In tropical and subtemperate regions of the world, farmers plant vegetables and other crops in a vast array of cropping systems that often involve more than one crop being grown on the same parcel of land in a year. Weed communities within each cropping system shift depending on physical and climatic factors and the specific crop and weed management practices employed. Modern weed management strategies involve combinations of crop production practices and specific weed control technologies intended to reduce weed competition, thereby shifting the competitive balance in favor of the crop. Weed research and, training efforts, therefore, must focus on the entire cropping system with emphasis on year-round and multi-year management of weed communities.

Fosamine [ethyl hydrogen (aminocarbonyl)phosphonate] acted as a chemical inhibitor of shoot growth for periods of up to 3 yr following a single summer application to honey mesquite [Prosopis juliflora (Swartz) DC. var. glandulosa (Torr.) Cockerell] trees. Anatomical study of field, greenhouse, and seedling mesquite indicated that the inhibition of primary and secondary growth caused by fosamine was due to a general cessation of nuclear activity in apical meristems and in the vascular and cork cambia of this species.

Napropamide [2-(α-naphthoxy)-N,N-diethylpropionamide] applied with simazine [2-chloro-4,6-bis(ethylamino)-s-triazine] reduced by 47 to 82% the severity of simazine symptoms on California privet (Ligustrum ovalifolium Hassk.), staghorn sumac (Rhus typhina L.), and ‘Moonlight’ broom (Cytisus X praecox Bean). Oryzalin (3,5-dinitro-N4,N4-dipropylsulfanilamide) had a similar but smaller effect (40% reduction) when applied with simazine on broom in a field experiment and an effect equal to napropamide with simazine on privet in a greenhouse experiment. Both napropamide and oryzalin prevented some of the growth reduction from simazine on privet, broom, and ‘Baltic’ ivy (Hedera helix L. ‘Baltica’) in field experiments. In the greenhouse study with privet, napropamide prevented much of the growth-reducing effects of simazine on both shoots and roots of privet. Oryzalin appeared to have a similar effect.

The selective action of chloroxuron [3-(p-(p-chlorophenoxy)phenyl)-1,1-dimethylurea] on tolerant soybean [Glycine max (L.) Merr. ‘Kent’] and susceptible tall morningglory [Ipomea purpurea (L.) Roth] was studied using root and foliar applications. In root uptake studies with carbonyl-14C-chloroxuron, soybean roots contained greater than 90% of the total absorbed radioactivity; whereas tall morningglory roots contained approximately 60% of the total absorbed radioactivity. Leaves of 1, 2, and 3-wk old soybean plants contained 0.8, 3.7, and 3.5% of the total radioactivity, respectively; whereas leaves of tall morningglory plants of corresponding age contained 29.1, 33.0, and 37.4% of the total radioactivity. Metabolism from root uptake of both species was similar; and metabolites were identified as approximately 85% chloroxuron, 13% 3-(p-(p-chlorophenoxy)phenyl-1-methylurea (Compound II), and 2.5% 3-(p-(p-chlorophenoxy)phenylurea (Compound III) in 3-wk old plants. In a nutrient solution containing 5.6 × 10−6M chloroxuron, soybean plants with roots intact were tolerant; whereas plants with roots detached were susceptible indicating the importance of differential translocation for selective action from root uptake. Following foliar spray and washing of 2-wk old plants, tall morningglory foliage absorbed about two times more radioactivity than soybean foliage. Further, 18 hr after foliar application, chloroplasts isolated from tall morningglory contained twice as much 14C-chloroxuron as soybean chloroplasts. Metabolite concentrations recovered from foliar treatment of both species were similar; and metabolites were identified as 88.1 to 92.2% chloroxuron, 6.4 to 8.6 % Compound II and 0.5 to 1.7% Compound III.

Nontoxic concentrations of 2,4-D [(2,4-dichlorophenoxy)acetic acid] increased the toxicity of ametryne [2-(ethylamino)-4-(isopropylamino)-6-(methylthio)-s-triazine], when applied to the roots of waterhyacinth [Eichornia crassipes (Mart.) Solms], when applied to the roots or foliage of corn (Zea mays L.), and to a lesser extent, when applied to the roots of soybean [Glycine max (L.) Merr.]. Nontoxic concentrations of 2,4-D frequently also increased the absorption of ametryne, water, and radioactive calcium by all three species. Mixtures containing toxic concentrations of 2,4-D with ametryne were not synergistic and sometimes the mixtures were less toxic than ametryne alone. Toxic concentrations of 2,4-D also decreased plant dry weight and the absorption of ametryne, water, and calcium. Low concentrations of ametryne increased the absorption of water and sometimes calcium while high concentrations decreased dry weight and the absorption of water and calcium.

Effects of difenzoquat on wild oats grown under controlled environmental conditions were studied. Seedling height and fresh weight were significantly reduced 5 days after postemergence treatment. Dose-dependent increase in cell membrane permeability was detected after a 12-h exposure to the herbicide. Scanning electron micrographs showed normal leaf hairs and cuticular wax but swollen guard cells 10 days after treatment. Ultrastructural changes occurred before the visible symptoms. The primary effect of difenzoquat appears to be the disruption of the tonoplast and plasmalemma. The tonoplast showed greater damage than the plasmalemma. Secondary effects included damage to mitochondria and chloroplasts. Mitochondria were swollen and often ruptured, but the effect did not increase with the duration of exposure. Chloroplasts became spherical in shape, and their contents were also affected. The changes included: accumulation and then disappearance of starch granules, swelling of frets, fusion of granal thylakoids, detachment and then rupture of the outer membrane of the envelope, and clumping of ribosomes. By contrast, natural senescence caused greater injury of the plasmalemma than the tonoplast, a marked increase in size of plastoglobuli, and loss of starch grains without early accumulation.

The persistence of 2,4-D [(2,4-dichlorophenoxy) acetic acid] and 2,4,5-T [(2,4,5-trichlorophenoxy)acetic acid] was studied in the chaparral environment. Chamise (Adenostoma fasciculatum H. & A.), grass and forbs, soil surface litter, and soil were sampled for up to 360 days after herbicide (4.5 kg/ha) application. Over 50% of the 2,4-D and 2,4,5-T recovered was found on soil surface litter while 18 to 31% was found on vegetation. Much less herbicide (0.1 to 0.2%) was found in soil (0 to 5 cm) than in foliage or litter. No herbicide residues were found below the 0 to 5 cm soil sample zone. Herbicide residues on foliage and litter decreased rapidly (up to 93%) within 30 days after application but remained constant thereafter until winter rainfall began. Residues of both herbicides were evident in chamise foliage (0.01 to 0.02%), surface litter (0.01 to 0.03%), and soil (0.01%) up to 360 days after application.

The soil weed seedbank is the source of future weed infestations. Seed predation can result in a large number of seed losses, thus contributing to weed biocontrol. Earthworms are important predators of seeds and seedlings and affect seeds and seedling survival after gut passage. A study was conducted to assess the ability of Pheretima guillelmi (Kinberg) to ingest and digest the seeds and seedlings of 15 main farmland weed species. Pheretima guillelmi ingested the seeds and seedlings of each weed species tested. The percentages of seeds and seedlings ingested were 96.7% to 100% and 21.7% to 94.2%, respectively. Pheretima guillelmi showed greater ingestion of seeds than seedlings for each species and digested the seeds and seedlings of each weed species tested to varying extents. The percentages of seeds and seedlings digested were less than 15% irrespective of the weed species. Passage through the gut of P. guillelmi affected the survival of seeds and seedlings. The germination of large crabgrass [Digitaria sanguinalis (L.) Scop.], green foxtail [Setaria viridis (L.) P. Beauv.], goosegrass [Eleusine indica (L.) Gaertn.], Chinese sprangletop [Leptochloa chinensis (L.) Nees], Malabar sprangletop [Leptochloa fusca (L.) Kunth], redroot pigweed (Amaranthus retroflexus L.), common purslane (Portulaca oleracea L.), barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.], and ricefield flatsedge (Cyperus iria L.) seeds egested by P. guillelmi decreased by 46%, 49%, 47%, 25%, 38%, 26%, 32%, 13%, and 15%, respectively, compared with their respective controls. In contrast to seed ingestion, ingestion of seedlings by P. guillelmi was fatal to individuals of all weed species; no seedlings survived passage through the gut. Our results indicate that predation of weed seeds and seedlings by P. guillelmi probably depletes the soil weed seedbank and that the introduction of P. guillelmi into fields is a potential strategy for weed biocontrol in farmland.

Latin America is a region of contrasts including mountains and valleys, deserts and jungles, forests and prairies, prosperous and poor, modern and traditional, small farms and huge ones, malnourished and well-fed, machines and manpower, herbicides and machetes. Any country from Mexico to Chile would provide examples of these contrasts. The countries share a common heritage of language, culture, religion, and education. Each is unique in its present developmental stage and future plans (6).

Tolerance of corn (Zea mays L. ‘B’), cotton (Gossypium hirsutum L. ‘coker 413’), soybean (Glycine max Merr. ‘Hardee’), turnip (Brassica rapa L. ‘Tendergreen’), sorghum (Sorghum bicolor (L.) Moench. ‘Georgia 615’), purple nutsedge (Cyperus rotundus L.), yellow nutsedge (C. esculentus L.), and johnsongrass (Sorghum halepense (L.) Pers.) to 2,6-dichlorobenzonitrile (dichlobenil) at 0, 0.14, 0.28, 0.56, 1.12, and 2.24 kg/ha in four Georgia soils was determined. Equivalent rates of dichlobenil generally were more toxic in Davidson clay loam which had the highest clay content. Crop tolerance was corn > sorghum > cotton > turnip. Purple and yellow nutsedge tolerance to dichlobenil was intermediate to that of the crops tested. Johnsongrass response was equivalent to that shown by sorghum.

Diethatyl [N-(chloroacetyl)-N-(2,6-diethylphenyl) glycine] applied posttransplant in peppers (Capsicum annuum L.) but preemergence to hairy galinsoga [Galinsoga ciliata (Raf.) Blake # GASCI] controlled the weed at 1.7 to 2.2 kg ai/ha. No serious crop injury or yield reduction was observed when diethatyl was applied to peppers at 9 kg/ha posttransplant or at bloom. Soil ridging, practiced to improve drainage and control the crown rot phase of Phytophthora blight (Phytophthora capsici L.), limited the duration of weed control by disrupting the herbicide-treated surface soil and exposing untreated soil. Diethatyl applied at 2.2 kg/ha posttransplant and again at pepper bloom, after the soil ridges were formed, controlled hairy galinsoga throughout the entire growing and harvest season.

Combinations of antagonistic herbicides can be helpful in the search for seed-applied chemical safeners to protect crop plants from herbicide injury. If a particular herbicide combination is selectively antagonistic so that the crop is not injured but weed control efficacy is not reduced, it should be possible to develop a new, more selective formulation of the herbicide which includes the antagonist or antidote. A promising new approach involves the use of early pretreatments of crop plants with subtoxic levels of a particular herbicide to increase crop tolerance to later, higher rates of that herbicide. When there are different mechanisms for herbicide detoxification in different plant species, it should also be possible to develop selective herbicide synergists that would provide equal efficacy at lower rates with greater crop tolerance. As our knowledge of herbicide metabolism and mode of action develops, it will be increasingly possible to use other chemicals to selectively synergize or safen herbicides to solve problems in important crop-weed situations.

Two, 4-dichloro-6-fluorophenyl-p-nitrophenyl ether (CFNP) is similar in herbicidal property and chemical structure to nitrofen (2,4-dichlorophenyl-p-nitrophenyl ether). CFNP inhibits the growth of the hypocotyl and young buds of germinating weeds. Henry Crabgrass (Digitaria adscendens H.B.K. Henr.) was controlled by CFNP at 0.5 kg/ha, however sunflower (Helianthus annuus L.) was tolerant to 16 kg/ha of CFNP. CFNP also effectively controlled largeseed dodder (Cuscuta indecora Choisy) which is a serious problem in sunflower. Although trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) did not control common dodder at 8 kg/ha, CFNP gave control at 0.5 kg/ha.

Napropamide [2-(α-naphthoxy)-N,N-diethylpropionamide] is a preemergence herbicide used for the control of most annual grasses and many annual broadleaf weeds. Its influence on the uptake and translocation of N, P, K, Ca, Mg, Cu, Fe, and Zn by four plant species was observed. Napropamide influenced, in some cases dramatically, the uptake and movement of the essential elements, but its influence was not consistent from one type of plant to another.

Five goals for agriculture are proposed: profitable production, sustainable production, environmentally safe production, satisfaction of human needs, and compatibility with a just social order. Four ways to view the origin of potentially conflicting values expressed in the five goals follow. In view of the five goals and differing value positions, the most pressing question for weed science is to what extent current methods of weed control are compatible with a more sustainable and environmentally sensitive agriculture.

Glasshouse experiments were conducted to determine the effects of trifluralin on root growth and mineral relations of corn seedlings. Root weight to shoot weight ratios of corn seedlings were positively correlated to concentrations of trifluralin in soil. Root length to shoot weight ratios, however, were inversely related to trifluralin concentrations. Phosphorous and potassium concentrations in shoot tissue were reduced 60 and 35%, respectively, by 0.25 mg trifluralin kg−1 soil. Growth inhibition due to trifluralin was partially overcome by supplementing soil with nutrients.

Experiments involving the application of nitralin [4-(methylsulfonyl)-2,6-dinitro-N,N-dipropylaniline] at 0.56, 1.12, or 1.68 kg/ha on each of three successive plantings of pickling cucumbers (Cucumis sativus L. ‘Explorer’) were conducted during a 2-yr period. Residues of nitralin in soil increased with rate and number of applications. Nitralin residues accumulated to higher concentrations in a soil with 2.9% organic matter than in a soil with 0.7% organic matter. However, cucumber plants were injured less in the soil with the highest concentrations of herbicide. Oats (Avena sativa L. ‘Carolee’) planted following cucumbers were injured by nitralin residues resulting from the highest application rates. Bioassays and chemical analyses suggested that no appreciable soil residues of nitralin remained during the succeeding year.

Laboratory and growth room experiments were conducted to determine effects of CGA-133205, oxabetrinil, and flurazole on grain sorghum hybrids ‘DeKalb 46’ and ‘DeKalb 48’ after the seed were treated with the herbicide safeners and stored for up to 24 wk at various relative humidity levels. Each safener treatment had an adverse effect on grain sorghum seed as reflected by fewer normal seedlings at 4 d when averaged over storage time and relative humidity levels. Flurazole and oxabetrinil increased the number of ungerminated seed of both hybrids, and 0.4 and 0.8 g ai kg–1 CGA-133205 increased ungerminated seed of DeKalb 46. At ≥ 8 wk after storage, a reduction in germination was observed for seed treated with flurazole or oxabetrinil compared to seed stored without safener. This reduction also occurred with 0.4 and 0.8 g kg–1 CGA-133205, but only at 24 wk after storage. Each safener treatment minimized reduction in seedling shoot fresh weight by herbicides compared to the no-safener check through 24 wk after storage.

Herbicide tests conducted over a period of 3 years indicated that 3-(3,4-dichlorophenyl)-l,l-dimethylurea (diuron) at 2.5 lb/A preemergence, or a,a,a-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) at 2.0 lb/A preemergence, incorporated, are safe and effective herbicides for weed control in gladiolus (Gladiolus grandiflorus). Applications of dimethylamine salt of 2,4-dichlorophenoxyacetic acid (2,4-D) applied as directed sprays at rates of 0.5 to 1.0 lb/A prior to spiking controlled late season weeds without apparent injury to the plants.