In New Mexico chile pepper production, pendimethalin is traditionally applied shortly after crop thinning, which is 9 to 10 wk after crop seeding. Pendimethalin applications before crop thinning may be a method for controlling early-season weeds in chile pepper; however, chile pepper tolerance to early-season applications of pendimethalin is poorly understood. We conducted a greenhouse study to evaluate young chile pepper responses to pendimethalin. We also conducted a field study to determine weed and chile pepper responses to early-season, postemergence-directed pendimethalin in combination with herbicides registered for preemergence applications. The greenhouse study included three treatments administered when chile pepper was at the four-leaf stage: (i) pendimethalin applied to foliage and soil, (ii) pendimethalin applied to soil only, and (iii) a nontreated control. The field study included four treatments: (i) preemergence applications of napropamide followed by postemergence-directed pendimethalin at 5 wk after crop seeding, (ii) preemergence applications of clomazone followed by postemergence-directed pendimethalin at 5 wk after crop seeding, (iii) postemergence-directed pendimethalin without preemergence herbicides, and (iv) nontreated, weed-free control. We conducted the field study at two sites that differed in soil texture. Pendimethalin application rates were maximum labeled rates for the specific soil. Results from the greenhouse study indicated that pendimethalin applied to foliage and soil stunted two of five cultivars, whereas pendimethalin applied to soil did not affect chile pepper height, fresh weight, dry weight, or root area. Results from the field study indicated that postemergence-directed pendimethalin did not affect chile pepper height or fruit yield, or cause visible symptoms of herbicide injury. Postemergence-directed pendimethalin reduced the densities of weeds, including junglerice. The results of this study indicate that postemergence-directed applications of pendimethalin at 5 wk after crop seeding do not cause crop injury or yield loss in chile pepper, while providing some weed control benefits.

Weed seeds with mechanical damage are more susceptible to mortality in soil than nondamaged seeds. In this study we introduce a colorimetric assay to distinguish mechanically damaged weed seeds from nondamaged weed seeds. Our objectives were to 1) compare steepates from mechanically damaged seeds against steepates from nondamaged seeds for their capacities to reduce resazurin—a nontoxic, water-soluble dye that changes color and light absorbance properties in response to pH; and 2) use light absorbance data from steepate-resazurin solutions to create classification trees for distinguishing damaged from nondamaged weed seeds. Species in this study included barnyardgrass, curly dock, junglerice, kochia, oakleaf datura, Palmer amaranth, spurred anoda, stinkgrass, tall morningglory, and yellow foxtail. Seeds of each species were subjected to mechanical damage treatments that collectively represented a range of damage severities. Damaged and nondamaged seeds were individually soaked in water to produce steepates that were combined with resazurin. Light absorbance properties of steepate-resazurin solutions indicated that for all species except kochia, damaged seeds reduced resazurin to greater extents than nondamaged seeds. Prediction accuracy rates for classification trees that used absorbance values as predictor variables were conditioned by species and damage type. Prediction accuracy rates were relatively low (66% to 86% accurate) for lightly damaged seeds, especially grass weed seeds. Prediction accuracy rates were high (91% to 99% accurate) for severely damaged seeds of specific broadleaf and grass weeds. Steepate-resazurin solutions that successfully separated seeds took no more than 32 h to produce. The results of this study indicate that the resazurin assay is a method for quickly distinguishing damaged from nondamaged weed seeds. Because rapid assessments of seed intactness may accelerate the development of tactics for reducing the number of weed seeds in soil, we advocate further development of resazurin assays by laboratories studying methods for weed seedbank depletion.

Propanil is an acylanilide herbicide introduced in the early 1960s to control dicotyledonous weeds and grasses, including Echinochloa species in cultivated rice. Since then, propanil has been used extensively in rice production in the United States and in several other countries. Propanil is an inhibitor of photosystem II, but rice is tolerant to propanil because of the presence of a high level of aryl acylamidase that catalytically degrades the compound to nonphytotoxic products, i.e., 3,4-dichloroaniline and propionic acid. About 10 yr ago, biotypes of barnyardgrass and junglerice were discovered to be resistant to propanil. The resistance mechanism of these two biotypes has been shown to be elevated levels of aryl acylamidase activity. Various strategies to combat propanil resistance and to more fully understand the biochemistry involved in this resistance have been investigated. These include studies on the interactions of herbicides and other chemicals with propanil, rotation of rice with other crops (consequently the use of other herbicide modes of action), and use of alternative herbicides in rice. Certain compounds, including some organophosphate insecticides, are potent inhibitors of aryl acylamidase, which can act as synergists with propanil to increase phytotoxicity. Another compound that lacks insecticidal or herbicidal activity, PPG-124, has been commercialized as a herbicide synergist for propanil. These chemical and biochemical interactions and other factors involved in propanil-resistant Echinochloa weeds are presented and discussed.

Junglerice is one of the most serious grass weeds of rice in the tropics. Greenhouse studies were conducted to evaluate growth and reproduction of junglerice in response to water stress. Plant height, biomass, and seed production of junglerice grown alone were reduced with increasing water stress. However, most stressed plants (irrigated at 12.5% of field capacity) still produced considerable biomass (8.5 g plant−1) and seeds (>1,600 seeds plant−1). When junglerice and rice were grown together under water-stressed condition, junglerice was taller than rice. The junglerice-to-rice biomass ratio also increased from 4.7 at 100% of field capacity to 7.6 at 12.5% of field capacity, indicating the greater junglerice vigor in water-stress conditions. In another study, the influence of the duration of water stress at intervals between 3 and 15 d on growth and seed production of junglerice was evaluated. Plant height, biomass, and seed production decreased with increasing water-stress duration. However, the weed produced an average of 400 seeds plant−1 in the most stressed treatment (i.e., when irrigation was applied at 15-d intervals). Water-stressed treatments did not affect germination of junglerice seeds in the laboratory. Growth and seed production of junglerice at all moisture levels ensures survival of the population in an unpredictable environment and contributes to the weedy nature of this species. The joint effect of enhanced weed competition and drought stress could severely harm crop yield; therefore, it is important to control such weeds in the early stages of crops and save stored moisture for the crops.

Twenty-one grass weeds have evolved resistance to herbicides in Latin America, particularly in rice, soybean, wheat, and orchards. Junglerice, the most widespread and economically important rice weed, evolved resistance to propanil, acetyl-coenzyme A carboxylase (ACCase)-inhibitor herbicides, quinclorac, and imazapyr in Central America, Colombia, and Venezuela. Some junglerice populations are resistant to at least three herbicide modes of action. Other herbicide-resistant (HR) rice weeds are barnyardgrass and gulf cockspur to quinclorac in Brazil, and saramollagrass to ACCase-inhibitor herbicides in Colombia and bispyribac in Venezuela. Populations of weedy rice resistant to imidazolinones are now emerging, most likely originated from gene flow from imidazolinone-resistant rice. Saramollagrass also became resistant to nicosulfuron in corn in Venezuela. Eight species associated with soybean are resistant to ACCase-inhibitor herbicides in Brazil (alexandergrass, goosegrass, and southern crabgrass) and Bolivia (Louisiana cupgrass, itchgrass, sudangrass, and two common wild sorghum species). Four more ACCase-inhibitor–resistant species (hedgehog dogtailgrass, wild oat, rigid ryegrass, and Italian ryegrass) are found in Chile infesting canola and wheat. ACCase-inhibitor–resistant hood canarygrass, littleseed canarygrass, and wild oat are important in wheat in Mexico. Resistance to acetolactate synthase (ALS)-inhibitor herbicides has been reported in itchgrass, goosegrass, and Mexican grass. Italian ryegrass populations resistant to glyphosate have been found in Chile and Brazil. Glyphosate resistance has also evolved in goosegrass in Bolivia and johnsongrass in Argentina. In general, little is done to prevent resistance evolution. An exception is the stewardship programs aiming to prevent gene flow from imidazolinone-resistant rice to weedy rice. Once resistance evolves, HR populations are mostly managed by shifting to herbicides with different modes of action and, in some cases, by slightly modifying agronomic practices. Propanil formulations containing a synergist are used to manage propanil-resistant junglerice. Increased no-till agriculture and planting of glyphosate-resistant crops are likely to select more glyphosate-resistant weeds.

Junglerice is one of the most serious grass weeds of rice in the tropics. Experiments were conducted in the laboratory and screenhouse to determine the influence of environmental factors on seed germination and seedling emergence of junglerice in the Philippines. In the laboratory, germination was stimulated by light, suggesting that seeds of this species are positively photoblastic. The tested temperatures (35/25, 30/20, and 25/15 C alternating day/night temperatures), however, did not influence germination. Germination in the laboratory was not affected by a soil pH range of 4 to 9, but was decreased by salinity (> 50 mM NaCl) and moisture stress (< −0.2 MPa osmotic potential). In the screenhouse, germination of junglerice was greatest (97%) for seeds at the soil surface, but emergence declined exponentially with increasing seed burial depth, and no seedlings emerged from seeds buried at 6 cm. In pots, seedling emergence declined markedly with the addition of rice residue to the soil surface at rates equivalent to 4 to 6 tonnes (t) ha−1. As germination of junglerice was strongly stimulated by light, and seedling emergence was optimal at shallow burial depths, this species is likely to be problematic in reduced tillage systems.

Weeds are a major biotic constraint to aerobic rice production in Asia. Research is needed on the effects of cultural practices on weed management in aerobic rice, including techniques such as planting pattern and competitive cultivars. Field experiments were conducted in Punjab, India, in the wet seasons of 2008 and 2009 to study the growth of weeds and two rice cultivars [PR 115 and Punjab (P.) Mehak 1] in relation to planting pattern (uniform rows [23-cm row spacing] and paired rows [15-, 30-, and 15-cm row spacings]) under aerobic conditions. Junglerice and rice flatsedge were the dominant weed species during the early stages of the crop, while Chinese sprangletop and large crabgrass were the predominant species during flowering stage of the crop. Weed dry matter was not affected by planting pattern of P. Mehak 1; however, for PR 115, weed dry matter was greater in rice grown in uniform rows (244 g m−2) than in paired rows (183 g m−2). Planting patterns did not affect weed-free crop growth and yield, but weeds tended to be more abundant in the uniform planting system, particularly under cultivar PR 115. Consequently, this cultivar grew and yielded better under the paired rows when weeds were present. The cultivar PR 115 had greater yield potential than P. Mehak 1, but growth and productivity of P. Mehak 1 were unaffected by the planting patterns, suggesting better competitive ability against weeds than PR 115. The results imply that yield of some aerobic rice cultivars may be improved by exploring competitiveness of rice cultivars through paired row planting patterns. There is a need to study plasticity changes for cultivars which respond with more competiveness in paired rows. The identified traits could be useful as selection criteria for screening weed-competitive cultivars in paired row pattern.

Junglerice is one of the world's most problematic C4 grass weeds present in dry-seeded rice in many countries. A screenhouse study was conducted to determine the effect of four rice planting densities (0, 100, 200, and 400 plants m−2) and four nitrogen (N) rates (0, 50, 100, and 150 kg ha−1) on the growth and morphological plasticity of junglerice. Junglerice plant height was reduced by 15 to 35%, tiller number by 54 to 77%, leaf number by 61 to 85%, leaf area by 69 to 90%, leaf biomass by 63 to 88%, stem biomass by 70 to 92%, and inflorescence biomass by 66 to 94% at rice planting densities ranging from 100 to 400 plants m−2 relative to the junglerice plants grown alone. However, all these growth parameters increased with increasing N rates. Junglerice biomass increased by 125 to 472%, whereas rice biomass increased by 122 to 285% with the application of 50 to 150 kg N ha−1. Additional N favored junglerice biomass production relative to rice. Rice crop interference (200 to 400 plants m−2) reduced junglerice growth and biomass and overshaded the junglerice plants when no N was applied. Increasing N application resulted in taller plants and higher biomass of junglerice, while it reduced root-shoot weight ratio. These results suggest that increasing N rate increased the competitive ability of the junglerice over rice regardless of crop planting density. Information generated in this study could be useful in devising appropriate combinations of planting density and fertilizer management strategies for cultural junglerice management, particularly in situations where junglerice species are more responsive than rice to N.