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   94249篇
  免费   8217篇
  国内免费   43篇
林业   3832篇
农学   3800篇
基础科学   632篇
  12251篇
综合类   13435篇
农作物   3354篇
水产渔业   6950篇
畜牧兽医   50499篇
园艺   1161篇
植物保护   6595篇
  2021年   718篇
  2020年   1158篇
  2019年   1861篇
  2018年   2247篇
  2017年   2519篇
  2016年   2357篇
  2015年   2022篇
  2014年   2304篇
  2013年   4429篇
  2012年   3281篇
  2011年   3923篇
  2010年   2802篇
  2009年   2516篇
  2008年   3634篇
  2007年   3255篇
  2006年   3211篇
  2005年   2733篇
  2004年   2644篇
  2003年   2654篇
  2002年   2448篇
  2001年   3448篇
  2000年   3390篇
  1999年   2559篇
  1998年   983篇
  1997年   1022篇
  1996年   865篇
  1995年   1022篇
  1994年   892篇
  1993年   894篇
  1992年   1844篇
  1991年   1920篇
  1990年   1856篇
  1989年   1856篇
  1988年   1607篇
  1987年   1594篇
  1986年   1627篇
  1985年   1527篇
  1984年   1226篇
  1983年   1033篇
  1982年   716篇
  1979年   1061篇
  1978年   820篇
  1975年   818篇
  1974年   899篇
  1973年   864篇
  1972年   806篇
  1971年   795篇
  1970年   803篇
  1969年   780篇
  1967年   734篇
排序方式: 共有10000条查询结果,搜索用时 16 毫秒
61.
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   
62.
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   
63.
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   
64.
Subgroup-specific peptide-based enzyme-linked immunosorbent assays from the G-protein of the ovine and bovine respiratory syncytial virus (RSV), respectively, were used to determine the prevalence of the ovine and bovine subgroup strains of RSV infections in cattle. A total of 1,102 bovine serum samples were obtained from 6 diagnostic laboratories located in the northwestern and the southeastern USA and were tested for antibody to either the bovine or ovine subgroups of RSV. Antibody to viruses from each subgroup was present in samples from each region and all states tested. The Southeast had a higher prevalence of the bovine subgroup strains (69.5%). Then did the Northwest (40.9%). The prevalence of the ovine strain was similar for the two regions (16.7% in the southeast, 14.9% in the northwest). The overall prevalence was 56.6% for the bovine strain and 15.9% for the ovine strain. These results suggest members of the ovine subgroup of RSV circulate in the cattle population but with less frequency than those viruses of the bovine subgroup.  相似文献   
65.
Summary Acute febrile diseases are characterized by specific and non-specific symptoms. The non-specific responses are presented under the headings: fever, inflammation and pain, experimental models for investigating febrile reactions, haematologic changes, blood biochemical changes, cardiovascular effects, changes in gastric function, and the effects of fever upon pharmacokinetics of drugs. It was the purpose of this review to describe present concepts of thermoregulation and fever, the associated reactions produced by bacterial pyrogens and the mechanisms of these reactions. The available data illustrate, that many questions have not yet been clearly answered. However, the entire field of research involving endogenous substances, such as interleukin-1, is now moving ahead with great speed. Furthermore, there is some evidence which suggests that fever and the associated lower plasma zinc and iron levels act together as a co-ordinated non-specific host defence mechanism. Since experimental fever has a distinct effect upon the pharmacokinetics of drugs, more attention should be given to this aspect.  相似文献   
66.
Prevention of renal carriage of leptospirosis in dogs by vaccination   总被引:1,自引:0,他引:1  
Dogs vaccinated with a Leptospira interrogans vaccine containing serogroups Canicola and Icterohaemorrhagiae and prepared from cultures grown in a protein-free medium resisted challenge with heterologous representatives of these serogroups. In contrast, control dogs were pyrexic and leptospires were isolated from the blood for nine days following Canicola challenge and six days after Icterohaemorrhagiae challenge. Leptospires were isolated from the urine of controls throughout the post challenge period and from kidneys at post mortem examination of six out of six and four out of six Canicola and Icterohaemorrhagiae challenged controls, respectively. There have been no reports of hypersensitivity reactions to this vaccine since its introduction in 1980.  相似文献   
67.
GRAY  S. J.; PRICE  COLIN 《Forestry》1990,63(2):161-175
Traditional forest economics deals with regularly-ordered activities.For many reasons, however, choices must often be made amongconflicting operational priorities. In these circumstances,maximizing net discounted revenue per hectare does not maximizeglobal profit. Priorities for action should be chosen in orderof urgency index ratio, defined as the cost of delaying an operationdivided by the units of limited resource needed to undertakethe operation. Forest managers tend to use informal and sometimesconflicting rules to assign priorities among uses of limitedresources. These priorities often differ from those indicatedby urgency index ratio. Urgency index ratio can be determinedby simple calculations based on readily-available data. Theconcept can be extended to include multiple constraints.  相似文献   
68.
Whereas the rise in temperature during the past 30–40 years has already had clear impacts on the phenology of fruit trees and pathogenic insects, there is a lack of such correlations for fungal pathogens. An examination of fruit rots indicates that pathogenic fungi react differentially to climate change due to their complex infection biology. The appearance of the black rot fungus Diplodia seriata in Northwestern Europe is best explained by rising temperatures during the vegetation period. An increase in fruit rot caused by Nectria galligena is anticipated when milder and more humid winter months favour canker formation on twigs and branches, thereby increasing inoculum for fruit infections. An increasing importance of Neofabraea alba and Glomerella cingulata and/or G.??acutata as storage rots of apples in Northern Germany cannot be safely correlated with the climate change at present. Research on fungi is currently being expanded at the OVB Jork in order to ensure a faster identification of new pathogens and a more thorough investigation of relevant features of their infection biology.  相似文献   
69.
Softening process and total antioxidant activity were evaluated in kiwifruit differently exposed to light intensity and harvested at two different times (October 10 and November 14). Fruit was stored for 2 months at 0 °C (S1) and then maintained for a week at ambient temperature (S2). The results showed that fruit harvested later presented a faster softening rate during storage than fruit harvested earlier, even if antioxidant activity did not change. Light-exposed fruit showed higher flesh firmness than that of shaded ones. Polygalacturonase activity was higher in kiwifruit maintained for a week at ambient temperature after cool storage and, in particular the highest value was recorded in fruit harvested later. The behaviour of β-galactosidase was different: it did not show changes in fruit harvested later and significantly decreased in light- and shade-exposed fruit harvested earlier.  相似文献   
70.
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