In Louisiana approximately 300,000 acres of sugarcane are grown each year for the manufacture of sugar. Another 125,000 acres are necessary for the rotation program, making approximately 425,000 acres devoted to the production of cane. Of these, 100,000 acres were estimated in 1950 to be so badly infested with Johnson grass (Fig. 1) that a control program was necessary to obtain economic yields of cane. In addition to this acreage, headlands, ditchbanks, and roadsides of many additional plantations were infested with Johnson grass and constituted a potential threat to the cane fields.

In the recent past, the various plant species which are lumped together in the general class of aquatics have received considerable attention from workers in the field of weed control because of their importance in affecting water usage in irrigation, water reservoirs, transportation, and recreational facilities.

Cotton and grain sorghum were treated with directed paraquat (1,1′-dimethyl-4,4′-dipyridylium cation) sprays in the field. Tall cotton was less susceptible to the herbicide than smaller plants. Some cotton varieties showed greater susceptibility to paraquat than others. In general, fiber quality was not affected by paraquat at ⅛ and ¼ lb/A, although there was some effect on the fiber coarseness of the more susceptible varieties. Increasing light intensity caused more leaf damage from cotton stem applications of paraquat. Directed paraquat applications invariably caused the outer sorghum leaf sheaths to show necrotic spots where treated. Treatments on 6-inch sorghum reduced yields, but plants treated when taller than 6 inches were not injured. Thirteen sorghum hybrid and open-pollinated varieties showed varying degrees of leaf and sheath burn due to paraquat treatments.

Preemergence applications of DCPA (dimethyl 2,3,5,6-tetrachloroterephthalate) controlled germinating seedlings of largeseed dodder (Cuscuta indecora Choisy) in alfalfa, and the control continued throughout the season. Applications of DCPA were necessary prior to germination of dodder seed. Under California conditions, applications of DCPA made during early winter (December) provided early season control but not satisfactory control throughout the season as was accomplished with the March treatments. Autoradiographic studies showed only limited movement of tracer in alfalfa plants.

Maleic hydrazide (MH) or 1,2–dihydropyridazine–3,6–dione is an organic plant growth regulator which has been found useful in preventing sucker formation in flue-cured tobacco, in preventing sprouting of stored vegetables, and in inhibiting the growth of some weeds and grasses. The compound can be synthesized from hydrazine and maleic acid or by reacting hydrazine hydrate and maleic anhydride. The commercial product, MH–30, is made by reacting MH with diethanolamine (DEA), and contains 58 per cent active ingredient, the remainder being classified as inert ingredients.

As part of a long-term experiment to determine the impacts of composted manure and straw amendments (replacing 50% of chemical fertilizer with composted pig manure, wheat straw return combined with chemical fertilizer, and setting no fertilizer and chemical fertilizer-only as controls) on rice-associated weeds in a rice (Oryza sativa L.)–wheat (Triticum aestivum L.) rotation system, species richness, abundance, density, and biomass of weeds were assessed during years 8 and 9. Fertilization decreased the species richness and total density of rice-associated weeds but increased their total biomass. The species richness and densities of broadleaf and sedge weeds decreased with fertilization, while species richness of grass weeds increased only with straw return and density was not significantly affected. The shoot biomass per square meter of grass and broadleaf weeds was significantly higher with fertilization treatments than with the no-fertilizer control, while that of sedge weeds declined with fertilizer application. With fertilization, the densities of monarch redstem (Ammannia baccifera L.) and smallflower umbrella sedge (Cyperus difformis L.) decreased, that of Chinese sprangletop [Leptochloa chinensis (L.) Nees] increased, and those of barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] and monochoria [Monochoria vaginalis (Burm. f.) C. Presl ex Kunth] were not significantly affected. Ammannia baccifera was the most abundant weed species in all treatments. Whereas composted pig manure plus fertilizer resulted in higher density of A. baccifera and lower shoot biomass per plant than chemical fertilizer only, wheat straw return plus chemical fertilizer caused lower density and shoot biomass of A. baccifera. Therefore, it may be possible that fertilization strategies that suppress specific weeds could be used as improved weed management program components in rice production systems.

Prior to planting of birdsfoot trefoil (Lotus corniculatus L.), stony soils were sprayed with ethyl N,N-dipropylthiocarbamate (EPTC) at 0, 2.0, or 4.0 lb/A and were tandem-disked or harrowed 0, 1, 2, or 3 times or rotary-tilled to incorporate the herbicide. Rotary tillage was the best incorporation method where stones were not numerous. Tandem disking was the most practical. Annual grasses were controlled by 2.0 lb/A EPTC and one incorporation. Higher rates of EPTC and more incorporations resulted in better annual broadleaf weed control and increased yields of birdsfoot trefoil.

Alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] at 3.4 and 6.7 kg/ha controlled weeds present in sweet potatoes (Ipomoea batatas L. ‘Jewel’, ‘Centennial’, ‘Caromex’) at two locations in North Carolina. Yield was not reduced from slight early season injury observed with the herbicide treatments. Most sweet potato root samples contained less than the detectable limit of 0.05 ppmw of alachlor. At one location, samples of the cultivars Jewel and Centennial that received 6.7 kg/ha contained 0.06 and 0.05 ppmw, respectively. Averaged over locations, residue levels in soil samples from plots treated at 6.7 kg/ha decreased from 1.55 ppmw initially to less than 0.05 ppmw by 16 weeks after application.

The etymology of herbicide has been traced to a U. S. trademark originally registered by William J. Reade of the Reade Manufacturing Company.

Laboratory and field experiments were conducted to describe the root system of honeyvine milkweed [Cynanchum laeve (Michx.) Pers.], and to study factors affecting the growth of plants from root fragments of shallow, ascending roots. The root system of a typical plant was composed of lateral roots radiating from a sparsely-branched, vertical taproot. Laterals tended to concentrate in a boundary area between a loamy surface soil and a zone of clay accumulation, this boundary area being below the zone of cultivation. Vertical taproots were found to a depth of 2 m, with sections collected at this and shallower depths demonstrating the ability to produce aerial shoots. In laboratory studies, root sections were killed either by freezing for 2 h or by drying at either 20 or 30 C for 24 h. The optimum temperature for shoot development from root sections was between 20 and 30 C. Shoot emergence and growth from buried root sections were generally unaffected by either depth of planting or root length. As many as 45 daughter shoots were produced from a single plant originating from seed, and 27 daughter shoots were developed from a root section 131 days after planting. The maximum distance daughter shoots were observed from original plants 131 days after planting was 111 cm.

The herbicide 4-amino-3,5,6-trichloropicolinic acid (picloram) was applied to several bermudagrasses (Cynodon spp.). Most of the grasses were susceptible to picloram; however, some were resistant and recovered within 16 months from the initial treatment. There were no detectable metabolites of 14C picloram in bermudagrass (C. dactylon (L.) Pers. ‘U-3’) or common Kentucky bluegrass (Poa pratensis L.). Also, there were no abnormal changes in cells of swollen nodes of picloram-treated bermudagrass that could explain the selective herbicidal injury of picloram on bermudagrass.

Experiments were conducted to develop systems for control of American germander [Teucrium canadense L. var. virginicum (L.) Eat.] in corn (Zea mays L. ‘Trojan TXS 105A’). Soil-incorporated, preemergence, and postemergence herbicide treatments were evaluated in greenhouse studies. Soil incorporated applications of EPTC (S-ethyl dipropylthiocarbamate) + R-25788 (N,N-diallyl-2,2-dichloroacetamide), atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] and alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] effectively reduced its growth. Preemergence applications of alachlor and atrazine did not control American germander. Postemergence applications of the dimethylamine salt of 2,4-D [(2,4-dichlorophenoxy)acetic acid] the butoxyethanol ester of 2,4-D and dicamba (3,6-dichoro-o-anisic acid) gave 20 to 41% control. Atrazine + crop oil gave 76 to 84% control. Soil-incorporated treatments of atrazine or EPTC + R-25788 controlled American germander better than alchlor when the rhizomes were planted at depths of 2.5 and 10 cm in greenhouse studies. In field trials American germander was a severe competitor in corn, especially when soil moisture was limited, and had the capability of substantially reducing corn yields when not controlled. EPTC + R-25788 and atrazine applied alone or in combinations as preplant incorporated treatments increased yield significantly and controlled 85 to 99% of the American germander 72 weeks after planting. Atrazine + oil was the only effective postemergence treatment.

Slender amaranth (Amaranthus viridis L.) and redroot pigweed (Amaranthus retroflexus L.) are increasingly problematic weeds of summer crops in Australia. Water is considered the most limiting factor in an agroecosystem, and water stress adversely impacts the growth and reproduction of plant species. The primary objective of this study was to determine the growth and fecundity of two Australian biotypes (Goondiwindi and Gatton) of A. viridis and A. retroflexus under water-stress conditions. Four water-stress treatments (100%, 75%, 50%, and 25% field capacity [FC]) at a 4-d irrigation interval were chosen. No difference was observed for growth and seed production between the two biotypes of both species when grown under varying soil moisture regimes. At 100% FC, A. viridis produced 44 g plant−1 aboveground biomass and 1,740 seeds plant−1. The maximum growth (46 g plant−1) and seed production (3,070 seeds plant−1) of A. retroflexus were observed at 100% FC. The growth and seed production of both species were reduced with increased water-stress levels. Both weeds responded to water stress by decreasing the shoot:root biomass ratio. However, A. viridis (290 seeds plant−1) and A. retroflexus (370 seeds plant−1) were able to produce a significant number of seeds per plant even at 25% FC. Results suggest that both weeds will produce seeds under water-limiting conditions. Therefore, management strategies are required to minimize the growth and survival of weeds in water-deficit conditions.

Seven herbicides were applied for selective weed control on three different seedling stands of five cool-season grasses. The herbicides were evaluated for their influence on weed control, grass establishment, and first crop grass seed production. These experiments indicate that 3,5-dibromo-4-hydroxybenzonitrile (bromoxynil) was outstanding in selective control of broadleaf weeds in seedling grass stands. Two other herbicides, 3,6-dichloro-o-anisic acid (dicamba) and 6,7-dihydrodipyrido[1,2-a:2′,1′-c]pyrazinediium ion (diquat) were valuable under most conditions. The remaining herbicides—(2,4-dichlorophenoxy)acetic acid (2,4-D), 2-sec-butyl-4,6-dinitrophenol (dinoseb), 2,3,6-trichlorobenzoic acid (2,3,6-TBA), and 1-butyl-3-(3,4-dichlorophenyl)-1-methylurea (neburon)—were satisfactory for selective weed control under some conditions, but their effects were not uniform for the different species and plantings.

Two experiments were conducted to examine woolly cupgrass seed reserve utilization and the effect of germination depth on seed reserve utilization, tiller number, and timing of tiller emergence. The first experiment included germination in light with water, in light with Hoagland's solution, and in darkness with water; the second included five planting depths of 1, 3, 5, 7, and 9 cm. Endosperm utilization and shoot and root growth over time were recorded. Endosperms lost weight rapidly until 6 d after germination (DAG), when weight loss slowed down. Endosperm weight loss ceased 8 or 10 DAG. Endosperm weight loss was fastest for germination in light with Hoagland's solution. Deep planting significantly decreased emergence rate and increased the time to emergence. Differences in residual endosperms may have affected the early development of woolly cupgrass seedlings. Germination from a depth of 9 cm depleted the endosperm by the time of emergence. Deep planting also decreased shoot dry weight and tiller number 15 d after planting (DAP). Because the first tiller of woolly cupgrass usually appeared 10 to 12 DAP, and the endosperm had been depleted before that, there was no endosperm to support the new tillers.

Using an oriental mustard root length bioassay, thiencarbazone bioavailability and dissipation in five Saskatchewan soils was investigated under laboratory conditions. Thiencarbazone bioavailability was assessed at 0 to 3.9 µg ai kg−1. Thiencarbazone concentrations corresponding to 50% inhibition (I50 values) obtained from dose-response curves varied from 0.56 to 1.71 µg kg−1. Multiple regression analysis indicated that organic carbon content (P = 0.018) and soil pH (P = 0.017) predicted thiencarbazone bioavailability. Thiencarbazone dissipation was examined in soils incubated at 23 C and moisture content of 85% field capacity. Thiencarbazone half-lives estimated from dissipation curves were 9 to 50 d, and organic carbon content (P = 0.002) and soil pH (P = 0.008) were significant factors affecting thiencarbazone dissipation. Thiencarbazone bioavailability decreases and dissipation rate is slower in Canadian prairie soils of high organic matter content and low soil pH. Because root length of oriental mustard plants also was reduced by ammonium, therefore ammonium-containing or -producing fertilizers can cause false positive results for thiencarbazone soil residues. Canaryseed roots had the same sensitivity to ammonium as oriental mustard roots but were not inhibited by thiencarbazone. Therefore canaryseed root length bioassay was effective in identifying inhibition caused by ammonium toxicity. Use of oriental mustard root and canaryseed root bioassays together can aid in interpreting bioassay results for detection of thiencarbazone residues.

Experiments were conducted in Sri Lanka to compare weed seedling emergence in three sugarcane plots of 0.1 ha planted in October 1995, January 1996, and April 1996. In each plot, weed seedling emergence was monitored for 20 wk in five permanent quadrats on each of three microsites: on ridges, in furrows, and on adjacent fallow land. Soil moisture (0 to 5 cm) and soil temperature (at 2.5 cm) were also recorded. Only crowfootgrass, swamp millet, and guineagrass (all grasses) occurred in all nine planting time-by-microsite combinations. About half of all seedlings emerging over the three planting times were swamp millet, and the next most frequent species was tropic ageratum. The composition of the emerged flora was similar on ridges and in furrows, but more seedlings emerged in the furrows than on the ridges. The highest number of emerged seedlings and of species occurred on adjacent fallow land. The major factor influencing seedling emergence appeared to be soil moisture.