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   299737篇
  免费   16699篇
  国内免费   686篇
林业   24237篇
农学   14149篇
基础科学   2981篇
  45464篇
综合类   40647篇
农作物   17346篇
水产渔业   17607篇
畜牧兽医   123365篇
园艺   6419篇
植物保护   24907篇
  2019年   3711篇
  2018年   4196篇
  2017年   4637篇
  2016年   4967篇
  2015年   4428篇
  2014年   5779篇
  2013年   16435篇
  2012年   7174篇
  2011年   9251篇
  2010年   7698篇
  2009年   8223篇
  2008年   8633篇
  2007年   7812篇
  2006年   8264篇
  2005年   7642篇
  2004年   7358篇
  2003年   7245篇
  2002年   6581篇
  2001年   7210篇
  2000年   6912篇
  1999年   6208篇
  1998年   4281篇
  1997年   4322篇
  1996年   4046篇
  1995年   4674篇
  1994年   4108篇
  1993年   3824篇
  1992年   5229篇
  1991年   5431篇
  1990年   5223篇
  1989年   5261篇
  1988年   4749篇
  1987年   4819篇
  1986年   4791篇
  1985年   5014篇
  1984年   4498篇
  1983年   4113篇
  1982年   3267篇
  1981年   3126篇
  1980年   3029篇
  1979年   4054篇
  1978年   3521篇
  1977年   3243篇
  1976年   3073篇
  1975年   3081篇
  1974年   3540篇
  1973年   3454篇
  1972年   3327篇
  1971年   3237篇
  1970年   3083篇
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
111.
The contributions presented at the EIFAC Symposium on Fisheries and Society in 2000 are reviewed. There have been considerable advances in many sectors of inland fisheries over the past decade. Nowadays the value of inland fisheries to society is generally better understood and increasingly recognized. However, it was recognized that the allocation of fishery resources and their management could still be made more effective through more extensive application of the recent approaches and techniques discussed during the Symposium and reviewed in the paper.  相似文献   
112.
Abstract. The ability of two nitrogen cycle models, of contrasting complexity, to predict N mineralization from a range of grassland soils in the UK, was evaluated. These were NCYCLE, a simple mass balance model of the N cycle in UK grasslands, and CENTURY, a more complex model simulating long-term C, N, P & S dynamics in grassland ecosystems. The models were tested using field measurements of net N mineralization from a range of grassland soils (differing in soil type, history & management practice), obtained over a 2 year period using a soil core incubation technique. This method was considered to measure the total net release of mineral N from the soil organic matter over a specified time, including N which may have been recycled several times. NCYCLE consistently under-estimated mineralization rates at all sites. By contrast, there was some correlation between CENTURY predictions of net N mineralization and field measurements. This may have reflected the different abilities of the two models to simulate N recycling. Neither model, however, was able to predict adequately the effect of cultivation and reseeding on net N mineralization.  相似文献   
113.
The aim of this study was to assess the PCB pollutionof the South-Western branch of Lake Como by using thezebra mussel (Dreissena polymorpha) as abioindicator since its recent reintroduction. Someenvironmental and physiological variables influencingaccumulation processes of organochlorine compounds inDreissena were investigated because climaticfactors may affect life cycle, reproductive stage andage. PCB levels were highest in the soft tissues ofDreissena close to the industrialized area ofComo (2298 ng g-1 of lipids) and slowly decrease goingnorth, reaching approximately half this value at theclosing section of the lake branch. Nonetheless, thePCB concentrations are well below the dangerous levelsfor the aquatic environment.  相似文献   
114.
Since the International Society of Veterinary Acupuncture (IVAS) was founded in 1974, acupuncture (AP) has received greater acceptance by veterinary professionals throughout the world. This article introduces some important animal diseases that respond well to AP therapy. These include resuscitation of small animals, treatment of anoestrous gilts and sows, bovine reproductive disease, canine vertebral problems and equine backpain, etc. Conventional medicine considers these to be difficult cases to treat. Veterinarians have become more aware of the benefits of AP especially for those diseases, thanks to the efforts of experienced practitioners and scientists, and the many published reports on veterinary AP that have introduced some good indications for AP therapy in veterinary practice. Possible mechanisms behind the effectiveness of AP are discussed. This article aims to introduce veterinarians to good indications for AP to initiate their interest in the practice of AP. Although this is a rapidly expanding field, a long march must begin with one step. We wish this article to be the shoes for such a march. For more information on veterinary AP, contact IVAS , P.O. Box 1478, Longmont, CO.. 80502, USA (http://www.ivas.org), or your national veterinary acupuncture society (http://www.komvet.at/ ivadkom/vapsocs.htm).  相似文献   
115.
116.
Two serological tests (indirect immunofluorescence and enzyme-linked immunosorbent assay) were developed for the detection of fetal antibody to Chlamydia psittaci. Fetal blood and thoracic fluid from 126 field cases of suspected ovine chlamydial abortion were examined using both tests. Placenta and fetal tissues (lung, liver, and kidney) from the same animals were also examined by the following conventional diagnostic methods: isolation in McCoy cells, detection of chlamydial lipopolysaccharide (LPS), modified Ziehl-Nielsen staining, and direct fluorescent antibody staining of chlamydia in frozen cryostat sections. Seventy cases were positive by fetal serology, and of these, 68 were also positive by isolation and/or LPS detection. The remaining 56 cases had negative fetal serology, and of these, 39 were positive by isolation and/or LPS detection. Results indicate that fetal serology, although less sensitive than either isolation in McCoy cells or detection of chlamydial LPS antigen, may be of particular use when placenta is not available.  相似文献   
117.
118.
    
Ohne Zusammenfassung  相似文献   
119.
120.
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