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   291篇
  免费   25篇
林业   17篇
农学   3篇
基础科学   1篇
  35篇
综合类   30篇
农作物   14篇
水产渔业   36篇
畜牧兽医   160篇
园艺   8篇
植物保护   12篇
  2024年   1篇
  2023年   4篇
  2022年   5篇
  2021年   9篇
  2020年   14篇
  2019年   13篇
  2018年   11篇
  2017年   15篇
  2016年   8篇
  2015年   8篇
  2014年   10篇
  2013年   17篇
  2012年   15篇
  2011年   22篇
  2010年   21篇
  2009年   17篇
  2008年   23篇
  2007年   15篇
  2006年   16篇
  2005年   15篇
  2004年   10篇
  2003年   11篇
  2002年   7篇
  2001年   4篇
  2000年   7篇
  1999年   1篇
  1998年   3篇
  1997年   2篇
  1996年   1篇
  1994年   2篇
  1991年   2篇
  1990年   1篇
  1985年   1篇
  1984年   1篇
  1975年   1篇
  1973年   1篇
  1972年   2篇
排序方式: 共有316条查询结果,搜索用时 15 毫秒
31.
The western barred bandicoot, Perameles bougainville, is an endangered Australian marsupial species. Routine histology of liver samples collected at necropsy from 19 of 20 (95%) western barred bandicoots revealed the sporadic to common occurrence of abnormal hepatocyte nuclei characterized by margination of chromatin and concomitant central pallor. Some abnormal hepatocyte nuclei were mildly to markedly enlarged and irregularly shaped. Periodic acid-Schiff reagent stained 131 of 142 (92%) of these abnormal hepatocyte nuclei. Positive staining was completely eliminated by diastase pretreatment. Transmission electron microscopy revealed that abnormal hepatocyte nuclei with marginated chromatin did not contain viral particles. Rather, glycogen beta-particles and alpha-rosettes were identified within some abnormal hepatocyte nuclei. Glycogen intranuclear inclusions were an incidental finding in western barred bandicoot hepatocytes.  相似文献   
32.
33.
34.
Heterogeneity in human responses and decision‐making can contribute to the resilience of social–ecological systems in the face of environmental, political and economic pressures. In fishery systems worldwide, the ability of harvesters to maintain a diverse portfolio of fishing strategies is important for building adaptive capacity. We used a case‐study approach to examine the complexity of factors that inhibit or promote diversification in fisheries of Alaska, one of the major fishing regions of the world. Through a combination of harvest records and literature review, we explored shifts in participation and portfolio diversity in Alaskan fisheries over three decades. The four case‐studies examined the responses of fishers, fleets and communities to multiple, intersecting pressures, including biological declines, market and price dynamics, fishery privatization and the 1989 Exxon Valdez oil spill. These cases illustrate how stressors acting at multiple scales can encourage or constrain opportunities for diversification, and that these opportunities may be spread inequitably across participants. Overall, we found evidence for reduced participation and increasing specialization in Alaskan commercial fisheries. While numerous factors explain these trends, policies like individual quota systems and the increasing cost of entry into fisheries are forcing consolidation at local to regional scales. A portfolio approach to managing fisheries that reduces barriers to diversification and includes broad representation of resource users and communities in management may help to maintain opportunity and choice for fishers.  相似文献   
35.
In the initial phase of a national project to map clay, sand and soil organic matter (SOM) content in arable topsoil in Sweden, a study area in south-west Sweden comprising about 100 000 ha of arable land was assessed. Models were created for texture, SOM and two estimated variables for lime requirement determination (target pH and buffering capacity), using a data mining method (multivariate adaptive regression splines). Two existing reference soil datasets were used: a grid dataset and a dataset created for individual farms. The predictor data were of three types: airborne gamma-ray spectrometry data, digital elevation from airborne laser scanning, and legacy data on Quaternary geology. Validations were designed to suit applicability assessments of prediction maps for precision agriculture. The predictor data proved applicable for regional mapping of topsoil texture at 50 × 50 m2 spatial resolution (root mean square error: clay = 6.5 %; sand = 13.2 %). A novel modelling strategy, ‘Farm Interactive’, in which soil analysis data for individual farms were added to the regional data, and given extra weight, improved the map locally. SOM models were less satisfactory. Variable-rate application files for liming created from derived digital soil maps and locally interpolated soil data were compared with ‘ground truth’ maps created by proximal sensors on one test farm. The Farm Interactive methodology generated the best predictions and was deemed suitable for adaptation of regional digital soil maps for precision agricultural purposes.  相似文献   
36.
It currently remains unclear if rotifers contain sufficient mineral levels to meet larval fish requirements. In this study, rotifers were enriched with a commercial enrichment (control), or with additional iodine, iodine and copper, or iodine, copper and manganese, and the effects of feeding these rotifers to Atlantic cod (Gadus morhua) larvae from 3 to 18 days post hatch were investigated. Rotifer enrichment with minerals was successful, but Mn enrichment also increased rotifer zinc levels. No differences were observed between treatments in larval growth or survival, or in the mRNA levels in the majority of the redox system genes analysed. Only Zn levels increased in cod larvae in response to mineral enrichment of rotifers. Apart from Zn, little evidence was found to suggest that cod larvae require increased concentrations above the control rotifer levels of the essential elements studied here.  相似文献   
37.
Meta‐analysis has been an integral tool for fisheries researchers since the late 1990s. However, there remain few guidelines for the design, implementation or interpretation of meta‐analyses in the field of fisheries. Here, we provide the necessary background for readers, authors and reviewers, including a brief history of the use of meta‐analysis in fisheries, an overview of common model types and distinctions, and examples of different goals that can be achieved using meta‐analysis. We outline the primary challenges in implementing meta‐analyses, including difficulties in discriminating between alternative hypotheses that can explain the data with equal plausibility, the importance of validating results using multiple lines of evidence, the trade‐off between complexity and sample size and problems associated with the use of model output. For each of these challenges, we also provide suggestions, such as the use of propensity scores for dealing with selection bias and the use of covariates to control for confounding effects. These challenges are then illustrated with examples from diverse subfields of fisheries, including (i) the analysis of the stock–recruit relationship, (ii) fisheries management, rebuilding and population viability, (iii) habitat‐specific vital rates, (iv) life‐history theory and (v) the evaluation of marine reserves. We conclude with our reasons for believing that meta‐analysis will continue to grow in importance for these and many other research goals in fisheries science and argue that standards of practice are therefore essential.  相似文献   
38.
Low intensity control burns are a standard fuel reduction management tool used in pine barrens ecosystems. Periodic disturbances through fire can be an important influence on the cycling of nutrients within the ecosystem. Previous studies have shown that the inorganic chemistry of leaf litter residues differs with increasing temperature. Our study compared chemical changes in white oak (Quercus alba), pitch pine (Pinus rigida) and black huckleberry (Gaylussacia baccata), characteristic of the New Jersey pine barrens, during thermal decomposition using FT-IR spectroscopy. Three replicates of senescent leaf material were ground and separately heated for 2 h at: 100, 200, 300, 400 and 550 °C. These temperatures are representative of the range seen in fuel reducing prescribed burns in the pine barrens. Unburned litter of each species was used as a control. An optimization process using varying amounts of KBr and oak litter was performed to develop favorable FT-IR spectral conditions for a sample to KBr ratio of 0.75%. Chemometric analysis of the FT-IR spectra using principal component analysis (PCA) was used to analyze the changes in carbohydrate chemistry of each litter plant species (leaf litter species) at each temperature. In general, it appears that there is clear separation of leaf litter species at the different combustion temperatures. Infrared spectroscopy illustrated that all three species shared wavenumbers characteristic of the primary components of leaves such as cellulose, lignin and hemicellulose. Results from the PCA indicated separation of litter species and species by combustion temperature. PC axis 1 corresponds to the effects of temperature on leaf litter species and PC axis 2 separates the leaf litter species. At the low temperatures (control-200 °C), oak, pine and huckleberry litter species separated from each other. Wavenumbers that contributed to the separation of species at low temperatures belonged to functional group stretching frequencies of outer surface waxes, basic sugars, fatty acids and aldehydes. It appears that oak had more IR bands specific to suberin content. Convergence of these species occurs at 300 °C. Complexity of chemical composition decreases at this particular temperature as is shown by the decrease in wavenumber richness when compared to litters at low and high temperatures. Oak, pine and huckleberry had similar IR spectra showing bands belonging to outer surface wax content, pectin, lignin and hemicellulose. With increasing temperatures (400-550 °C), differences between litter species increased slightly. Plant material was reduced to similar composition due to thermal decomposition, which consisted of inorganic materials such as carbonate, phosphate and sulfate ions and possible fused aromatics.  相似文献   
39.
BackgroundCirculating plasma N-terminal pro-brain natriuretic peptide (NT-proBNP) concentration facilitates emergency diagnosis of congestive heart failure (CHF) in people. Its utility to discriminate between dyspneic cats with CHF vs. primary respiratory disease requires further assessment. Our objectives were to determine if NT-proBNP (1) differentiates dyspneic cats with CHF vs. primary respiratory disease; (2) increases with renal insufficiency; (3) correlates with left atrial dimension, radiographic cardiomegaly, and estimated left ventricular filling pressure (E/Ea).MethodsNT-proBNP was measured in 167 dyspneic cats (66 primary respiratory disease, 101 CHF) to evaluate (1) relationship with clinical parameters; (2) ability to distinguish CHF from primary respiratory disease; (3) optimal cut-off values using receiver operating characteristic (ROC) curve analysis.ResultsNT-proBNP (1) was higher (median and inter-quartile [25th–75th] percentile) in CHF (754 pmol/L; 437, 1035 pmol/L) vs. primary respiratory disease (76.5 pmol/L; 24, 180 pmol/L) cohorts (P < 0.001); (2) positively correlated in CHF cats with increased inter-ventricular septal end-diastolic thickness (ρ = 0.266; P = 0.007) and LV free wall thickness (ρ = 0.218; P = 0.027), but not with radiographic heart size, left atrial size, left ventricular dimensions, E/Ea ratio, BUN, creatinine, or thyroxine; (3) distinguished dyspneic CHF cats from primary respiratory disease at 265 pmol/L cut-off value with 90.2% sensitivity, 87.9% specificity, 92% positive predictive value, and 85.3% negative predictive value (area under ROC curve, 0.94).ConclusionsNT-proBNP accurately discriminated CHF from respiratory disease causes of dyspnea.  相似文献   
40.
A food consumption revolution is taking place in Russia. After decades of severe constraints on food consumption options under the communist regime Russian consumers are now adopting new food products—including seafood products – at a high pace. Since Russian consumers have previously had very limited seafood consumption choices, the market can be seen as an interesting laboratory for investigating consumer responses to products that have previously not been available. Among imported seafood products are both wild and farmed species. Furthermore, Russian imports include both traditional species such as herring, and ‘new’ species such as pangasius. We analyze market integration among seafood products using Russian monthly import prices from 2002 to 2007 on several products, such as herring, salmonids and pangasius. We find that pangasius compete in the white fish segment, and is a price leader. In the salmonids market, farmed salmon trout appears to be the price leader, both in the fresh and frozen market segment.  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