Graduate Thesis Or Dissertation

 

Modeling purple nutsedge (Cyperus rotundus L.) growth : preliminary evaluation of light intensity, soil moisture and herbicide effects Public Deposited

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  • Purple nutsedge [Cyperus rotundas L.) has been recognized as one of the most troublesome perennial weeds of agricultural lands in tropical and some temperate regions. This research sought to determine the effects of timing of herbicides, shading, and soil moisture on plant population growth and tuber production of purple nutsedge through field and greenhouse experiments. The results of these experiments were used to validate a purple nutsedge population matrix model constructed with observed and reported data. Purple nutsedge control options were evaluated with model simulations. Glyphosate reduced shoot number, tuber number, and tuber viability of purple nutsedge, and the herbicide efficacy was higher when applied from 2 to 4 weeks after shoot emergence as compared to the first 2-week growth period. The more effective period for the herbicide coincided with the tuber initiation phase of purple nutsedge growth. Metolachlor caused only temporary suppression of purple nutsedsge. Sunlight intensity by 30%, 47%, 63%, and 90% caused in successively greater reductions in shoot number, tuber number, leaf area, and total dry weight of purple nutsedge. Shading decreased partitioning of plant biomass into tubers and increased partitioning into leaves. These responses remained essentially the same irrespective of timing of shading from early emergence through the first 4 weeks of plant growth. Depletion of available soil moisture from 25% to 75% also reduced the number and dry weights of shoots and tubers produced. However, proportional biomass allocation to shoots, leaves, and tubers and relative growth and net assimilation rates remained unaltered with soil moisture depletion, suggesting that purple nutsedge is fairly well adapted to low soil moisture levels. The importance of intraspecific competition on population regulation of purple nutsedge was evident from model simulations. Model predictions of maximum population size closely agreed with reported plant and tuber densities of purple nutsedge. Model simulations of proportional changes in population size of purple nutsedge, as influenced by soil moisture depletion or shading, also closely followed the field results. Model simulations indicated that seasonal application of herbicides resulting in 90% shoot kill will provide a successful level of control and that herbicide efficiency will be higher when shoots are killed during the second to fourth week of the growing period than from earlier applications. However, model simulations showed that a better strategy than using a highly effective, short duration herbicide is to provide a moderate level of purple nutsedge control extending through the growing season.
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