermal and ultraviolet light degradation losses compared to losses from open Petri dishes. Loss of the ester was greater when applied in diesel oil to Petri dishes than in either water or paraffin oil. The salt of picloram leached most after 12.5 cm simulated rainfall in soil columns to the 17.5 to 30-cm level (907 μg), followed by the acid (360 μg), and last the isooctyl ester (0 μg). However, considerable acid (161 μg) was recovered at the 32.5 to 45-cm depth from the isooctyl ester treatment exposed to wet soils for 3 days, indicating hydrolysis of the ester to acid.

Ivyleaf morningglory [Ipomoea hederacea (L.) Jacq. # IPOHE], planted in 1984 and 1985 the first of each month from April through August, began emerging in cotton (Gossypium hirsutum L. ‘Acala SJ-2’) 1 week after planting. Morningglory, at a density of 1 plant/2 m of row, emerging in April and May became so competitive to cotton by harvest in September that the total crop was lost. June plantings, although less competitive than earlier plantings, still reduced yields by 11%. Later plantings did not reduce yields of cotton. Morningglory plantings that produced seed (April through July) began flowering within 7 weeks, and viable seed was collected for the first time 9 weeks after planting. Ivyleaf morningglory plants that emerged in April and May produced about 11 000 seed/plant by cotton harvest. This compared to 3000 and 93 seed/plant for morningglory that emerged in June and July, respectively. The earliest that morningglory might be expected to produce seed in cotton planted in early April would be late May to early June, and morningglory emerging as late as the first of July could produce some seed by cotton harvest.

The phytotoxicity of 1,1-dimethyl-3-(α,α,α-trifluoro-m-tolyl)urea (fluometuron) and three of its analogs was studied on cotton (Gossypium hirsutum L.), foxtail (Setaria sp.), and redroot pigweed (Amaranthus retroflexus L.). Fluometuron was the most active compound, and the demethylated derivative and aniline analog were the least phytotoxic. The activity of the monomethyl analog of fluometuron, when applied via nutrient solution and to clay soil, was about 50% and 70% of the parent compound, respectively. Photosynthesis in cotton and redroot pigweed was inhibited by fluometuron, to a lesser degree by the monomethyl analog, and not at all by the demethyl and aniline analogs. Cotton was most tolerant, and only slight differences were found between the susceptibility of foxtail and redroot pigweed. These differences in response to fluometuron and its metabolites may contribute to the differential response between cotton and weeds.

Genetics can be a powerful adjunct to just about any kind of physiological study, including weed physiology or weed/herbicide interactions. Making, mapping, and reverting mutations is simple and straightforward. Making mutants can be as simple as isolating variant individuals from the “wild”, as uncomplicated as doing seed mutagenesis in your laboratory, or as sneaky as recovering mutants as sectors in whole plants. The overall principles for successful development of a protocol for seed mutagenesis of weeds are described and potential problem areas noted. These generalities are illustrated with a specific case history, that of chlorsulfuron. Although chlorsulfuron is accurately described as an inhibitor of the synthesis of branched chain amino acids, careful physiological examination suggests that it kills plant cells, not by starvation for amino acids, but by active toxicity of a metabolite, α-amino butyric acid, produced from a precursor available for diversion in cells with inhibited acetolactate synthase (EC 4.1.3.18, ALS). The story of dominant resistance due to an altered ALS enzyme is well known; analysis using additional mutants fleshes out the story of how chlorsulfuron works. Such analysis has the potential to help unravel other problems in weed physiology.

Three herbicides, 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine), 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine), and 3-tert-butyl-5-chloro-6-methyluracil (terbacil), applied annually at 0.45, 0.90, and 1.80 kg/ha for 2 or 3 years to establish forage alfalfa (Medicago sativa L. ‘Ranger’), effectively controlled downy brome (Bromus tectorum L.), tumble mustard (Sisymbrium altissimum L.), and prickly lettuce (Lactuca serriola L.). However, downy brome control decreased to less than half and broadleaf control ceased 18 months after the final herbicide application. Atrazine injured alfalfa most. Usually, annual applications of 0.45 kg/ha did not reduce yield. Greenhouse and field bioassay data indicated that the herbicides leached below the 0 to 32.5 cm soil zone and persisted. Residues were absorbed by roots of crops planted 2 to 3 years after the last application. Injury was similar whether the last application was made 18 or 30 months previously. The order of crop tolerance was sweet corn (Zea mays L. ‘Rugosa’) > potatoes (Solarium tuberosum L. ‘Norgold’) > winter wheat (Triticum aestivum L. ‘Nugaines’) > bush beans (Phaseolus vulgaris L. ‘Humilis’) ≅ sugar beets (Beta vulgaris L. ‘U. & I. hybrid No. 8′).

Greenhouse studies were conducted to study the competition between soybeans [Glycine max (L.) Merr. ‘Vansoy’] and barnyardgrass [Echinochloa crus-galli (L.) Beauv.]. The competition was initiated or terminated when soybeans were 2, 3, 4, 5, 6, 7, 8, 9, and 10 weeks old. The reproductive efficiency of soybeans was not lowered by 4 to 7 weeks of competition after planting of soybeans. Competition between the two species for the entire growing season resulted in a significant decrease in plant height of soybeans. The total dry matter per plant was negatively correlated with an increase in the length of competition. Competition during the first 5 to 9 weeks of the life cycle of soybeans was more deleterious to soybean productivity than the last 5 to 9 weeks of its life cycle.

The phytotoxicity of soil-applied terbutryn [2-(tert-butylamino)-4-(ethylamino)-6-(methylthio)-s-triazine] to wheat (Triticum aestivum Vill.) was significantly affected by soil moisture and soil temperature. Distribution coefficients (Kd) provided a better indication of the phytotoxicity of terbutryn to wheat than any single measured parameter contributing to herbicide adsorption by the soil. Soil temperatures and soil moisture levels suitable for good plant growth tended to enhance the phytotoxicity of terbutryn. No phytotoxic levels of terbutryn to wheat were detected in Teller sandy loam after 20 weeks of incubation at above 10C and 14% soil moisture by weight. However, phytotoxicity to wheat was observed in air-dry terbutryntreated soil after an incubation period of 20 weeks, regardless of incubation temperature. Significant quantities of terbutryn may remain in the field under dry soil conditions.