Italian ryegrass (Lolium multiflorum Lam.) is a serious and widespread weed problem in grain fields of western Oregon and Washington. Field studies were conducted to measure effects of Italian ryegrass interference on winter wheat (Triticum aestivum L.) yields. Wheat cultivars used were ‘Nugaines,’ ‘Druchamp,’ ‘Hyslop,’ and ‘Yamhill.’ Ryegrass densities ranging from 0 to approximately 100 plants/m2 were established by hand distribution on the soil surface, followed by harrowing. Urea fertilizer was applied in the spring at 56, 112, and 168 kg of N/ha. As ryegrass densities increased, wheat grain yields decreased. In one experiment, wheat grain yield was reduced by 4,100 kg/ha when the ryegrass density was increased from 0.7 to 93 plants/m2. Interference caused greater percentage reductions in wheat yields at 168 kg N/ha than at 56 kg N/ha when ryegrass densities were high. The percentage of yield reduction tended to be higher in short wheat cultivars, Nugaines and Hyslop, than in tall cultivars, Druchamp and Yamhill. When no weed control measures were used, ryegrass infestations in the 1970 crop caused wheat yield reductions in the 1971 crop. This was due to higher ryegrass densities in 1971 in plots where the heaviest ryegrass densities occurred in 1970. When ryegrass was controlled with diuron [3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea] in 1971, no carry‐over effects from previous ryegrass infestations could be measured. Results obtained in these studies illustrate the importance of good ryegrass control in winter wheat management systems.

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In 1987, two applications of the methyl ester of diclofop failed to effectively control Italian ryegrass in some growers' fields. As a result of these observations, studies were initiated to determine if the surviving ryegrass is more tolerant to diclofop than previously untreated ryegrass. Growth chamber studies on five accessions of ryegrass demonstrated that collections from two growers' fields were highly tolerant of diclofop. Diclofop GR50(rate required to reduce shoot weight by 50%) values of accessions A and B and commercial ryegrass, when diclofop was applied to seedlings at the two-leaf stage, were 2.9, 3.4, and 0.0056 kg ai/ha, respectively. Diclofop-tolerant biotypes were not more tolerant to postemergence barban or preemergence-incorporated triallate plus postemergence diuron or metribuzin. These herbicides are possible alternative treatments in fields where diclofop is no longer effective.

Dinitroaniline herbicides are absorbed readily by roots and emerging shoots, but shoot exposure is more phytotoxic. Translocation within the plant varies by specific herbicide but commonly is minor. Dinitroaniline herbicides injure plants by binding to tubulin, a dimer protein in the ceil that polymerizes to form microtubules (MTs). MTs form the major part of the mitotic apparatus, including spindle fibers, which enable chromosomes to separate during cell division. Dinitroaniline herbicides prevent tubulin from polymerizing into MTs, thus arresting mitosis. This leads to abnormal cells with more than the normal complement of chromosomes and, frequently, lobed nuclei. MTs also are responsible for orienting cell wall microfibrils in such a way that they prevent lateral enlargement of cells. Treatment with dinitroaniline herbicides leads to disorientation of the microfibrils, leading to one of the common symptoms—spherical cells instead of rectangular ones. Studies on the metabolism of trifluralin in plants have shown that amination, dealkylation, and cyclization all can occur. However, metabolites often amount to a small percentage of the original herbicide. In general, trifluralin seems quite stable within the plant.

Summary. Significant stimulation of root or shoot growth of oats (Avena sativa L.) resulted from pre‐emergence applications of eleven herbicides at sublethal doses. Two additional herbicides stimulated cucumber (Cucumis sativus L.) root growth. Several types of herbicides with different mechanisms of lethal action were represented. Three herbicides failed to stimulate plant growth significantly in these studies, although all of them have been observed to cause growth stimulation in other experiments. Studies on oat seedling respiration, photosynthesis, protein content, free amino acid content, and total available soluble carbohydrates failed to provide any consistent explanation for stimulation. These results provide further evidence of the validity of the Arndt‐Schulz hypothesis which states that all poisons are stimulatory at sublethal concentrations. Stimulation de la croissance des plantes par des concentrations sublétales d'herbiddes Résumé. Une stimulation significative de la croissance des racines ou des tiges de I'avoine (Avena sativa L.) a été provoquée par l'application en pré‐levée de onze herbicides à des doses sublétaies. Deux autres herbicides ont stimulé la croissance des racines de concombre. Plusieurs types d'herbicides possédant des mécanismes diffrénts d'action létale étaient représentés. Trois herbicides n'ont pas provoqué de stimulation significative de la croissance au cours de cette étude, bien qu'ils aient provoqué une stimulation dans d'autres expériences. Des recherches sur la respiration, la photosynthése, la teneur en protéines, en amino‐acides libres et le total disponible des hydrates de carbone solubles dans la plantule d'avoine n'ont fourni aucune explication logique de la stimulation. Ces résultats apportent une nouvelle preuve de la validité de l'hypothése d'Arndt‐Schulz selon laquelle tous les poisons ont un effet stimulant à des concentrations sublétaies. Förderung des Pflanzenwachstums durch Herbizide in sublethalen Konzentrationen Zusammenfassung. Die Vorauflaufanwendung von elf Herbiziden in sublethalen Dosen förderte das Wurzel‐ und Sprosswachstum von Hafer (Avena sativa L.) signifikant. Zwei weitere Herbizide stimulierten das Wurzelwachstum bei der Gurke (Cucumis sativus L.). Es waren Herbizide aus verschiedenen Gruppen mit unterschiedlichem Mechanismus der lethalen Wirkung vertreten. Drei Herbizide förderten in diesen Studien das Wachstum nicht signifikant, obwohl eine solche fördernde Wirkung in anderen Experimenten beobachtet worden war. Untersuchungen der Atmung von Haferkeimlingen, der Photosynthese, des Proteingehaltes, des Gehaltes an freien Aminosäuren und der Gesamtmenge an löslichen Kohlenhydraten erbrachten keine einheitliche Erklärung für die Förderung. Diese Ergebnisse stützen die Gültigkeit der Arndt‐Schulz Hypothese, die besagt, dass alle Gifte in sublethalen Dosen stimulierend wirken.