Quizalofop controlled 2- to 3-leaf and 5- to 6-leaf red rice better than fluazifop-P, haloxyfop, or sethoxydim applied alone. Red rice control increased when acifluorfen was tank mixed with haloxyfop, fluazifop-P, or sethoxydim. Antagonism was most severe when imazaquin was tank mixed with any of the grass herbicides. The efficacy of sethoxydim, fluazifop-P, and haloxyfop was reduced when applied with chlorimuron or bentazon. In greenhouse experiments, all of the grass herbicides, except for fenoxaprop, reduced fresh weight of red rice 71% or more. Based on fresh weight reduction, acifluorfen, fomesafen, and lactofen only reduced the activity of fluazifop-P, while imazaquin decreased the activity of all grass herbicides.

Greenhouse studies were conducted to evaluate sensitivity of red rice ecotypes from Arkansas, Louisiana, Mississippi, and Texas to soil- and foliar-applied herbicides. Ecotype response to herbicides was variable. Molinate at 4.48 kg ai/ha controlled red rice ecotypes 92 to 100% and cultivars ‘Lemont,’ ‘Mars,’ and ‘Maybelle’ 39 to 63%. Red rice control with thiobencarb at 4.48 kg ai/ha was at least 89%. The tank mixture of fenoxaprop at 0.08 kg ai/ha plus fluazifop-P at 0.10 kg ai/ha controlled red rice and rice cultivars more than for either herbicide applied alone. Red rice and rice cultivar control with paraquat at 0.70 kg ai/ha was 83 to 100%. Blackhulled red rice TX 4 was less sensitive to paraquat and glufosinate than other ecotypes and cultivars. Glufosinate at 1.12 kg ai/ha was required to provide at least 94% control of ecotype TX 4.

Four years of field experiments were used to assess the competitive ability of red rice, a weedy variety of rice, and ‘Mars’, a cultivar of rice. Red rice was the dominant competitor each of the 4 yr, and an average of one red rice plant reduced Mars grain yield per plant equal to the effect of four Mars plants. Intervarietal competition was more important than intravarietal competition for Mars, whereas the reverse was true for red rice. The product of the coefficients for intravarietal competition did not significantly exceed the product of the coefficients for intervarietal competition, indicating that the two varieties were competing for the same resources. Using the reciprocal yield model coefficients from 1989, grain yield losses of Mars, due to red rice densities of 4, 16, 25, and 300 plants m−2, were predicted to approximate 13, 37, 48, and 92%, respectively, at a fixed cultivar density of 100 plants m−2.

Growth and development of red rice (strawhull) and ‘Lemont’ and ‘Newbonnet’ rice were compared in 1987 and 1988 under noncompetitive field conditions at Stuttgart, AR. Growth and developmental differences were greater between red rice and rice than between rice cultivars. Compared to rice, red rice was taller, produced more culms m−2 and aboveground dry weight, had higher leaf area indices, and had a greater flag leaf area. Also, it had lower leaf to stem ratios late in the season, had greater crop growth rate early in the season but less late in the season, and produced a lower grain weight than rice. Compared to Newbonnet, Lemont plants were shorter at 60 d after emergence or later, produced more culms m−2, had a greater leaf area index, and produced higher grain weight than Newbonnet. Also, Lemont and Newbonnet produced comparable leaf to stem ratios, crop growth rates, and flag leaf areas.

Preplant incorporation of alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide] at 3.6 kg ai/ha, metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] at 2.2 kg ai/ha, and tank mixtures of imazaquin {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid} with either alachlor at 0.14 + 2.2 kg ai/ha or metolachlor at 0.14 + 1.1 kg ai/ha controlled >90% red rice (Oryza sativa L. # ORYSA) in soybeans [Glycine max (L.) Merr.]. Also, FMC-57020 [2-(2-chlorophenyl)methyl-4,4-dimethyl-3-isoxazolidinone] at 1.7 kg ai/ha applied preplant incorporated controlled > 80% of the red rice. DPX-Y6202 {2-[4-[(6-chloro-2-quinoxalinyl)oxy]-phenoxy]-propionic acid, ethyl ester} was the best and most consistent postemergence herbicide treatment for red rice control when applied once at 0.28 kg ai/ha or sequentially at 0.14 kg ai/ha each time. Haloxyfop {2-[4-[[3-chloro-5-(trifluoromethyl)-2-pyridinyl]oxy] phenoxy] propanoic acid} applied at 0.16 kg ai/ha followed by another application at 0.14 kg ai/ha gave excellent red rice control in 2 of 3 years. Soybeans were uninjured by the herbicide treatments and yield was higher from treated than untreated soybeans.

The weed control spectrum of clomazone, the influence of clomazone on the performance of other herbicides, and the performance of tank mixtures containing clomazone were evaluated from 1985 through 1987. Clomazone applied preemergence at 280 g ai/ha controlled 90 to 100% of barnyardgrass, giant foxtail, goosegrass, large crabgrass, seedling johnsongrass, prickly sida, and velvetleaf. Clomazone at reduced rates was more effective applied preemergence than preplant-incorporated for 10 of 23 weed species. Clomazone enhanced imazaquin activity on entireleaf morningglory and sicklepod in 1986. A tank mixture of clomazone, imazaquin, and metribuzin at half the labeled rates reduced soybean injury, increased soybean yield, and controlled weeds as well as the mixture at labeled rates.

A cropping system of soybeans and rice grown in alternate years for 4 yr controlled red rice when combined with effective herbicide treatments applied to the soybean crop. Effective herbicide treatments in soybeans were preplant incorporated applications of alachlor, metolachlor, or alachlor plus trifluralin each followed by postemergence over-the-top applications of bentazon plus mefluidide, postemergence directed applications of paraquat, or sequential applications of postemergence over-the-top bentazon plus mefluidide and postemergence directed paraquat. Control of red rice in soybeans reduced red rice infestations in the subsequent rice crop. Red rice infestations were lower in rice grown the fourth year than in rice grown the second year. Postemergence over-the-top or directed herbicide treatments used alone or combined with preplant incorporated applications of trifluralin did not control red rice in soybeans with subsequent red rice infestations in the rice crop.

Substituted quinolinecarboxylic acids, including quinclorac (BAS 514H), are a new class of highly selective auxin herbicides, which are chemically similar to naturally occurring compounds isolated from plants and soils. Quinclorac is used in rice to control important dicot and monocot weeds, particularly barnyardgrass. The herbicide has also been developed for application in turfgrass areas, spring wheat, and chemical fallow. Quinclorac is readily absorbed by germinating seeds, roots, and leaves and is translocated in the plant both acropetally and basipetally. By mimicking an auxin overdose, quinclorac affects the phytohormonal system in sensitive plants. The compound stimulates the induction of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase activity and thus promotes ethylene biosynthesis. In susceptible dicots, increased levels of ethylene trigger an accumulation of abscisic acid (ABA), which, as part of the intrinsic auxin activity of quinclorac, plays a major role in growth inhibition and the induction of epinasty and senescence. In sensitive grasses, such as barnyardgrass species, large crabgrass, broadleaf signalgrass, and green foxtail, quinclorac leads particularly to an accumulation of tissue cyanide, formed as a co-product during increased ACC and ethylene synthesis. This causes phytotoxicity characterized by the inhibition of root and particularly shoot growth with tissue chlorosis and subsequent necrosis. These effects were not observed in tolerant rice and a resistant biotype of barnyardgrass. No significant differences in uptake, translocation, or metabolism of quinclorac between resistant and sensitive grasses were found. Hence, a target-site-based mechanism of selectivity is suggested. The induction process of the ACC synthase activity plays the primary role in the selective herbicide action of quinclorac. This is a common effect of auxin herbicides and auxins, which lead to the accumulation of cyanide and/or ABA depending on the plant species and tissues, the compound concentration in the tissue, and their biological activity.

Field experiments were conducted to investigate methods of controlling red rice (Oryza sativa L. ♯ ORYSA) in drill-seeded rice (O. sativa). Treatments included the rice cultivar ‘Mars', coated with calcium peroxide (CaO2) at 40% (w/w) and a crop protectant, R-33865 (O,O-diethyl-O-phenyl phosphorothioate) at 0.5 and 1% (v/w). Molinate (S-ethyl hexahydro-1H-azepine-1-carbothioate) at 6.7 kg ai/ha was applied preplant incorporated (ppi). The land was flooded (2.5 to 5 cm deep) after seeding with rice (100 kg/ha, 2.5 cm deep), and the water was maintained throughout the growing season. CaO2, with or without molinate, increased rice grain yield 50% and increased rice culm density fivefold above untreated rice. Molinate applied ppi controlled 96% of the red rice. Rice seed coated with only CaO2 or with CaO2 plus R-33865 at 0.5%, each combined with ppi molinate, produced 5690 and 6030 kg/ha of grain, respectively. These high yields were associated with red rice control by molinate and good stands of rice provided by O2 supplied by CaO2. R-33865 applied to rice seed at 1% (v/w) injured rice by reducing rice culm densities 41%, compared with rice without protectant.

A study was conducted in 1994 and 1995 at two Mississippi locations to evaluate preplant incorporated (PPI) and preemergence (PRE) applications of alachlor, clomazone, SAN 582, metolachlor, pendimethalin, and trifluralin, and postemergence (POST) applications of AC 263,222 and imazethapyr alone or followed by clethodim late postemergence (LPOST) for red rice control in soybean. Applications of 110 g ai/ha clethodim increased red rice control when following any earlier herbicide application at one location that harbored a high natural infestation. In 1 yr at one location, red rice seedhead suppression from PPI and PRE herbicide applications alone was greater than 95% due to high activity from herbicides and drought conditions during red rice seedhead development. Early postemergence (EPOST) applications of 30 g ae/ha AC 263,222 suppressed at least 95% of red rice seedheads, regardless of year, location, or clethodim LPOST application. At one location, any treatment where 110 g/ha clethodim followed an earlier herbicide application suppressed red rice seedheads at least 95%. Compared to the nontreated control, only AC 263,222 injured soybean (30%) and reduced soybean yield (200 kg/ha).

Plant characteristics of red rice ecotypes obtained from Arkansas, Louisiana, Mississippi, and Texas, including 11 strawhulled, five blackhulled, two goldhulled, and one brownhulled type, were evaluated under field conditions. Most ecotypes were uniform and stable but manifested considerable genetic variability. Red rice plants had pubescent leaves, were taller with lighter green color, and produced more tillers and panicles per plant than rice cultivars ‘Lemont,’ ‘Mars,’ and ‘Maybelle.’ Most ecotypes were highly susceptible to seed shattering starting about 14 d after anthesis when seed moisture was more than 25%. Seeds of most ecotypes were highly dormant at harvest. Rice cultivars had a larger flag leaf and more total leaf area per plant at anthesis and produced more seeds per panicle than red rice. Some red rice ecotypes had plant characteristics closely related to cultivated rice suggesting natural hybridization with rice.

Glyphosate was evaluated at 0.8, 1.3, and 1.7 kg ae/ha applied at the two-leaf, four-leaf, or two- to three-tiller growth stage for red rice control. In addition, red rice seedheads were counted concurrently with soybean harvest at each of three locations to assess treatment effect on seedhead reduction. Field studies were conducted at Starkville, MS, in 1994 and 1995 and Shaw, MS, in 1995. A significant rate response was not observed for red rice control 2 and 4 wk after treatment (WAT) or for seedhead reductions. Glyphosate controlled red rice 88, 91, and 88% 2 WAT when applied to two-leaf, four-leaf, or two- to three-tiller red rice, respectively. Due to subsequent seedling emergence, control from glyphosate applications to two- or four-leaf red rice 4 WAT was 51 and 84%, respectively. Red rice treated at the two- to three-tiller stage was controlled 91% 4 WAT. When compared to the nontreated control, seedheads were reduced 97% by two- to three-tiller applications, compared to 87 and 56% reductions from four- and two-leaf applications, respectively.

Both alachlor at 3.6 kg ai/ha and metolachlor at 2.8 kg ai/ha applied preplant incorporated consistently controlled red rice 82% and 90%, respectively, over a 3-yr period. Averaged across years, only mefluidide at 0.14 kg ai/ha plus bentazon at 0.84 kg ai/ha, haloxyfop at 0.28 kg ai/ha, and quizalofop at 0.14 kg ai/ha applied postemergence controlled red rice comparable to the preplant herbicide treatments. High rates of sethoxydim and fluazifop (0.56 kg ai/ha), haloxyfop (0.28 kg ai/ha), and quizalofop (0.14 kg ai/ha) were needed to obtain soybean yields equivalent to alachlor and metolachlor.

Field experiments were conducted in 1986, 1987, and 1988 to evaluate imazethapyr rate and time of application on postemergence control of 24 weed species. Contour graphs were developed that predicted imazethapyr rates required for various levels of weed control based upon weed leaf number at application. Rates below the labeled rate (70 g ha−1) provided 90% or greater control of common cocklebur, smallflower morningglory, and smooth pigweed if applied to 3 true-leaf or smaller weeds and of barnyardgrass, seedling johnsongrass, and Palmer amaranth if applied while weeds were in the cotyledon or 1 true-leaf stage. A rate of 70 g ha−1 provided 90% control of large crabgrass in the 1 true-leaf stage. Entireleaf morningglory, red rice, pitted morningglory, and velvetleaf are not susceptible enough to imazethapyr for 90% or greater control to be obtained with rates lower than 70 g ha−1 at the 1 true-leaf growth stage. These data demonstrate how control data can be used for developing effective reduced-rate herbicide recommendations based on weed leaf number.

Red rice at 20 plants m−2 was grown with two rice cultivars for 0, 40, 60, 80, 100, or 120 days after emergence. Red rice interference for 120 days after rice emergence reduced straw dry weights of Lemont and Newbonnet 58 and 34%, respectively. Grain yield of Lemont and Newbonnet was reduced 86 and 52%, respectively, by red rice interference for 120 days after emergence. Regression analyses indicated that red rice interference reduced straw dry weights of Newbonnet and Lemont 25 and 50 kg ha−1 day−1, respectively, for interference durations of 40 to 120 days after emergence. Grain yield of Newbonnet and Lemont was reduced 60 and 93 kg ha−1 day−1, respectively, for interference durations of 60 to 120 days. Negative linear relationships occurred between interference durations of red rice and plant height, panicles m−2, spikelets panicle−1, filled grains panicle−1, or panicle length of both cultivars. However, all parameters were reduced more for Lemont than for Newbonnet as interference duration increased. Head rice (whole kernels) and total milled rice yields of both cultivars were reduced by season-long red rice interference. Red rice straw dry weight and number of culms m−2 were greater when red rice was grown with Lemont than when grown with Newbonnet.

Glufosinate applied postemergence alone and in mixture with pendimethalin, thiobencarb, quinclorac, propanil, bensulfuron, bentazon, acifluorfen, or triclopyr was evaluated on bialaphosresistant (BAR) rice and red rice in field studies. Glufosinate at 2.2 kg ai/ha alone was less phytotoxic (6%) to BAR-transformed rice than when it was applied in combination with 0.4 kg ai/ha triclopyr (59%) or 0.6 kg ai/ha acifluorfen (22%). Rice yield with glufosinate alone was similar to the weed-free check the first year, but 13% less than the weed-free check the second year. For the glufosinate plus triclopyr mixture, rice yield was reduced by 39 and 76% compared with glufosinate alone in 1994 and 1995, respectively. Red rice control was 92% with either 3.4 kg ai/ha propanil or 0.6 kg/ha acifluorfen mixed with 0.6 kg/ha glufosinate, which was greater than for glufosinate alone and the other combinations. Propanil or acifluorfen mixed with glufosinate reduced red rice plant height, panicle maturity, and 100-seed weight 16, 31, and 24%, respectively, compared to glufosinate alone and 30, 48, and 43%, respectively, compared to the nontreated weedy check.

Pot experiments were conducted in the field at Stuttgart, AR, during 1982 and 1983 to evaluate growth and morphological differences between strawhull and blackhull red rice (Oryza sativa L. ♯ ORYSA) biotypes collected from Arkansas, Louisiana, and Texas. All red rice biotypes were compared with rice (Oryza sativa L.) cultivars ‘Lebonnet’ and ‘Nortai’. Growth and morphological differences were greatest between cultivars and red rice biotypes, less between blackhull and strawhull types, and least among collections within blackhull or strawhull. Cultivars emerged slower, were shorter, tillered less, produced less straw and fewer panicles/plant, had a lower leaf area index, and had less grain shattering than most of the red rice biotypes. Blackhull red rice biotypes tillered 27% more, produced 18% more straw, and had later maturity than strawhull. Blackhull red rice from Arkansas emerged earlier, tillered 6 to 38% more, and produced 8 to 38% more panicles per plant than other red rice biotypes, whereas blackhull red rice from Texas was 11 to 26% taller at maturity than other biotypes.

Field research was conducted in 1985 and 1986 to compare the efficacy of fluazifop and quizalofop on production of panicles and seeds of red rice. Single and first sequential treatments were applied to red rice in the early-tillering, midtillering, and panicle initiation stages of growth. Sequential treatments were applied 14 days after each earlier application to red rice in the midtillering, late-tillering, and early-heading growth stages, respectively. Both herbicides were applied singly or sequentially at 70, 140, and 280 g/ha. Sequential applications of fluazifop and quizalofop at 280 g/ha caused the greatest reduction of red rice panicle and seed production. Fluazifop at 280 g/ha applied sequentially reduced panicle production 75 to 80% and seed production 80%; 140 g/ha applied sequentially reduced seed production 83%. Quizalofop at 280 g/ha applied sequentially reduced panicle production 75 to 100% and seed production 91%. Sequential applications of either herbicide applied to red rice plants in the panicle initiation and early-heading growth stages were the most effective treatments.

In 1986 and 1987, imazethapyr at 0.11 or 0.14 kg ai/ha and metolachlor plus imazaquin at 1.4 plus 0.16 kg/ha, applied preplant incorporated, controlled greater than 85% of red rice in soybeans. Red rice was controlled similarly by standard herbicide treatments of alachlor at 2.8 kg ai/ha, metolachlor at 2.2 kg ai/ha, and alachlor plus imazaquin at 2.5 plus 0.14 kg ai/ha. Eighty-four percent or more of red rice was controlled with single postemergence herbicide treatments of DPX-Y6202-31 at 0.14 kg ai/ha or imazethapyr at 0.11 kg ai/ha or by sequential postemergence treatment of DPX-Y6202-31 (0.07 plus 0.07 kg ai/ha) or clethodim (0.1 plus 0.1 kg ai/ha). Soybean injury was insignificant, and yields from treated plots were superior or equal to those from untreated plots.

Field studies were conducted from 1992 to 1994 to evaluate herbicides applied preplant incorporated (PPI), preemergence (PRE), and postemergence (POST) for red rice and Echinochloa spp. control in soybean. Metolachlor PPI at 3.4 kg ai/ha controlled red rice late season 90 to 92%. Alachlor at 4.5 kg ai/ha and SAN 582H at 2.2 or 3.4 kg ai/ha, PPI or PRE, metolachlor plus imazaquin at 2.8 + 0.14 kg ai/ha PRE, and quizalofop-P POST at 0.07 kg ai/ha provided 83 to 95% red rice control in at least 2 of 3 yr. The addition of imazaquin to metolachlor or pendimethalin did not improve red rice control. Early-season Echinochloa spp. control with trifluralin, pendimethalin, and pendimethalin + imazaquin applied PPI; metolachlor, SAN 582H at 2.2 or 3.4 kg/ha, and metolachlor + imazaquin applied PPI or PRE; alachlor, AC 263,222 + imazaquin, and AC 263,222 + imazethapyr applied PRE; and sethoxydim and quizalofop-P applied POST was 90 to 100% in at least 2 of 3 yr. However, Echinochloa spp. control decreased for all treatments later in the season. Pendimethalin applied PPI at 2.2 kg ai/ha or in mixture with imazaquin at 1.7 + 0.14 kg ai/ha injured soybean 14 to 34% in 2 yr. Trifluralin PPI, SAN 582H at 2.2 or 3.4 kg/ha PPI or PRE, imazaquin PPI, metolachlor + imazaquin PPI or PRE, and AC 263,222 + imazethapyr injured soybean 12 to 41% in at least 1 of 3 yr.