首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   441篇
  免费   8篇
林业   21篇
农学   3篇
  52篇
综合类   70篇
农作物   7篇
水产渔业   13篇
畜牧兽医   259篇
园艺   5篇
植物保护   19篇
  2022年   3篇
  2021年   3篇
  2020年   4篇
  2019年   5篇
  2018年   3篇
  2017年   9篇
  2016年   6篇
  2015年   4篇
  2013年   9篇
  2012年   10篇
  2011年   24篇
  2010年   10篇
  2009年   7篇
  2008年   18篇
  2007年   27篇
  2006年   16篇
  2005年   15篇
  2004年   16篇
  2003年   10篇
  2002年   13篇
  2001年   9篇
  2000年   23篇
  1999年   11篇
  1998年   4篇
  1997年   4篇
  1993年   4篇
  1992年   8篇
  1991年   11篇
  1990年   8篇
  1989年   5篇
  1988年   11篇
  1987年   13篇
  1986年   12篇
  1985年   4篇
  1984年   8篇
  1983年   5篇
  1981年   3篇
  1980年   3篇
  1979年   4篇
  1978年   5篇
  1977年   3篇
  1975年   5篇
  1973年   3篇
  1972年   3篇
  1971年   5篇
  1970年   8篇
  1969年   6篇
  1968年   3篇
  1927年   2篇
  1885年   2篇
排序方式: 共有449条查询结果,搜索用时 15 毫秒
101.
102.
Landscape ecology belongs to the macroecological spatial studies and its theory focuses on spatial heterogeneity and ecological holism. Landscape rehabilitation based on ecological planning and design can be used satisfactorily to restore degraded mining land to make it productive and reestablish a stable ecological equilibrium that is coordinated with its surroundings in order to attain ecological holism. Degraded mining land can be classified into diverse types, with different rehabilitation methods possible for each of the classifications. In addition, degraded mining lands are also associated with various types of landscape. Accordingly, landscape rehabilitation goals can only be realized by working out reasonable macrolandscape patterns and establishing suitable microecological conditions based on the landscape ecology. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   
103.
104.
Three genetic manipulation strategies are described for enhancing the effectiveness of secondary metabolites of crop plants in reducing pest and disease damage. Targets for gene transfer studies are secondary metabolites of oilseed rape, lepidopteran sex pheromones and drimane anti-feedants. Initial work towards isolation of genes for the biosynthesis of drimane anti-feedants is described.  相似文献   
105.
The accelerating rate of change in urban areas calls for more rapid and cost-effective surveys of derelict land. A review of the use of remote sensing1 for this task reveals that, although aerial photography has been used for urban analysis since the 1940s, it has not been widely adopted for surveys of derelict land, despite demonstrations of its utility. Other forms of remote sensing are similarly shown to have been little-used and to have had patchy success. The hitherto restricted utility of remote sensing is found to be due to a combination of limitations of the forms of imagery available and the methods of analysis employed. Recent changes in available remote sensing imagery, notably the higher resolution now provided by the SPOT satellite, coupled with advances in digital image-processing techniques point to improved opportunities for using remote sensing to monitor derelict land. As a demonstration of these developments, a texture image based on scene variance is derived from a SPOT image of part of north Staffordshire. All derelict and neglected land parcels larger than 1.25 ha are delimited on this image. Remote sensing techniques are not yet ready to supplant detailed ground surveys but they have a growing role to play in monitoring various aspects of land dereliction.  相似文献   
106.
A gas-liquid chromatographic (GLC) method is described for determining residues of Bayer 73 (2-aminoethanol salt of 2',5-dichloro-4'-nitrosalicylanilide) in fish muscle, aquatic invertebrates, mud, and water by analyzing for 2-chloro-4-nitroaniline (CNA), a hydrolysis product of Bayer 73. Bayer 73 residues are extracted from fish muscle tissue, invertebrates, and mud with acetone-formic acid (98+2), and partitioned from water samples with chloroform. After sample cleanup by solvent and acid-base partitioning, the concentrated extract is hydrolyzed with 2N NaOH and H2O2 for 10 min at 95 degrees C. The CNA is then partitioned into hexane-ethyl ether (7+3) and determined by electron capture GLC. Average recoveries were 88% for fish, 82% for invertebrates, 82% for mud, and 98% for water at 3 or more fortification levels.  相似文献   
107.
选用35头处于干奶期40d、泌乳期相近和上胎平均日产奶量20kg的中国荷斯坦奶牛,采用完全随机区组设计分为7组,即:对照组、亚硒酸钠组(7.5mgSe/d、15mgSe/d和22.5mgSe/d)和赛乐硒组(7.5mgSe/d、15mgSe/d和22.5mgSe/d),研究赛乐硒和无机硒对奶牛泌乳性能和乳硒含量的影响。结果表明:在泌乳期30d、60d、90d赛乐硒组较对照组显著提高奶牛产奶量、乳蛋白率和乳干物质含量,有提高乳脂率和乳糖含量的趋势。试验前各组乳硒差异不显著,试验期赛乐硒组较无机硒组、对照组显著提高乳硒含量。  相似文献   
108.
Heterogeneous environments make it difficult to apply consistent selection pressure because often it is difficult to identify a single or a few superior genotypes across all sets of conditions. However, when the target system is characterized by heterogeneity of environmental stress, varieties developed in high-yielding conditions may fail to satisfy farmers’ needs. Although this type of system is often found in marginal environments of developing countries, heterogeneous environmental conditions are also a feature of organic and low-external-input systems in developed countries. To meet the needs of these systems, breeding programs must decentralize selection, and although decentralized selection can be done in formal breeding programs, it is more efficient to involve farmers in the selection and testing of early generation materials. Breeding within these target systems is challenging, both genetically and logistically, but can identify varieties that are adapted to farming systems in marginal environments or that use very few external inputs. A great deal has been published in recent years on the need for local adaptation and participatory plant breeding; this article reviews and synthesizes that literature.  相似文献   
109.
The sensitivity of the normally innervated iris sphincter to its neuro-transmitter, acetylchloline, and to relatd agents varies inversely with the preexisting physiological stimulus background, that is, the environmental light intnsity. This normal variability suggests the existence of a negative feedback mechahnism whereby sensitivity of the effector cell is modutlated by a product of neuronal activity.  相似文献   
110.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号