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   475篇
  免费   15篇
林业   18篇
农学   27篇
基础科学   6篇
  165篇
综合类   37篇
农作物   26篇
水产渔业   36篇
畜牧兽医   126篇
园艺   10篇
植物保护   39篇
  2023年   6篇
  2022年   9篇
  2021年   24篇
  2020年   24篇
  2019年   28篇
  2018年   21篇
  2017年   16篇
  2016年   23篇
  2015年   13篇
  2014年   14篇
  2013年   18篇
  2012年   32篇
  2011年   37篇
  2010年   15篇
  2009年   17篇
  2008年   24篇
  2007年   26篇
  2006年   13篇
  2005年   17篇
  2004年   20篇
  2003年   14篇
  2002年   12篇
  2001年   3篇
  2000年   3篇
  1999年   2篇
  1998年   3篇
  1997年   1篇
  1996年   4篇
  1995年   3篇
  1994年   2篇
  1993年   1篇
  1992年   5篇
  1990年   2篇
  1989年   1篇
  1988年   4篇
  1987年   1篇
  1986年   3篇
  1985年   3篇
  1984年   8篇
  1983年   3篇
  1982年   2篇
  1981年   4篇
  1980年   3篇
  1975年   2篇
  1973年   1篇
  1971年   1篇
  1964年   1篇
  1956年   1篇
排序方式: 共有490条查询结果,搜索用时 125 毫秒
81.
Alpine blue-sow-thistle (Cicerbita alpina (L.) Wallr.) is a perennial herb distributed all along the entire Alpine Arc, the shoots of which are still collected in the wild in several areas of Northern-Eastern Italy for food purposes. Our paper deals with experiments of domestication of this species carried out in the surroundings of Trento and lasted from 2004 to 2010. The cultivation was carried out transplanting seedlings, obtained by wild seed of a unique accession. One experimental trial (2004/2008) was carried in the location Frisanchi, located at 1,078 m a.s.l. In this trial the theses compared 4 harvests, 1 week spaced, carried out respectively in the 2nd, 3rd and 4th year after the transplant. Of the total quantity harvested after 4 years, the most shoots (60) and the highest dried weight per m2 (14 g) were obtained from shoots collected 2 or 3 years after being transplanted. However, comparing the shoots yielded in the last year of the trial only (2008), the best results were obtained from 3 or 4-year-old transplants (42 shoots/m2; 8.2 g dry weight). Nevertheless there four times harvesting of shoots weakened the plants’ development and a high failure rates was recorded. In addition to the real experimental trial another small cultivation was carried out in an higher location, Monte Bondone at 1,500 m a.s.l., in which 5 years after the transplant (2010) only 2 harvests were carried out. In this experiment the yield recorded was very encouraging (10.5 g m2) and also the quality of the shoots collected was good.  相似文献   
82.
To date, most of the experimental and modelling research on the mobilization mechanisms of autochthonous colloidal‐sized soil particles has focused on single rainfall events. However, in the field, natural events interrupted by rainless periods of various durations follow one another. Some studies have shown that the amount of leached particles is much affected by the chronology of rainfall events. In this paper, we propose a model to compute the amount of mobilized particles during the transient flow regime of long series of successive rainfall events. Particles that can be mobilized are assumed to be located at the surface of preferential flow paths in contact with flowing water. As water passes, particles are mobilized, thus uncovering new particles and renewing the stock of mobilizable particles. We also report experimental data on the impact of long series of rainfall events on particle mobilization in undisturbed calcareous soil columns under controlled conditions. The model provides a framework to understand the variation of mobilization observed during the transient flow regime of these rainfall events. These variations resulted in part from the interplay between macropore water content at the onset of rain and the chronology of previous rainfall events. Additionally, the model provides a feedback loop between particle mobilization and minute macropore structure modifications. Once coupled with a particle transport model this feature may be useful to model soil structure changes during long series of successive rainfall events.  相似文献   
83.
In a previous survey concerning cows of reproductive age, we demonstrated that oocytes isolated from ovaries with <10 medium antral follicles of 2 to 6 mm in diameter (low ovaries; Lo) show less developmental competence than oocytes collected from ovaries with >10 medium antral follicles (high ovaries; Hi). The aim of the present study was to evaluate whether a defective endothelial nitric oxide synthase/nitric oxide (eNOS/NO) system and vasculature in healthy medium antral follicles is likely to reduce oocyte competence from Lo ovaries. Thus, experiments were conducted to 1) immunolocalize eNOS protein during folliculogenesis; 2) quantify eNOS protein/vasculature in the follicle wall; and 3) verify if NO donor, S-nitroso acetyl penicillamine (SNAP) administration during in vitro maturation affects developmental competence of oocytes isolated from Lo ovaries. Endothelial nitric oxide synthase protein was detected in granulosa and theca cells, as well as in blood vessels from primordial to antral follicles. Quantitative analysis indicated that in medium antral follicles from Lo ovaries, eNOS protein expression and vasculature were reduced (P < 0.05). The addition of SNAP improved blastocyst and hatching rates of oocytes from Lo ovaries, promoting a percentage similar to oocytes from Hi ovaries, and reduced the percentage of apoptotic nuclei in in vitro-produced blastocysts (P < 0.05). Results from our study suggest that in bovine ovaries with small mid antral follicle number, a defective eNOS/NO system is related to a reduced follicle vasculature and may affect oocyte quality, thus inducing a premature decline of fertility.  相似文献   
84.
Magnetic resonance and computed tomography features of 4 cases of canine congenital vertebral anomalies (CVAs) are discussed. Two of the cases represent unusual presentations for such anomalies that commonly affect screw-tail or toy breeds. Moreover, the combination of CVAs and a congenital peritoneo-pericardial diaphragmatic hernia has never before been imaged.  相似文献   
85.
Neosporosis is of alarming economic concern in the cattle industry. The effectiveness of diagnostic tests for detecting specific antibodies against Neospora caninum is hampered by potential cross-reaction with other coccidia. Use of a single specific antigen might improve test specificity. An indirect enzyme-linked immunosorbent assay (ELISA) was developed using the truncated protein NcSRS2 expressed in Escherichia coli. The ELISA results were compared with those of the indirect fluorescence antibody test (IFAT). Receiver Operating Characteristic (ROC) and Tests in the Absence of a Gold Standard (TAGS) analysis revealed an assay having 96% specificity and 95% sensitivity when applied to 145 positive and 352 negative sera from two distinct cattle populations. Using OD ≤ 0.095 as the cut-off point, the assay's negative and positive predictive values ranged from 98.8% to 50.8% and from 58.8% to 99.1%, respectively, depending on neosporosis prevalence in a given area. The novel ELISA-NcSRS2 format described in the present report constitutes a specific and sensitive method for detecting N. caninum in cattle.  相似文献   
86.
One of the aims of the Water Framework Directive 2000/60/CE is to assess the ecological status of water bodies in Europe in relation to priority contaminants, including some persistent organic pollutants (POPs). Recognizing the benefits of measuring hydrophobic compounds in biota tissues rather than in water, we used the European eel Anguilla anguilla in the present study as a bioindicator species for monitoring POPs in freshwater/brackish environments. We present the results of a contamination analysis of eels carried out in three Italian sites representative of different pollution levels: the last part of Tevere River, which flows through a very urbanized and industrialized area, Caprolace Lake and Lesina Lagoon, inside two different protected national parks. A very high pollution variability was recorded within each group of eels, in particular for those caught at Caprolace Lake. Due to this high variability, inter-population comparisons were carried out only between eels collected in Tevere River and Lesina Lagoon. PCBs values in Lesina Lagoon were two orders of magnitude lower than those measured in Tevere River, while no significant differences were found for levels of DDT compounds. Our results confirm the suitability of the eel as an indicator of persistent bioaccumulative pollutants in brackish environments and rivers where it can be easily caught, but we are sceptical of its use in deep lakes where its capture might be problematic considering standard operation procedures for fish sampling.  相似文献   
87.
88.
89.
There are many definitions of irrigation system efficiency that are applied over a range of scales. Many traditional definitions considered only the water diverted as the water volume of concern. Considering also the water consumed in defining effective irrigation efficiency is a shift from the classical definition of system efficiency. In this paper, equations are derived for calculating the following system performance measures: the irrigation consumptive use coefficient, irrigation system efficiency, irrigation water and soil salinities, relative yield, and productivity of consumed, diverted and beneficially used water. The expressions are based on quite general assumptions and are valid for systems with a single water source and layouts composed of (or simplified to) irrigation units arranged in a row. The aim of these expressions is to illustrate how system performance is affected by the reuse of water which depends on the system’s hydraulic connections and the irrigation unit performance. Illustrations of the model are provided for systems in series and in parallel. Testing and refinement by removing some of the general assumptions underlying the model will be needed to develop practical applications that can be more confidently applied for comparison and improvement of irrigation systems.  相似文献   
90.
Prostate cancer (PCa) is the leading cause of cancer death in men, and its treatment is commonly associated with severe adverse effects. Thus, new treatment modalities are required. In this context, natural compounds have been widely explored for their anti-PCa properties. Aquatic organisms contain numerous potential medications. Anticancer peptides are less toxic to normal cells and provide an efficacious treatment approach via multiple mechanisms, including altered cell viability, apoptosis, cell migration/invasion, suppression of angiogenesis and microtubule balance disturbances. This review sheds light on marine peptides as efficacious and safe therapeutic agents for PCa.  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