全文获取类型
收费全文 | 273篇 |
免费 | 29篇 |
专业分类
林业 | 19篇 |
农学 | 7篇 |
51篇 | |
综合类 | 26篇 |
农作物 | 15篇 |
水产渔业 | 22篇 |
畜牧兽医 | 149篇 |
园艺 | 5篇 |
植物保护 | 8篇 |
出版年
2023年 | 5篇 |
2022年 | 4篇 |
2021年 | 12篇 |
2020年 | 9篇 |
2019年 | 16篇 |
2018年 | 13篇 |
2017年 | 11篇 |
2016年 | 14篇 |
2015年 | 11篇 |
2014年 | 9篇 |
2013年 | 17篇 |
2012年 | 16篇 |
2011年 | 26篇 |
2010年 | 15篇 |
2009年 | 9篇 |
2008年 | 11篇 |
2007年 | 19篇 |
2006年 | 12篇 |
2005年 | 13篇 |
2004年 | 8篇 |
2003年 | 9篇 |
2002年 | 8篇 |
2001年 | 4篇 |
2000年 | 3篇 |
1999年 | 4篇 |
1998年 | 3篇 |
1995年 | 1篇 |
1994年 | 2篇 |
1993年 | 1篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1990年 | 2篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1983年 | 2篇 |
1980年 | 4篇 |
1955年 | 1篇 |
排序方式: 共有302条查询结果,搜索用时 15 毫秒
61.
62.
63.
Jaramillo TF Jørgensen KP Bonde J Nielsen JH Horch S Chorkendorff I 《Science (New York, N.Y.)》2007,317(5834):100-102
The identification of the active sites in heterogeneous catalysis requires a combination of surface sensitive methods and reactivity studies. We determined the active site for hydrogen evolution, a reaction catalyzed by precious metals, on nanoparticulate molybdenum disulfide (MoS2) by atomically resolving the surface of this catalyst before measuring electrochemical activity in solution. By preparing MoS2 nanoparticles of different sizes, we systematically varied the distribution of surface sites on MoS2 nanoparticles on Au(111), which we quantified with scanning tunneling microscopy. Electrocatalytic activity measurements for hydrogen evolution correlate linearly with the number of edge sites on the MoS2 catalyst. 相似文献
64.
Methicillin-resistant Staphylococcus pseudintermedius among dogs admitted to a small animal hospital
Nienhoff U Kadlec K Chaberny IF Verspohl J Gerlach GF Kreienbrock L Schwarz S Simon D Nolte I 《Veterinary microbiology》2011,150(1-2):191-197
The aim of this study was to determine the frequency of carriage of methicillin-resistant Staphylococcus pseudintermedius (MRSP) among dogs admitted to a small animal hospital during a 17-month period, to characterize these isolates and to initially screen for possible factors associated with MRSP carriage. Swabs were taken from the nose/pharynx and the perineum as well as from wounds and skin infections (if present) of 814 dogs before entering the small animal hospital. A questionnaire for background information was completed. The staphylococcal species and methicillin resistance were confirmed pheno- and genotypically. The identified MRSP isolates were characterized by SCCmec typing, testing for susceptibility to 25 antimicrobial agents and SmaI-directed pulsed-field gel electrophoresis. A first screening for possible risk factors for MRSP carriage was performed by means of unifactorial contingency tables and CART analysis. Sixty (7.4%) dogs were positive for MRSP. All MRSP isolates harboured a type II-III SCCmec cassette and showed extended resistance to antimicrobial agents. Fifteen different SmaI patterns were observed. The major factors that clustered with MRSP carriage were former hospitalization and antibiotic treatment within the last six months before sampling. This study showed that only a minor part of the sampled dogs carried multi-resistant MRSP isolates. The facts that prior hospitalization and/or antibiotic therapy are potential associated factors for MRSP carriage underline the necessity of a judicious use of antibiotics in small animal medicine. 相似文献
65.
Ana Eguiguren Enrico Pirotta Kristina Boerder Maurício Cantor Godfrey Merlen Hal Whitehead 《水产资源保护:海洋与淡水生态系统》2021,31(6):1466-1481
- Sperm whales have occupied the waters off the Galápagos Islands, Ecuador, for at least the past 200 years. During the 19th century, they were the target of intensive whaling that severely depleted the population. In recent times, after commercial whaling ended, sperm whales in the region remain vulnerable to multiple threats, especially potential entanglement in fishing gear, which may hinder their ability to recover from the whaling era.
- As a highly mobile, long-lived species, long-term analysis of the habitat use of sperm whales is necessary to establish effective conservation and management strategies. Here, contemporary (1985–2014) and historical (1830–1850) sperm whale habitat use off the Galápagos Islands was analysed and contrasted to the extent of the Galápagos Marine Reserve (GMR). Contemporary habitat use and its variability over time were modelled as a function of geographic, oceanographic, and topographic variables using generalized additive models.
- The fine-scale habitat (<50 km) used by sperm whales was associated with topographic (i.e. depth and slope) and oceanographic characteristics (i.e. relative sea surface temperature and standard deviation of sea surface temperature), but these preferences varied over time.
- While historical and contemporary data indicate that sperm whale habitat primarily occurred within the boundaries of the GMR, in recent years, whales were found up to 30.1% of the time outside the GMR, potentially overlapping with commercial fisheries operating in the area.
- The dynamic nature of the relationship of this nomadic species with its habitat highlights the need of large-scale conservation efforts across the Eastern Tropical Pacific region, including the wide-scale enforcement of regulations requiring the use of Automatic Identification System in fishing vessels, the promotion of on-board fisheries observer programmes, the development of adaptive management strategies, and international collaboration to identify and mitigate threats.
66.
Karin Hultin Jäderlund DVM Kerstin Hansson DVM Anna-Lena Berg DVM pHd Anders Sjöström MD pHd Kristina Narfström DVM PHD 《Veterinary radiology & ultrasound》2003,44(5):581-588
To evaluate the age of fontanelle closure of normal kittens and the size of their lateral ventricles, 50 ultrasonographic examinations using the bregmatic fontanelle as an acoustic window were made. Seven kittens, laboratory animals, were included in the study. To verify the location of the lateral ventricle, two of the kittens were sacrificed as neonates. In one of them ink was injected prior to autopsy into one lateral ventricle under sonographic guidance. In a longitudinal study of five of the kittens, the skull depth and the depth of the central part of the lateral ventricle reproduced in a longitudinal view could be measured up to the age of about 5 months. During that period, the skull depth increased from a median value of 1.95 cm (1.92-1.98) in a seven-day-old cat to 2.58 cm (2.52-2.59) in a 154-day-old cat, while the afore-mentioned ventricle values increased from 0.3 mm to 1.1 mm. 相似文献
67.
Jessica M. Price Janet Silbernagel Kristina Nixon Amanda Swearingen Randy Swaty Nicholas Miller 《Landscape Ecology》2016,31(5):1093-1115
Context
Broad-scale land conservation and management often involve applying multiple strategies in a single landscape. However, the potential outcomes of such arrangements remain difficult to evaluate given the interactions of ecosystem dynamics, resource extraction, and natural disturbances. The costs and potential risks of implementing these strategies make robust evaluation critical.Objectives
We used collaborative scenario modeling to compare the potential outcomes of alternative management strategies in the Two Hearted River watershed in Michigan’s Upper Peninsula to answer key questions: Which management strategies best achieve conservation goals of maintaining landscape spatial heterogeneity and conserving mature forests and wetlands? And how does an increase in wildfire and windthrow disturbances influence these outcomes?Methods
Scenarios were modeled using the VDDT/TELSA state-and-transition modeling suite, and resulting land cover maps were analyzed using ArcGIS, FRAGSTATS, and R statistical software.Results
Results indicate that blending conservation strategies, such as single-ownership forest reserves and working forest conservation easements in targeted areas of the landscape, may better achieve these goals than applying a single strategy across the same area. However, strategies that best achieve these conservation goals may increase the sensitivity of the landscape to changes in wildfire and windthrow disturbance regimes.Conclusions
These results inform decision-making about which conservation strategy or combination of strategies to apply in specific locations on the landscape to achieve optimum conservation outcomes, how to best utilize scarce financial resources, and how to reduce the financial and ecological risks associated with the application of innovative strategies in an uncertain future.68.
69.
70.