The germination responses of seeds from 11 weed species to various chemical and physical treatments were studied. Germination of nine species was promoted by gibberellic acid and inhibited by abscisic acid, suggesting that germination of some weed seeds may be controlled by appropriate levels of these substances. Most of the weeds that germinated better after a hot-water soak treatment responded to sonication and infrared treatment, indicating that temperature was a factor in the germination response obtained with these methods, although physical disruption of the seed coat also was involved.

It is difficult to quantify the mechanism(s) responsible for competition-induced yield loss using traditional experimental techniques. A technique using yield and 13C discrimination (Δ) for wheat, a C3 plant, has been developed to separate total yield loss (TYL) into yield loss due to N (YLNS) and water (YLWS) stresses. The objective of this research was to determine whether the Δ approach could be used in corn, a C4 plant, to separate TYL into YLNS and yield loss due to a combination of water and light stresses (YLWLS). The field study had a factorial design using five corn densities and five N rates and was conducted in western Nebraska in 1999 and 2000. Relationships for YLNS and YLWLS with TYL were derived from only a portion of the yield and Δ data collected in 1999 and validated based on the remaining data collected in 1999 and 2000. In 1999, 20 to 40% of TYL was due to YLWLS, whereas in 2000, a dry year, YLWLS accounted for 60 to 80% of the TYL. Results from using the Δ-based approach were consistent with analysis of variance results. For example, calculated YLWLS values were related to measured YLWLS by the equation: calculated YLWLS = 19 + 0.91 (measured YLWLS) (r 2 = 0.95; P < 0.01). The Δ approach, based on a plant's physiological response to the environment, can be used to separate and quantify competition-induced YLNS and YLWLS in corn.

Greenhouse and on-farm tests were conducted in North Carolina in 1979 and 1980 to evaluate sicklepod (Cassia obtusifolia L. # CASOB) management in soybeans [Glycine max (L.) Merr.]. All postemergence herbicide applications gave better sicklepod control when applied following vernolate [S-dipropylthiocarbamate) preplant incorporated than when applied following alachlor [2-chloro-2,6-diethyl-N-(methoxymethyl)acetanilide] preemergence. This resulted from decreased fatty alcohols and hydrocarbons in the epicuticle of vernolate-treated sicklepod. When applied sequentially to vernolate, toxaphene (chlorinated camphene, 67 to 69% chlorine) plus an oil concentrate and acifluorfen {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid} applied postemergence to the first true-leaf stage of sicklepod resulted in 95 and 90% control, respectively. Linuron [3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea] and metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one] applied alone or as tank mixes with 2,4-DB [4-(2,4-dichlorophenoxy)butyric acid] provided greater than 90% sicklepod control when applied postemergence-directed to soybeans. Metribuzin was more injurious to soybeans than linuron.

Research was conducted at Rosemount, Minnesota in 1979 and 1980 to evaluate the effect of soil moisture on quackgrass [Agropyron repens (L.) Beauv. # AGRRE] interference in soybeans [Glycine max (L.) Merr.]. Four treatments were studied: 1) nonirrigated soybeans, 2) irrigated soybeans, 3) nonirrigated soybeans, quackgrass-infested, and 4) irrigated soybeans, quackgrass-infested. Percent N, P, and K of soybeans was not affected by quackgrass interference. Irrigation did not increase the yield of quackgrass-free soybeans. The yield of quackgrass-infested soybeans was increased by irrigation when soil moisture was limited, but yields were always less than quackgrass-free soybeans. Quackgrass was nearly the same height or taller than the soybeans at all stages of soybean development and offered substantial competition for light throughout the growing season. This research indicates that an adequate supply of soil moisture can reduce quackgrass interference in soybeans, but cannot eliminate it, since quackgrass apparently continues to compete with soybeans for light.

Various environmental factors were tested under laboratory conditions to determine their effects on germination of seeds of prickly sida (Sida spinosa L. ♯3 SIDSP). Neither freezing and thawing nor moist chilling at 5 C promoted seed germination. However, increasing the incubation temperature and subjecting seeds to wet-dry cycles enhanced germination; high temperatures were more effective than alternate wetting and drying. Shifting seeds from a lower to a higher temperature regime increased germination. Seeds shifted from 15/6, 20/10, 25/15, or 30/15 C to higher regimes of 20/10, 25/15, 30/15, 35/20, or 40/25 C germinated to greater percentages than did seeds kept continuously at the lower thermoperiods. With an increase in length of time seeds were at a lower temperature, there was an increase in the percentage that germinated after they were moved to a higher regime.

When localized treatments of α,α,α-trifluoro-2,6-di-nitro-N,N-dipropyl-p-toluidine (trifluralin) and 4-(methylsulfonyl)-2,6-dinitro-N,N-dipropylaniline (nitralin) were applied to the shoot zone of sorghum (Sorghum bicolor (L.) Moench, var. RS-610) and cucumber (Cucumis sativus L., var. Wis. SMR-15) before seedling emergence, the most susceptible sites were the sorghum coleoptilar node and the cucumber hypocotyl hook. The longer before emergence that the sorghum shoot was exposed to the herbicides, the greater was the injury. However, if herbicide concentration was sufficient, the growing point just above the sorghum coleoptilar node was inhibited by both herbicides when applied after seedling emergence. Maximum uptake of 14C-trifluralin and 14C-nitralin occurred through the coleoptilar node and hypocotyI hook of sorghum and cucumber, respectively. Rate of entry, uptake, and translocation of 14C-trifluralin was greater than that of 14C-nitralin in sorghum and cucumber shoots.

Weed management could be more efficient and require less herbicide if growers could afford to estimate the composition, density, and distribution of weed seed banks. Spatial distribution of a weed seed bank will affect the accuracy of both mean estimates and interpolated maps of density. Consequently, information about the general characteristics of spatial distributions of seeds in a seed bank is needed to identify the most efficient strategies for sampling. Seed banks were sampled on 8.4-m square grids in eight irrigated corn fields to identify the common features of distributions of seed banks of annual weeds. Spatial dependence was described with correlograms for four to eight species in each field. Spatial dependence was detected for 36 of 45 distributions, and seed counts were correlated to an average distance of 25 to 150 m for a distribution. Seed banks of different species and fields had common features of spatial correlation: spatial pattern accounted for less than half of the total variability of seed counts, spatial correlation decreased rapidly over short distances, and ranges of spatial dependence varied with direction. For half of the distributions, the maximum range of spatial dependence was at least twice as long as the minimum range. Seed counts were correlated for the longest distances in the direction of the crop row for 16 distributions, and the distance was longer in the direction of the crop row than across rows for 26 of the 36 samples. Researchers should be able to design more efficient sampling plans for growers if the common features of spatial dependence are considered. For seed banks like these, the accuracy of maps and estimates of seed bank density may be improved by collecting multiple cores around each sampling location to mitigate the effect of short-scale spatial variability. In addition, sampling may be more efficient with grids and interpolation methods that account for ranges that are 1.5 to 2 times longer in the direction of the crop row than perpendicular to the row. With a 55- by 30-m sampling grid, adjacent observations would be correlated, and maps could be made for 80% of these seed banks. More closely spaced observations would be needed to describe the rapid decline in spatial correlation with distance for a more accurate or finer-scale map. Whether sampling seed banks for making management decisions will be cost-effective is not clear. However, potential methods to sample and map seed bank distributions more efficiently have not been exhausted.

Rice by-products were evaluated in the greenhouse for herbicidal activity on various weeds and crops. Rice by-products and corn gluten meal (CG) were applied at 0, 125, 250, 500, and 750 g m−2 preemergence (PRE) and preplant incorporated (PPI). The efficacy of rice by-products and CG in reducing weed emergence and shoot weight of broadleaf species was in the order of medium-grain fatty rice bran (MF) > long-grain fatty rice bran (LF) > CG > defatted rice bran (DF) > long-grain hull (LH) > medium-grain hull (MH). For reducing grass emergence, MF = CG > LF > DF > LH > MH, and for shoot weight reduction, CG > MF > LF > LH > DF = MH. Palmer amaranth and ivyleaf morningglory were the most susceptible weeds (91 and 82%) followed by sicklepod, hemp sesbania, and prickly sida (65 to 70%). Velvetleaf was the most tolerant broadleaf weed. Grasses were not as susceptible to rice bran as broadleaf weeds. In general, MF was the best material for reducing weed emergence and its efficacy was not affected by application method. Cotton and corn were the most tolerant direct-seeded crops to MF (6% reduction in plant stand), and soybean, Italian ryegrass, tomato, and rice had intermediate tolerances (30 to 86% stand reduction). Mustard, cucumber, and lettuce were the most susceptible crops (71 to 98% reduction in plant survival). The minimum effective rate was 250 g m−2 MF PPI or PRE.

Three Avena fatua (wild oat) populations resistant to imazamethabenz, flamprop, and fenoxaprop-P were identified from the northwest agricultural region of Manitoba, Canada. These populations were identified after producer reports of failure of imazamethabenz to provide satisfactory control in the field. Although these A. fatua populations had previously been exposed to other herbicides, primarily ACCase inhibitors, imazamethabenz had never before been applied. In growth room experiments, resistant (R) plants were 7.2 and 8.7 times more resistant to imazamethabenz and flamprop, respectively, than susceptible (S) plants, as measured by the ratio of dosages required to inhibit shoot dry matter accumulation by 50% (GR50 R/S). The three populations did not differ significantly (P < 0.05) in levels of resistance to imazamethabenz. Similarly, the populations did not differ in levels of resistance to flamprop. The populations differed in their response to fenoxaprop-P; levels of resistance for two populations were 2.0-fold, while the remaining population was 2.9-fold. An experiment conducted in 1995 in one of the infested fields confirmed multiple herbicide resistance, with A. fatua panicle numbers in August being 36, 128, and 44% of untreated controls at recommended dosages of imazamethabenz, flamprop, and fenoxaprop-P, respectively. Three additional populations of A. fatua with multiple herbicide resistance from other areas of Manitoba were identified in a 1996 field experiment. For the six A. fatua populations in the 1996 experiment with multiple herbicide resistance, panicle numbers expressed as a percentage of the untreated controls varied from 44 to 77% for imazamethabenz, 57 to 83% for flamprop, and 43 to 88% for fenoxaprop-P (commercially recommended dosage of each herbicide). Multiple herbicide resistance in A. fatua is not rare; screening of A. fatua seed samples from across Manitoba and Saskatchewan has identified a number of additional R populations. The evolution of herbicide resistance in the absence of direct selection is a very serious development as producers with multiple herbicide resistance in A. fatua are left with a very limited number of herbicide options for selective control in crops commonly grown in western Canada.

Vineyard weed communities were examined under the influence of an organic weed control practice, soil cultivation with a Clemens cultivator, and applications of the herbicide glyphosate. Experimental treatments (winter–spring glyphosate, spring cultivation, fall–spring cultivation, fall cultivation–spring glyphosate) were carried out in a California wine grape vineyard for 3 yr. Cultivation alone was not as effective as glyphosate, based on lower weed biomass in the glyphosate-only treatment in 2 of 3 yr. However, given that two passes with the Clemens cultivator decreased weed biomass relative to one pass, it is possible that additional passes could bring about further reductions. Pairing fall cultivation with glyphosate was as effective at reducing weed biomass as two glyphosate applications in 2 of 3 years, suggesting that substituting a glyphosate application with cultivation may be an effective method of reducing herbicide use in vineyards. Canonical correspondence analysis revealed significant treatment effects on community structure. Weed composition in the spring cultivation treatment was significantly different from that of all other treatments. Based on our findings of high relative abundance of field bindweed and sowthistle species, which are problematic vineyard weeds that grow into the vine canopy and disrupt canopy management practices, it is possible that either the presence of soil disturbance or the absence of herbicides favored these species.

Growth room studies were conducted to determine the physiological basis of reduced glyphosate efficacy under low soil nitrogen using velvetleaf, common ragweed, and common lambsquarters as model species. Glyphosate dose–response experiments of weeds grown under low (1.5 mM) and high (15 mM) soil N were conducted. Velvetleaf and common lambsquarters grown under low N required 169 g ae ha−1 glyphosate for a significant reduction in biomass, but only 84 g ae ha−1 were required when grown under high N. However, when common ragweed was grown under low or high soil N there was no significant difference in response to glyphosate at all doses tested. The reduced glyphosate efficacy on velvetleaf and common lambsquarters under low N was primarily due to decreased herbicide translocation to the meristem. It appears that low N may decrease the net assimilation of carbon in plants, resulting in a decrease in the net export of sugars and hence glyphosate from mature leaves. Understanding the relationship between soil N and herbicide efficacy may help explain observed weed control failures with glyphosate and may contribute to our knowledge of the occurrence of weed patchiness in fields. This is the first report illustrating a physiological basis for decreased glyphosate efficacy under low soil N in selected weed species.

Oxygen uptake and oxidative phosphorylation were inhibited by 10–4 M of α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) in isolated mitochondria of corn (Zea mays L., var. Dixie 18), Sorghum (Sorghum vulgare Pers., var. Early Hegari) and Soybean (Glycine max L., var. Lee). Little or no inhibition was detectable at 10–5 and 10–6 M in corn and sorghum. Data for 4-(methylsulfonyl)2,6-dinitro-N,N-dipropylaniline (nitralin) were more variable but similar. The effects on these processes do not seem to be related to the selectivity of trifluralin since it caused a greater reduction in oxygen and phosphate uptake in resistant species (soybean) than the susceptible species (sorghum).

The need for practical revegetation strategies for managing medusahead-infested rangeland is substantial and unmet. Our objective was to test the potential for using the single-entry approach (simultaneous application of herbicide and seed with one entry) developed for other invasive-weed infested systems for revegetating medusahead-infested rangeland. Since areas infested with medusahead are frequently burned and burning may help control medusahead and provide a more suitable seedbed for desired species, we also tested the single-entry approach in conjunction with burning. Our primary hypotheses were that (1) the combination of imazapic and seeding would provide best establishment of desired species and (2) burning would enhance the efficacy of imazapic on medusahead and enhance seedling establishment. Treatments included three seeding rates (none, 13.2, or 25.0 kg ha−1 of an even mixture of all species), two herbicides (0 and 52 g a.i. ha−1 imazapic; with and without), and two burning regimes (burned, not burned) applied mid-October 2006 on two sites. In late July, 2008 through 2010, plant density and biomass were sampled. We found that the simultaneous application of imazapic and seeding resulted in establishment of desired species where they were absent. Imazapic application without seeding increased perennial bunchgrass density where enough residual plants existed before treatment. Burning tended to improve the control of medusahead with imazapic and promoted desired species. Even though follow-up treatments may be necessary, applying imazapic and seeding in a single-entry approach may provide a more practical initial revegetation strategy than multi-entry approaches on rangeland devoid of desired residual species. This procedure should reduce the costs of treatment application and make revegetating annual grass-infested rangeland more affordable.

Red rice is a major weed problem in rice production of the southern United States and other rice-producing countries. One hundred thirty red rice accessions from 26 rice-growing counties in Arkansas were tested for tolerance to imazethapyr in seed- and whole-plant response bioassays. The red rice accessions were compared with imazethapyr-resistant (ClearfieldTM) rice cultivars (‘CL121’, ‘CL161’, and ‘CL-XL8’) and conventional rice cultivars (‘Bengal’, ‘Dongjin’, ‘Drew’, and ‘Wells’). Red rice accessions 79, 84, and 118 showed 17-, 48-, and 37-fold more tolerance to imazethapyr, respectively, than the standard susceptible red rice accession (82) in whole-plant bioassays. The imazethapyr-resistant rice cultivars, CL121, CL161, and CL-XL8 were 41-, >177-, and 48-fold more resistant to imazethapyr, respectively than the susceptible standard. The imazethapyr-tolerant red rice and ClearfieldTM cultivars were generally cross tolerant to other acetolactate synthase (ALS; EC 4.1.3.18) inhibiting herbicides such as imazapyr, imazaquin, imazamox, and pyrithiobac. The tolerance level of red rice or rice to imidazolinone herbicides was highest with imazaquin and lowest with imazapyr. The imazethapyr-tolerant red rice accessions and ClearfieldTM rice were susceptible to glufosinate and glyphosate. The ALS enzyme of tolerant red rice accessions was less sensitive to imazethapyr than the susceptible standard, but tolerance at the enzyme level was less than at the whole-plant level. Therefore, tolerance of red rice to imazethapyr may involve other mechanisms besides an insensitive target site. We learned that a few imazethapyr-tolerant red rice populations existed probably before ClearfieldTM rice was introduced, supporting the hypothesis that evolution of herbicide-resistant red rice populations can happen with intensive herbicide selection pressure.

AlertInf is a recently developed model to predict the daily emergence of three important weed species in maize cropped in northern Italy (common lambsquarters, johnsongrass, and velvetleaf). Its use can improve the effectiveness and sustainability of weed control, and there has been growing interest from farmers and advisors. However, there are two important limits to its use: the low number of weed species included and its applicability only to maize. Consequently, the aim of this study was to expand the AlertInf weed list and extend its use to soybean. The first objective was to add another two important weed species for spring-summer crops in Italy, barnyardgrass and large crabgrass. Given that maize and soybean have different canopy architectures that can influence the interrow microclimate, the second objective was to compare weed emergence in maize and soybean sown on the same date. The third objective was to evaluate if AlertInf was transferable to soybean without recalibration, thus saving time and money. Results showed that predictions made by AlertInf for all five species simulated in soybean were satisfactory, as shown by the high efficiency index (EF) values, and acceptable from a practical point of view. The fact that the algorithm used for estimating weed emergence in maize was also efficient for soybean, at least for crops grown in northeastern Italy with standard cultural practices, encourages further development of AlertInf and the spread of its use.

Various combinations of crops, herbicides, and tillage were evaluated for field bindweed (Convolvulus arvensis L.) control in western South Dakota. Four crop rotations (continuous wheat (Triticum aestivum L.), wheat-fallow, wheat-sorghum (Sorghum vulgare Pers.), and wheat-sorghum-fallow) were modified by the application of (2,4-dichlorophenoxy)acetic acid (2,4-D) and/or the use of post-harvest tillage. Intensive cultivation and 2,4-D treatment were used to aid in the control of field bindweed in forage crops grown on a long-term basis and in conjunction with small grain rotations. Treatment with 2,4-D in a grain crop and with non-selective herbicides after harvest were tested. Crops were planted the succeeding 2 or 3 years and observed for effect of chemical residue. The established plants could be essentially eliminated while utilizing adapted crop rotations. The use of 2,4-D alone or in combination with cultivation made it possible to reduce the stand of field bindweed (20.7 to 22.2 shoots/sq yd) 90% or more in 3 years in all rotations. A ¾-lb/A rate of 2,4-D in June prevented seed production, killed susceptible plants, and weakened the remaining plants, but a follow-up treatment of 2,4-D in the fall, post-harvest cultivation, or post-harvest treatment with herbicides such as 2,3,6-trichlorobenzoic acid (2,3,6-TBA), 3,6-dichloro-o-anisic acid (dicamba), or 4-amino-3,5,6-trichloropicolinic acid (picloram) was necessary to kill them.

The effects of rainfall and temperature upon herbicidal activity of 2-chloro-N,N-diallylacetamide (CDAA), 2-chloro-4,6-bis-(ethylamino)-s-triazine (simazine), and 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine), for the control of foxtail (Setaria spp.) were determined. CDAA spray was effective if applied to warm soil and if ½ inch of rain fell during the first week after application, unless a heavy rain fell immediately after application. The effectiveness of granules was not impaired by a heavy rain immediately after treatment or by a delay of 2 weeks before receiving ½ inch of rain. Simazine and atrazine sprays and granules were effective if 1 inch of rain fell within 15 days after application of simazine or if ½ to 1 inch fell within 15 to 20 days afterwards for atrazine. Mechanical incorporation aided in activation of simazine when rainfall was slightly below minimum required for activation.

Studies were undertaken to determine effects of scarification, temperature, stratification, pH, and osmotic potential on seed germination; to determine the effect of planting depth on emergence; to describe the emergence sequence; and to quantify seedling development of bigroot morningglory. Mechanical and chemical scarification caused increased germination but cool, moist stratification did not break dormancy. Optimum germination occurred at 20 and 25 C and with alternating temperatures of 20/10 and 30/20 C. The optimum pH range for germination was between 6 and 8.5. Increasing the solution osmotic potential decreased germination linearly from 90% at −0.08 MPa to 0% at −0.8 MPa. Plant establishment was best at 2 to 4 cm and decreased at greater seeding depths. Seedling emergence was observed and recorded and found not to fit the classical models of hypogeal or epigeal emergence. Only 3% of 5-day-old seedlings sprouted after being clipped to the root crown. When seedlings were 11 days old at clipping, over 85% of the plants sprouted. These results indicate that bigroot morningglory has the potential to successfully reproduce by seed and that the rapid development of the plant should be considered when designing control programs.

Greenhouse and field experiments were performed to investigate diclofop rate and mowing height interactions on goosegrass control and ‘Tifgreen’ and ‘Tifdwarf’ bermudagrass tolerance. In greenhouse experiments, greatest goosegrass control was achieved with diclofop when plants were maintained at 1.3 cm. Increased diclofop rates were required to suppress goosegrass mowed higher than 1.3 cm or unmowed. Greater than 90% goosegrass control was achieved with the combination of 1.3-cm mowing height and 0.6 kg ai ha–1 of diclofop. In field experiments, a minimum of 2 weeks was necessary for complete herbicidal activity. Diclofop at 1.1 kg ha–1 provided >90% control of goosegrass mowed between 1.9 to 2.5 cm. The addition of nonionic surfactant (0.25% by vol) to diclofop did not influence control. The addition of metribuzin (0.1 kg ai ha–1) to diclofop resulted in initial increased control, but it was transient The addition of MSMA (2.2 kg ai ha–1) to diclofop reduced goosegrass control an average of 18% compared to diclofop treatments alone. Tifdwarf bermudagrass was more sensitive to diclofop compared to Tifgreen. Seven to 14 days were required for Tifdwarf to recover from initial injury. This injury, although significant, was acceptable for bermudagrass used for golf greens. Clipping weights following treatment were also less for Tifdwarf than Tifgreen.

Field experiments were used to assess how distance mediates the nontarget effect on crepe myrtle by two chrysomelid beetles that were introduced to the United States in 1992 for biological control of purple loosestrife. Previous laboratory tests in Germany and concurrent tests in Oregon showed that although the control organisms can feed on crepe myrtle, they cannot complete development. Therefore, we predicted that negative effects on crepe myrtle would decrease with distance from the purple loosestrife stand. To test this prediction, cohorts of both plant species were transplanted at increasing distances (0, 5, 15, 30, and 50 m) from the colonization source. We found that leaf damage inflicted by the beetles was negatively correlated with increasing distance. Damage was significantly lower at each distance for crepe myrtle plants than for purple loosestrife plants, with a mean difference of 22% and a 95% confidence interval ranging from 12 to 31%. Extensive defoliation of crepe myrtle was limited to within 30 m of the edge of the loosestrife stand. Plant yield was negatively correlated with damage: the closer plants were to the purple loosestrife stand, the greater the suppression of biomass in both plant species. However, loosestrife biomass decreased significantly more quickly than crepe myrtle biomass, with a mean difference in slopes of 0.035 and a 95% confidence interval ranging from 0.022 to 0.048. Our results suggest that release of the Galerucella beetles in North America poses little risk to crepe myrtle. Beetles can feed but cannot complete their life cycle on crepe myrtle, and damage to crepe myrtle approaches zero approximately 50 m from the beetle colonization source.