Greenhouse and laboratory studies were conducted to determine the physiological basis for selectivity of nicosulfuron and primisulfuron in 5 plant species. Differential sensitivity of the species was quantified by determining GR50 values (herbicide rate required to reduce plant growth 50%) for each species/herbicide combination. GR50 data indicated the following levels of sensitivity: corn—tolerant to both herbicides; seedling johnsongrass—sensitive to both herbicides; barnyardgrass—sensitive to nicosulfuron and tolerant to primisulfuron; giant foxtail—sensitive to nicosulfuron and tolerant to primisulfuron; and eastern black nightshade—tolerant to nicosulfuron and sensitive to primisulfuron. Studies using 14C-radiolabeled herbicides were conducted to determine whether differential herbicide absorption, translocation, or metabolism contributed to whole plant responses. Nicosulfuron and primisulfuron selectivity in corn, johnsongrass, barnyardgrass, and giant foxtail was primarily due to differential herbicide metabolism rate. Tolerant species metabolized the herbicide more rapidly and extensively than sensitive species. Differential herbicide absorption, translocation, or metabolism did not explain differential sensitivity of eastern black nightshade to the herbicides. Further studies indicated that the tolerance of eastern black nightshade to nicosulfuron and its sensitivity to primisulfuron was directly related to lower sensitivity of the acetolactate synthase (ALS) to nicosulfuron than to primisulfuron. Eastern black nightshade translocated very little (3%) of the nicosulfuron applied. The ALS sensitivity of johnsongrass and eastern black nightshade was similar in the presence of nicosulfuron. A combination of a higher ALS level and less herbicide translocation contributes to tolerance of eastern black nightshade and to sensitivity of johnsongrass to nicosulfuron.

Microsomes (100,000 g pellet containing mixed membrane fractions but primarily endoplasmic reticulum) were isolated from shoots of corn, shattercane, and woolly cupgrass grown from naphthalic anhydride treated or untreated seed to determine if metabolism of bentazon, nicosulfuron, and primisulfuron could be demonstrated in the preparations. Corn is tolerant of all three herbicides, shattercane is tolerant of bentazon, and woolly cupgrass is tolerant of bentazon and primisulfuron. Naphthalic anhydride treatment was required for detectable bentazon, nicosulfuron, and primisulfuron hydroxylation in corn microsomes and for bentazon hydroxylation in woolly cupgrass microsomes. Bentazon hydroxylation was low, but detectable, in microsomes from shattercane shoots without naphthalic anhydride treatment. Naphthalic anhydride-treated corn microsomes hydroxylated 292, 120, and 52 pmol mg−1 protein min−1 of bentazon, nicosulfuron, and primisulfuron, respectively. Primisulfuron (19 pmol mg−1 protein min−1), but not nicosulfuron, was hydroxylated in woolly cupgrass microsomes. Neither nicosulfuron nor primisulfuron was hydroxylated in shattercane microsomes. Bentazon and primisulfuron inhibited nicosulfuron hydroxylation in corn microsomes. Bentazon, but not nicosulfuron, also inhibited primisulfuron hydroxylation in the corn microsomes. This indicates that the three herbicides can interact at the same cytochrome P-450(s) in corn. Primisulfuron hydroxylation was not inhibited by either bentazon or nicosulfuron in woolly cupgrass microsomes. This suggests that the cytochrome P-450(s) for primisulfuron hydroxylation are different between corn and woolly cupgrass. Also, bentazon hydroxylation in corn and shattercane microsomes was inhibited by the cytochrome P-450 inhibitor tetcyclasis, while that in woolly cupgrass was not. Again, this suggests a difference in the cytochrome P-450(s) responsible for bentazon metabolism among the species. Although absolute conclusions comparing in vitro microsomal activities to whole plant herbicide tolerance cannot be made because it is unknown whether the same cytochrome P-450(s) are studied in microsomes from naphthalic anhydride-treated tissue as are responsible for in vivo herbicide metabolism, there was a broad correlation between metabolism of a particular herbicide in microsomes of a species and the species' tolerance of that herbicide.

An enzyme-directed approach has led to finding specific inhibitors of imidazoleglycerolphosphate dehydratase (IGPD), which catalyzes a dehydration reaction of imidazoleglycerol phosphate (IGP) to form imidazoleacetol phosphate in the biosynthesis of histidine. These newly synthesized IGPD inhibitors showed wide-spectrum, postemergence herbicidal activity at application rates ranging from 0.05 to 2 kg ai ha−1. IGPD from wheat germ was purified to apparent homogeneity and characterized, and IGPD cDNAs were isolated from Arabidopsis and wheat cDNA libraries. Both the purified plant enzyme and the recombinant enzymes, which have been prepared by expressing the isolated IGPD cDNAs in the baculovirus/insect cell system, were used for establishment of an in vitro enzyme assay for inhibitor evaluation. Enzyme inhibitors were designed and synthesized by considering the structure of the substrate, IGP, and the inhibition activities were enhanced by a systematic structure—activity relationship study in vitro. Three triazole phosphonates, IRL 1695, IRL 1803, and IRL 1856, inhibited the reaction of the enzyme with Ki values of 40 ± 6.5 nM, 10 ± 1.6 nM, and 8.5 ± 1.4 nM, respectively, and were highly cytotoxic to cultured plant cells. Plant cell growth inhibition was completely reversed only by l-histidine, proving that the cytotoxicity was primarily caused by the inhibition of histidine biosynthesis. These new IGPD inhibitors have demonstrated that the specific inhibition of plant IGPD is lethal to plants. Further, treatment of Arabidopsis with IRL 1803 affected gene expression of eight enzymes involved in the biosynthesis of aromatic amino acids, histidine, lysine, and purines. These effects were reversed by the addition of l-histidine, indicating that the changes in gene expression were caused by the inhibition of l-histidine biosynthesis. These results indicate that higher plants, like microorganisms, are capable of cross-pathway metabolic regulation.

Weeds spread through movement of seeds and vegetative reproductive propagules. Pollen movement can spread weedy traits, such as herbicide resistance, between related weed and crop species. Weed seeds can spread short or long distances by natural plant dehiscence mechanisms, wind, water, animals, and man&s activities. This symposium paper is a practical review of short-distance spread of weed seeds in and between nearby arable fields and noncrop lands, examining some of the causes of spread and subsequent effects on crop production. Pollen movement, as it affects the spread of herbicide resistance, also is considered a component of short-distance weed spread. Specific weed management options can be used to reduce man-caused weed seed spread within and between nearby fields, thus reducing potential crop yield losses. Long-term management will be more difficult for weed seed spread by natural dispersal mechanisms.

WSSA members were surveyed in 1993–1994 to determine their perceptions of the contribution of weed biology to weed management. A questionnaire was included in the society newsletter, from which 152 responses were returned by mail or collected at the 1994 annual meeting. Over half the respondents felt that the overall contribution of weed biology to weed management had been moderate to high. Aspects of population dynamics and competition emerged as the areas that respondents felt should have the greatest impact on weed management in the future. The areas of computer modeling, interactions between weeds and other pests, and seedbank dynamics were predicted to show the greatest increases in importance in the future. The relative importance of taxonomy and weed identification was expected to decrease. Allelopathy, morphology and anatomy, and genetics and evolution were considered least likely to be important to weed management.

The effect of target site mutation for acetolactate synthase (ALS)-inhibitor resistance on ALS activity was evaluated in a sulfonylurea-resistant (R) biotype of prickly lettuce with a proline173 to histidine substitution in Domain A of the ALS enzyme. I50 values for ALS inhibition by several ALS-inhibitor herbicides were determined for R and susceptible (S) biotypes. Results from both a standard ALS assay and a chloroplast assay for ALS activity showed that the R biotype also was cross-resistant to representatives of the imidazolinone (imazethapyr) and triazolopyrimidine (flumetsulam) families, but was not cross-resistant to the pyrimidinyl oxybenzoate (4,6-dimethoxypyrimidin-2-y 1-oxy-2-benzoic acid) tested. The Km (pyruvate) was similar for ALS extracted from the R and S biotypes, suggesting that mutation for resistance did not alter pyruvate binding on the enzyme. However, specific activity of ALS from the R biotype was 57% less than specific activity of ALS from the S biotype, suggesting that the resistance mutation may affect enzyme function, expression, or stability. ALS from the R biotype was less sensitive to inhibition by the branched chain amino acids, valine, leucine, and isoleucine, than ALS from the S biotype. Reduced sensitivity to feedback inhibition was correlated with 70, 40, and 9% higher concentrations of valine, leucine, and isoleucine, respectively, on a per seed basis in R vs. S seed.

SCB-1, a semicarbazone, is a phytotoxin with both preemergence and postemergence activity against broadleaves and grasses. It effectively displaced [3H]naptalam bound to a solubilized plasma membrane fraction, with the concentration required for 50% dissociation being 3.5 nM. The intrinsic dissociation constant (Kd) for binding of [3H]naptalam to zucchini plasma membranes was estimated to be 9.7 nM. Binding of [3H]naptalam to the solubilized protein fraction from the plasma membrane was rapid, time dependent, saturable, and reversible. The Kd for binding was estimated at 4 nM. Based on the maximum binding of [3H]naptalam, concentration of the binding protein was calculated to be 23.6 and 10.8 pmol mg−1 protein in the plasma membrane and solubilized fraction, respectively. SCB-1 also blocked the efflux of [14C]IAA, resulting in more than 800% accumulation of radiolabel in zucchini squash hypocotyl segments at 5 μM. This accumulation was quenched by excess cold IAA and 2,4-D. Collectively, these results suggest that SCB-1 is a potent inhibitor of auxin efflux and that it competes with naptalam for binding to the plasma membrane protein.

Greenhouse and field experiments were conducted to study competition between wheat and rigid ryegrass or between wheat and cowcockle using additive series and growth analysis. Wheat was the dominant competitor with either weed. One wheat plant was as competitive as 1 1 or 19 rigid ryegrass plants in greenhouse and field experiments, respectively. One wheat plant was as competitive as three to 24 cowcockle plants, depending on environmental conditions. Dry weight of roots, leaves, stems, and spikes or capsules responded similarly to the effects of competition between wheat and rigid ryegrass or cowcockle. Shoot dry weight was the easiest, fastest, and least expensive component to measure competition. Growth analyses of individual plants showed that wheat had a greater leaf area, shoot and root dry weight, and absolute growth rate than rigid ryegrass or cowcockle, particularly early in the season. A range of 120 to 240 wheat plants m−2 can minimize rigid ryegrass or cowcockle competition and achieve an acceptable grain yield in semiarid areas in Morocco.

In the first of 2 field studies, weed biomass and soybean seed yield were used to evaluate 16 soybean genotypes for competitive ability against 12 weed species at Rosemount, MN, in 1992 and 1993. The yield and ranking of soybean genotypes often varied with the weed species. Grass weed species reduced yields the most, and small-seeded broadleaf weeds reduced yields the least across years. ‘Parker’ was highly competitive, as it suppressed weed biomass and produced high soybean yield. ‘Kato,’ ‘Kasota,’ ‘Dawson,’ and ‘Glenwood’ minimized weed biomass and maintained soybean yield while in competition with grass weeds but yielded poorly relative to other soybean genotypes in weed-free conditions. ‘Lambert’ produced high soybean yield in weed-free conditions, but yield dropped markedly when in competition with grass weeds. ‘Grande,’ ‘Heifeng 25,’ and ‘Norman’ soybeans were poor competitive genotypes in weedy situations and low yielding in weed-free conditions. A 2nd field study conducted at Rosemount and St. Paul, MN, during 1993 evaluated 16 soybean genotypes under 4 levels and durations of weed pressure for weed competitiveness. Parker, ‘Sturdy,’ and M89-794 were most competitive in suppressing weed biomass and producing high yields. Lambert yielded fairly well but allowed high weed biomass. M89-1743, M89-1006, ‘Archer,’ and ‘Ozzie’ yielded poorly and did not sup press weed biomass production. No relationship was found between weed competitiveness and soybean canopy area, height, and volume measured 30–45 d after planting (DAP).

Exotic annual weeds on western U.S. rangelands, including yellow starthistle, owe at least part of their invasion success to prolific seed production. Occurrence of flowering in relation to soil moisture and development of competing vegetation heavily influences reproductive output. Yellow starthistle flowers much later than associated Mediterranean invaders, which avoid summer drought by early flowering and senescence. This study used successive planting dates and four densities to investigate the influence of temperature, photoperiod, and intraspecific competition on reproductive phenology in a population of yellow starthistle from Lewiston, ID. A base temperature for seed germination of 2 C was determined by experiments at constant temperatures. Under the conditions of this study, density, photoperiod, and vernalization did not regulate reproductive development, as measured by onset of bud, flowering, and achene dispersal stages. A thermal time model adequately predicted phenology in this population of yellow starthistle. Plants emerging between October and July required about 1,240 degree days for 50% of the plants to reach bud stage, and an additional 500 and 900 degree days to flowering and achene dispersal, respectively. The relatively long time from emergence to mature achenes in this species influences management options such as prescribed fire, livestock grazing, and management of competing vegetation. Insensitiviry to photoperiod and lack of a vernalization requirement allow late germinating plants to reproduce if moisture is adequate.

To predict weed emergence and help farmers make weed management decisions, we constructed a mathematical model of seed germination for green and redroot pigweed based on temperature and water potential (moisture) and expressing cumulative germination in terms of thermal time (degree days). Empirical observations indicated green pigweed germinated at a lower base temperature than redroot pigweed but the germination rate of redroot pigweed is much faster as mean temperature increases. Moisture limitation delayed seed germination until 23.8 C (green pigweed) or 27.9 (redroot pigweed); thereafter, germination was independent of water potential as mean temperatures approached germination optima. Our germination model, based on a cumulative normal distribution function, accounted for 80 to 95% of the variation in seed germination and accurately predicted that redroot pigweed would have a faster germination rate than green pigweed. However, the model predicted that redroot pigweed would germinate before green pigweed (in thermal time) and was generally less accurate during the early period of seed germination. The model also predicted that moisture limitation would increase, rather than delay, seed germination. These errors were related to the mathematical function chosen and analyses used, but an explicit interaction term for water potential and temperature is also needed to produce an accurate model. We also tested the effect of mean temperature on shoot elongation (emergence) and described the relationship by a linear model. Base temperatures for shoot elongation were higher than for seed germination. Shoot elongation began at 15.6 and 14.4 C for green and redroot pigweed, respectively; they increased linearly with temperature until the optimum of 27.9 C was reached. Elongation was dependent on completion of the rate-limiting step of radicle emergence and was sensitive to temperature but not moisture; hence, elongation was sensitive to a much smaller temperature range. Beyond mathematical changes, we are testing our model in the field and need to link it to ecophysiological, genetic, and spatially explicit population processes for it to be useful in decision support for weed management.

Economic assessment of weed management strategies in rice is dependent upon a quantitative estimate of the yield impact of a given weed population. To assist rice producers in making such assessments, a mathematical model was developed to predict rice yield reduction as a function of weed density and duration of interference. The nonlinear empirical model was a unique 3-dimensional adaptation of the Richards equation with 4 parameters. Using published data, individual parameter values were fitted for each of 6 weed species interfering with either conventional or semi-dwarf statured rice cultivars. The functional form of the equation produced surfaces that were qualitatively consistent with available data and experience regarding rice-weed biology. Hence, predictions from the model should be useful and reliable in assessing the economic impact of weeds and in determining the feasibility of alternative weed control treatments for various field scenarios.

Field studies were conducted at Dekalb and Urbana, IL, in 1995 and 1996 to evaluate the effectiveness of sethoxydim for giant foxtail control in sethoxydim-resistant (SR) corn. Experiments studied sequential and total postemergence applications of grass herbicide standards compared to sethoxydim. Additional studies evaluated the compatibility of sethoxydim with postemergence broadleaf herbicides. Metolachlor plus atrazine and metolachlor followed by dicamba plus atrazine gave at least 88% control of giant foxtail at both locations in both years. Metolachlor plus flumetsulam plus clopyralid provided 90% or greater grass control over all experiments, with the exception of only 75% control at Dekalb in 1995 due to a heavy giant foxtail infestation. In comparison, flumetsulam plus clopyralid followed by postemergence applications of sethoxydim or nicosulfuron provided the same level of grass control as preemergence metolachlor, except at Dekalb in 1995 where control was 72% for both sethoxydim and nicosulfuron. Sequential applications of sethoxydim increased control of giant foxtail compared to a single sethoxydim application in 1995. Sethoxydim applied alone controlled giant foxtail 8% better than nicosulfuron at Urbana in 1996. Postemergence sethoxydim applied alone provided 87% or better control of giant foxtail. Sethoxydim performance was consistent when applied with flumetsulam plus clopyralid plus 2,4-D (NAF-73), halosulfuron plus dicamba, and bromoxynil. The efficacy of sethoxydim was reduced in combination with dicamba plus atrazine in three of the four trials, and bentazon plus atrazine as well as primisulfuron plus prosulfuron in all trials. Sethoxydim outperformed nicosulfuron in combinations with bromoxynil at Urbana. These studies indicate sethoxydim has excellent potential to be used in corn for postemergence control of giant foxtail.

Attempts to discover new active ingredients and target sites within the aromatic pathway have resulted in the synthesis of potent enzyme inhibitors, but no herbicides. As an aid in identifying a new target for inhibitor design and screening, we have determined the mode of action of a compound (6-methyl anthranilate) that exhibits noncommercial levels of herbicidal activity. Our evidence suggests that 6-methyl anthranilate is converted in vivo, by traversing the tryptophan biosynthetic sequence, to 4-methyl tryptophan, which inhibits anthranilate synthase. Inhibitors synthesized by design and those found by target-based screening converged on analogs of tryptophan and anthranilate. None, however, was more herbicidal than 6-methyl anthranilate.

Studies were conducted in Florida to evaluate interference of common cocklebur with peanut. Peanut yield reduction ranged from 0 to 88% for common cocklebur densities of 0–32 plants 8 m−1 of peanut row, and predicted loss was similar under normal moisture conditions. When moisture levels were above normal, the impact on yield was 9–24% less than when soil moisture was normal. Common cocklebur caused peanut yield loss if allowed to interfere for more than the first 2 wk after crop emergence, and peanut had to be common cocklebur free for at least 12 wk to prevent a yield reduction. These results show common cocklebur to be more competitive with peanut than other weeds evaluated previously.

Knowledge of weed biology is essential for development of both economically and environmentally acceptable weed management systems. Weed biology relates to plant attributes such as morphology, seed dormancy and germination, physiology of growth, competitive ability, and reproductive biology. Concepts of population biology such as seedbank dynamics for annuals and root reserves, dormancy, and longevity of vegetative propagules for perennials can be used to predict weed infestations better and to evaluate sustainable management strategies. Integrated approaches that give priority to depletion of root reserves or seedbanks through interfering with dormancy or germination requirements have great potential to enhance weed management strategies in the future.

Field studies were conducted at the Main Experiment Station, Fayetteville, AR, in 1994 and 1995 to evaluate the effect of postemergence herbicides applied at late-season timings on sicklepod seed production and viability. AC 263,222, dicamba, glyphosate, glufosinate, and paraquat were applied at five rates (zero; labeled rate; one-half, one-fourth, and one-eighth the labeled rate) to three sicklepod growth stages (bud formation, flowering to 9-cm pods, and 15- to 30-cm pods). The greatest reduction in seed production occurred when herbicides were applied at the flowering to 9-cm pod growth stage. All herbicides applied at one-half the labeled rate to sicklepod at bud formation and at the flowering to 9-cm pod growth stage reduced seed production greater than 80%, except glufosinate. Regardless of herbicide, rate, or application timing, viability of remaining seed was ≥ 90% with all treatments.

Downy brome is one of the most troublesome winter annual weeds in winter wheat-fallow rotations in the central Great Plains. A 3-yr seed burial study was initiated to determine how long downy brome seed remained germinable when placed on the soil surface or 2.5 cm deep at four different times in four environments. Only 1 to 7% of the downy brome seed survived after 1 yr on the soil surface in chemical fallow and stubble mulch when deposited in August, but survival varied in September, October, and November. In 1970, a year with low fall and winter precipitation, 36 to 46% of the seed placed on the soil surface of chemical fallow in September, October, and November survived, compared with 1 to 8% for stubble mulch tillage. Early spring tillage covered more seed with soil, and downy brome seed survival decreased. When fall and winter precipitation was normal, stubble mulch and chemical fallow had 1 to 20% germinable seed remaining. Induced dormancy existed in some years. More downy brome seed survived when placed on the soil surface of crested wheatgrass sod (14 to 50%) than on smooth brome sod (0 to 36%). No differences existed among environments when downy brome seed was buried 2.5 cm deep. Only 0.4% of downy brome seed buried 2.5 cm survived after 1 yr when averaged across all environments.

The establishment of common milkweed in corn, soybean, and wheat and rotations of corn-soybean and soybean-wheat was determined in field studies at Rosemount and Morris, MN. Common milkweed seedling density 115 d after initial seedling emergence was 500, 15,000, and 31,300 seedlings ha−1 in corn, soybean, and wheat, respectively, in 1990; 500, 7,300, and 15,600 in 1991 at Rosemount and 2,900, 26,500, and 37,000 in 1990; and 2,200, 6,700, and 8,100 in 1991 at Morris in the year of seedling establishment. Common milkweed seedling defoliation by mon arch butterfly larva was 1,31, and 0% at Rosemount and 20, 78, and 0% at Morris for corn, soybean, and wheat, respectively. In the 2nd and 3rd rotational years, corn-soybean rotations had fewer shoots ha−1 115 d after shoot emergence than soybean wheat rotations. Seedling densities 115 d after initial seedling emergence were greater in corn-soybean rotations where soybean rather than corn was the rotational crop and in soybean-wheat rotations where wheat rather than soybean was the rotational crop immediately following common milkweed seeding. Conventional tillage treatments had lower common milkweed shoot density than conservation tillage in all years of all rotations except in the initial count, when soybean was the first rotational crop in which shoot density was greater in conventional than in conservation tillage treatments. Cyanazine, imazethapyr, or bromoxynil applications in corn, soybean, or wheat rotational crops, respectively, reduced common milkweed shoot density.

The story of agriculture is the story of weed interference. After millennia of weed control we still have weeds. This situation has led many growers to observe that “the weeds always win.” One of the most important reasons weeds are so successful is their biodiversity. Biodiversity is an inevitable consequence of the struggle an individual weed species undergoes in the presence of neighbors, and by occupying a physical space in an agroecosystem. Weeds have evolved in response to cropping system practices by adapting and occupying niches left available in agroecosystems. Forces created by our cropping practices over evolutionary time have led to the weed diversity we observe today. Diversity underlies weed management in several important ways. A plant experiences diversity among its neighbors in at least five different ways. Weeds have adapted to selection in agroecosystems in several ways: (1) genetic variants within a species; (2) somatic polymorphism of plant parts; (3) success in diverse habitat microsites; (4) temporal adaptations within the community; and (5) floristic diversity of a community at higher levels than the species. Herein, weed diversity is discussed in this broader context, in terms of population behaviors that emerge as a consequence of the activities of individual components at lower levels of organization. Diversity is also discussed in terms of its implications for weed management. The potential exists to develop management strategies based on differences in weed and crop diversity. These strategies might be developed by characterization of weedy genetic and phenotypic diversity; enhancement of crop, cropping system, and agroecosystem diversity; and characterization of the spatial distribution of weed populations.