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Broad-leaved weeds have generally been dismissed as less competitive than grass weeds in oilseed rape and therefore their competitive effect on the crop has been ignored. This has limited the development of cost-effective strategies for their control. A survey identified the main weeds of winter oilseed rape in north-east Scotland as chickweed, annual meadowgrass, volunteer barley, mayweed spp., field pansy and shepherd's purse. Of these, only the effect of volunteer barley has been previously researched. Field trials over five years showed that the effects of broad-leaved weeds on the growth and yield of oilseed rape varied between seasons, but clearly demonstrated that these weeds can be very competitive and can cause substantial yield losses. The start of crop stem extension in spring was identified as the critical time for broad-leaved weed competition in oilseed rape. Unless the crop is free of weed competition at this time or is sufficiently vigorous to outgrow weeds, yield losses will result. However, autumn proved to be the optimum time to control broad-leaved weeds and early treatments were found to be most efficacious. The variation in crop response between seasons was largely attributable to seasonal weather and its effects on crop vigour and weed growth. Seasonal weather also influenced the efficacy of reduced doses of herbicides. Metazachlor gave good weed control at reduced doses in vigorous crops, but not in less vigorous crops in poor growing seasons. This variability makes it difficult to predict crops responses to weed competition and the justification for herbicides. Thus a semi-prophylactic approach is proposed, based on a reduced dose of metazachlor applied early with the option of a reduced dose follow-up post-emergence treatment in autumn, if necessary. This is a practical approach which could be adopted on many farms and would benefit the farmer in terms of improved cost-effectiveness and go some way towards satisfying the demand for reduced pesticide inputs.

This research programme sought to establish whether crop varieties had different competitive abilities against weeds and what the characteristics of competitive cultivars might be. A series of experiments studied the relative competitiveness with fat hen (<I>Chenopodium album</I>) of four spring wheat varieties Alexandria, Tonic, Canon and Baldus selected to represent different heights, tillering potentials and leaf angles. Additional experiments examined aspects of seed production, seed germination and seedling establishment of fat hen. In the glasshouse, all the wheat varieties were of similar competitive ability and no plant character was related to crop aggressivity index. In field experiments, the variety ranking of aggressiveness against fat hen was Alexandria > Tonic > Canon > Baldus. Crop aggressivity was positively related to crop plant height, leaf area and tillers per plant when grown in mixture with fat hen, and negatively related to fat hen height, leaf area and dry matter per plant. Physiological attributes such as absolute and relative growth rates, net assimilation rate, specific leaf area and leaf area ratio of wheat were rarely related to crop aggressivity at any single harvest but changed with time. Relationships of aggressivity with wheat characters measured in monocultures were generally poor. Crop aggressivity was positively related to relative loss of weed dry matter, and negatively related to relative loss of wheat dry matter and grain yield. The most competitive variety was associated with the lowest weed seed production and seed return to the soil. Wheat varieties were less competitive when fat hen emerged 15 days earlier but were highly competitive when fat hen emerged 15 days later. Light interception by crop canopies in mixture was positively related to crop aggressivity. It is suggested that plant breeders should use greater height, greater leaf area, more droopy leaves and greater number of tillers as selection criteria for competitive wheat cultivars.

Laboratory experiments were conducted to characterise the germination of some tropical weed seeds. Field experiments in Sri Lanka characterised the seed bank and monitored seedling emergence after sugarcane planting. In laboratory experiments, final germination, median germination time and rate of germination were determined on a Temperature Gradient Plate and used to define values for optimum temperature (T_o), base temperature (T_b), and upper temperature limit for germination (T_m). Final germination, median germination time and rate of germination were also evaluated to study germination response to water availability. Field studies were conducted in Sri Lanka to estimate the seed rain and the bank and also to monitor seedling emergence after sugarcane planting. Taylor's Power Law (TPL) parameters were used to describe the spatial distribution of seeds in the seed bank. The Principal Components Analyses (PCA) was used to summarise the variation between samples and species. The Shannon-Weaver diversity index was used to estimate species diversity in the seed bank. Chi-square analyses (x²) were performed to assess the species similarity of the seed bank and the emerged vegetation. Seedling emergence in the field was examined for relationships with rainfall, soil moisture and soil temperatures. The seed rain fluctuated with time. The higher seed rain in fallow land increased the density of the seed bank particularly near the surface. Ploughing substantially reduced the seed bank (by about 80%) and mixed seeds more evenly through the ploughed layer. The majority of seedlings (98%) emerged from the top 4.0cm of soil. Hot and humid conditions favoured the germination and emergence of weed seedlings. This was seen not just in comparison of plantings in October 1995, January 1996 and April 1996 but also more seedlings emerged in the furrows than on ridges. There was no similarity of the species composition of the seed bank and the seedling population arising. Also, the abundance of common species in the seed bank and emerged vegetation was poorly correlated. There was a little relationship between seedling emergence and soil temperature in this investigation.

Graduation date: 1981

Thesis (M.S.)--University of Wisconsin--Madison, 1952. / Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 25-26).

Thesis (Ph. D.)--University of Wisconsin--Madison, 1984. / Typescript. Vita. Includes bibliographies.

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84 pp. / First Edition Published 2001 / The noxious weed problem in the western United States has been described as, a biological forest fire racing beyond control because no one wants to be fire boss. Indeed, when small weed infestations are left unchecked, they can grow exponentially and spread across the land much like a slow-moving biological wildfire. However, land consumed by fire usually recovers and is often more productive than before the fire occurred. On the other hand, land consumed by noxious weeds may be irreversibly changed and never again reach its full biological potential. Reviewed 12/2016, First Edition Published 2001

Exotic invasive aquatic weeds such as alligator weed (Alternanthera philoxeroides) threaten native ecosystems by interfering with native plant communities, disrupting hydrology, and diminishing water quality. Development of new tools to combat invaders is important for the well being of these sensitive areas. Integrated pest management offers managers an approach that combines multiple control methods for better control than any one method used exclusively. In a greenhouse and field study, we tested the effects of selective herbicide application frequency, native competitor plant introduction, and their integration on alligator weed. In the greenhouse study, alligator weed shoot, root, and total biomass were reduced with one herbicide application, and further reduced with two. Alligator weed cumulative stem length and shoot/root ratio was only reduced after two applications. In the greenhouse, introduction of competitors did not affect alligator weed biomass, but did affect shoot/root ratio. The interaction of competitor introduction and herbicide did not significantly influence alligator weed growth in the greenhouse study. In the field, alligator weed cover was reduced after one herbicide application, but not significantly more after a second. Introduction of competitor species had no effect on alligator weed cover, nor did the interaction of competitor species and herbicide application. This study demonstrates that triclopyr amine herbicide can reduce alligator weed biomass and cover, and that two applications are more effective than one. To integrate selective herbicides and native plant introduction successfully for alligator weed control, more research is needed on the influence competition can potentially have on alligator weed growth, and the timing of herbicide application and subsequent introduction of plants.