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   334篇
  免费   14篇
林业   37篇
农学   13篇
基础科学   3篇
  133篇
综合类   21篇
农作物   16篇
水产渔业   12篇
畜牧兽医   76篇
园艺   6篇
植物保护   31篇
  2024年   1篇
  2023年   1篇
  2022年   1篇
  2021年   5篇
  2020年   10篇
  2019年   10篇
  2018年   4篇
  2017年   9篇
  2016年   9篇
  2015年   9篇
  2014年   13篇
  2013年   21篇
  2012年   30篇
  2011年   16篇
  2010年   11篇
  2009年   21篇
  2008年   20篇
  2007年   30篇
  2006年   19篇
  2005年   11篇
  2004年   17篇
  2003年   15篇
  2002年   15篇
  2001年   8篇
  2000年   7篇
  1999年   3篇
  1998年   4篇
  1997年   2篇
  1996年   1篇
  1995年   3篇
  1994年   2篇
  1993年   4篇
  1992年   2篇
  1991年   2篇
  1990年   1篇
  1989年   5篇
  1988年   2篇
  1977年   1篇
  1970年   1篇
  1939年   2篇
排序方式: 共有348条查询结果,搜索用时 328 毫秒
1.
2.
Intravenous (i.v.) bolus administration of xylazine (XYL) (0.5 mg/kg) immediately followed by a continuous rate infusion (CRI) of 1 mg kg−1 hr−1 for 2, 4, and 6 hr produced immediate sedation, which lasted throughout the duration of the CRI. Heart rate decreased and blood pressure increased significantly (p > .05) in all horses during the first 15 min of infusion, both returned to and then remained at baseline during the duration of the infusion. Compartmental models were used to investigate the pharmacokinetics of XYL administration. Plasma concentration–time curves following bolus and CRI were best described by a one-compartment model. No differences were found between pharmacokinetic estimates of the CRIs for the fractional elimination rate constant (Ke), half-life (t1/2e), volume of distribution (Vd), and clearance (Cl). Median and range were 0.42 (0.15–0.97)/hr, 1.68 (0.87–4.52) hr, 5.85 (2.10–19.34) L/kg, and 28.7 (19.6–39.5) ml min−1 kg−1, respectively. Significant differences were seen for area under the curve ( ) (p < .0002) and maximum concentration (Cmax) (p < .04). This indicates that with increasing duration of infusion, XYL may not accumulate in a clinically relevant way and hence no adjustments are required in a longer XYL CRI to maintain a constant level of sedation and a rapid recovery.  相似文献   
3.
4.
According to current scientific opinion the risk of human infection with H5N1 via preparation and consumption of poultry meat is negligible.This opinion has not yet been challenged by a formal risk assessment, due to the lack of empirical data. We have developed a scenario pathway model as a conceptual framework for a formal assessment of the H5N1 risk to humans through consumption of poultry meat and parameterise the model using information derived from expert opinions. The aim of this study was to investigate whether the notion of an overall negligible risk via the oral infection route is consistent with ad hoc data and expert opinions on the relevant parameters of the model.The model is mainly based on expert opinion. A stochastic Monte-Carlo simulation was conducted which took into consideration (amongst others) the exposure and infection of chicken (broiler and layer), turkeys, ducks and geese, the probabilities of detection prior to slaughter, virus survival and contamination during slaughter, as well as during the cutting and preparation of meat in commercial plants and in private households, respectively. The empirical consumption pattern for poultry meat in Germany was taken into account in the simulation.The results show that the risk for the individual consumer is practically zero whereas up to 23 cases per year in Germany might occur if the upper (more pessimistic) ranges of the expert opinions apply.The finding of a low but non-negligible risk to the population is discussed in relation to the epidemiological information available from recent outbreaks in South East Asia.  相似文献   
5.
Transplantation of ovarian tissue has high potential for female gamete conservation. However, optimal timing of oocyte recovery for in vitro maturation and fertilization is still critical. Therefore the aim of the present study was to use high-resolution transcutaneous ultrasonography to monitor follicular development within xenografted ovarian tissue. Ovarian cortex fragments (n=44) from domestic cats were transplanted into athymic nude rats (n=12). Graft development in the animals was assessed weekly by high frequency ultrasound (10-22 MHz) under two different FSH regimes. Blood collection for serum estradiol determination and vaginal smears were performed simultaneously. The xenografts were removed at different time points according to the ultrasound findings. The survival rate of the transplants 4 weeks after surgery was 54.5% and antral follicular growth was observed within 10 grafts from 5 different hosts (8.6 +/- 6.43 follicles per graft). Early follicle antrums could be detected from 0.4 mm onwards. The growth rate of the antral cavity was calculated from weekly measurements (0.56 +/- 0.44 mm per week). Although vaginal cells and estradiol levels followed a cyclic pattern, no correlation was found between follicular diameter, estradiol and keratinized vaginal cells. We recovered 5, 1 and 4 cumulus oocyte complexes from three different individuals during weeks 19, 21, and 23 respectively. Extrusion of a polar body (1 oocyte) and germinal vesicle break down (7 oocytes) indicated progression of maturation after in vitro culture. We conclude that ultrasonography und provided a reliable method to examine xenograft survival and follicular development within the grafts. Furthermore, this technique is suitable for assessment of the efficiency of hormonal treatment and narrowing of the optimal time frame for oocyte retrieval. To our knowledge, this is the first report of the in vivo development of early antral follicles in mammalian species.  相似文献   
6.
Background: Nitrogen losses is an economic problem for wheat production and a high risk to the environment. Therefore, improved N fertilizer management is a key to increasing the N efficiency and minimizing N losses. To increase N efficiency, enhanced fertilizers such as urea combined with urease inhibitor can be used. Aims: The aim of present study was to evaluate the effects of different N forms on grain storage protein subunits in winter wheat and to examine whether the observed changes correlate with parameters of baking quality. Methods: The investigation was performed over two consecutive years at two locations in Germany. Protein subunits were analyzed by SDS‐PAGE. Results: Protein concentrations were similarly increased after fertilization with ammonium nitrate and urea + urease inhibitor. Analysis of the individual storage protein fractions indicated that both fertilizers specifically enhanced ω‐gliadins and HMW glutenins, but the effect was more pronounced in the ammonium nitrate treatment. Application of urea + urease inhibitor had greater influence on the protein composition and resulted in higher specific baking volume as well as the best fresh keeping ability, in comparison with urea treatment. Conclusion: Considering that the urea + urease inhibitor treatment resulted in almost comparable improvements of NUE and baking quality, with the additional benefit of reduced N losses in combination with easy handling, urea + urease inhibitor can be recommended as a viable alternative to both urea alone and ammonium nitrate treatments. This opens up an opportunity for the reduction of N loss in wheat production when use of urea is preferred.  相似文献   
7.
Based on recent findings in the literature, we developed a process‐oriented conceptual model that integrates all three process groups of organic matter (OM) stabilization in soils namely (1) selective preservation of recalcitrant compounds, (2) spatial inaccessibility to decomposer organisms, and (3) interactions of OM with minerals and metal ions. The model concept relates the diverse stabilization mechanisms to active, intermediate, and passive pools. The formation of the passive pool is regarded as hierarchical structured co‐action of various processes that are active under specific pedogenetic conditions. To evaluate the model, we used data of pool sizes and turnover times of soil OM fractions from horizons of two acid forest and two agricultural soils. Selective preservation of recalcitrant compounds is relevant in the active pool and particularly in soil horizons with high C contents. Biogenic aggregation preserves OM in the intermediate pool and is limited to topsoil horizons. Spatial inaccessibility due to the occlusion of OM in clay microstructures and due to the formation of hydrophobic surfaces stabilizes OM in the passive pool. If present, charcoal contributes to the passive pool mainly in topsoil horizons. The importance of organo‐mineral interactions for OM stabilization in the passive pool is well‐known and increases with soil depth. Hydrophobicity is particularly relevant in acid soils and in soils with considerable inputs of charcoal. We conclude that the stabilization potentials of soils are site‐ and horizon‐specific. Furthermore, management affects key stabilization mechanisms. Tillage increases the importance of organo‐mineral interactions for OM stabilization, and in Ap horizons with high microbial activity and C turnover, organo‐mineral interactions can contribute to OM stabilization in the intermediate pool. The application of our model showed that we need a better understanding of processes causing spatial inaccessibility of OM to decomposers in the passive pool.  相似文献   
8.
Succinate dehydrogenase inhibitors (SDHIs) have played a crucial role in disease control to protect cereals as well as fruit and vegetables for more than a decade. Isoflucypram, the first representative of a newly installed subclass of SDHIs inside the Fungicide Resistance Action Committee (FRAC) family of complex II inhibitors, offers unparalleled long‐lasting efficacy against major foliar diseases in cereals. Herein we report the chemical optimization from early discovery towards isoflucypram and the first hypothesis of its altered binding mode in the ubiquinone binding site of succinate dehydrogenase. © 2020 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.  相似文献   
9.
In the 1970s unexpected forest damages, called “new type of forest damage” or “forest decline”, were observed in Germany and other European countries. The Federal Republic of Germany and the German Federal States implemented a forest monitoring system in the early 1980s, in order to monitor and assess the forest condition. Due to the growing public awareness of possible adverse effects of air pollution on forests, in 1985 the ICP Forests was launched under the convention on long-range transboundary air pollution (CLRTAP) of the United Nations Economic Commission for Europe (UN-ECE). The German experience in forest monitoring was a base for the implementation of the European monitoring system. In 2001 the interdisciplinary case study “concept and feasibility study for the integrated evaluation of environmental monitoring data in forests”, funded by the German Federal Ministry of Education and Research, concentrated on in-depths evaluations of the German data of forest monitoring. The objectives of the study were: (a) a reliable assessment of the vitality and functioning of forest ecosystems, (b) the identification and quantification of factors influencing forest vitality, and (c) the clarification of cause-effect-relationships leading to leaf/needle loss. For these purposes additional data from external sources were acquired: climate and deposition, for selected level I plots tree growth data, as well as data on groundwater quality. The results show that in particular time series analysis (crown condition, tree growth, and tree ring analysis), in combination with climate and deposition are valuable and informative, as well as integrated evaluation of soil, tree nutrition and crown condition data. Methods to combine information from the extensive and the intensive monitoring, and to transfer process information to the large scale should be elaborated in future.
Sabine AugustinEmail:
  相似文献   
10.
Gegenstand der Untersuchungen sind zwei Dauerfeldversuche, die im Jahr 1984 in Rauischholzhausen (Deutschland) und im Jahr 1996 in Wroc?aw‐Swqjec (Polen) mit vergleichbarer Methode und Zielstellung angelegt wurden. Ausgewertet wurden hier die Zuckerrübenerträge im Mittel der dreijährigen Rotation der Jahre 1997–1999 in Abhängigkeit von organischen und mineralischen Düngungsmaßnahmen. Die erzielten Ergebnisse unterstreichen die Bedeutung der organischen Düngung für die Ertragssicherung im Zuckerrübenanbau. Die untersuchten organischen Dünger Stalldung, Stroh und Biokompost verbesserten die Ertragswirksamkeit mineralischer N‐Gaben.

Die ertragssteigernde Wirkung organischer Dünger war bei fehlender oder niedriger mineralischer N‐Düngung größer als bei hohen N‐Gaben. Auf stark lehmigem Sandboden (Wrozlaw) wurden höhere Ertragseffekte erzielt als auf schwerem Lehmboden (Rauischholzhausen). Die Blatterträge wurden durch die angewendeten Düngungsmaßnahmen stärker beeinflußt als die Rübenerträge. Stroh und Biokompost zeigten die gleichen Ertragseffekte wie Stalldung.  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