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11.
Anoda cristata is a troublesome annual broad-leaved weed in summer crops in the rolling Pampa in Argentina; seeds are the only source of regeneration of this species. Seed persistence or depletion is the result of survival and loss processes, including predation. The objective of this study was to determine survival at two burial depths in undisturbed soil and predation rates of A. cristata seeds in soyabean crops in different rotations and tillage systems. Survival was discontinuous and decreased to 25% after 35 months, after which no further reduction in survival was observed to the end of the experiment at 96 months. No differences in seed survival between seeds placed on the soil surface and buried 5 cm below the soil surface were found at 80 months, but at later times survival was lower for seeds placed on the soil surface. Predation rates ranged between 0.3% day−1 and 6.7% day−1. Of the models tested, a polynomial regression of the rate of predation with time gave the best representation of seed predation. From January to July, predation was higher in non-tillage plots in the wheat/soyabean rotation. There was no significant difference in predation rates between tillage systems in the soyabean monoculture and no difference between planting densities. Higher crop residue levels in non-tillage plots in the wheat/soyabean rotation was the dominant factor influencing seed predation, probably because such habitat favours the presence of seed predators. 相似文献
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合理调整我国人民的食物结构是提高全民族身体素质,推进社会文明进步,具有深远的战略意义的一项国策。调整国民食物结构是一个科学性,政策性很强,社会涉及面广,地区人群民食习惯差别大的极为复杂的动态系统工程。作者针对山西省情实际,从追朔我国饮食传统,膳食民习,食物资源及演变趋向探求,提出合理调整山西人民食物结构的策略与途径。 相似文献
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采用颍次分布X~2检验、Iwao平均拥挤度m~*=a+βx和Taylor幂法则S~2=am~b等8种方法,分析了七块麦田中七星瓢虫幼虫的空间分布型,认为其分布型为负二项分布,且聚集的原因是由于个体之间存在着相互吸引,并确定了代表性较强的棋盘式抽样技术,最佳抽样数量为:Q=28.7088/+0.7552。 相似文献
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拓扑简化可以为渐进网格带来更好的效果,但目前几乎所有的渐进网格方法都不支持拓扑简化,而且edge collapse和vertex split操作有时会产生非流形.针对这些问题,提出了基于图形旋转系统的渐进网格法.以基于图形旋转系统的数据结构和操作实现了渐进网格,用图形旋转系统的操作集合构建了edge collapse和vertex split操作.在此基础上,通过扩展新操作即可进行任意拓扑变化,从而实现拓扑简化, 相似文献
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Field experiments were conducted to characterize the demography of Abutilon theophrasti and Setaria faberi in a conventionally managed 2‐year (maize/soya bean) rotation, and in 3‐year (maize/soya bean/triticale + red clover) and 4‐year (maize/soya bean/triticale + lucerne/lucerne) rotations managed with 72% and 79% lower herbicide inputs respectively. Rates of weed seedling recruitment, seedling survival and adult plant fecundity were determined for populations in each phase of each rotation and used to calculate annual rates of weed population change, Δ. In both years of the study, Δ for A. theophrasti populations declined or remained stable in all three rotation systems. Despite greater rates of seedling survival and fecundity in maize and soya bean in the 3‐ and 4‐year rotations, increases in Δ for A. theophrasti populations were prevented in these systems because of low fecundity in triticale and low seedling survival and fecundity in lucerne. For Setaria faberi populations, Δ remained stable in the 2‐year rotation, increased in the 3‐year rotation in both years, and increased in the 4‐year rotation in 1 year. The results of this study indicate that when herbicide use is reduced, rotations that include triticale and lucerne can facilitate the suppression of A. theophrasti. Rotations that include lucerne can contribute to restraining S. faberi population growth, given adequate levels of seedling mortality in this crop. 相似文献
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论述宁夏农区耕地轮作牧草发展牧草种子业、草产业和生产绿肥的可行性,自然条件、利益、市场机遇,并推荐了适应宁夏地区种植的优良牧草种。 相似文献