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   447篇
  免费   29篇
林业   72篇
农学   9篇
基础科学   1篇
  64篇
综合类   39篇
农作物   8篇
水产渔业   10篇
畜牧兽医   261篇
园艺   2篇
植物保护   10篇
  2021年   3篇
  2020年   5篇
  2019年   11篇
  2018年   9篇
  2017年   6篇
  2016年   14篇
  2015年   6篇
  2014年   11篇
  2013年   32篇
  2012年   12篇
  2011年   19篇
  2010年   11篇
  2009年   13篇
  2008年   16篇
  2007年   23篇
  2006年   11篇
  2005年   15篇
  2004年   28篇
  2003年   17篇
  2002年   15篇
  2001年   11篇
  2000年   7篇
  1999年   4篇
  1998年   10篇
  1997年   15篇
  1996年   11篇
  1995年   3篇
  1994年   3篇
  1992年   4篇
  1991年   5篇
  1990年   9篇
  1989年   6篇
  1988年   6篇
  1987年   6篇
  1986年   5篇
  1983年   3篇
  1981年   3篇
  1980年   3篇
  1979年   3篇
  1978年   3篇
  1972年   4篇
  1971年   6篇
  1970年   3篇
  1969年   3篇
  1967年   3篇
  1966年   4篇
  1965年   6篇
  1956年   3篇
  1932年   4篇
  1911年   6篇
排序方式: 共有476条查询结果,搜索用时 15 毫秒
201.
202.
In animals and plants, social structure can reduce conflict within populations and bias aggression toward competing populations; however, for bacteria in the wild it remains unknown whether such population-level organization exists. Here, we show that environmental bacteria are organized into socially cohesive units in which antagonism occurs between rather than within ecologically defined populations. By screening approximately 35,000 possible mutual interactions among Vibrionaceae isolates from the ocean, we show that genotypic clusters known to have cohesive habitat association also act as units in terms of antibiotic production and resistance. Genetic analyses show that within populations, broad-range antibiotics are produced by few genotypes, whereas all others are resistant, suggesting cooperation between conspecifics. Natural antibiotics may thus mediate competition between populations rather than solely increase the success of individuals.  相似文献   
203.
204.
Objective: To determine the clinical efficacy of abdominal fluid to peripheral blood ratios of creatinine and potassium concentrations to diagnose uroperitoneum in dogs.
Design: Records of 13 dogs with confirmed uroabdomen were retrospectively analyzed. Prospective evaluation of 8 dogs with nonrenal ascites provided data for a control population.
Setting: Veterinary Medical Teaching Hospital.
Animals: Client owned dogs.
Interventions: None
Measurements and Main Results: Abdominal fluid potassium (mEq/L) and creatinine concentrations (mg/dl) were recorded. Peripheral blood potassium and creatinine concentrations were also recorded. Ratios were calculated based on these values. An abdominal fluid creatinine concentration to peripheral blood creatinine concentration ratio of > 2:1 was predictive of uroabdomen in dogs (specificity 100%, sensitivity 86%). An abdominal fluid potassium concentration to peripheral blood potassium concentration of > 1.4:1 is also predictive of uroabdomen in dogs (specificity 100%, sensitivity 100%). All dogs with uroabdomen had an abdominal fluid creatinine concentration that was at least 4 times normal peripheral blood levels.
Conclusion: Abdominal fluid to peripheral blood potassium and creatinine ratios provide a means to diagnose uroperitoneum in dogs without elevated peripheral blood creatinine.  相似文献   
205.
Numerous saturated and aromatic hydrocarbons, but not polar compounds, originating from plants and microorganisms (biomarkers) have been reported in sediments, coals, and petroleum. Here we describe natural product terpenoids found in two fossil conifers, Taxodium balticum (Eocene) and Glyptostrobus oregonensis (Miocene). A similar terpenoid pattern is also observed in extant Taxodium distichum. The preservation of characteristic terpenoids (unaltered natural products) in the fossil conifers supports their systematic assignment to the Cypress family (Cupressaceae sensu lato). The results also show that fossil conifers can contain polar terpenoids, which are valuable markers for (paleo)chemosystematics and phylogeny.  相似文献   
206.
A 13-hour-old Aberdeen-Angus was involuntarily recumbent since birth. Congenital nutritional muscular dystrophy was suspected based on clinical findings, increased serum creatine kinase, and decreased serum vitamin E and selenium levels. Recovery followed after supportive therapy and parenteral vitamin E and selenium. Reports of this disease in newborn calves are unusual.  相似文献   
207.
Anticoagulant effects of low-molecular-weight heparins in healthy cats   总被引:4,自引:0,他引:4  
BACKGROUND: Low-molecular-weight heparin (LMWH) has potential benefit in cats at risk for thromboembolic disease. However, LMWH pharmacokinetics has not been characterized in the cat. Drug effect with LMWH may be evaluated with analysis of factor Xa inhibition (anti-Xa) or thromboelastography (TEG). HYPOTHESIS: Administration of LMWH at previously recommended dosages and schedules to healthy cats will result in inhibition of factor Xa and hypocoagulable TEG. ANIMALS: In vivo research with heparin was performed in 5 purpose-bred cats. METHODS: In a prospective study with randomized crossover design, heparin or placebo was administered. Treatments were unfractionated heparin (UFH), 250 IU/kg q6h; dalteparin, 100 IU/kg q12h; enoxaparin, 1 mg/kg q12h; or 0.9% saline, 0.25 mL/kg q6h. Each drug was administered for 5 consecutive days followed by a minimum washout of 14 days. Baseline and post-treatment analyses included anti-Xa, TEG, and prothrombin time/activated partial thromboplastin time. RESULTS: Mean anti-Xa activity 4 hours after enoxaparin (0.48 U/mL) approached the human therapeutic target (0.5-1.0 U/mL); however, mean trough anti-Xa activity was below detection limits. Mean anti-Xa activity 4 hours after dalteparin was lower, and only 1 cat attained therapeutic target at a single time point. Cats receiving UFH attained target anti-Xa activity and changes in TEG at trough and 4 hours. CONCLUSIONS: Cats have rapid absorption and elimination kinetics with LMWH therapy. On the basis of pharmacokinetic modeling, cats will require higher dosages and more frequent administration of LMWH to achieve human therapeutic anti-factor Xa activity of 0.5-1 U/mL. Peak anti-Xa activity is predicted at 2 hours after administration of LMWH.  相似文献   
208.
INDEPTH geophysical and geological observations imply that a partially molten midcrustal layer exists beneath southern Tibet. This partially molten layer has been produced by crustal thickening and behaves as a fluid on the time scale of Himalayan deformation. It is confined on the south by the structurally imbricated Indian crust underlying the Tethyan and High Himalaya and is underlain, apparently, by a stiff Indian mantle lid. The results suggest that during Neogene time the underthrusting Indian crust has acted as a plunger, displacing the molten middle crust to the north while at the same time contributing to this layer by melting and ductile flow. Viewed broadly, the Neogene evolution of the Himalaya is essentially a record of the southward extrusion of the partially molten middle crust underlying southern Tibet.  相似文献   
209.
Four groups of six cattle were vaccinated from two to five times at 6 month intervals with two different trivalent FMD vaccines licensed in the given year. The FMDV type A strains in the vaccines designated A5F and A5B were closely related. Three months after the last vaccination the cattle were challenged by contact with animals inoculated with the original field strain A5B. The inoculated animals developed typical FMD symptoms with vesicles in the mouth and on the feet. Those cattle which had received vaccines that did not contain strain A5B also became severely sick, even after five vaccinations. Animals vaccinated twice with type B containing vaccine were also not completely protected. A safe protection can obviously only be achieved for fairly short periods of time if vaccine and challenge strain are homologous. It is proposed to change the rules of licensing, to speed up the procedure to vaccinate in cases of outbreaks. The need for further research, especially into improving vaccines, is stressed.  相似文献   
210.
102 fattened pigs with known genetic origin were tested on a conveyor-belt 1--4 days before slaughtering. 64 animals run in the first passage in each case 154 m in a 4 minutes (0.64 m/s), compared to it the 38 animals of the second passage run in each case 770 m in 20 minutes. Measurements of heart frequency and breathing and body temperature were taken before and at intervals until 30 minutes after movement. After slaughtering the values of pH, colour and drip loss of the M. longissimus dorsi were measured. It turned out a difference in the meat quality with a high part of PSE-meat in the first passage (36%), caused by season. The criterion "recovery time of heart action after load" was able to value the stress susceptibility in dependence on meat quality p.m. Animals with a longer recovery time exhibited a lower pH45 value, a lighter colour and a higher drip loss of meat. But only after a certain stress intensity (2. passage) the assertion was given.  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