Graduate Thesis Or Dissertation
 

Seed yield and yield components of Pennfine perennial ryegrass (Lolium perenne L.) as influenced by time and rate of spring nitrogen and chemical dwarfing

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  • Seed yield in perennial ryegrass is the product of yield components that develop during the life of the plant. Crop yield potential is defined by the number of fertile tillers, spikelets per spike, and florets per spikelet. It has been shown that perennial ryegrass realizes only a small percentage of the potential as harvested seed, and is an inherently poor seed producer as cultivars have been selected primarily for vegetative production or desirable turf characteristics. The Willamette Valley of Oregon produces nearly all of the perennial ryegrass seed grown in the United States. Environmental conditions during plant growth control potential yield development and the efficiency with which it is used. To increase the efficiency of seed production, it is important to understand the effects specialized cultural management techniques have in this producing region. In these studies on perennial ryegrass, the effects of varying the amount and time of spring applied nitrogen (N), and the effects of the plant growth retardant paclobutrazol were investigated under field conditions using the cultivar Pennfine. In addition, the influence of high levels of early spring N, combined with growth retardant use was investigated. Increased tiller densities, the result of higher N level, can result in a greater seed yield potential. However, as tiller densities become greater, the components of seed yield on individual tillers become smaller. In addition, earlier and more severe lodging result in greater tiller mortality and harvest dry weight losses. Split spring applications totaling 120 to 150 kg N ha⁻¹ appeared to provide a better balance between compensating yield components, while maintaining an economic seed yield. Spring application of paclobutrazol delayed the onset and severity of lodging, resulting in an increased number of fertile tillers in the stand, and a greater number of potential seed sites per unit area. Earlier dates of application had a greater effect in reducing lodging in 1983, but no difference was observed between two spring growth stages in 1984. Reduced lodging appears to enhance seed set, resulting in a greater number of seeds recovered at harvest and a higher floret site utilization (FSU). Harvest index was also increased with paclobutrazol. Application of paclobutrazol significantly increased seed yield, a result of improved seed recovery due to more seeds per spike. However, the increased yield potential resulting from higher N rate was not recovered by application of paclobutrazol where severe lodging conditions occurred prior to the completion of seed filling. Under more normal environmental conditions, 120 kg N ha⁻¹, followed with a growth retardant application, had a significantly greater seed yield than the other treatments.
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