The introduction of herbicide-resistant crops and postemergence herbicides with a wide action spectrum shifted the research focus from how to when crops should be treated. To maximize net return of herbicide applications, the evolution of weed–crop competition over time must be considered and its effects quantified. A model for predicting the yield trend in relation to weed removal time, considering emergence dynamics and density, was tested on data from glyphosate-resistant soybean grown in cropping systems in Italy and Argentina. Despite an ample variation of weed emergence dynamics and weed load in the four trials, the model satisfactorily predicted yield loss evolution. The estimated optimum time for weed control (OTWC) varied from about 18 d after soybean emergence in Argentina to 20 to 23 d in Italy, with time windows for spraying ranging from 14 to 28 d. Within these limits a single glyphosate application ensures good weed control at low cost and avoids side effects like the more probable unfavorable weed flora evolution with double applications and the presence of residues in grains. Despite the apparent simplicity of weed control based on nonselective herbicides, the study outlines that many variables have to be considered to optimize weed management, particularly for the time evolution of the infestation and, subsequently, a proper timing of herbicide application.

The two isomers of diallate [S-2,3-dichloroallyl)diisopropylthiocarbamate] can be isolated by preparative scale gas chromatography using a Carbowax 20M liquid phase. on the basis of IR and NMR spectroscopic and other evidence the isomer with the shorter retention time is assigned the trans configuration and that with the longer retention time is assigned the cis configuration.

Total nonstructural carbohydrate (hereinafter referred to as TNC) storage in roots and utilization of TNC by yellow rocket (Barbarea vulgaris R. Br.), white cockle (Lychnis alba Mill.), and hoary alyssum (Berteroa incana (L.) DC) was studied through a 1-year life cycle in plants seeded July 8, July 22, and August 5, 1966. For yellow rocket, TNC storage was most rapid during September and reached maximum levels by November 2. Younger plants stored TNC more efficiently than did older plants, but total grams of TNC per plant were greater for older plants than for younger plants. No plants bolted during the seeding year. Rapid declines in TNC of the roots began in late March, and minimum TNC levels coincided with the completion of flowering in mid-June for all plants the year after seeding. For white cockle, TNC storage was most rapid during September and early October, and reached maximum levels for all plants by November 2. There was no apparent winter injury to any of the plants. Total growth in 1967 was greatest for plants seeded July 8 and least for plants seeded August 5. Flowering began June 2 and seed were mature June 29. Adventitious buds were present on the roots of all plants on all sampling dates from September 7, 1966, through July 7, 1967. Minimum levels of TNC during spring growth were reached in mid-June. For hoary alyssum, TNC storage was most rapid during late September and October and reached maximum levels by the middle of November. TNC levels were higher in plants seeded July 22 or August 5 than in those seeded July 8. Plants seeded July 8 were flowering profusely by the middle of September, those seeded July 22 were flowering October 6, but those seeded August 5 did not flower in the seeding year. In 1967, all plants were in full bloom May 26. Minimum TNC levels were reached at full bloom and there was no accumulation of TNC as of July 7.

Asia Minor bluegrass (Polypogon fugax Nees ex Steud.) is a problem grass weed of winter crops in China, where a population has become resistant to aryloxyphenoxypropionate (APP) herbicides. The mechanism of resistance is due to an Ile-2041-Asn mutation of the ACCase gene. Screen house experiments were conducted to study the growth, fecundity characteristics, and competitive ability of this aryloxyphenoxypropionate-resistant (APP-R) biotype compared with a susceptible (APP-S) biotype. When grown under noncompetitive conditions, the APP-R P. fugax developed more rapidly than the APP-S plants, with earlier tiller and panicle emergence and seed shedding; the APP-R P. fugax set seeds nearly 12 d earlier than the APP-S biotype. APP-R and APP-S biotypes had similar aboveground dry weight before the flowering stage. Fecundity of the APP-R biotype was similar to the APP-S biotype (8.57 g seeds plant−1 and 0.17 g seeds panicle−1 versus 8.22 g seeds plant−1 and 0.13 g seeds panicle−1, respectively). Ultimately, the relatively slower-developing APP-S P. fugax had 50% more shoot dry weight than the APP-R plants. Relative competitiveness among the APP-R and APP-S P. fugax biotypes was investigated through replacement series experiments. No difference in competitive ability was measured between APP-R and APP-S biotypes on the basis of shoot dry weight before the tillering stage. These results indicate that there is no apparent fitness penalty for the APP-R P. fugax. The shorter growth cycle of APP-R with no apparent fitness penalty suggests that growers will need begin weed control earlier and possibly include vegetative crops with an even shorter growth cycle in their rotations.

The beetle Oberea erythrocephala, whose larvae mine stems and roots of leafy spurge (Euphorbia esula L. # EPHES), was introduced into Oregon, Montana, and Wyoming between 1980 and 1984. Although it was not recovered in Oregon and Wyoming, it became established at two of three release sites in Montana and appears to be accepting leafy spurge plants at a fourth.

Buthidazole {3-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl}-4-hydroxyl-1-methyl-2-imidazolidinone} was incubated for 30 days and then leached through Norfolk loamy sand, Lakeland loamy sand, Davidson clay, and Alamance silt loam soils. Similar amounts of the herbicide leached through all of the soils, but distribution in the soils varied greatly. 14C-buthidazole distribution in the Lakeland and Norfolk sandy soils was relatively uniform throughout the soil columns. In the Alamance silt loam and Davidson clay, much greater amounts of buthidazole were found in the upper soil zones than in the lower zones. Six metabolites of buthidazole (dihydroxy, desmethyl dihydroxy, dehydrate, methylurea, urea, and amine) were distributed in varying amounts in each of the soils.

Wild watermelon (Citrullus vulgaris Schrad. var. citroides Bailey) is a widespread problem in cotton (Gossypium hirsutum L.) and grain sorghum [Sorghum bicolor (L.) Moench] planted on sandy soils. Preplant herbicide treatments did not suppress wild watermelon emergence, dry matter, or melon production. Preemergence application of fluometuron [1,1-dimethyl-3-(α,α,α-trifluoro-m-tolyl)urea] suppressed growth and controlled the weed. However, propazine [2-chloro-4,6-bis(isopropylamino)-s-triazine] controlled and nearly eliminated melon production. Although postemergence application of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] was effective, most postemergence treatments gave erratic control, probably because of differences in vigor from seedlings emerging from varying depths. There was no difference in emergence from seed planted 2.5 to 10 cm deep. However, emergence was lower from seed planted 20 cm deep and seedlings were less vigorous from seed buried 10 cm or deeper.

Three poisonous varieties of timber milkvetch (Astragalus miser Dougl. ex Hook.) were grown under different temperature regimes in growth chambers. Miserotoxin, the poisonous chemical in timber milkvetch, was in greater concentration in plants grown at higher day temperatures (32 C) than at 24 C. In the field, nitrogen fertilizers did not affect miserotoxin synthesis in any variety, but significantly increased plant nitrogen in two varieties. Excluding light from Yellowstone milkvetch [var. hylophilus (Rydb.) Barneby] for 2 weeks significantly lowered miserotoxin concentration. Miserotoxin level in timber milkvetch was reduced when photosynthesis (a) was blocked by subjecting plants to prolonged darkness; (b) was disrupted by treating plants with 2,4,5-T [(2,4,5-trichlorophenoxy)acetic acid] or silvex [2-(2,4,5-trichlorophenoxy)propionic acid]; (c) was reduced by lowered day temperature; (d) was terminated by senescence and bleaching.

Pitted morningglory and entireleaf morningglory treated with 14C-imazaquin translocated 14C to areas above and below the treated leaf. Pitted morningglory absorbed and translocated more 14C from 14C-imazaquin than entireleaf morningglory. Translocation of 14C from root-supplied 14C-imazaquin was similar in both species 1 d after treatment, with 14C moving rapidly to the shoots. Entireleaf morningglory metabolized slightly more imazaquin than pitted morningglory in treated leaves. Greater tolerance of entireleaf morningglory than pitted morningglory to postemergence applications of imazaquin is attributed to reduced absorption and translocation and increased metabolism of the herbicide in the entireleaf morningglory.

I am honored by your invitation to participate in a symposium on this timely topic. Our twenty-first annual meeting on weed science and technology comes during a critical period of challenging readjustment for agricultural production in the world, especially in the United States. To focus perspective on this subject, I have compiled a list of factors, trends, and economic forces that can exert a strong impact on the future of agriculture, food production, herbicide technology, and integrated crop-management programs during the next 20 yr.

The management of Eurasian watermilfoil, an inimical aquatic weed in the Tennessee Valley, involves the use of water level management and herbicidal treatment with 2,4-D [(2,4-dichlorophenoxy)acetic acid]. The possible use of other chemicals to make 2,4-D more effective was tested by measuring growth of Eurasian watermilfoil under laboratory conditions with combinations of sublethal levels of 2,4-D and partially inhibitory concentrations of AlCl3, NH4Cl, Na2AsO2, BaCl2, Na2B4O7, CuSO4, HgCl2, Pb(NO3)2, NaCl, and ZnSO4. At the levels tested, BaCl2, Pb(NO3)2, and ZnSO4 were only additive in effect with 2,4-D. Other compounds produced synergism at some concentrations and mutual antagonism at other levels. The greatest synergism was obtained by using HgCl2, AlCl3, and NaCl at 0.2 μM, 10.0 μM, and 200.00 mM respectively.

Jointed goatgrass is a troublesome weed in winter wheat in the Pacific Northwest of the United States. Wheat and jointed goatgrass (JGG) can cross and produce hybrids in the field that can serve as a potential bridge for gene migration between the two species. To determine the potential for gene movement it is important to be able to identify hybrids in the field. To study the effect of wheat genotype on hybrid phenotype, reciprocal crosses were made between JGG and two common wheat cultivars: ‘Brundage 96’, ‘Hubbard’, a common-type advanced breeding line: ‘87–52814A’, and a club wheat cultivar: ‘Rhode’. Hybrids and parents were measured for plant height, spike length, flag leaf length, flag leaf width, and number of spikelets. Reciprocal effects were nonsignificant for all characteristics measured, indicating that hybrid morphology was not affected by the direction of the cross. Hybrids were different from their wheat parents for spike length, plant height, and flag leaf width. Hybrids produced from each of the wheat parents were uniform in phenotypic characters. Spikes were intermediate in circumference (size) from crosses between JGG and common wheat lines; however, club wheat × JGG crosses resulted in spikes that were more similar to common wheat. Spike size and flag leaf width for all hybrids also were intermediate between their parents. Hybrids differed in spike size and awn characteristics because of unique characteristics of the wheat parent. Based on these results, it should be possible to identify hybrids in the field accurately, regardless of the wheat parent or direction of the cross unless the parent is a club wheat.

Because of interest in monitoring crop response to weed interference, greenhouse experiments were conducted to evaluate interference of purple and yellow nutsedge on the growth, development, and spectral response of cotton and soybean. Cotton fresh weight was reduced 9 to 42% compared with the control when grown with yellow and purple nutsedge. Fresh weight of soybean was reduced 27 to 60% when it emerged simultaneously with yellow nutsedge and 45 to 63% when it emerged 7 d after yellow nutsedge. Soybean fresh weight was reduced 30 to 35% when it emerged simultaneously with purple nutsedge and 44 to 72% when it emerged 7 d after purple nutsedge. Reflectance data were analyzed using wavelet transformation techniques with the HAAR mother wavelet. Nine extracted features from the cotton and soybean leaf reflectance measurements were used to classify single-leaf cotton and soybean reflectance measurements to predict whether cotton or soybean was growing in the presence or absence of purple and yellow nutsedge. After training the system, the ability to separate leaf reflectance measurements of crops growing weed free from those growing in the presence of purple and yellow nutsedge was tested using cross-validation with the nearest mean classifier. Cross-validation accuracy results for cotton were 62 to 70%. Cross-validation accuracy for soybean and yellow nutsedge was similar, regardless of emergence, and ranged from 60 to 71%. Features extracted from the soybean reflectance measurements were not as effective in classifying soybean leaf reflectance measurements based on the presence or absence of purple nutsedge. A decrease in accuracy was observed for both simultaneous and delayed soybean emergence in purple nutsedge fresh weight categories from more than 2,560 g to more than 3,420 g. Overall, the system correctly classified soybean emerging simultaneously with purple nutsedge 58 to 74% and soybean emerging 7 d after purple nutsedge 53 to 67%. These results indicate the potential of differentiating crops under stress using spectral reflectance, although refinements to the system must be made before it is field ready.

Derivative of 2,4-dichlorophenoxyacetic acid (2,4-D), type of bottom sorbent, and application concentration affected the rate and quantity of 2,4-D released into water from attapulgite granules. All three factors were temperature dependent. Temperature, granule size, and solution pH affected the rate during short exposure periods, but not the total quantity of release of herbicide during 256 hours. Generally, decrease in temperature was associated with a decrease in rate of release of 2,4-D. Decrease in granule size slightly increased the rate of release. Increased acidity reduced the rate of release of 2,4-D acid. Types of attapulgite granules were not significantly different.

Rates of subaqueous release of eleven granular herbicides were measured at concentrations of 10, 15, and 20 μg ai/ml. Compounds tested were 2,4-D and its butoxyethanol ester (BE), propylene glycol butyl ether ester (PGBE), and isooctyl ester (IOE) derivatives, tris-(2,4-dichlorophenoxyethyl)phosphite, the IOE of 2,4,5-trichlorophenoxyacetic acid, the BE, PGBE, and potassium salt of 2-(2,4,5-trichlorophenoxy)propionic acid (silvex), the sodium salt of 2,3,6-trichlorobenzoic acid, and 2,3,6-trichlorophenylacetic acid (fenac). The rate of release of herbicides from granules varied from complete release of silvex potassium salt in 4 hours to less than 10 per cent release of 2,4-D IOE in 256 hours.

Witchweed (Striga asiatica), a parasitic plant with brilliant-red flowers and hairy bright-green stems and leaves, was identified in the Western Hemisphere for the first time in 1956, although it had been present in the United States for several years. Figure 1 shows severe corn damage caused by witchweed. In a 1959 survey conducted by the United States Department of Agriculture witchweed was found on approximately 250,000 acres of land in 15 counties in North Carolina and 7 counties in South Carolina. It attacks many species in the family Gramineae. Witchweed jeopardizes the production of corn, sorghum, sugarcane, and probably other gramineous crops in any climatological regions of the United States where it may be adapted. This study attempted to obtain some indication of the possible limits of its spread.

Cattails (Typha spp) are common and troublesome emergent aquatic weeds throughout the United States. They interfere with proper utilization and maintenance of irrigation and drainage canals. They further interfere with flow of water in channels where water velocity is low, and cause deposition of silt. In reservoirs, farm ponds, marshes, and lake margins, cattails waste large quantities of water, crowd out plants which produce food for wildlife and often interfere with fishing, boating, and other uses of these bodies of water.

The sodium salt of 2,2–dichloropropionic acid (dalapon) has found a place in chemical weed control practices because of its effectiveness against grasses. The nature of its action is understood only poorly.

The herbicides n-propyl ethyl-n-butylthiolcarbamate (PEBC), PEBC + 2,3-dichloroallyl diisopropylthiol-carbamate (DATC), PEBC + ethyl N,N-di-n-propylthiolcarbamate (EPTC), and PEBC + 2,3,3-trichloroallyl diisopropyldithiolcarbamate (CP-22819) among others produced effective control of lambsquarters (Chenopodium album), pigweed (Amaranthus retroflexus), Setaria spp. and other weed species without undue injury to sugar beet seedlings. PEBC + DATC also gave control of wild oat (Avena fatua), and increased the control of lambsquarters; however, this latter effect was inversely related to the control of other broad-leaved weeds. Control equal to PEBC was produced by PEBC + EPTC with 2.5 lb. less total active ingredient per acre. The herbicide 3,6-endoxohexahydrophthalic acid (endothal) gave inferior control of weeds. None of the herbicides tested effectively controlled kochia (Kochia scoparia).

Crop production on most lands of the western United States requires the use of supplemental water which is delivered through established irrigation canal distribution systems. These systems frequently support heavy growths of aquatic plants which prevent or impede the passage of water. Because a reduction in the carrying capacity of a canal makes it necessary to deprive potential productive land of the water required to grow a satisfactory crop, it is imperative to keep the waterways open.