Integrated weed management strategies for delaying herbicide resistance in wild oats

Les biotypes de folle avoine (Avena fatua) résistants aux herbicides infestent les principales régions céréalières de l'Ouest américain et de l'Ouest canadien. Cet article passe en revue les stratégies de lutte intégrée contre les mauvaises herbes qui peuvent être utilisées pour empêcher ou retarder le développement de la résistance aux herbicides chez la folle avoine. Une stratégie de lutte intégrée contre la folle avoine, destinée à retarder ou à empêcher le développement de la résistance, devrait être basée sur la prévention de l'introduction des grains de folle avoine dans le sol. Deux façons d'y arriver sont d'empêcher l'immigration de graines dans le champ à partir de sources externes, et de réduire ou éliminer la production de graines par la folle avoine déjà présente au champ. Il est de plus en plus évident que le recours à l'utilisation continuelle d'herbicides comme seul moyen de lutte contre les mauvaises herbes n'éliminera pas la folle avoine ni les autres graines de mauvaises herbes de la banque de graines du sol. Au contraire, tout porte à croire que cette pratique va sélectionner des biotypes résistants aux herbicides utilisés, particulièrement là où des herbicides ayant le même mode d'action sont utilisés de façon continue. Il est essentiel, cependant, que les herbicides soient considérés seulement comme une composante d'un système intégré global incluant la lutte culturale et d'autres stratégies de gestion, et que les principes agronomiques soient considérés lors du développement de ce système.Herbicide-resistant biotypes of wild oats (Avena fatua) infest most major cereal producing regions in the western United States and Canada. This paper reviews potential integrated weed management strategies that can be used to prevent or delay selection of herbicide-resistant wild oats plants. An integrated wild oats management strategy to delay or prevent the development of herbicide resistance should be based on preventing the movement of wild oats seed into the soil. Two ways to achieve this are by preventing the immigration of seed into the field from external sources, and by reducing or eliminating seed production by wild oats already in the field. It is becoming increasingly clear that reliance on continuous herbicide useas the sole means of weed control will fail to eliminate wild oats and other weed seed from the soil seedbank. On the contrary, evidence is mounting that this practice will select for biotypes that are resistant to the herbicides used, especially where herbicides of the same mode of action are used continuously. It is essential, therefore, that herbicides be considered as just one component of an overall integrated System together with cultural control and other management strategies, and that agronomic principles be considered when developing this System

Water conservation under reduced tillage systems

Water is important for dryland crop production. Seldom is rainfall sufficient or adequately distributed during a growing season so that dryland crops can produce to their fullest potential. It is necessary to have stored water available in the soil to supplement inadequate growing season rainfall for economical crop production. Stored water is especially important in the Inland Pacific Northwest of north central Oregon, southeastern Washington, and northern Idaho, where 65% of annual precipitation occurs during the six-month (Sept. 1 to Feb. 28) winter period and 30% during the four-month (March 1 to June 30) growing season. Stored water is also important in the Eastern Idaho Plateau where the low annual precipitation is nearly evenly distributed over the months of the year. The water balance equation tells us that change in water content in the soil - precipitation + inflow - runoff + upward flow - drainage - evapotranspiration (ET). Any cultural practice that decreases runoff or ET can result in increased water in the soil. To store adequate quantities of water, deep soils (> 60 inches) with good infiltration and water holding capacity are required. Summer fallow has long been the traditional practice for storing water in soils for later use by crops. Fallow periods vary from 14 to 15 months where winter small grains are seeded to 21 months where spring small grains are seeded. Water storage efficiency for fallow is low, ranging from 10 to 35% in the Great Plains and the Southwest; to 30-37% in eastern Idaho and northern Utah; to 40-45% of precipitation in the Inland Pacific Northwest (Evans and Lemon, 1957). Good water conservation yields increased crop production, stability of production, and increased water use efficiency. Soil tillage and residue management play significant roles in collection and storage of precipitation in the soil. Our objectives are to discuss insights in water conservation gained under the STEEP (Solutions to Economic and Environmental Problems) program (Oldenstadt et al., 1982) during these last ten years and problems that remain. New research information will be discussed under topics of crop residues, conservation tillage systems, fallow and models

Low-energy fixed points of random Heisenberg models

The effect of quenched disorder on the low-energy and low-temperature properties of various two- and three-dimensional Heisenberg models is studied by a numerical strong disorder renormalization group method. For strong enough disorder we have identified two relevant fixed points, in which the gap exponent, omega, describing the low-energy tail of the gap distribution, P(Delta) ~ Delta^omega is independent of disorder, the strength of couplings and the value of the spin. The dynamical behavior of non-frustrated random antiferromagnetic models is controlled by a singlet-like fixed point, whereas for frustrated models the fixed point corresponds to a large spin formation and the gap exponent is given by omega ~ 0. Another type of universality classes is observed at quantum critical points and in dimerized phases but no infinite randomness behavior is found, in contrast to one-dimensional models.Comment: 11 pages RevTeX, eps-figs included, language revise

Water conservation under reduced tillage systems

Water is important for dryland crop production. Seldom is rainfall sufficient or adequately distributed during a growing season so that dryland crops can produce to their fullest potential. It is necessary to have stored water available in the soil to supplement inadequate growing season rainfall for economical crop production. Stored water is especially important in the Inland Pacific Northwest of north central Oregon, southeastern Washington, and northern Idaho, where 65% of annual precipitation occurs during the six-month (Sept. 1 to Feb. 28) winter period and 30% during the four-month (March 1 to June 30) growing season. Stored water is also important in the Eastern Idaho Plateau where the low annual precipitation is nearly evenly distributed over the months of the year. The water balance equation tells us that change in water content in the soil - precipitation + inflow - runoff + upward flow - drainage - evapotranspiration (ET). Any cultural practice that decreases runoff or ET can result in increased water in the soil. To store adequate quantities of water, deep soils (> 60 inches) with good infiltration and water holding capacity are required. Summer fallow has long been the traditional practice for storing water in soils for later use by crops. Fallow periods vary from 14 to 15 months where winter small grains are seeded to 21 months where spring small grains are seeded. Water storage efficiency for fallow is low, ranging from 10 to 35% in the Great Plains and the Southwest; to 30-37% in eastern Idaho and northern Utah; to 40-45% of precipitation in the Inland Pacific Northwest (Evans and Lemon, 1957). Good water conservation yields increased crop production, stability of production, and increased water use efficiency. Soil tillage and residue management play significant roles in collection and storage of precipitation in the soil. Our objectives are to discuss insights in water conservation gained under the STEEP (Solutions to Economic and Environmental Problems) program (Oldenstadt et al., 1982) during these last ten years and problems that remain. New research information will be discussed under topics of crop residues, conservation tillage systems, fallow and models