We developed a technique to measure the projected leaf area of plants any time during the day or night. “Projected leaf area” is the leaf area seen from directly above, and is the area most likely to intercept herbicide spray from an over-the-top application. The technique involves photographing a plant and a reference square of known area from directly overhead using a light-tight box containing a flash unit. Negative images thus obtained are reflected (by means of a slide projector and mirror) onto filter paper lying on a glass stage. The projected leaf area is determined by planimetry of the negative image of the leaves and the reference square. The projected leaf area can be compared to the actual total leaf area after one determines the latter with a conventional area meter.

The responses of whitebrush (Aloysia lycioides Cham.) and several associated species to herbicides were evaluated in the Central Basin and South Plains of Texas. At Llano, hexazinone [3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4(1H,3H)-dione], picloram (4-amino-3,5,6-trichloropicolinic acid), and tebuthiuron {N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N,N′-dimethylurea} were the most effective herbicides for whitebrush control. Picloram was the most effective herbicide on Lindheimer pricklypear (Opuntia lindheimeri Engelm.), tasajillo (Opuntia leptocaulis DC.), Texas colubrina [Colubrina texensis (Torr. & Gray) Gray], and Texas persimmon (Diospyros texana Scheele). Hexazinone, karbutilate [tert-butylcarbamic acid ester with 3(m-hydroxyphenyl)-1,1-dimethylurea], and tebuthiuron controlled live oak (Quercus virginiana Mill.). Tebuthiuron pellets were slightly more effective on whitebrush when applied broadcast than when applied in rows 1.8 to 6.0 m apart. Roller chopping alone did not kill whitebrush nor improve effectiveness of tebuthiuron but did improve access to the land and visibility for 3 yr. Picloram applied either as the undiluted liquid formulation or as granules was the most effective herbicide for controlling a spectrum of species, including whitebrush. Picloram pellets were generally more effective on Lindheimer pricklypear and Texas persimmon when applied broadcast than when applied in rows. However, tebuthiuron was the most effective herbicide on blackbrush acacia (Acacia rigidula Benth.) and huisache [Acacia farnesiana (L.) Willd.], especially when applied in rows rather than broadcast.

Low species diversity accompanied high populations of Canada thistle [Cirsium arvense (L.) Scop.]. Two perennial grasses and two rushes grew with Canada thistle; annual plants did not. Canada thistle litter, ground roots, and ground foliage added to soil in greenhouse bio-assay tests reduced growth of redroot pigweed (Amaranthus retroflexus L.) and green foxtail [Setaria viridis (L.) Beauv.] more than cucumber (Cucumis sativis L.) or barley (Hordeum vulgare L.). The addition of nutrients did not mask the toxic effect. Ethanolic extracts of Canada thistle roots and foliage were similar in their ability to reduce radicle growth of barley, cucumber, green foxtail, and redroot pigweed in petri dish studies. There was no significant difference between water controls and controls adjusted to the average pH and osmotic potential of the extracts. Germination of barley and cucumber seed was not affected. Comparisons between ethanolic extractions and soil incorporation of plant residues with presumably non-allelopathic plants revealed that cucumber and barley extracts reduced redroot pigweed radicle growth whereas barley and green foxtail extracts increased green foxtail radicle growth. These effects were not observed when these plant residues were mixed in soil.

Afterripening requirements and the possible occurrence of endogenously controlled germination rhythms in downy brome (Bromus tectorum L.) were investigated in the laboratory. Afterripening temperatures from 0 to 50 C, maintained for periods of 4 to 28 days, had little effect on the percentage and rate of downy brome germination when seeds were incubated at 15 and 20 C. However, at 30 C germination temperature, the percentage and rate of germination were significantly increased by short periods (4 to 7 days) of afterripening at 50 C. Similar results occurred at 20 to 40 C afterripening temperatures after 14 to 28 days exposure. In general, high temperature afterripening conditions (40 to 50 C) initially increased downy brome germination at 30 C incubation temperature, but prolonged exposure tended to decrease germination. No endogenously controlled germination rhythms were observed in downy brome caryopses.

Six substituted dinitrobenzamine herbicides, including butralin [4-(1,1-dimethylethyl)-N-(1-methylpropyl)-2,6-dinitrobenzenamine], dinitramine (N4,N4-diethyl-α,α,α-trifluoro-3,5-dinitrotoluene-2,4-diamine), fluchloralin [N-(2-chloroethyl)-2,6-dinitro-N-propyl-4-(trifluoromethyl)aniline], pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine], profluralin [N-(cyclopropylmethyl)-α,α,α-trifluoro-2,6-dinitro-N-propyl-p-toluidine], and trifluralin (α,α,α,-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine), were evaluated as directed broadcast sprays applied and soil incorporated at time of last cultivation (layby) of cotton (Gossypium hirsutum L. ‘Acala SJ-2’). At the rates used, all herbicides provided more than 90% control of annual grasses. During 1976 and 1977, control of redroot pigweed (Amaranthus retroflexus L.) and smooth pigweed (Amaranthus hybridus L.) ranged from 90 to 100% and none of the herbicides differed significantly in their effect. However in 1975, butralin at 1.1 kg/ha did not improve pigweed control when compared to the untreated control. None of the herbicides consistently controlled black nightshade (Solanum nigrum L.) and except pendimethalin, none caused detectable cotton injury. In 2 of 3 yr, pendimethalin caused enlarged growth of the cotton stem in the cotyledonary node area contacted by the herbicide spray. Stem breakage following wind occurred in about 5% of the cotton plants; however, the injury was not manifested by reduced yield. None of the herbicides influenced cotton yield. Residues from soil samples collected 4 months after herbicide application reduced growth of Japanese millet [Echinochloa crus-galli (L.) Beauv. var. frumentacea (Link) Wright]3 and grain sorghum [Sorghum bicolor (L.) Moench] 24 to 49%.

Sicklepod (Cassia obtusifolia L.) leaves and leaflets changed their orientation throughout 24-h periods. These endogenously controlled rhythmic movements resulted in dramatic changes in the percent projected leaf area (PPLA), the leaf area observed from directly above and expressed as a percentage of the total actual leaf area. The PPLA of sicklepod varied more than seven-fold throughout a 24-h period, being greatest during the day and least at night. Control of sicklepod with linuron [3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea] applied postemergence at intervals throughout 24-h periods was greater from daytime applications than from late evening, night, and early morning applications. Percent control was highly correlated with PPLA, suggesting that rhythmic leaf movements and the resulting change in PPLA could be an important factor in determining the efficacy of postemergence herbicide applications on sicklepod by altering the area of the leaf surface most likely to intercept and retain herbicide spray from an over-the-top application. In addition to endogenously controlled leaf movements, we observed leaf movements caused by simulated cloud cover (80% shading). Shading increased the PPLA of sicklepod, suggesting that more leaf surface might be exposed to an over-the-top spray during cloudy periods than during sunny periods.

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.

We describe a small, light-weight, battery-powered timer that emits an audible signal at 1-s intervals. This audible timer is used to monitor ground speed when applying chemicals to plots, thereby eliminating the need for manual manipulation of a stop watch.

Studies were conducted in 1977 and 1978 to evaluate the impact of surface irrigation water on the dissemination of weed seed in the North Platte River Project from May through September each year. Seventy-seven different plant species were found in irrigation water, with germination averaging 26%. Two to five times more weed seed were found in two irrigation canals than in the North Platte River. As water moved through the canals, the weed seed content of the water increased dramatically. The majority of seed collected was found floating on the water surface. Amounts of seed collected varied with time with the largest amounts being collected in June and July and gradually declining in August and September. Samples also were taken to determine the amount of seed entering the field with irrigation water during the growing season. During the 1978 growing season irrigation water disseminated 48,400 seed per ha in the sampled fields.

Roots of 10- and 14-day-old seedlings and excised leaves of cotton (Gossypium hirsutum L. ‘Coker 310’), jimsonweed (Datura stramonium L.), peanuts (Arachis hypogaea L. ‘Florigiant’), and prickly sida (Sida spinosa L.) were treated with ring-labeled 14C-tetrafluron {N,N-dimethyl-N′-[3-(1,1,2,2-tetrafluoroethoxy)phenyl] urea} in liquid culture. Herbicide absorption and subsequent translocation were limited in peanuts, intermediate in cotton, and very high in jimsonweed and prickly sida. Absorption of 14C-tetrafluron increased with time in 10- and 14-day-old seedlings. After 6 h, 10-day-old jimsonweed had absorbed as much as 50% of the initial 10-μg dose, whereas peanuts absorbed only 10%. Almost 100% uptake was observed after 48 h with 14-day-old seedlings of jimsonweed and prickly sida. Methanol-extracted 14C increased with time, and was higher for the weeds than for the crops. Only a small fraction of methanol-insoluble radioactive material was obtained, with the largest value in 14-day-old cotton at 48 h (3.0%). Autoradiographs demonstrated more rapid translocation of radioactivity from roots to shoots in jimsonweed and prickly sida than in cotton and peanuts. Limited absorption and translocation of tetrafluron in seedlings of peanut and cotton, and the rapid absorption and subsequent accumulation of high concentrations in seedlings of jimsonweed and prickly sida, are proposed to explain the selective action observed.

Field experiments were conducted over a 2-yr period to study the comparative morphological development of cogongrass [Imperata cylindrica (L.) Beauv.] ecotypes from 15 different locations throughout the three main regions of Iraq. Plant height and density of stands varied more than 2-fold. The number of flowering heads varied 11-fold for plants collected from the different locations. Mature leaf-blades varied in width from 4.7 to 6.9 mm. These distinct variations indicate the existence of different ecotypes of cogongrass in Iraq.

The overall objective of research at ICRISAT (International Crops Research Institute for the Semi-Arid Tropics) is to develop effective, economically feasible, improved weed-management systems for major crops and cropping systems of the Semi-Arid Tropics (SAT). Initially, emphasis has been on surveys, ecological studies, weed management in crops, and herbicide screening. The aim was to identify possible cultural, mechanical, biological, and chemical inputs, and to determine their relative feasibility for small farmers. Systems research has involved manipulation of cropping systems to obtain better weed management and identification of the most effective techniques. Herbicide-related studies are designed to increase productivity rather than reduce the cost of hand weeding. Research on individual crops has been aimed at determining the extent of crop-weed competition in major crops. In collaboration with economists, operations research in farmers' fields was initiated to observe the success of farmers' weed-control methods. The primary program emphasis will be the evaluation of weed-management principles that may have wide applicability throughout the SAT. This paper outlines the approaches to weed-management research at ICRISAT and gives a conceptual framework of future research needs.

We studied the effectiveness of glyphosate [N-(phosphono-methyl)glycine] for the control of Orobanche spp. in the field when applied to the crop foliage prior to the emergence of the parasite. The best Orobanche control in October-sown carrots (Daucus carota L.) parasitized by Orobanche crenata Forsk. and O. aegyptiaca Pers. and in December-sown broad bean (Vicia faba L.) and peas (Pisum sativum L.) parasitized by O. crenata was obtained by spraying twice, 2 weeks apart, in late January and in February, respectively. Carrot root yield remained unaffected by glyphosate at rates up to 200 g/ha. In heavily infested fields, carrots were irreversibly damaged by early Orobanche parasitism. The highest yields of pods of broad beans occurred after two or three applications of 150 g/ha glyphosate. Peas were seriously damaged by 150 g/ha. Two or three sprays of 50 g/ha glyphosate resulted in the highest green-kernel yield of pea where the soil was not heavily infested. Glyphosate effectively reduced O. cernua Loeffl. emergence in non-irrigated tomatoes (Lycopersicum esculentum Mill.), but was very phytotoxic to the crop. Glyphosate was highly effective in controlling Orobanche spp.; however, marginal selectivity may be sufficient in some crops and not in glyphosate-susceptible ones.

A study of the C4 plant itchgrass (Rottboellia exaltata L.f.) grown under 100%, 60%, 25% and 2% sunlight revealed differences in the anatomy and cytology of the foliar mesophyll and vascular bundles associated with shading. In the bundle sheath, shading caused a reduction in thickness of the cell walls, shrinkage of plastids, rearrangement of plastid thylakoids, a reduction in starch deposits and vacuolization of the cytoplasm. In general, plastids and mitochondria retained membrane integrity but underwent stromal deterioration. Shading effects on cytoplasm were similar for mesophyll and bundle sheath cells. Mesophyll chloroplasts lost starch grains and the peripheral reticulum tended to decrease with greater shading. Grana were well developed at all irradiances, although the chloroplasts themselves decreased in size with shading. Movement of mesophyll chloroplasts away from bundle sheath-mesophyll borders was pronounced at 60% sunlight and was progressively greater at the two lower irradiances. These observations suggest that the structural relationships thought to be necessary for the intercellular transfer of C4 acids and the functioning of the C4 photosynthetic pathway were disrupted by shading.

Two series of experiments were conducted from 1976 through 1978 on Marvyn loamy sand in central Alabama to determine the competitive relationship of yellow nutsedge (Cyperus esculentus L.) with cotton (Gossypium hirustum L. ‘Stoneville 213’). In the first series of experiments yellow nutsedge was left undisturbed or removed from plots to give periods of competition of 0, 2, 4, 6, 8, 10, and 25 weeks. Seed cotton yield was reduced 2 of 3 yr with full season (25 week) competition, but was unaffected by shorter periods of competition in all years. Main stem diameter of cotton was also reduced 2 of 3 yr when competing full season while main stem height was reduced in only 1 of 3 yr with full season competition. In the second series of experiments yellow nutsedge densities were monitored in 48 individual plots and regression analysis showed yields of seed cotton were consistently reduced with full-season competition. Yellow nutsedge shoot densities ranged from zero to 93 shoots/m2 (measured 13 cm on either side of the drill) and approximately 18 kg/ha of seed cotton were lost for each additional yellow nutsedge plant per m2.

The effects of temperature, relative humidity (RH) and light on absorption and translocation of 14C-glyphosate [N-(phosphonomethyl)glycine] in common bermudagrass [Cynodon dactylon (L.) Pers. ‘Common’] and the effect of temperature and stage of growth on glyphosate activity in horsenettle (Solanum carolinense L.) were determined. 14C-glyphosate was used to evaluate the absorption and translocation of foliar-applied herbicide in bermudagrass plants grown at 22 or 31 C and 35 or 80% RH and absorption in excised leaf tips and stem sections. Autoradiography of bermudagrass indicated that downward translocation of 14C was extensive with large accumulations in new roots and rapidly growing stolons. RH of 35% decreased 14C-glyphosate translocation to the untreated foliage of plants grown at 31 C as compared to 80% RH. RH of 35% also decreased the translocation of 14C to the rhizomes and roots of plants grown at 22 C. The absorption of 14C was greater in excised leaf tips than stem sections and was greater in the light than in the dark. Horsenettle plants grown at 32 C had shoots which were more rapidly killed by glyphosate but had more regrowth than plants treated at 13 C. Shoots of plants treated while blooming were also more rapidly killed than those treated in the prebloom stage of growth, but the regrowth was not significantly different.

R-25788 (N,N-diallyl-2,2-dichloroacetamide) and H-31866 [N-allyl-N-(3,3-dichloroallyl)dichloroacetamide] were more effective than CDAA (N,N-diallyl-2-chloroacetamide) in preventing yield reductions to corn (Zea mays L. ‘NB-611’) from alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] or metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] in the greenhouse. A CGA-43089 [α-(cyanomethoximino)-benzacetonitrile] seed treatment (1.25 g/kg) was more effective than a R-25788 tank mix in preventing yield reductions to grain sorghum [Sorghum bicolor (L.) Moench ‘G-623 GBR’] from alachlor or metolachlor in the greenhouse. Absorption of 14C-alachlor by sorghum seedlings grown in petri dishes, and absorption, translocation, and metabolism of 14C-metolachlor by sorghum seedlings grown in soil, were not affected by CGA-43089 seed treatment. Forage sorghum [Sorghum bicolor (L.) Moench ‘Rox Orange’] was used to simulate shatter cane [Sorghum bicolor (L.) Moench] in field plots. In the absence of Rox Orange, alachlor and metolachlor reduced sorghum grain yields. This yield reduction was prevented by a CGA-43089 seed treatment, but not by a R-25788 tank mix with herbicides. In plots seeded with 10,000 Rox Orange seed/57 m2, grain yields of sorghum increased as alachlor or metolachlor plus CGA-43089 rates increased. There was no grain yield response to any herbicide treatment in plots seeded with 50,000 Rox Orange seed/57 m2.

Two methods of obtaining air samples were used over 4-month periods in 1976 and 1977 to quantitate trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) vapors arising from a silt loam. With the first method, only the air from an enclosure covering 0.37 m2 of soil was sampled. The second method used air probes to sample the air above the field. Rainfall and resultant soil moisture were related to trifluralin vaporization from field soil. Soon after trifluralin incorporation (0.84 kg/ha) and, while the soil was relatively dry, there was very little herbicide vaporization. Trifluralin vapors accumulated rapidly during a period in which several centimeters of rain fell, but with continued precipitation the vapor density declined. The influence of rainfall and resultant soil moisture upon vaporization became less apparent as the soil concentration of the herbicides declined. Less than 0.5% of the applied trifluralin was detected as vapors during each 4-month period.

Glyphosate [N-(phosphonomethyl)glycine] applied to cranberry (Vaccinium macrocarpon Ait.) as a spray at 4 kg/ha in March controlled aster (Aster subspicatus Nees) 77 to 87%. Glyphosate swab treatment with 6% solution in July or August, 1978 controlled spirea (Spiraea douglasii C.) 90%. A single application of terbacil (3-tert-butyl-5-chloro-6-methyluracil) in April, 1976 at 2 kg/ha controlled inflated rush (Carex vesicaria L.) 70% for 3 yr. Phytotoxicity was not observed on ‘McFarlin’ cranberry vines from any of these glyphosate or terbacil applications. Phytotoxicity did result from April, July, or August spray applications of glyphosate. Swab treatments did not cause any injury to cranberry.

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).