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The problems associated with short growing seasons has led to the development of leafy-reduced stature (LRS) corn hybrids. These hybrids have more leaf area above the ear, more rapid leaf area development, shorter stature, earlier maturity, and better responses to high plant populations and narrow spacings than conventional hybrids. Plants grown in a reduced light environment are limited in carbon assimilation and this, in turn, results in reductions in growth and development. A way to supplement the availability of photosynthate is injection of sucrose into plant stems. The objective of this thesis was to determine the ability of LRS corn plants to compete with weeds, and the reactions of weed species to the shade, including the relationships between weed growth (increase in biomass) and development (shape) under shaded conditions. Three years of field experiments (LRS and more conventional corn hybrids with both transplanted and naturally growing weeds) and two years of greenhouse work [weeds alone, C3 (lamb's quarters and velvetleaf) and C4 (redroot pigweed) in full sun or deep (75%) shade injected with 15% sucrose or not] were conducted. Yield reductions due to weed pressure were lower for LRS than other hybrids. Biomass production by both transplanted and naturally occurring weeds was up to 85% less under corn canopies than when grown without competition from corn. The biomass of C4 weeds was more reduced by competition with corn plants than that of C3 weeds. In spite of quick and early leaf development, leaves and other plant parts of LRS were not damaged excessively by mechanical (rotary hoeing) weed control. Both C3 and C4 weed plants produced more dry matter when injected with sucrose. Dry weights of sucrose injected shaded plants were not different from full sun uninjected plants. However, sucrose injection did not alter shading effects on development (distribution of biomass). Dry matter production and photosynthetic rates of C4 weeds were more reduced

Dwarf apple tree growth and yield under several non-herbicidal ground management systems were compared. Six ground cover systems were established in two newly planted orchards. After two growing seasons, trees in the straw mulch and geotextile mulch treatments had larger trunk cross-sectional areas and more shoot growth than trees in the red fescue and insectary plant cover treatments. Growth of trees under the manure mulch and cultivation treatments was intermediate. When four of the systems were established along rows of five-year old trees in an existing orchard, the increase in trunk cross-sectional area over two growing seasons was greatest for trees in the manure mulch and straw mulch followed by tose in geotextile mulch, trees in the red fescue grew least. The use of a mulch, such as straw or geotextile, has a favourable effect on growth and has potential for use in organic orchards.

Control of Taraxacum officinale (dandelion) and other broadleaf weeds in turfgrass has been readily achieved with phenoxy herbicides, but the herbicide option has been revoked in many regions, necessitating alternative weed control strategies. One biological alternative is Sclerotinia minor, an Ascomycete fungus. The impact of S. minor on broadleaf weed dynamics and biotic interactions were studied in a turfgrass environment. The goal was to maximize effectiveness of a S. minor formulation as a biocontrol of dandelion using an ecological approach. S. minor efficacy was not affected by turf microenvironments and was similarly efficacious with spring or fall application. All accessions from a worldwide collection of dandelion and 32 turfgrass broadleaf species were susceptible to S. minor. Biocontrol efficacy was inversely correlated with dandelion age, but efficacy on all ages was enhanced in the presence of grass competition. When combined with regular mowing at 7-10 cm, the S. minor suppressive effect on dandelion was similar to the herbicide effect, particularly in the following season. Weed suppression was less with close mowing at 3-5 cm due to increased dandelion seedling recruitment. While spring herbicide application was effective to suppress dandelion population, the S. minor treatment has no residual activity, necessitating a second application to suppress seedling recruits. Root regrowth after S. minor infection was minimal and was further reduced in superior quality turf after season-long mowing, and after spring applications. S. minor infected dandelion seeds, reduced the dandelion seedbank, and reduced dandelion seedling emergence by 98%. S. minor did not affect the emergence or the total biomass of cool season temperate turfgrass species. Turfgrass quality was improved following S. minor application and populations of other broadleaf weeds were also controlled by S. minor. Understanding the biotic interactions within the turfgrass environment has rewardingly lead to successful integration of the S.minor biocontrol with the common management tools of mowing and over-seeding to achieve excellent control of dandelion and a healthy thriving turf.

Field trials were conducted to evaluate the effect of quizalofop-ethyl on quackgrass plants in a potato cropping sequence. Fall and summer applications were compared for their quackgrass control potential. Season-long quackgrass control was obtained with quizalofop-ethyl at 96 g/ha following summer application. An increase in the rate of quizalofop-ethyl did not further improve control. Yields with quizalofop-ethyl at 96 g/ha were similar to standard treatments sethoxydim and fluazifop-butyl at recommended rates. Quackgrass control following a summer application was not maintained through to the following season. Fall applications did not result in adequate control of quackgrass the following season at any of the quizalofop-ethyl rates tested.

Lettuce plants (Lactuca sativa L. var. Ithaca) were transplanted on different beige and black paper mulches and on a coextruded white/black polyethylene mulch in organic soil in 1997 and 1998. Non-weeded and weeded control plots were also used. Plant mortality in 1997 for plants grown on bare soil was 10 to 15% greater than that of mulched plants. Head weight was found to be 3.6 times (1997) and 12 times (1998) greater for lettuce grown on mulch compared with a weeded control. The transmission, reflection and absorption of light of the white/black polyethylene mulch and black paper mulch remained stable over the course of the experiment while those of the other papers changed over time. Paper mulch decomposition occurred during the experiment. Although, black paper mulch was the only mulch that gave complete control of weed growth, weed populations present under the beige papers were too small in terms of the size and number of weeds to cause lettuce yield reduction.

Bibliography: leaves 209-254. / xv, 254 leaves : ill. (chiefly col.) ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Experiments detailed in this thesis, evaluate the impact of various herbicides and herbicide mixtures on the growth, nodulation and nitrogen fixation of annual pasture legumes, the efficacy of the herbicides for weed control, and the potential role of mechanical defoliation to replace herbicides, leading to lower cost and more sustainable farming systems. / Thesis (Ph.D.)--University of Adelaide, Dept. of Plant Science, 1996

Examensarbete. / Delrapport i FoU-projekt S 01-3284/08 "Skötsel av vegetation inom banområde"

Thesis--Virginia Polytechnic Institute and State University. / Also available via the Internet.

Kenaf (Hibiscus cannabinus L.) was introduced in 2005 as a fibre crop on a commercial scale in the KwaZulu-Natal Province of South Africa. No herbicides have yet been registered for use in this crop. The purpose of this study was to determine the tolerance of kenaf to a total of five pre-emergence and four post-emergence herbicides under semi- and fully controlled conditions. The herbicides were chosen based on their potential safety for use in Hibiscus spp. as well as on the weed spectra they are registered for in other crops. Several additional factors were also taken into consideration, such as: temperature, soil depth and timing of herbicide application. Four pot trials were conducted to determine the separate and combined effects of herbicide, temperature, planting depth and application timing. During the first trial the effects of five pre-emergence herbicides and four post-emergence herbicides were researched. The pre-emergence herbicides were: S-dimethenamid, imazethapyr, fluometuron/prometryn, pendimethalin, S- metolachlor and the post-emergence herbicides were: bentazone, 2,4-DB, monosodium methanearsonate and pyrithiobac sodium. The trials were conducted under either semi-controlled conditions in a glasshouse or in growth cabinets under fully controlled conditions at the Hatfield Experimental Farm of the University of Pretoria. All experiments were conducted with a Hutton soil with 22% clay. Each trial lasted about 40 days to allow for maximum phytotoxicity damage manifestation on the kenaf seedlings. Measurements that were taken included plant height, herbicide damage, weed control efficiency, fresh plant weight, dried plant weight, and dried root weight. The data were subjected to analysis of variance (ANOVA) to determine the statistical likelihood of damage to plants from the herbicides. In Trial 2, 3 and 4 the interaction effects of herbicide and plant depth, herbicide and temperature, and herbicide and application timing were researched respectively. Neither planting depth nor application timing affected the kenaf seedlings negatively, but low temperature in combination with the application of herbicides during germination of seed and seedling emergence had serious deleterious effects on the young kenaf seedlings. Based on the findings the majority of the herbicides can be included in further field trials on Hibiscus cannabinus L. with the exception of S-dimethenamid and fluometuron/prometryn which caused substantial injury to the kenaf seedlings. Copyright / Dissertation (MInstAgrar)--University of Pretoria, 2011. / Plant Production and Soil Science / unrestricted