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   474篇
  免费   15篇
林业   135篇
农学   12篇
  103篇
综合类   36篇
农作物   16篇
水产渔业   37篇
畜牧兽医   123篇
园艺   7篇
植物保护   20篇
  2020年   7篇
  2019年   6篇
  2018年   8篇
  2017年   6篇
  2016年   10篇
  2015年   8篇
  2014年   12篇
  2013年   11篇
  2012年   18篇
  2011年   30篇
  2010年   10篇
  2009年   15篇
  2008年   28篇
  2007年   20篇
  2006年   22篇
  2005年   17篇
  2004年   19篇
  2003年   22篇
  2002年   12篇
  2001年   7篇
  2000年   7篇
  1997年   4篇
  1992年   4篇
  1989年   4篇
  1986年   3篇
  1984年   5篇
  1983年   4篇
  1982年   4篇
  1977年   4篇
  1975年   3篇
  1974年   5篇
  1972年   6篇
  1969年   3篇
  1968年   3篇
  1967年   5篇
  1966年   3篇
  1965年   4篇
  1962年   3篇
  1956年   4篇
  1948年   7篇
  1941年   3篇
  1934年   3篇
  1933年   3篇
  1932年   12篇
  1927年   7篇
  1919年   6篇
  1918年   5篇
  1915年   4篇
  1904年   5篇
  1902年   3篇
排序方式: 共有489条查询结果,搜索用时 15 毫秒
451.
452.
When the isolated teat of a cow was examined with an 8.5 MHz linear array transducer in a vertical plane, the teat canal appeared as a thin, white line, bordered on each side by parallel, thick, grey-black bands. In a horizontal plane a comparable image was obtained. In a sheep, images of comparable quality were obtained with a 12 MHz transducer. Histological studies of the tissues whose removal led to the disappearance of this characteristic ultrasonographic appearance showed that it was associated with the stratified keratinised squamous epithelium with distinct papillae. The content of keratin in the stratum corneum was apparently responsible for the bright zone; the stratum lucidum was not visible, and the surrounding dark, less echoic area was associated with the stratum granulosum. Doppler echography in live animals confirmed this designation. The outer layers of the teat wall were more echogenic.  相似文献   
453.
454.
Usutu virus (USUV), a flavivirus of the Japanese encephalitis virus complex, was for the first time detected outside Africa in the region around Vienna (Austria) in 2001 by Weissenb?ck et al. [Weissenb?ck, H., Kolodziejek, J., Url, A., Lussy, H., Rebel-Bauder, B., Nowotny, N., 2002. Emergence of Usutu virus, an African mosquito-borne flavivirus of the Japanese encephalitis virus group, central Europe. Emerg. Infect. Dis. 8, 652-656]. USUV is an arthropod-borne virus (arbovirus) circulating between arthropod vectors (mainly mosquitoes of the Culex pipiens complex) and avian amplification hosts. Infections of mammalian hosts or humans, as observed for the related West Nile virus (WNV), are rare. However, USUV infection leads to a high mortality in birds, especially blackbirds (Turdus merula), and has similar dynamics with the WNV in North America, which, amongst others, caused mortality in American robins (Turdus migratorius). We hypothesized that the transmission of USUV is determined by an interaction of developing proportion of the avian hosts immune and climatic factors affecting the mosquito population. This mechanism is implemented into the present model that simulates the seasonal cycles of mosquito and bird populations as well as USUV cross-infections. Observed monthly climate data are specified for the temperature-dependent development rates of the mosquitoes as well as the temperature-dependent extrinsic-incubation period. Our model reproduced the observed number of dead birds in Austria between 2001 and 2005, including the peaks in the relevant years. The high number of USUV cases in 2003 seems to be a response to the early beginning of the extraordinary hot summer in that year. The predictions indicate that >70% of the bird population acquired immunity, but also that the percentage would drop rapidly within only a couple of years. We estimated annually averaged basic reproduction numbers between R (0)=0.54 (2004) and 1.35 (2003). Finally, extrapolation from our model suggests that only 0.2% of the blackbirds killed by USUV were detected by the Austrian USUV monitoring program [Chvala, S., Bakonyi, T., Bukovsky, C., Meister, T., Brugger, K., Rubel, F., Nowotny, N., Weissenb?ck, H., 2007. Monitoring of Usutu virus activity and spread by using dead bird surveillance in Austria, 2003-2005. Vet. Microbiol. 122, 237-245]. These results suggest that the model presented is able to quantitatively describe the process of USUV dynamics.  相似文献   
455.
In Europe, rabies in bats is caused by European Bat Lyssavirus (EBLV) type 1 (EBLV-1) or type 2 (EBLV-2) which form two distinct genotypes (gt 5 and 6) within the genus Lyssavirus of the family of Rhadoviridae. Spill-over infections of EBLV in humans have caused fatal rabies encephalitis and highlighted the relevance of this wildlife disease for public health. The vast majority of the 831 European bat rabies cases reported between 1977 and 2006 were identified as EBLV-1. Only few virus isolates originating from Switzerland, The Netherlands and the United Kingdom were characterized as EBLV-2. Here we report the first EBLV-2 case detected in Germany in a Daubenton's bat (Myotis daubentonii) in August 2007. The bat showed clinical signs of disorders of the central nervous system and subsequently tested positive for rabies. The virus was isolated and characterized as EBLV-2 based on its antigen pattern and by nucleotide sequencing. Phylogenetic analysis indicated an association to EBLV-2 isolates from Switzerland which correlates with the origin of the bat close to the Swiss border.  相似文献   
456.
In autumn 2011, a novel species of the genus Orthobunyavirus of the Simbu serogroup was discovered close to the German/Dutch border and named Schmallenberg virus (SBV). Since then, SBV has caused a large epidemic in European livestock. Like other viruses of the Simbu serogroup, SBV is transmitted by insect vectors. Adult ruminants may show a mild transient disease, while an infection during a critical period of pregnancy can lead to severe congenital malformation, premature birth or stillbirth. The current knowledge about the virus, its diagnosis, the spread of the epidemic, the impact and the possibilities for preventing infections with SBV is described and discussed.  相似文献   
457.
The concept of green infrastructure has been recently taken up by the European Commission for ensuring the provision of ecosystem services (ESS). It aims at the supply of multiple ESS in a given landscape, however, the effects of a full suite of management options on multiple ESS and landscape multifunctionality have rarely been assessed. In this paper we use European floodplain landscapes as example to develop an expert based qualitative conceptual model for the assessment of impacts of landscape scale interventions on multifunctionality. European floodplain landscapes are particularly useful for such approach as they originally provided a high variety and quantity of ESS that has declined due to the strong human impact these landscapes have experienced. We provide an overview of the effects of floodplain management options on landscape multifunctionality by assessing the effects of 38 floodplain management interventions on 21 relevant ESS, as well as on overall ESS supply. We found that restoration and rehabilitation consistently increased the multifunctionality of the landscape by enhancing supply of provisioning, regulation/maintenance, and cultural services. In contrast, conventional technical regulation measures and interventions related to extraction, infrastructure and intensive land use cause decrease in multifunctionality and negative effects for the supply of all three aspects of ESS. The overview of the effects of interventions shall provide guidance for decision makers at multiple governance levels. The presented conceptual model could be effectively applied for other landscapes that have potential for a supply of a high diversity of ESS.  相似文献   
458.
Spermatozoa of Leuciscus cephalus have the enzymatic outfit for glycolysis but lack lipase, phospholipase and glucosidase activities. Therefore, they are not able to utilize lipids and polysaccharides as energy resources. During motility they use monosaccharides as energy reservoirs: the intracellular glucose, galactose and fructose levels decrease significantly while lactate levels increase.  相似文献   
459.
By use of sodium dodecyl sulfate polyacrylamide gel electrophoresis, ovarian fluid proteins and main proteins of unfertilized eggs were qualitatively and quantitatively investigated in the brown trout, Salmo trutta, to see whether some of them were correlated with the rate of embryos reaching the eyed embryo stage. In the ovarian fluid, 12 types of proteins in the range of 39–166 kDa were detected whereby three proteins were lipoproteins and two were glycoproteins. Ovarian fluid proteins with a molecular weight of 85, 68, 62 and 39 kDa were negatively correlated with the percentage of eyed stage embryos. The statistical significance of the relations was low in simple and multiple regression models (R2≤0.534) indicating that the relations were influenced and superposed by other factors. Therefore, ovarian fluid proteins give only poor information about maturity and quality of eggs. In the eggs, nine major types of proteins in the range of 95–15 kDa were identified. The 95 kDa protein was a lipoprotein, the 85 and the 62 kDa protein were glycoproteins, and the 15 kDa protein was a phosphoprotein. The 95, 85, 77 and 39 kDa protein were positively correlated with embryo survival to the eyed embryo stage. The explanatory effect of the multiple regression model was very high (R2=0.961) indicating that distinct egg proteins are closely related with egg quality.  相似文献   
460.
Glucokinase (GK) plays a key role in whole-body glucose homeostasis by catalyzing the phosphorylation of glucose in cells that express this enzyme, such as pancreatic beta cells and hepatocytes. We describe a class of antidiabetic agents that act as nonessential, mixed-type GK activators (GKAs) that increase the glucose affinity and maximum velocity (Vmax) of GK. GKAs augment both hepatic glucose metabolism and glucose-induced insulin secretion from isolated rodent pancreatic islets, consistent with the expression and function of GK in both cell types. In several rodent models of type 2 diabetes mellitus, GKAs lowered blood glucose levels, improved the results of glucose tolerance tests, and increased hepatic glucose uptake. These findings may lead to the development of new drug therapies for diabetes.  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