Cover crops and living mulches bring many benefits to crop production. Interest in winter annual cover crops such as winter rye and hairy vetch for ground cover and soil erosion control has been increasing in the last 30 yr in some areas. The integration of cover crops into a cropping system by relay cropping, overseeding, interseeding, and double cropping may serve to provide and conserve nitrogen for grain crops, reduce soil erosion, reduce weed pressure, and increase soil organic matter content (Hartwig and Hoffman 1975). Hairy vetch has increased availability of nitrogen to succeeding crops, increased soil organic matter, improved soil structure and water infiltration, decreased water runoff, reduced surface soil temperature and water evaporation, improved weed control, and increased soil productivity (Frye et al. 1988). More recent research with perennial living mulches, such as crownvetch (Hartwig 1983), flatpea, birdsfoot trefoil, and white clover (Ammon et al. 1995), has added a new dimension to the use of ground covers that eliminates the need to reseed each year. Cropping systems with the use of ground covers have been worked out for vineyards, orchards, and common agronomic crops, such as corn, small grains, and forages. Legume cover crops have the potential for fixing nitrogen, a portion of which will be available for high-nitrogen–requiring crops such as corn. In areas where excess nitrogen is already a problem, the use of ground covers may provide a sink to tie up some of this excess nitrogen and hold it until the next growing season, when a crop that can make use of it might be planted (Hooda et al. 1998). Even legumes tend to use soil nitrogen rather than fixing their own, if it is available. It is these possibilities that provide the incentive for looking at the effect of various kinds of cover crops on soil erosion, nitrogen budgets, weed control, and other pest management and environmental problems.

Nomenclature: Crownvetch, Coronilla varia L. ‘Penngift’; birdsfoot trefoil, Lotus corniculatus L.; corn, Zea mays L.; flatpea, Lathrus sylvestrus L.; hairy vetch, Vicia villosa Roth; white clover, Trifolium repens L.; winter rye, Secale cereale L.

Herbicides that target the enzyme acetolactate synthase (ALS) are among the most widely used in the world. Unfortunately, these herbicides are also notorious for their ability to select resistant (R) weed populations. Now, there are more weed species that are resistant to ALS-inhibiting herbicides than to any other herbicide group. In most cases, resistance to ALS-inhibiting herbicides is caused by an altered ALS enzyme. The frequent occurrence of weed populations resistant to ALS inhibitors can be attributed to the widespread usage of these herbicides, how they have been used, the strong selection pressure they exert, and the resistance mechanism. In several cropping systems, ALS-inhibiting herbicides were used repeatedly as the primary mechanism of weed control. These herbicides exert strong selection pressure because of their high activity on sensitive biotypes at the rates used and because of their soil residual activity. Several point mutations within the gene encoding ALS can result in a herbicide-resistant ALS. From investigations of numerous R weed biotypes, five conserved amino acids have been identified in ALS that, on substitution, can confer resistance to ALS inhibitors. Substitutions of at least 12 additional ALS amino acids can also confer herbicide resistance in plants and other organisms but, to date, have not been found in R weed populations. Mutations in ALS conferring herbicide resistance are at least partially dominant, and because the gene is nuclear inherited, it is transmitted by both seed and pollen. Furthermore, in many cases there is apparently a negligible fitness cost of the resistance gene in the absence of herbicide selection. Although resistance to ALS-inhibiting herbicides has been a bane to weed management, it has spurred many advances within and beyond the weed science discipline. As examples, resistance to ALS-inhibiting herbicides has been exploited in the development of herbicide-resistant crops, studies of weed population dynamics, and in developing protocols for targeted gene modification. Resistance to ALS-inhibiting herbicides has greatly affected weed science by influencing how we view the sustainability of our weed management practices, what we consider when developing and marketing new herbicides, and how we train new weed scientists.

The broadleaf auxinic herbicide clopyralid was applied to three varieties of corn (Pioneer 36B08, Pioneer 3730, and Pioneer 3559) to determine whether it was phytotoxic to this crop. The effects of clopyralid on the growth and development of corn were compared with those induced by the auxinic herbicides dicamba, 2,4-D, picloram, and fluroxypyr. When compared with the other auxinic herbicides, clopyralid, applied as a foliar spray at the three- and six-leaf stages of development, caused the least damage to all three varieties of corn. Among the auxinic herbicides tested, fluroxypyr and dicamba caused severe damage to the three varieties, whereas picloram and 2,4-D had significant detrimental effects on the growth and development of varieties 36B08 and 3730. Similar results were also obtained when corn seeds were germinated in petri dishes containing increasing concentrations of the auxinic herbicides. In addition to correlating these growth and development effects with auxinic herbicide–induced physiological changes in corn, we compared the effects of clopyralid and dicamba on proton efflux from isolated protoplasts of the three varieties. Results of these biophysical studies are consistent with those from our growth and developmental studies and confirm that clopyralid is least effective in eliciting a response in corn when compared with dicamba. We conclude that clopyralid does not cause the deleterious effects seen with other auxinic herbicides when sprayed under optimal environmental conditions, i.e., high humidity and temperature.

Nomenclature: Clopyralid; 2,4-D; dicamba; fluroxypyr; picloram; corn, Zea mays L. ‘Pioneer 36B08’, ‘Pioneer 3730’, ‘Pioneer 3559’.

Glyphosate is a broad-spectrum herbicide that has been used extensively for more than 20 yr. The first glyphosate-resistant weed biotype appeared in 1996; it involved a rigid ryegrass population from Australia that exhibited an LD₅₀ value approximately 10-fold higher than that of sensitive biotypes. We have characterized gene expression levels and glyphosate sensitivity of 5-enolpyruvylshikimate 3-phosphate synthase (EPSPS), the target enzyme for glyphosate inhibition, in sensitive and resistant lines derived from this population. Restriction fragment length polymorphism analyses were also performed to examine the distribution of EPSPS gene variants and the gene copy number. A two- to threefold increase in basal EPSPS messenger RNA (mRNA) and enzyme activity levels was observed in the most resistant lines analyzed; however, differences among lines in the sensitivity of EPSPS to glyphosate were not apparent. Induction of EPSPS was observed within 48 h after application of 1.5 kg ae ha⁻¹ of glyphosate. This was reflected in elevated levels of both EPSPS mRNA and enzyme activity. Similarly, 3-deoxy-d-arabino-heptulosonate 7-phosphate synthase mRNA levels increased after glyphosate treatment; however, basal and induced transcript levels were comparable for sensitive and resistant lines in this case. The restriction fragment length polymorphism analyses showed no evidence for gene amplification or cosegregation of a specific EPSPS gene variant with glyphosate resistance. EPSPS expression in lines exhibiting an intermediate level of resistance was indistinguishable from that in glyphosate-sensitive lines, suggesting that the mechanism could, at least in part, be non–target-based.

Nomenclature: Glyphosate; rigid ryegrass, Lolium rigidum Gaudin LOLRI.

Experiments were conducted to determine the inheritance of resistance in a Wisconsin accession of eastern black nightshade to acetolactate synthase (ALS) inhibitors. ALS-inhibitor–susceptible (S) and ALS-inhibitor–resistant (R) plants were crossed (S × R), and inheritance was characterized in F₁ and F₂ generations. Inheritance was characterized further in progeny of reciprocal crosses (R × S) and backcrosses (BCs) (S × F₁ and R × F₁). In dose–response experiments, three- to four-leaved F₁ plants were intermediate in response to imazethapyr compared with parent R and S plants. The imazethapyr ED₅₀ value (the effective dose that reduced shoot dry biomass by 50% compared with nontreated plants) was 99.6 ± 13.6, 32.5 ± 8.9, 24.5 ± 3.3, and 0.6 ± 0.1 g ae ha⁻¹ for R, F₁ (R × S), F₁ (S × R), and S plants, respectively. Similarly, the response of in vivo ALS activity in F₁ plants to imazethapyr was intermediate compared with that of parent R and S plants. To differentiate among phenotypes in F₂ generations and in BC generations, the response of three- to four-leaved plants to imazethapyr applied in a single dose was scored 21 d after treatment as R (no imazethapyr symptomology), S (total plant necrosis), or intermediate (I, severely stunted plants with chlorotic and twisted leaves at the apical meristem). R, I, and S phenotypes segregated in a 1:2:1 ratio in the F₂ generation. S and I phenotypes segregated in a 1:1 ratio in progeny from F₁ BCs to the S parent. Similarly, R and I phenotypes segregated in a 1:1 ratio in progeny from BCs to the R parent. Eastern black nightshade resistance to ALS inhibitors is associated with a single, nuclear, incompletely dominant allele, which codes for an insensitive form of the target ALS enzyme.

Nomenclature: Imazethapyr; eastern black nightshade, Solanum ptycanthum L. SOLPT.

A survey conducted in a seven-county region of eastern Oregon during the summers of 1998, 1999, and 2000 characterized the occurrence of jointed goatgrass × wheat hybrids and the features of weed infestations promoting hybridization. During the survey, 93 infested sites were visited with jointed goatgrass collected from 57 sites and hybrids collected from 45 sites. Thirteen collection sites were located in uncultivated areas. Observations of jointed goatgrass infestations in and around cropped fields suggested that jointed goatgrass successfully escapes control where weed populations persist in fencerows, access roads, scablands, draws, and roadsides. Most jointed goatgrass and hybrid populations were located in winter wheat fields but were also found in five spring grain fields. Of the 754 hybrid plants collected, 44% contained backcross seed. For all 3 yr, a 1% backcross hybrid seed–production rate was found. A parentage analysis of a subsample of the total hybrid collection showed that the majority were F₁ hybrids and that jointed goatgrass was most often the female parent. This observational study has established that F₁ hybrids are common in jointed goatgrass–infested wheat fields. Their capability for backcross seed production suggests the potential development of advanced backcross forms that resemble jointed goatgrass. The survey results offer valuable input for the risk assessment of gene flow potential between jointed goatgrass and herbicide-resistant wheat.

Nomenclature: Jointed goatgrass, Aegilops cylindrica Host AEGCY; wheat, Triticum aestivum L.

Experiments were carried out to investigate weed seed production in widely spaced spring wheat crops that received aggressive mechanical weed control (hoeing and harrowing) compared with that in narrowly spaced crops receiving less aggressive mechanical control (harrowing only). Three species (wild buckwheat [Polygonum convolvulus], ladysthumb [Polygonum persicaria], and common chickweed [Stellaria media]) were studied in three row-spacing treatments (10, 20, and 30 cm) and two sowing densities (140 and 180 kg ha⁻¹). Average seed production per surviving plant was up to three times higher in the 30-cm treatments compared with the 10-cm treatments. Taking into account the 40–50% weed mortality resulting from control in the 30-cm treatments, seed production per seedling was still higher in the 30-cm treatments than in the 10-cm treatments. Differences in wheat yield were not found among treatments. From the perspective of long-term weed population management, using a narrow row spacing would be more effective in spring wheat on the basis of experiments with the weed species considered here.

Nomenclature: Ladysthumb, Polygonum persicaria L., POLPE; wild buckwheat, Polygonum convolvulus L., POLCO; common chickweed, Stellaria media (L.) Vill., STEME; wheat, Triticum aestivum L.

No-tillage field studies were conducted in 1999 and 2000 at Columbia, MO, to determine the interaction of grass weed interference and side-dressed N fertilization on corn and weed growth, corn yield, and the N content of the soil and plant biomass at various intervals early in the growing season and at harvest. Ammonium nitrate (112 kg N ha⁻¹) was applied before planting. A herbicide was applied to the entire experimental area before planting to control winter vegetation and to reduce broadleaf weed emergence. A mixture of large crabgrass, giant foxtail, and barnyardgrass was allowed to reinfest the experiment after corn planting and was sprayed with glyphosate when the weeds were 8, 15, 23, or 31 cm tall. Each removal date treatment was duplicated; one series received 45 kg N ha⁻¹ side-dressed when the corn was approximately 60 cm tall, and the other series received no additional N. Corn dry weight and the N content of the corn biomass early in the growing season were similar between the weed-free control and treatments with grass interference up to 23-cm height. Side-dressed N applications before grass weeds were controlled did not increase the early-season N content of the corn biomass in 1999, but the N content of the grass weeds increased by 11 kg ha⁻¹. In 2000, side-dressed N increased the N content of the corn and grass biomass by 13 and 18 kg ha⁻¹, respectively. This suggests that grass weeds should be controlled before the side-dressed N applications to ensure that the N applied can be used by the corn rather than by the grass weeds. Grass weed interference beyond 15-cm height reduced corn yield by at least 1.13 Mg ha ⁻¹ and N content in the corn biomass at corn harvest by at least 30 kg ha⁻¹.

Nomenclature: Glyphosate; corn, Zea mays (L.) ‘Dekalb 626RR’; giant foxtail, Setaria faberi (L.) Herrm. SETFA; large crabgrass, Digitaria sanguinalis (L.) Scop. DIGSA; barnyardgrass, Echinochloa crus-galli (L.) Beauv. ECHCG.

Studies quantifying weed seed production as a function of weed density are expensive and difficult, and lack of these data is a common limitation in modeling weed population dynamics over time. Observed empirical and theoretical relationships between crop yield loss curves and weed seed production curves led us to the hypothesis that there should be a strong relationship between the shapes of these two curves. Data from literature sources were evaluated to test this hypothesis for hyperbolic curves and to determine if the data describing the crop yield loss caused by weeds could provide estimates of the shape parameter of a hyperbolic equation for describing density dependence in weed reproduction. For each of 162 data sets, a shape parameter (N50) and a scale parameter (U ) were estimated for an increasing hyperbolic model both for absolute crop yield loss as a function of weed density (N50_YL, U_YL) and for weed yield (either total biomass yield or seed yield) as a function of weed density (N50_WY, U_WY). N50_YL was strongly correlated with N50_WY across all data sets, with an apparent 1:1 relationship between the two. This relationship suggests that the shape parameter of the yield loss model may substitute for the shape parameter of a hyperbolic model describing the density-dependence of weed seed production. This substitution will be most useful in weed population modeling situations where data describing crop yield loss as a function of weed density are already available, but data describing weed seed production as a function of weed density are not available.

Nomenclature: barnyardgrass, Echinochloa crus-galli (L.) Beauv. ECHCG; black medic, Medicago lupulina L. MEDLU; catchweed bedstraw, Galium aparine L. GALAP; common cocklebur, Xanthium strumarium L. XANST; common ragweed, Ambrosia artemisiifolia L. AMBEL; corn poppy, Papaver rhoeas L. PAPRH; downy brome, Bromus tectorum L. BROTE; eastern black nightshade, Solanum ptycanthum Dun. SOLPT; field violet, Viola arvensis Murr. VIOAR; giant foxtail, Setaria faberi Herrm. SETFA; green foxtail, Setaria viridis (L.) Beauv. SETVI; hemp sesbania, Sesbania exaltata (Raf.) Rydb. ex A.W. Hill SEBEX; johnsongrass, Sorghum halepense (L.) Pers. SORHA; jointed goatgrass, Aegilops cylindrica Host AEGCY; kochia, Kochia scoparia (L.) Schrad. KCHSC; littleseed canarygrass, Phalaris minor Retz. PHAMI; redstem filaree, Erodium cicutarium (L.) L'Her. ex Ait. EROCI; round-leaved mallow, Malva pusilla L. MALNE; smooth pigweed, Amaranthus hybridus L. AMACH; spurred anoda, Anoda cristata (L.) Schlecht. NAVCR; velvetleaf, Abutilon theophrasti Medicus ABUTH; wild oat, Avena fatua L. AVEFA; wild poinsettia, Euphorbia heterophylla L. EPHHL; wild-proso millet, Panicum miliaceum L. PANMI; barley, Hordeum vulgare L.; chili, Capsicum annuum L.; dry bean, Phaseolus vulgaris L.; field pea,