The nonionic surfactants Tween 20 [oxysorbic (20 POE) polyoxyethylene sorbitan monolaurate] and Tween 80 [oxysorbic (20 POE) polyoxyethylene sorbitan monooleate] at concentrations of 0.05, 0.1, 0.2, and 0.4% v/v stimulated germination of barnyardgrass [Echinochloa crus-galli (L.) Beauv. # ECHCG] seeds in petri dishes. Stimulation occurred under both 30/20 and 25/15 C diurnal temperature cycles whether or not light was supplied during the 8-h high-temperature portion of the cycle. Barnyardgrass responded most to treatment under environmental conditions in which untreated seeds germinated the least. A 0.1% surfactant concentration often stimulated germination of weed seeds as effectively as did higher concentrations. Germination of redroot pigweed [Amaranthus retroflexus L. # AMARE] seeds was inhibited by Tween 80 in the light under both the low- and high-temperature regimes and by Tween 20 at low temperature in the light. Germination of tumble pigweed (Amaranthus albus L. # AMAAL) seeds was inhibited by both surfactants in the high-temperature regime whether or not light was supplied. Common purslane (Portulaca oleracea L. # POROL) seeds were insensitive to treatment.

Field studies were conducted from 1986 through 1989 to determine the feasibility of using clomazone in a winter wheat-corn-fallow rotation. Clomazone at 1.1 kg ai/ha did not control emerged downy brome in autumn nor did it control redroot pigweed and tumble pigweed that emerged after a spring application of paraquat at 0.6 kg ai/ha. However, clomazone plus atrazine at 0.6 plus 2.2 kg ai/ha provided weed control in winter wheat stubble from October to corn harvest 12 mo later. Corn yields from this treatment were equal to or greater than the conventional practice of applying atrazine plus glyphosate at 2.2 plus 0.4 kg ae/ha in the fall followed by a spring application of metolachlor plus dicamba at 2.8 ai plus 0.3 kg ae/ha. When kochia densities were high, clomazone plus atrazine controlled triazine-resistant kochia better than metolachlor plus dicamba. Atrazine at 2.2 kg/ha applied in autumn followed by clomazone at 0.8 kg/ha in April generally caused more corn injury than when the same rate of clomazone was applied in autumn with the atrazine. The percent of bleached corn plants varied with planter, year, time of application, and rate of clomazone applied. The optimum time to apply clomazone at 0.6 kg/ha was in autumn with atrazine. This treatment caused some corn bleaching but produced the highest corn yield over 3 yr. In a dry autumn and spring, less clomazone injury occurred with a planter equipped with row cleaners than those without. The injury pattern associated with planters was inconsistent when precipitation was average and no injury occurred in a wet autumn and spring.

In a semiarid Mediterranean site in central Spain, field experiments were conducted on a Calcic Haploxeralf (noncalcic brown soil), which had been managed with three crop rotations and two tillage systems (no-tillage and conventional tillage) since 1987. The crop rotations consisted of barley→vetch, barley→sunflower, and a barley monoculture. The study took place in two growing seasons (1992–1994) to assess the effects of management practices on the weed seedbank. During this period, spring weed control was not carried out in winter crops. In the no-tillage system, there was a significant increase in the number of seeds of different weed species: anacyclus, common purslane, corn poppy, knotted hedge-parsley, mouse-ear cress, spring whitlowgrass, tumble pigweed, venus-comb, and Veronica triphyllos. Conversely, the presence of prostrate knotweed and wild radish was highest in plots under conventional tillage. These results suggest large differences in the weed seedbank as a consequence of different soil conditions among tillage systems, but also the necessity of spring weed control when a no-tillage system is used. With regard to crop rotations, the number of seeds of knotted hedge-parsley, mouse-ear cress, and spring whitlowgrass was greater in the plots under the barley→vetch rotation. Common lambsquarters dominated in the plots under the barley→sunflower rotation, whereas venus-comb was the most frequent weed in the barley monoculture. Larger and more diverse weed populations developed in the barley→vetch rotation rather than in the barley→sunflower rotation or the barley monoculture.

Field experiments were conducted from 1987 to 1990 to investigate the combination of cultural and chemical methods for control of Amaranthus spp. in corn. The tolerance of corn hybrids to herbicides or herbicide combinations was dependent on application rate. The early season threshold level for a mixed population of Amaranthus spp. occurred 3.5 wk after emergence. Density of Amaranthus spp. from 155 to 495 plants/m2 caused corn yield reduction of 50%. A quadratic model was derived relating percentage of yield reduction vs. duration of weed competition. The results emphasize the importance of using herbicides, cultural practices, and competition thresholds for an integrated weed control approach of Amaranthus spp. in corn.

The control of puncturevine in southern peas with imazethapyr and imazaquin and the potential for carryover of these herbicides to spinach was evaluated in Arkansas. Imazethapyr (0.07 kg ai ha−1) and imazaquin (0.14 kg ha−1) applied to southern peas either PPI tank-mixed with trifluralin (0.84 kg ha−1) or PRE or POST following trifluralin controlled puncturevine at least 70%. Imazaquin and imazethapyr remaining in soil after use caused injury and yield reduction to spinach planted 3 to 4 mo after herbicide application in the fall after southern pea harvest. In spring spinach planted 8 to 9 mo after herbicide application, the only injury observed was from soil residues of imazethapyr applied POST in one of two years, with no yield reduction. In rate by timing experiments, imazethapyr at 0.07 kg ha−1 applied PPI, PRE, and 3 and 6 d after puncturevine emergence controlled puncturevine at least 95% at the 2-wk rating. At the 4-wk rating, imazethapyr at 0.07 kg ha−1 applied PRE provided this level of control. Imazaquin at 0.07 and 0.14 kg ha−1 controlled puncturevine greater than 93% when applied up to 12 d after emergence. Puncturevine control was less with both herbicides at lower rates and as application timing increased.

Greenhouse and field studies were conducted to examine weed control with, and potato tolerance to, rimsulfuron or rimsulfuron + metribuzin applied POST. In greenhouse trials, hairy nightshade control with rimsulfuron applied POST at 9 to 35 g/ha was ≥ 90%, but cutleaf nightshade control was < 30%. In field studies, rimsulfuron at 18 to 35 g/ha provided ≥ 85% control of hairy nightshade, redroot pigweed, tumble pigweed, and shepherd's-purse; 75 to 82% control of volunteer oats; and 25 to 87% control of common lambsquarters. Common lambsquarters control was increased to 92 to 100% when rimsulfuron at 18 to 35 g/ha was applied in combination with metribuzin at 280 g/ha. Hairy nightshade and redroot pigweed control usually were not improved by adding metribuzin to the rimsulfuron spray mixture and tumble pigweed and shepherd's-purse control were improved only slightly. Volunteer oat control with rimsulfuron sometimes was antagonized by the addition of metribuzin to the spray mixture. In weed-free trials, yield of potatoes grown for the fresh market or to produce seed tubers was not reduced by rimsulfuron at rates as high as 70 g/ha.

Spring small grains were not as competitive with barnyardgrass and witchgrass as winter wheat. Winter wheat grain yields were greater than spring barley or spring wheat in 1986, 1987, and 1988 and oat in 1986 and 1988. Barnyardgrass, stinkgrass, and witchgrass control with glyphosate plus 2,4-D plus atrazine at 0.6 plus 0.8 plus 1.7 kg ha−1 was usually less when the herbicides were applied to stubble of spring small grain versus winter wheat due to the advanced weed growth at treatment Barnyardgrass and witchgrass were more difficult to control than stinkgrass, redroot pigweed, tumble pigweed, kochia, and tumble thistle. No-till corn planted into winter wheat stubble had fewer barnyardgrass and witchgrass than corn planted into spring wheat stubble. The addition of metolachlor plus atrazine at 1.7 plus 0.6 kg ha−1 eliminated differences among small grain cultivars in weed control in corn. Corn grain yields from winter wheat plots were greater than other small grains in 1989 because of better weed control and more crop residue.

Experiments were conducted to determine adaptation characteristics to reduced irradiance of velvetleaf, common lambsquarters, eastern black nightshade, tumble pigweed, and soybean. Plants were grown to the 5- to 8-leaf stage in the greenhouse at ambient radiation (850 μE·m–2·s–1), and 26 and 13% of ambient radiation. Tumble pigweed, a C4 plant, had the highest light-saturated photosynthetic rates at all growth irradiances, while common lambsquarters had the highest rates of the four C3 species. All species adjusted to reduced irradiance by decreasing light-saturated photosynthesis, leaf respiration rates, root:shoot ratios, and leaf densities, while increasing leaf area ratios (LAR)3. As growth irradiance was reduced, support tissues (roots, stems, and petioles):leaf ratios did not change for common lambsquarters or velvetleaf, increased for soybean, and decreased for eastern black nightshade and tumble pigweed, indicating superior adaptation of the latter two species for efficient light harvesting at reduced growth irradiances. Of these five species, eastern black nightshade had the lowest respiration rate, the highest LAR, and the lowest support:leaf ratio for optimum adaptation to shaded environments.

Acifluorfen, lactofen, chlorimuron, thifensulfuron, imazethapyr, and imazaquin were evaluated for control of Palmer amaranth, common waterhemp, redroot pigweed, and tumble pigweed at three application timings in field and greenhouse experiments. Results from field studies indicated that most herbicides provided greater than 90% control of common waterhemp, redroot pigweed, and tumble pigweed regardless of time of application. Palmer amaranth was the most difficult species to control, and only thifensulfuron and imazethapyr provided greater than 80% control at all application timings. In the greenhouse experiment, herbicides were applied when pigweeds averaged 10 cm, 20 cm, and 30 cm in height. Results were similar to the field experiment, except that common waterhemp was more difficult to control.

Relationships between production of cotton (Gossypium birsutum L. ‘Westburn M’) and full-season interference from tumble pigweed (Amaranthus albus L. ♯ AMAAL) at densities ranging from 0 to 64 plants/10 m of row were measured in three replicated experiments in the field. Dry weight of harvested tumble pigweed increased by 0.149 to 0.402 kg/plot for each additional weed/10 m of row. Intraspecific competition among tumble pigweed plants was observed at the higher weed densities. Cotton plant height was reduced in all three experiments at the 32 or 64 weed densities, but harvesting difficulties were not encountered in these studies. The threshold density where initial lint yield reductions occurred ranged from 4 to 16 tumble pigweed plants/10 m of row in the three experiments. Lint yields were reduced from 8 to 11 kg/ha for each additional tumble pigweed plant/10 m of row. Interference from tumble pigweed did not significantly affect cotton fiber length, uniformity, strength, or micronaire of hand-harvested bolls.

Thirteen hard red winter wheat cultivars were evaluated for their ability to suppress summer annual weeds in grain production systems near North Platte, NE, from 1993 through 1997. ‘Turkey’, a 125-yr-old landrace selection, suppressed both broadleaf and grass weeds more than other cultivars. Some relatively new cultivars, such as ‘Arapahoe’, ‘Jules’, ‘Pronghorn’, and ‘Vista’ suppressed summer annual grasses almost as well as Turkey. Total weed density was negatively correlated with number of winter wheat stems/m2, mature winter wheat height, and lodging. Weed density after wheat harvest was positively correlated with delay in winter wheat seeding date and was negatively correlated with precipitation 0 to 30 d after winter wheat seeding, during tillering, tillering to boot stage, and heading to maturity stage. Mean air temperature 0 to 30 d after wheat seeding was positively correlated with weed density. In the spring, weed density was positively correlated with temperatures during the tillering stage, tillering to boot stage, and heading to maturity stage. Stinkgrass and witchgrass densities were positively correlated with severity of wheat leaf rust. The highest grain-producing cultivars included three medium height cultivars ‘Alliance’, Arapahoe, and ‘Niobrara’. Alliance wheat produced 53% more grain than Turkey, and the other two produced 43% more grain.

A survey of 174 fields was conducted to investigate performance of herbicides applied after winter wheat harvest on weeds across western and southern Nebraska during August and September 1998. Glyphosate plus 2,4-D plus atrazine was applied on 32%, glyphosate plus 2,4-D or dicamba on 24%, paraquat plus atrazine on 23%, glyphosate on 8%, ICIA0224 plus 2,4-D or atrazine on 10%, and atrazine plus 2,4-D on 3% of the fields. These treatments controlled 85 to 100% of the weeds except atrazine plus 2,4-D, which controlled 30%. The frequency of occurrence of the most prevalent summer annual grasses was as follows: green foxtail, 65%; barnyardgrass, 46%; stinkgrass, 41%; witchgrass, 39%; and longspine sandbur, 36%. The most common broadleaf weeds and their frequency were redroot pigweed, 32%; tumble pigweed, 30%; tall waterhemp, 28%; and kochia, 25%. Virginia groundcherry, 22%; common milkweed, 11%; yellow woodsorrel, 9%; and field bindweed, 6% were the most common perennial weeds. The five most difficult weeds to control were yellow nutsedge, spotted spurge, Virginia groundcherry, common milkweed, and toothed spurge, with control ratings of 0, 3, 17, 26, and 33%, respectively. These weeds were not controlled with glyphosate or mixtures containing glyphosate. Only 35% of the fields were treated before summer annual grasses had headed. Late applications required higher herbicides rates for effective control.