The combination of glyphosate and 2,4-D at various rates was evaluated for controlling existing weeds at planting in no-till corn and soybeans. Herbicide combinations in soybeans also included paraquat plus 2,4-D, linuron, or diuron. Standard treatments included glyphosate (0.6 and 1.1 kg ae/ha) and paraquat (0.3 and 0.6 kg ai/ha), and 2,4-D (0.6 kg ae/ha) alone. For corn, the addition of 2,4-D to glyphosate did not improve weed control, although the addition of 2,4-D to paraquat did improve horseweed control. Corn yield with the herbicide combinations was higher than that for the nonselective herbicides alone. Although initial weed control was good in soybeans, weed regrowth in all paraquat alone treatments was substantial, especially with horseweed. The addition of 2,4-D to paraquat improved control of horseweed and tall morningglory. The addition of linuron or diuron to paraquat improved horseweed and common ragweed control, whereas the addition of 2,4-D to glyphosate improved the control of tall morningglory but not the other weed species. Generally, after 4 weeks, all glyphosate treatments provided better horseweed control than all paraquat treatments. Paraquat plus either linuron or diuron and glyphosate alone or in combination with 2,4-D gave the highest soybean yields.

Studies were conducted to determine whether photosynthesis-inhibiting herbicides atrazine or bifenox, and shade affect the number, weight, senescence, and N fixation activity of nodules on the roots of the perennial legume crownvetch. Atrazine and bifenox were applied at rates of 2.24 kg ai/ha to shaded and unshaded plots. The shade was varied during the growing season to simulate changing irradiance levels beneath a corn canopy. Atrazine and bifenox treatments reduced nodule number to 13 and 42% of the untreated control, respectively, in 1980, and 18 and 35% in 1981. Shade treatments reduced nodule number to a low of 40% of the control in 1980 and 50% in 1981. Combined effects of herbicides and shade on nodule numbers were more than additive. Nodule fresh weights were reduced an average of 37% by herbicide treatments and 39% by shade treatments. Sloughed nodule numbers decreased in the herbicide and shaded treatments, suggesting that the reduction in nodule numbers was due to fewer nodules being produced. Nodule numbers were reduced a greater percentage by herbicides and shade than was herbage dry matter production. Specific nodule activity (SNA) did not differ in nodules from the atrazine, bifenox, or shade treatments on the six sampling dates in 1980 or on two of three sampling dates in 1981.

Metabolites from 906 microbial isolates were evaluated for herbicidal properties. These included 266 isolates of Streptomyces, 502 isolates of non-Streptomyces actinomycetes representing 18 genera, 28 unidentified aerobic actinomycetes, 70 fungi, and 40 isolates of eubacteria. Metabolites from 72 isolates significantly inhibited germination of cress seeds. In terms of generic specificity and frequency, about 18% of all the Streptomyces and Nocardiopsis isolates and 13% of Actinoplanes isolates were toxic to cress (Lepidium sativum L.) seeds. Among other inhibitors were three isolates of Actinomadura and one isolate each of Micromonospora, Micropolyspora, Strep to sporangium, Streptoverticillium, and Bacillus, and two isolates of unidentified actinomycetes. The toxigenic fungi included two isolates of Penicillium and one isolate each of Aspergillus, Scopulariopsis, and Paecilomyces species. About half the isolates toxic to cress were toxic to barnyardgrass (Echinochloa crus-galli L. Beauv. # ECHCG) seeds. of the isolates found effective in the secondary screening on the potted weeds, eight belonged to the genus Streptomyces and one was identified as Scopulariopsis brumptii. None of the examined isolates of Rhodococcus, Nocardia, Oerskovia, Thermoactinomyces, Thermomonospora, and six other genera of actinomycetes showed any appreciable toxicity to the seeds of either species.

Tebuthiuron {N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N,N′-dimethylurea} in various formulations and placements was applied to honey mesquite (Prosopis glandulosa Torr. # PRCJG), huisache [Acacia farnesiana (L.) Willd. # ACAFA], live oak (Quercus virginiana Mill. # QUEVI), Macartney rose (Rosa bracteata J. C. Wendl. # ROSBC), and Texas whitebrush [Aloysia gratissima (Gillies & Hook.) Troncoso # ALYLY]. There was little difference in a species response to the various tebuthiuron formulations. Pellets (20%) at 2 g ai/tree base reduced the canopy 85% and killed 50% of the honey mesquite. At 1 g ai/plant, two briquettes at 0.5 g ai each or pellets (5.14 g, 20%) killed 37 and 80% of the huisache, respectively. Pellets were no more effective at 2 g than at 1 g ai on huisache. Basal treatments of 0.25, 0.5, 0.5, 1, and 2 g ai tebuthiuron/tree killed from 8 to 22, 48 to 62, 62 to 85, and 92% of the live oak, respectively. Pellets at 2 g ai placed at the base reduced the canopy 83% and killed 59% of the Macartney rose. On Texas whitebrush, 0.25, 0.5, 1, and 2 g ai tebuthiuron/tree killed from 68 to 85, 75 to 100, 90, and 100% of the plants, respectively. Tebuthiuron persisted mainly in the upper 30 cm of soil for at least 15 months.

Laboratory experiments were conducted to evaluate the biodegradation of 14C-labeled butylate, cycloate, EPTC, pebulate, and vernolate in three butylate-history soils that had received three to eight applications of butylate under field conditions. After 20 days, biodegradation of butylate and EPTC was accelerated and had no lag phase in all three butylate-history soils. Butylate-adapted microorganisms were cross-adapted for EPTC and degraded EPTC as readily as butylate. Biodegradation of butylate and EPTC in Dothan soil without a butylate history exhibited a lag phase of 6 days after which 14CO2 was evolved at an exponential rate. This indicated that enhanced biodegradation was induced after one application of butylate or EPTC. Butylate-adapted microorganisms were cross-adapted for vernolate and pebulate in Dothan and pebulate in Wagram soils with a butylate history. Biodegradation of vernolate and pebulate was not enhanced in Varina butylate-history soil. After 20 days, there was no cross-adaptation for cycloate in any soil. These findings indicated that biodegradation of carbamothioates was influenced by soil type and previous carbamothioate use and that caution should be exercised in use of EPTC on fields of previous butylate use.

Field experiments were conducted during a 3-yr period to study the effects of three soybean [Glycine max (L.) Merr. ‘Hill’] row spacings, four periods of sicklepod (Cassia obtusifolia L. # CASOB) competition, and timing of fungicide application for control of seed and foliar diseases of soybeans on soybean yields and growth of sicklepod Competition of sicklepod for 4 weeks reduced soybean yields in 2 of the 3 yr. Competition for the entire season significantly reduced the yield of soybeans regardless of row spacing. Average soybean yields were higher with 51-cm row spacings than with 102-cm row spacings regardless of fungicide treatment or period of sicklepod competition. Foliar applications of benomyl significantly increased soybean yields in only 1 of 3 yr. Sicklepod plants did not grow taller than soybeans until more than 96 days after emergence. Greatest rate of increase in weights of individual sicklepod plants and growth of axillary branches was in the period from 72 to 96 days after emergence. During this time, there was about an 8-fold increase in the number of leaves produced by sicklepod plants. The most rapid daily increase in height of sicklepod was in the period from 96 to 100 days after emergence when more than 50% of the total growth occurred. Height and weight of sicklepod plants were less when soybeans were grown in 25-cm row spacings than in 102-cm row spacings. No differences were noted in die internal microflora of soybean seed collected from fold plots with regard to the presence of sicklepod, duration of sicklepod interference, or timing of fungicide application.

Two-level fractional factorial design experiments were conducted in growth chambers to determine the influence of environmental factors and adjuvants on the foliar penetration of the sodium salt of acifluorfen in velvetleaf. Relative humidity (RH) was found to have the greatest effect on penetration. Temperature was the second most important factor. Although soil moisture significantly affected penetration, this effect was much less than that produced by RH and temperature. Diammonium phosphate (DAP) was the most effective adjuvant tested. The use of DAP at 1.12 kg/ha significantly increased penetration. The effect of DAP was less than either RH or temperature. Oxysorbic (5 POE) surfactant also significantly increased penetration. Glycerol and oxtoxynol (7 to 8 POE) surfactant had no effect on penetration. In separate experiments, acifluorfen penetration at low and high RH and with the addition of DAP was measured to verify these results. Acifluorfen penetration into velvetleaf increased approximately ninefold with changes in RH from low to high. Penetration increased sevenfold when DAP was added. DAP was by far the most effective adjuvant increasing acifluorfen penetration into velvetleaf under conditions of low RH. Oxysorbic (5 POE) surfactant slightly increased penetration under these conditions.

Acifluorfen {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid} applied alone and in mixtures with propanil [N-(3,4-dichlorophenyl)propanamide] reduced rough grain yield of rice (Oryza sativa L. ‘Lebonnet’ or ‘Bond’) by 7% when applied at the late-booting or heading growth stages, averaged over several rates of application. Earlier applications did not reduce rice yield. The highest rate (0.6 kg ai/ha) of acifluorfen alone and in mixtures with propanil at 3.4 kg ai/ha reduced grain yields more than lower rates (0.1 or 0.3 kg ai/ha). Triclopyr {[(3,5,6-trichloro-2-pyridinyl)oxy] acetic acid} alone and in mixtures with propanil reduced rough grain yields of rice by 18% when applied at the late-booting growth stage, averaged over several rates of applications, but did not affect yields when applied at the early-tillering, jointing, or early-booting growth stages. Whole-grain milling yield and germination of rice seed were not affected by acifluorfen or triclopyr alone or in mixtures with propanil, regardless of rate or growth stage of application.

Growth of Douglas fir [Pseudotsuga menziesii (Mirb.) Franco] was increased by controlling grasses and broadleaf herbs with eight herbicide regimes during the first 3 yr after planting on a well-drained moist site in the Oregon Coast Range. The greatest growth occurred if weeds were controlled in the same growing season that tree seedlings were transplanted to the field; smaller increments came from second- and third-year weed control. Growth increases attributable to early weed control continued through the fifth year, indicating that conditions during establishment strongly influenced later growth. Plots with no herbaceous vegetation had more available soil water than those with competing vegetation, and tree seedlings on these plots experienced less water stress. Irrigation in the third year increased stem diameter of seedlings in that year but had no effect thereafter. Increases in average seedling stem volume at 5 yr after transplanting were linearly related (r2 = 0.77) to the difference in observed xylem potential during the first three growing seasons after transplanting and the xylem potential at which photosynthesis ceased, −2 MPa.

Field experiments were conducted to evaluate postemergence combinations of imazethapyr, imazquin, or chlorimuron with low rates of imazapyr for Johnsongrass and pitted morningglory control. Imazapyr applied alone at rates up to 4 g ai/ha gave little or no control of either weed species. However, the addition of imazapyr to various rates of imazethapyr or imazaquin synergistically increased both johnsongrass and pitted morningglory control 8 weeks after treatment. The rates of imazethapyr or imazaquin required for significant enhancement of johnsongrass control were higher than those required for pitted morningglory control. No synergistic increases in control of either weed species were noted with mixtures of imazapyr and chlorimuron. Although not synergistic in every case, the mixtures of imazapyr at 4 g/ha with imazethapyr, imazaquin, or chlorimuron gave johnsongrass and pitted morningglory control equal to or better than the next higher rate of these herbicides applied alone. Imazapyr did not increase soybean injury or decrease yield provided by chlorimuron, imazaquin, or imazethapyr.

This research was conducted near North Platte, NE, over an 18-yr period to determine the feasibility of using herbicides to replace tillage as the weed control method in a winter wheat (Triticum aestivum L.)-sorghum [Sorghum bicolor (L.) Moench.]-fallow rotation. Five tillage treatments [two tillage and three reduced or no-till treatments] were used on the same plots during the duration of this experiment on a Holdrege silt loam (Typic Argiustolls). Herbicides effectively replaced tillage for weed control. The no-till plots treated with atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine] after wheat harvest had higher sorghum and winter wheat yields, higher crop residue remaining on the soil surface, and lower weed yields than tilled plots. The most difficult weeds to control were volunteer wheat and barnyardgrass [Echinochloa crus-galli (L.) Beauv. # ECHCG]. Soil surface pH decreased over time because of increased use of nitrogen. The pH in nontilled plots was significantly lower than in tilled plots due to lack of soil mixing. Exchangeable calcium was the predominant cation leached from the top 5 cm but showed accumulation between the 5- to 12.5-cm depth. Organic matter content showed little change over time.

Two leaf-spotting pathogens, [Bipolaris setariae (Saw.)] and [Piricularia grisea (Cke.) Sacc.], were isolated from severely infected goosegrass plants. Pathogenicity tests conducted at 28 C showed that both fungi were 100% effective in infecting goosegrass when given a 72-h dew period. Dew period temperature and duration requirements were tested by inoculating 2-week-old plants with conidial suspensions of each fungus and incubating them in dew chambers (100% relative humidity). Disease index increased as dew period duration increased for both fungi and at all temperatures tested. Infection occurred at all temperatures with an optimum of 24 to 28 C for B. setariae and 28 C for P. grisea. In host range tests, representative plants of the Fabaceae, Malvaceae, Poaceae, and Solanaceae were inoculated with suspensions of either 20 000 or 60 000 spores/ml of each fungus and placed in growth chambers at 28 C. Infection was limited to members of the Poaceae. Sorghum showed a hypersensitive response to B. setariae. Both cultivars of corn developed light symptoms in response to both fungi at 20 000 and 60 000 spores/ml.

An experiment involving six winter wheat (Triticum aestivum L.) cultivars, an early-April herbicide application on wheat and on four dates after wheat harvest, and the growth of a subsequently planted corn (Zea mays L.) crop was conducted at North Platte, NE. ‘Centurk 78’ suppressed barnyardgrass [Echinochloa crus-galli (L.) Beauv. # ECHCG] more than ‘Bennett’ and ‘Eagle’ in the growing wheat and after wheat harvest in July, but there were no differences in weed yield among cultivars in corn planted 11 months later. Herbicides applied to the tillering wheat in early April improved weed control in wheat and the subsequent corn crop. Also, herbicides were applied 5, 25, 45, and 300 days after wheat harvest. Weed growth increased and soil water decreased as spraying dates were delayed. Herbicides applied 5 days after harvest did not maintain adequate weed control in the corn planted 11 months after wheat harvest and low corn yield resulted. Plots receiving herbicides 300 days after wheat harvest had the least soil water in the fall after wheat harvest but the best weed control in corn and highest corn yields because of better weed control in corn.