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   3159篇
  免费   148篇
  国内免费   2篇
林业   58篇
农学   112篇
基础科学   25篇
  343篇
综合类   410篇
农作物   122篇
水产渔业   160篇
畜牧兽医   1792篇
园艺   58篇
植物保护   229篇
  2021年   30篇
  2020年   39篇
  2019年   36篇
  2018年   43篇
  2017年   47篇
  2016年   45篇
  2015年   38篇
  2014年   46篇
  2013年   157篇
  2012年   103篇
  2011年   99篇
  2010年   58篇
  2009年   65篇
  2008年   120篇
  2007年   109篇
  2006年   106篇
  2005年   105篇
  2004年   82篇
  2003年   95篇
  2002年   113篇
  2001年   112篇
  2000年   98篇
  1999年   64篇
  1998年   42篇
  1997年   42篇
  1996年   45篇
  1995年   45篇
  1994年   31篇
  1992年   45篇
  1991年   63篇
  1990年   69篇
  1989年   56篇
  1988年   56篇
  1987年   59篇
  1986年   61篇
  1985年   57篇
  1984年   52篇
  1983年   50篇
  1982年   36篇
  1981年   30篇
  1979年   31篇
  1978年   33篇
  1977年   37篇
  1976年   31篇
  1975年   29篇
  1974年   36篇
  1973年   35篇
  1972年   35篇
  1971年   33篇
  1970年   34篇
排序方式: 共有3309条查询结果,搜索用时 687 毫秒
171.
Primary pulmonary neoplasia is relatively uncommon in cats and generally has a poor prognosis. In this multicenter, retrospective study of 57 cats with pulmonary neoplasia, the most frequent presenting signs were anorexia/inappetence (39%) and cough (37%). The pulmonary tumors were considered to be incidental findings in 9% cats. In computed tomographic (CT) images, primary pulmonary tumors appeared as a pulmonary mass in 55 (96%) cats and as a disseminated pulmonary lesion without a defined mass in two (4%) cats. Most pulmonary tumors were in the caudal lobes, with 28 (49%) in the right caudal lobe and 17 (30%) in the left caudal lobe. CT features associated with pulmonary tumors included mass in contact with visceral pleura (96%), irregular margins (83%), well‐defined borders (79%), bronchial compression (74%), gas‐containing cavities (63%), foci of mineral attenuation (56%), and bronchial invasion (19%). The mean (range) maximal dimension of the pulmonary masses was 3.5 cm (1.1–11.5 cm). Additional foci of pulmonary disease compatible with metastasis were observed in 53% cats. Pleural fluid was evident in 30% cats and pulmonary thrombosis in 12% cats. The histologic diagnoses were 47 (82%) adenocarcinomas, six (11%) tumors of bronchial origin, three (5%) adenosquamous cell carcinomas, and one (2%) squamous cell carcinoma. In this series, adenocarcinoma was the predominant tumor type, but shared many features with less common tumor types. No associations were identified between tumor type and CT features. Prevalence of suspected intrapulmonary metastasis was higher than in previous radiographic studies of cats with lung tumors.  相似文献   
172.
173.
174.
Protein haze formation in white wine is dependent on the presence of both wine protein and other unknown wine components, termed factor(s) X. The ability to reconstitute protein haze upon heating artificial model wine solutions (500 mg/L thaumatin, 12% ethanol, 4 g/L tartaric acid) to which candidate components were added was employed to identify factor(s) X. No protein haze was formed in the absence of additives. The individual or combined addition of caffeic acid, caftaric acid, epicatechin, epigallocatechin-O-gallate, gallic acid, or ferulic acid at typical white wine concentrations did not generate protein haze. However, PVPP fining of commercial wines resulted in a reduction in protein haze, suggesting that phenolic compounds may play a modulating role in haze formation. To elucidate the nature of the unknown factor(s) wine was fractionated and fractions were back-added to model wine and tested for their essentiality. Wine fractions were generated by ultrafiltration, reverse-phase chromatography, and mixed-mode anion-exchange and reverse-phase chromatography. The only purified fraction containing the essential component(s) was free of phenolic compounds, and analysis by mass spectrometry identified sulfate anion as the dominant component. Reconstitution with KHSO4 using either commercially available thaumatin or wine proteins confirmed the role of sulfate in wine protein haze formation. The two main wine proteins, thaumatin-like protein and chitinase, differed in their haze response in model wines containing sulfate. Other common wine anions, acetate, chloride, citrate, phosphate, and tartrate, and wine cations, Fe(2+/3+) and Cu(+/2+), when added at typical white wine concentrations were not found to be essential for protein haze formation.  相似文献   
175.
Field-trapping experiments with synthetic 2,7-nonadiyl dibutyrate, the female-produced sex pheromone of the orange wheat blossom midge, Sitodiplosis mosellana (Géhin), demonstrated that pheromone traps were highly attractive to males and caught very few non-target organisms. Different formulations of pheromone were tested to identify the optimum release rate and dispenser type for use in pheromone traps in the UK. Key findings were that racemic pheromone was as effective as enantiomerically pure (2S,7R)-2,7-nonadiyl dibutyrate, that release rates higher than 0.5 microg day(-1) were not necessary and that the optimal formulation was a 1 mg pheromone loading in a rubber septum. Pheromone traps gave a reliable indication of peak midge emergence, onset of flight and abundance of midges throughout the season. A strong correlation between maximum trap catch and crop infestation levels was obtained.  相似文献   
176.
OBJECTIVE: Previous research on nutrition labelling has mainly used subjective measures. This study examines the effectiveness of two types of nutrition label using two objective measures: eye movements and healthiness ratings. DESIGN: Eye movements were recorded while participants made healthiness ratings for two types of nutrition label: standard and standard plus the Food Standards Agency's 'traffic light' concept. SETTING: University of Derby, UK. SUBJECTS: A total of 92 participants (mean age 31.5 years) were paid for their participation. None of the participants worked in the areas of food or nutrition. RESULTS: For the standard nutrition label, participant eye movements lacked focus and their healthiness ratings lacked accuracy. The traffic light system helped to guide the attention of the consumer to the important nutrients and improved the accuracy of the healthiness ratings of nutrition labels. CONCLUSIONS: Consumers have a lack of knowledge regarding how to interpret nutrition information for standard labels. The traffic light concept helps to ameliorate this problem by indicating important nutrients to which to pay attention.  相似文献   
177.
OBJECTIVE: To examine the effectiveness of two methods of increasing fruit and fruit juice intake in pregnancy: midwives' advice and vouchers exchangeable for juice. DESIGN: Pregnant women were randomly allocated to three groups: a control group, who received usual care; an advice group, given advice and leaflets promoting fruit and fruit juice consumption; and a voucher group, given vouchers exchangeable for fruit juice from a milk delivery firm. Dietary questionnaires were administered at ~16, 20 and 32 weeks of pregnancy. Serum beta-carotene was measured at 16 and 32 weeks. SETTING: An antenatal clinic in a deprived area. SUBJECTS: Pregnant women aged 17 years and over. RESULTS: The study comprised 190 women. Frequency of fruit consumption declined during pregnancy in all groups, but that of fruit juice increased substantially in the voucher group. Serum beta-carotene concentration increased in the voucher group, from 106.2 to 141.8 micromol l(-1) in women with measurements on both occasions (P = 0.003), decreased from 120.0 to 99.8 micromol l(-1) in the control group (P = 0.005), and was unchanged in the advice group. CONCLUSIONS: Pregnant women drink more fruit juice if they receive vouchers exchangeable for juice supplied by the milk delivery service. Midwives' advice to eat more fruit has no great effect. Providing vouchers for fruit juice is a simple method of increasing its intake in a deprived population and may be useful for other sections of the community.  相似文献   
178.
Incorporation of stearic acid into canola oil to produce trans-free structured lipid (SL) as a healthy alternative to partially hydrogenated fats for margarine formulation was investigated. Response surface methodology was used to study the effects of lipozyme RM IM from Rhizomucor miehei and Candida rugosa lipase isoform 1 (LIP1) and two acyl donors, stearic acid and ethyl stearate, on the incorporation. Lipozyme RM IM and ethyl stearate gave the best result. Gram quantities of SLs were synthesized using lipozyme RM IM, and the products were compared to SL made by chemical catalysis and fat from commercial margarines. After short-path distillation, the products were characterized by GC and RPHPLC-MS to obtain fatty acid and triacylglycerol profiles, 13C NMR spectrometry for regiospecific analysis, X-ray diffraction for crystal forms, and DSC for melting profile. Stearic acid was incorporated into canola oil, mainly at the sn-1,3 positions, for the lipase reaction, and no new trans fatty acids formed. Most SL products did not have adequate solid fat content or beta' crystal forms for tub margarine, although these may be suitable for light margarine formulation.  相似文献   
179.
180.
Acanthocephalan and spargana parasites were identified within a body wall mass during exploratory surgery in a wild green tree snake. Acanthocephalan parasites have not previously been reported in this species. Surgical excision, the treatment of choice, could not be achieved because of the extensive infiltration of the coelomic cavity.  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