Weed removal experiments in dry edible beans were conducted during 1992 and 1993 at Crookston and Staples, MN. Ten manual weed removal treatments were studied to determine when a natural infestation of weeds first reduced dry bean yield, and when weed removal could be discontinued without further loss of seed yield. Major weeds in order of average biomass production on weedy check plots at dry bean harvest over locations and years were wild mustard, foxtail spp., redroot pigweed, common ragweed, wild buckwheat, hairy nightshade, and common lambsquarters. Hairy nightshade also emerged late in the growing season and could negatively affect harvest efficiency and stain navy beans. Weed removal treatments had little effect on dry bean stands or 100-seed weights of harvested dry bean seed. The critical period for weed control in dry beans was 3 to 5 or 6 weeks after planting (WAP). Thus, weed control practices should begin no later than 3 WAP and continue until at least 5 or 6 WAP for maximum dry bean yields.

The relative sensitivity of various crops and weeds to celery root residue incorporated into soil was determined in greenhouse experiments. A range of concentrations was used to determine the percentage of celery residue needed to cause a 50% reduction in plant growth (I50). The most sensitive weed was spiny amaranth and the least sensitive was common purslane. The most sensitive crop was radish and the least sensitive was celery. Selectivity was calculated as crop I50/weed I50 and varied from a maximum of 13 for celery and spiny amaranth to a low of 0.4 for radish and common purslane. Selective weed suppression might be obtained when celery residue is present.

Field studies were conducted to study the interaction of sethoxydim or fluazifop-P with clopyralid and/or ethametsulfuron applied as tank mixtures to canola. Control of the indicator species barley, broadbean, and wild mustard with the tank mixtures was comparable to, or sometimes better than, that attained with each herbicide alone. Canola tolerated all herbicides applied individually and all tank mixtures of these herbicides except fluazifop-P and ethametsulfuron. Tank mixtures of fluazifop-P and ethametsulfuron, with or without clopyralid included in the mixture, suppressed early growth of canola in two of four tests and reduced seed yield in one. Sethoxydim, clopyralid, and ethametsulfuron combined in tank mixtures provide an effective POST alternative for selective control of grass and broadleaf weeds in canola.

Barnyardgrass and many broadleaf weeds were severely stunted by POST applications of isoxaben. Most grasses were only moderately affected by isoxaben at rates up to 1 kg/ha. POST activity was enhanced by soil interception, but evident with foliar interception alone. Cell swelling in stems and in interveinal leaf lamina was observed after foliar application. A radiotracer study in redroot pigweed showed that only a small amount (3.3% of applied) of isoxaben entered the leaf following foliar application, and that little (0.08% of applied) isoxaben migrated beyond the point of application. The potential of isoxaben as a POST herbicide may be limited, in part, by poor absorption and translocation.

Both gibberellic acid and the substituted phthalimide AC-94377 [1-(3-chlorophthalimido)cyclohexanecarboximide] actively promoted germination of dormant seeds in five species out of nine. The species that responded to both compounds included wild oat (Avena fatua L.), wild mustard [Brassica kaber (DC.) L.C. Wheeler], curly dock (Rumex crispus L.), field pennycress (Thlapsi arvense L.) and tansy phacelia (Phacelia tanacetifolia L. Benth.). on a weight-per-weight basis, the ability of AC-94377 to stimulate germination was equal to, or greater than, GA3. The other two substituted phthalimides tested, AC-92803 [2-(3-chlorophthalimido)-2-methyl-2-isobutyl-acetamide] and AC-99524 [1-tetrahydrophthalimido-cyclohexane-carboximide], had either slight or no stimulatory activity.

Applications of the substituted phthalimide growth regulators AC-94377 [1-(3-chlorophthalimido)cyclohexanecarboximide] and AC-99524 [1-tetrahydrophthalimido-cyclohexanecarboximide] to the soil surface stimulated germination and emergence of dormant wild mustard seed (Sinapsis arvensis L. ♯4 SINAR) shallowly planted in soil in greenhouse experiments. Surface applications of AC-94377 enhanced the germination and emergence of dormant wild mustard seed planted 0.6 cm deep in soil from nine locations as the rates were raised from 0.4 to 3.7 kg ai/ha. Surface application of AC-94377 increased emergence rates and also extended the period of wild mustard emergence. Increasing quantities of wheat (Triticum aestivum L.) straw on the soil surface reduced the effect of surface-applied AC-94377 on shallowly buried dormant wild mustard seed. When dormant wild mustard seed were buried 1.3, 2.5, or 3.8 cm deep, surface applications of AC-94377 also stimulated greater emergence than the dormant controls from these depths, strongly suggesting that the compound moved from the surface to the depth of planting.

In growth chamber studies, we measured the movement of cotyledons and leaves or leaflets in seedlings of nine weed species to determine: the magnitude of movement, whether movements followed a rhythmic pattern, and if rhythmic movements were endogenously controlled. Seedlings were always entrained under a 15-h light:9-h dark regime. Measurements were made at 3-h intervals for 3 to 4 days under four light regimes: alternating 15-h light:9-h dark; continuous light; one 15-h light:9-h dark span, followed by continuous light; and a 15-h light:9-h dark regime, in which the dark span was advanced 9 h (phase shift) when measurements were begun. No clearly defined pattern of leaf movement was found in wild mustard [Brassica kaber (DC.) L. C. Wheeler var. pinnatifida (Stokes) L. C. Wheeler]. Daily rhythmic leaf movements that did not appear to be endogenously controlled were found in redroot pigweed (Amaranthus retroflexus L.) and black nightshade (Solanum nigrum L.). Circadian rhythmic leaf movements that appeared to be under endogenous control were found in jimson-weed (Datura stramonium L.), common lambsquarters (Chenopodium album L.), common cocklebur (Xanthium pensylvanicum Wallr.), prickly sida (Sida spinosa L.), sicklepod (Cassia obtusifolia L.), and coffee senna (Cassia occidentalis L.).

Field experiments were conducted to study the effect of fall and spring pre-plant incorporated applications of trifluralin (α,α,α-tri-fluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) and metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazine-5(4H)-one] alone and in combinations on crop tolerance, seed yield, and weed control in fababeans (Vicia faba L. ‘Diana’). Trifluralin applied either in the fall or the spring resulted in acceptable control of green foxtail [Setaria viridis (L.) Beauv.] and wild oat (Avena fatua L.) while metribuzin resulted in excellent control of wild mustard [Brassica kaber (DC.) L.C. Wheeler var. pinnatifida (Stokes) L.C. Wheeler]. In 2 out of 3 yr, a tank-mixture of trifluralin plus metribuzin applied in the fall at 1.4 and 0.4 kg/ha, respectively, resulted in significantly larger seed yield than the same combination applied at 1.1 and 0.3 kg/ha in the spring. Although the fall treatments generally resulted in higher seed yields, there was no consistent difference in weed densities between fall and spring applications. In both field and growth room studies, trifluralin reduced the injury to fababeans from metribuzin. A postemergence application of metribuzin in the spring at 0.3 kg/ha following a fall-application of trifluralin resulted in good wild mustard control but caused considerable crop damage. Dinoseb (2-sec-butyl-4,6-dinitrophenol) and bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2dioxide] controlled wild mustard effectively with no injury to the crop.

The initial metabolism of acetochlor [2-chloro-N-(ethoxymethyl)-N-(2-ethyl-6-methylphenyl)acetamide] in tolerant and susceptible plants was examined to determine if metabolism is a factor in the selective phytotoxicity of this herbicide. Six crop and ten weed species were used. In every case acetochlor was converted initially to thioether conjugates. In thirteen of the species, initial conjugation was with glutathione (GSH). The homoglutathione (hGSH) conjugate was formed in three legumes. Tolerant plant seedlings metabolized acetochlor more readily than susceptible plant seedlings. Based on these results, acetochlor detoxification is a primary factor in the selective phytotoxicity of this herbicide.

The influence of weed seed burial depth on seedling emergence is a common and important topic in undergraduate weed science courses. A laboratory exercise was developed at Iowa State University to actively demonstrate changes in weed seedling emergence with increased seed-burial depth. Twenty-five ivyleaf morningglory and wild mustard seeds were sown in pots in monoculture, and the number of emerging seedlings was counted for 3 wk. The results from two semesters were analyzed to determine whether the experiment was successful in meeting its intended outcomes. Assessment of student perceptions and attitudes indicated that the activity increased students' understanding of weed biology; improved students' critical thinking, reading, and writing skills; and improved students' ability to make scientific observations, interpret graphical information, and analyze and summarize research data.

Mapping weed infestations in an annual crop has implications not only for site-specific herbicide applications but also for planning future management strategies and understanding weed ecology. A controlled laboratory experiment, involving detached leaves, was conducted to investigate the potential to discriminate two crop and five weed species using hyperspectral and multispectral remote sensing. Stepwise discriminant function analyses showed that reflectance in the visible and “red-edge” regions of the spectrum were consistently required for species discrimination. The seven species were correctly identified 90 and 89% of the time using the hyperspectral and multispectral data, respectively, and the classification rules derived from discriminant function analyses. Errant species prediction with the hyperspectral data resulted in a grass being predicted as a grass and a broadleaf as a broadleaf. However, for multispectral data, incorrect classifications were more serious because errant predictions sometimes resulted in a grass being classified as a broadleaf and vice-versa. Further studies using plants at a variety of growth stages, from a variety of environments, and at the canopy level are warranted.