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   7081篇
  免费   330篇
  国内免费   6篇
林业   487篇
农学   148篇
基础科学   36篇
  1316篇
综合类   1224篇
农作物   228篇
水产渔业   559篇
畜牧兽医   2716篇
园艺   197篇
植物保护   506篇
  2023年   59篇
  2022年   75篇
  2021年   147篇
  2020年   151篇
  2019年   179篇
  2018年   181篇
  2017年   151篇
  2016年   177篇
  2015年   162篇
  2014年   185篇
  2013年   264篇
  2012年   476篇
  2011年   601篇
  2010年   296篇
  2009年   241篇
  2008年   494篇
  2007年   521篇
  2006年   466篇
  2005年   436篇
  2004年   413篇
  2003年   381篇
  2002年   387篇
  2001年   70篇
  2000年   51篇
  1999年   65篇
  1998年   51篇
  1997年   50篇
  1996年   44篇
  1995年   49篇
  1994年   43篇
  1993年   43篇
  1992年   30篇
  1991年   34篇
  1990年   32篇
  1989年   34篇
  1988年   28篇
  1987年   26篇
  1986年   24篇
  1985年   35篇
  1984年   26篇
  1983年   27篇
  1982年   20篇
  1981年   15篇
  1980年   23篇
  1979年   23篇
  1978年   14篇
  1977年   15篇
  1975年   11篇
  1974年   11篇
  1973年   17篇
排序方式: 共有7417条查询结果,搜索用时 15 毫秒
991.
Prion diseases such as scrapie in small ruminants, bovine spongiform encephalopathy (BSE) in cattle and Creutzfeldt-Jakob disease (CJD) in man, are fatal neurodegenerative disorders. These diseases result from the accumulation of misfolded conformers of the host-encoded prion protein (PrP) in the central nervous system. To date naturally-occurring PrP free animals have not been reported. Here we describe healthy non-transgenic animals, Norwegian Dairy Goats, lacking prion protein due to a nonsense mutation early in the gene. These animals are predicted to be resistant to prion disease and will be valuable for research and for production of prion-free products.  相似文献   
992.
Bartonella vinsonii subsp. berkhoffii is a recognized cause of endocarditis in dogs and human patients and has been associated with cardiac arrhythmias, myocarditis, granulomatous lymphadenitis, polyarthritis, and granulomatous rhinitis in dogs. Little is known regarding the mode of transmission or cellular localization of this bacteria following infection of a canine host. The aim of the current study was to determine whether erythrocytes may serve as a site of infection by B. vinsonii subsp. berkhoffii. In the study, we successfully demonstrate the invasion of canine erythrocytes by a B. vinsonii subsp. berkhoffii genotype III strain using an in vitro model system. Dog erythrocytes were incubated with B. vinsonii subsp. berkhoffii after which tubes were treated with gentamicin at 12, 24, and 48 h post-inoculation. After gentamicin elimination of extracellular bacteria, there was a gradual increase in intra-erythrocytic bacteria, as assessed by colony forming units per ml, at each collection time point. The largest recovery of intracellular bacteria occurred at 48 h post-infection. These results suggest that canine erythrocytes may serve in the maintenance of bacteremia due to B. vinsonii subsp. berkhoffii within an infected host.  相似文献   
993.
The suspensory ligament is difficult to image accurately, partly because it contains ligamentous fibers, as well as noncollagenous adipose and muscle tissue in the normal horse. Our hypothesis was that magnetic resonance (MR) imaging would be more accurate than ultrasonography in identifying the size of the suspensory ligament and the presence and size of noncollagenous tissues within the ligament. Eleven horses were used for ultrasonographic and MR imaging and histologic evaluation of the rear suspensory ligament. The origin and body of the normal suspensory ligament had a heterogenous appearance on MR images with two separate islands of mixed signal intensity evident throughout its otherwise hypointense cross-sectional area. Histologically, there were isolated islands of muscle, adipose, loose connective tissue and dense collagenous partitions, organized in two separate bundles that extended through the full length of the suspensory ligament origin and body to the level of its bifurcation. Comparison of MR images with corresponding histologic sections confirmed that islands of heterogenous signal intensity in normal suspensory ligaments correlated well with these bundles. Using ultrasonography, it was impossible to distinguish these islands from surrounding dense collagenous tissue consistently. MR imaging determined the cross-sectional area of the suspensory ligament more accurately than ultrasonography. Based upon these results, MR imaging is superior to ultrasonography for assessment of the suspensory ligament. The appearance associated with normal ligament anatomy needs to be understood before MR signal variation can be considered as indicative of disease in the suspensory ligament.  相似文献   
994.
The future of radiography will be digital. In exotic pet radiography, where some of the animals have a very low body weight and anatomic structures can be small, detail rendition plays an important feature in image quality. Veterinarians should be familiar with the technical principles, image quality criteria, and radiation exposure issues associated with the various types of digital systems currently available. This article discusses basic principles of digital radiography, technical solutions, and selected parameters characterizing detectors, processing, and monitors. An overview of reported experiences is given, and results from experimental clinical studies are reviewed to evaluate the current options and limitations in applying digital radiography to exotic pet medicine.  相似文献   
995.
ABSTRACT: Genetic resources available for Arabidopsis thaliana make this species particularly attractive as a model for molecular genetic studies of guard cell homeostasis, transport and signalling, but this facility is not matched by accessible tools for quantitative analysis of transport in the intact cell. We have developed a reliable set of procedures for voltage clamp analysis of guard cells from Arabidopsis leaves. These procedures greatly simplify electrophysiological recordings, extending the duration of measurements and scope for analysis of the predominant K+ and anion channels to that achieved previously in work with Vicia and tobacco guard cells.  相似文献   
996.
Brewers grains, a by-product of beer production, are often used as a livestock feed. Because brewers grains provide protein, fiber, and energy, they can be useful in a variety of diets. Protein in brewers grains can meet a significant portion of supplemental protein requirements; in addition, they provide fiber and needed bulk in the diets of ruminants and horses. Brewers grains and other brewers by-products have also been fed to pigs, sheep, and poultry. Currently, the primary market for wet brewers grains is as a dairy cattle feed; however, some may be fed to beef cattle in feedlots. Brewers grains have historically been marketed wet or dry, but wet brewers grains currently make up the majority of the marketed product. Brewers grains provide protein, energy, and fiber in livestock diets, but product variability can influence their utilization and necessitate a testing program to determine nutrient content.  相似文献   
997.
In birds of prey, vascular disease can be caused by a number of infectious or noninfectious agents and is in all likelihood underdiagnosed. This review attempts to provide a discussion of the available literature regarding vascular diseases in raptors by describing a number of conditions or etiological agents (e.g., atherosclerosis, aneurysms, bacterial and septicemic conditions, thromboembolic disease, and viral, fungal, parasitic, and miscellaneous diseases) and the vascular pathology that may occur with each.  相似文献   
998.
最近,在恶劣的天气条件下,北京和中、东部地区很多城市都经历了严重的空气污染。数个空气质量监测点都出现PM2.5爆表的情况。这种前所未有的严重污染引发了国内外媒体的广泛关注和报道,很多市民也纷纷通过微博表  相似文献   
999.
1000.
Crop protection seldom takes into account soil heterogeneity at the field scale. Yet, variable site characteristics affect the incidence of pests as well as the efficacy and fate of pesticides in soil. This article reviews crucial starting points for incorporating soil information into precision crop protection (PCP). At present, the lack of adequate field maps is a major drawback. Conventional soil analyses are too expensive to capture soil heterogeneity at the field scale with the required spatial resolution. Therefore, we discuss alternative procedures exemplified by our own results concerning (i) minimally and non-invasive sensor techniques for the estimation of soil properties, (ii) the evidence of soil heterogeneity with respect to PCP, and (iii) current possibilities for incorporation of high resolution soil information into crop protection decisions. Soil organic carbon (SOC) and soil texture are extremely interesting for PCP. Their determination with minimally invasive techniques requires the sampling of soils, because the sensors must be used in the laboratory. However, this technique delivers precise information at low cost. We accurately determined SOC in the near-infrared. In the mid-infrared, texture and lime content were also exactly quantified. Non-invasive sensors require less effort. The airborne HyMap sensor was suitable for the detection of variability in SOC at high resolution, thus promising further progress regarding SOC data acquisition from bare soil. The apparent electrical conductivity as measured by an EM38 sensor was shown to be a suitable proxy for soil texture and layering. A survey of arable fields near Bonn (Germany) revealed widespread within-field heterogeneity of texture-related ECa, SOC and other characteristics. Maps of herbicide sorption and application rate were derived from sensor data, showing that optimal herbicide dosage is strongly governed by soil variability. A phytoassay with isoproturon confirmed the reliability of spatially varied herbicide application rates. Mapping areas with an enhanced leaching risk within fields allows them to be kept free of pesticides with related regulatory restrictions. We conclude that the use of information on soil heterogeneity within the concept of PCP is beneficial, both economically and ecologically.  相似文献   
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