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Martin M. Williams II, Rick A. Boydston, and Adam S. Davis

Research in dent corn has found significant variation in crop/weed competition for light among hybrids. However, little has been published on the extent of variation in sweet corn competitive ability. Field studies were conducted under weed-free conditions to quantify canopy development and light environment among three sweet corn hybrids and to determine associations among canopy characteristics to crop yield. An early-season hybrid (Spirit) and two midseason hybrids (WHT2801 and GH2547) were grown at experimental sites located near Urbana, Ill., and Prosser, Wash., in 2004 and 2005. Maximum leaf area index (LAI) and intercepted photosynthetically active radiation (PAR) was typically highest for GH2547 and lowest for Spirit. Most differences in vertical LAI among hybrids was observed above 60 and 150 cm in Illinois and Washington, respectively, with WHT2801 and GH2547 having leaf area distributed higher in the canopy than Spirit. Both number and mass of marketable ears were positively correlated with maximum relative growth rate (correlation coefficients 0.60–0.81), leaf area duration (0.68–0.79), total LAI (0.56–0.74) at R1, and intercepted PAR (0.74–0.83) at R1. Differences in canopy properties and interception of solar radiation among Spirit, WHT2801, and GH2547 lead us to hypothesize that variation in weed-suppressive ability exists among hybrids. Future testing of this hypothesis will provide knowledge of interactions specific to sweet corn useful for developing improved weed management systems.

Open access

Toshio Shibuya, Ryosuke Endo, Yoshiaki Kitaya, and Mizuki Tsuchida

During transplant production, young plants are generally grown densely to improve the productivity per unit growing area ( Marr and Jirak, 1990 ). Growing plants at high density stimulates stem elongation due to increased competition for light

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James F. Cahill and Eric G. Lamb

light by stunting initial shoot growth ( Cahill, 1999 , 2002 ). In addition, there are two general strategies of competition in plants ( Goldberg, 1990 ). An effect competitor is a plant that is good at denying neighbors access to limiting resources

Open access

Craig J. Frey, Xin Zhao, Jeffrey K. Brecht, Dustin M. Huff, and Zachary E. Black

United States, production declined by nearly 40% during the same timeframe ( USDA-NASS, 2016 ). A major factor of the U.S. fresh-market tomato production decline has been increased competition from Mexico, as U.S. imports of Mexican tomatoes nearly

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David W. Marshall and Preston K. Andrews

Washington is the leading producer of apples in the United States. North-central and south-central Washington and the Columbia Basin are the major production regions within the state. The climate of these production regions is characterized by cold winters and hot, dry summers with high levels of light intensity. The principal varieties produced are still `Delicious', `Golden Delicious', and `Granny Smith'; however, `Fuji', `Gala', and `Braeburn' have been planted widely since 1988. Despite increasing levels of production and lower prices beginning in 1986, apple prices have recovered relatively well in recent years due to aggressive exports to southeast Asia and Mexico. Increased international competition has resulted in a trend towards higher-density orchards using dwarfing rootstock so that earlier production can be achieved. Evaluation of the performance of new varieties in Washington's climatic conditions has increased. Although not the focus of this article, several social and environmental issues are facing the Washington apple industry, including increasing restrictions on chemical usage, competition for a limited water resource, regulation of ground water quality, pending labor relations legislation, and increasing urbanization pressures.

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Michael K. Bomford

Polycultures are thought to offer yield advantages over monocultures when net competition between plants of different species is less than that between plants of the same species. Planting density and crop ratios may both alter these competitive effects. To observe such effects, dicultures of basil (Ocimumbasilicum L.), brussels sprout (Brassica oleracea L.), and tomato (Lycopersicumesculentum Mill.) were grown organically at a range of ratios and densities (1–47 plants/m2) over two field seasons. Relative land output (RLO) values were calculated from field data and from modeled yield-density-ratio surfaces. Both methods showed yield advantages from polyculture at high planting densities (RLO = 2.20 @ top density), but not at low densities. Dicultures offered a 19% yield advantage, on average. Competition for resources was compared by measuring canopy light interception and soil moisture content, showing tomato to be the most competitive crop, followed by brussels sprout, then basil. Diculture yield advantages were most pronounced when individuals of a less competitive species outnumbered those of a more competitive species. Yield advantages were 36% and 20% for dicultures dominated by basil and brussels sprout, respectively.

Dicultures dominated by tomato offered no yield advantage. The results are discussed in terms of the current ecological understanding of plant interactions, and possible advantages to be derived from small-scale intercropping, popularly termed companion planting.

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Claudio M. Dunan, Philip Westra, and Frank D. Moore III

A simulation model was built as a decision aid for management of five weed species in direct seeded irrigated onion (Allium cepa L.). The model uses the state variable approach and simulations are driven by temperature and sunlight as photosynthetically active radiation (PAR). It predicts yield reduction caused by competition for PAR according to the ratio of crop leaf area index (LAI) to weed LAI and respective light extinction coefficients (k). Input variables are plant density by species and average number of leaves by species. Number of leaves per plant is used by the model to provide an estimate of initial leaf area per plant. The model calculates initial species LAIs by multiplying species density times average leaf area per plant. The model accurately describes competitive interactions, taking into account respective plant densities, time of emergence, and time of weed removal. It permits economic evaluation of management factors such as handweeding, chemical weed control, herbicide phytotoxicity due to early application, and control of weed flushes during the season. The model is also used to evaluate mechanisms of plant competition for sunlight. In a sensitivity analysis, onion yield loss was more sensitive to weed PAR interception than to PAR use efficiency, the latter a species-dependent constant in the model.

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Carlos L. Ballaré and Ana L. Scopel

Our understanding of how plants use light signals to detect and respond to the proximity of neighbors has increased dramatically over the last few years. At the same time, the explosion of biotechnological techniques has opened a variety of opportunities to manipulate the photosensory systems of cultivated plants. Therefore, the idea is beginning to emerge that plastic responses of cultivated plants to population density could he deliberately changed by engineering genotypes with altered photomorphogenic behavior. This talk will provide a review of recent developments in the area of seedling photomorphogenesis, which will be used as a platform to evaluate the realism of current models of plant competition and the agricultural implications of interfering with plant photophysiology.

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Richard L. Harkess and Robert E. Lyons

Combinations of seeding rate, spacing, and weed control treatments were evaluated for their effect on the performance of the Virginia Tech transplanted meadow technique. The treatments consisted of seeding at 112 or 56 g·90 m−2; within-row transplant spacing of 30, 45, or 60 cm; and mulching, oryzalin application, or no weed control measures. Plant competition alone was insufficient, whereas oryzalin was the most effective for weed control but also reduced the plant stand and floral display. Mulch provided effective weed control with maximum floral display. Close transplant spacing within rows resulted in quick site coverage initially, but this advantage disappeared after 8 weeks compared to wider spacing. Seeding rate did not affect site coverage until the meadow reached maturity at 12 weeks. The lower seed rate allowed more lodging, resulting in a more open appearance and greater canopy light transmission. Chemical name used: 4-(dipropylamino)-3,5-dinitrobenzenesulfonamide (oryzalin).

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Jeffrey F. Derr

Weed management is an important concern for apple producers. Weeds compete with fruit trees for water, nutrients, light and pollination by insects. Weed competition can dramatically reduce apple tree (Malus domestica Borkh.) growth and yield. Weed control practices can impact rodent populations, and insect and disease management in orchards. Use of cultivation can increase soil erosion. Mulches are too expensive for use in orchards and can increase rodent problems. Weeds are generally controlled within the row using herbicides while a grass sod is often used in row middles for erosion control. The most commonly used postemergence herbicides in apples are glyphosate, paraquat, and 2,4-D. Simazine is the most commonly used preemergence herbicide.