全文获取类型
收费全文 | 663篇 |
免费 | 50篇 |
专业分类
林业 | 53篇 |
农学 | 14篇 |
基础科学 | 1篇 |
183篇 | |
综合类 | 30篇 |
农作物 | 27篇 |
水产渔业 | 106篇 |
畜牧兽医 | 267篇 |
园艺 | 13篇 |
植物保护 | 19篇 |
出版年
2022年 | 3篇 |
2021年 | 8篇 |
2020年 | 11篇 |
2019年 | 14篇 |
2018年 | 26篇 |
2017年 | 16篇 |
2016年 | 28篇 |
2015年 | 21篇 |
2014年 | 29篇 |
2013年 | 72篇 |
2012年 | 46篇 |
2011年 | 29篇 |
2010年 | 21篇 |
2009年 | 12篇 |
2008年 | 39篇 |
2007年 | 34篇 |
2006年 | 14篇 |
2005年 | 31篇 |
2004年 | 38篇 |
2003年 | 22篇 |
2002年 | 15篇 |
2001年 | 29篇 |
2000年 | 24篇 |
1999年 | 8篇 |
1998年 | 8篇 |
1997年 | 8篇 |
1996年 | 7篇 |
1995年 | 22篇 |
1994年 | 6篇 |
1993年 | 3篇 |
1992年 | 8篇 |
1991年 | 11篇 |
1990年 | 7篇 |
1989年 | 8篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1984年 | 3篇 |
1983年 | 4篇 |
1979年 | 3篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 3篇 |
1972年 | 2篇 |
1971年 | 1篇 |
1970年 | 3篇 |
1969年 | 3篇 |
1968年 | 1篇 |
1960年 | 1篇 |
1941年 | 1篇 |
排序方式: 共有713条查询结果,搜索用时 15 毫秒
681.
Hanne K. Sjølie Dennis Becker Daniel Håbesland Birger Solberg Berit Hauger Lindstad Stephanie Snyder Mike Kilgore 《Small-Scale Forestry》2016,15(1):29-43
Norway has set ambitious targets for increasing bioenergy production. Forest residue extraction levels are currently very low, but residues have the potential to be an important component of the wood energy supply chain. A representative sample of Norwegian nonindustrial private forest owners having at least 8 ha (20 acres) of productive forest land was surveyed about their willingness to supply logging residues for wood energy production. About 59 % responded that they were willing to do so. Logistic regression analyses revealed that the following factors were positively associated with the likelihood of being willing to supply logging residues: total forest area, education level, living in a region with active timber markets and a history of forest production, and having positive perceptions of residue extraction and forestry’s role in mitigating climate change. Four variables were negatively associated with the likelihood to supply residues: living on property, being older than 65 years, having family or friends who are opposed to residue extraction, and having negative perceptions of residue extraction. The study provides insight regarding nonindustrial forest owners’ attitudes towards extraction of forest residues that may aid policy-makers designing effective means to meet national bioenergy production goals. 相似文献
682.
683.
Sediment cores from two lakes, connected by a limed river, were analyzed for organic matter, calcium (Ca), iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), lead (Pb), cadmium (Cd), and ion exchange capacity. The distribution of trace metals in the pristine upper lake is governed primarily by changes in the long distance atmospheric transported pollutants. In the lake down stream of the liming station the distribution of trace metals in sediments deposited before liming started (1985) is similar to that in the pristine lake. Liming of the connecting river raises the pH value by about 1.8 units from 5.1 enhancing the accumulation rate of trace metals in the downstream lake. Iron enriched acid insoluble particles are likely to be important carriers of trace metals. The liming agent itself is the most important source for Pb, while increased accumulation of Zn and Cd is caused primarily by enhanced scavenging of dissolved metals. For Cu the two sources are of about equal importance. 相似文献
684.
Frössling J Ohlson A Björkman C Håkansson N Nöremark M 《Preventive veterinary medicine》2012,105(3):202-208
Financial resources may limit the number of samples that can be collected and analysed in disease surveillance programmes. When the aim of surveillance is disease detection and identification of case herds, a risk-based approach can increase the sensitivity of the surveillance system. In this paper, the association between two network analysis measures, i.e. 'in-degree' and 'ingoing infection chain', and signs of infection is investigated. It is shown that based on regression analysis of combined data from a recent cross-sectional study for endemic viral infections and network analysis of animal movements, a positive serological result for bovine coronavirus (BCV) and bovine respiratory syncytial virus (BRSV) is significantly associated with the purchase of animals. For BCV, this association was significant also when accounting for herd size and regional cattle density, but not for BRSV. Examples are given for different approaches to include cattle movement data in risk-based surveillance by selecting herds based on network analysis measures. Results show that compared to completely random sampling these approaches increase the number of detected positives, both for BCV and BRSV in our study population. It is concluded that network measures for the relevant time period based on updated databases of animal movements can provide a simple and straight forward tool for risk-based sampling. 相似文献
685.
Larval abundance across the European eel spawning area: An analysis of recent and historic data 下载免费PDF全文
Håkan Westerberg Michael J. Miller Klaus Wysujack Lasse Marohn Marko Freese Jan‐Dag Pohlmann Shun Watanabe Katsumi Tsukamoto Reinhold Hanel 《Fish and Fisheries》2018,19(5):890-902
The abundance and distribution of leptocephalus larvae of the European eel (Anguilla anguilla, Anguillidae) were examined using ten historic and recent Sargasso Sea expeditions that were selected on the basis of having the largest number of sampling stations and highest catches. The surveys cover the period 1920–2014. Station data were recalculated to the same unit of larval density per unit area, and the irregular station positions were transformed to a regular spatial grid to allow calculation of comparable measures of abundance of the youngest (0+) leptocephalus cohort. The result is that the mean and maximum densities of 0+ leptocephali after 2007 on average have decreased by 70%–80% from the densities during the period before the drastic decrease in glass eel recruitment, which started in the 1980s. This is of the same magnitude as the change in spawning stock, if the total continental commercial landings are used as a proxy. In the same period, the glass eel recruitment in Europe has decreased by more than 95%. The conclusion is that a major cause for the recruitment decrease may be an increased leptocephalus mortality during the oceanic phase or a large geographic shift in glass eel arrival. Combining the survey data, the spatial distribution of 0+ leptocephali was concentrated south of the northernmost front in the Subtropical Convergence Zone, but high densities were also found far south of the front in the western part of the distribution area and leptocephali were present also north of the average frontal position. 相似文献
686.
EFFECTS OF TWO DIFFERENT ANESTHETIC PROTOCOLS ON CARDIAC FLOW MEASURED BY TWO DIMENSIONAL PHASE CONTRAST MAGNETIC RESONANCE IMAGING 下载免费PDF全文
Randi Drees Rebecca A. Johnson Rebecca L. Stepien Alejandro Munoz Del Rio Christopher J. François 《Veterinary radiology & ultrasound》2015,56(2):168-175
Companion animals are routinely anesthetized or heavily sedated for cardiac MRI studies, however effects of varying anesthetic protocols on cardiac function measurements are incompletely understood. The purpose of this prospective study was to compare effects of two anesthetic protocols (Protocol A: Midazolam, fentanyl; Protocol B: Dexmedetomidine) on quantitative and qualitative blood flow values measured through the aortic, pulmonic, mitral, and tricuspid valves using two‐dimensional phase contrast magnetic resonance imaging (2D PC MRI) in healthy dogs. Mean flow per heartbeat values through the pulmonary artery (Qp) and aorta (Qs) were compared to right and left ventricular stroke volumes (RVSV, LVSV) measured using a reference standard of 2D Cine balanced steady‐state free precession MRI. Pulmonary to systemic flow ratio (Qp/Qs) was also calculated. Differences in flow and Qp/Qs values generated using 2D PC MRI did not differ between the two anesthetic protocols (P = 1). Mean differences between Qp and RVSV were 3.82 ml/beat (95% limits of agreement: 3.62, ?11.26) and 1.9 ml/beat (?7.86, 11.66) for anesthesia protocols A and B, respectively. Mean differences between Qs and LVSV were 1.65 ml/beat (?5.04, 8.34) and 0.03 ml/beat (?4.65, 4.72) for anesthesia protocols A and B, respectively. Mild tricuspid or mitral reflux was seen in 2/10 dogs using 2D PC MRI. No aortic or pulmonic insufficiency was observed. Findings from the current study indicated that these two anesthetic protocols yield similar functional measures of cardiac blood flow using 2D PC MRI in healthy dogs. Future studies in clinically affected patients are needed. 相似文献
687.
688.
Retrospective evaluation of thoracic computed tomography findings in dogs naturally infected by Angiostrongylus vasorum 下载免费PDF全文
689.
Prevalence of piscine orthoreovirus and salmonid alphavirus in sea‐caught returning adult Atlantic salmon (Salmo salar L.) in northern Norway 下载免费PDF全文
A S Madhun C H Isachsen L M Omdal A C B Einen S Mæhle V Wennevik E Niemelä T Svåsand E Karlsbakk 《Journal of fish diseases》2018,41(5):797-803
Heart and skeletal muscle inflammation (HSMI) caused by piscine orthoreovirus (PRV) and pancreas disease (PD) caused by salmonid alphavirus (SAV) are among the most prevalent viral diseases of Atlantic salmon farmed in Norway. There are limited data about the impact of disease in farmed salmon on wild salmon populations. Therefore, the prevalence of PRV and SAV in returning salmon caught in six sea sites was determined using real‐time RT‐PCR analyses. Of 419 salmon tested, 15.8% tested positive for PRV, while none were positive for SAV. However, scale reading revealed that 10% of the salmon had escaped from farms. The prevalence of PRV in wild salmon (8%) was significantly lower than in farm escapees (86%), and increased with fish length (proxy for age). Sequencing of the S1 gene of PRV from 39 infected fish revealed a mix of genotypes. The observed increase in PRV prevalence with fish age and the lack of phylogeographic structure of the virus could be explained by virus transmission in the feeding areas. Our results highlight the need for studies about the prevalence of PRV and other pathogens in Atlantic salmon in its oceanic phase. 相似文献
690.
Little is known about the size selectivity of cod in static fishing gears such as pots and traps. In this field study, floating fishing pots were equipped with 40, 45 and 50 mm square mesh escape windows in order to estimate the size selectivity of cod at different mesh sizes. Relationships between selectivity parameters and mesh size, as well as an optimal mesh size for the escape window with respect to current minimum landing size of cod in the Baltic Sea were sought. The results show that the floating pot is not only species selective when used in the Baltic Sea cod fishery, the implementation of an escape window reduced the proportion of undersized bycatch in the pots by more than 90%. The estimated length at 50% retention was found to be a direct function of fish body length (girth) and mesh size of the escape window, while the estimated selection range remained unchanged regardless of mesh size. Optimal mesh size, with respect to the length at 50% retention and current minimal landing size of 38 cm in the Baltic, was determined to be 45 mm. Strong indications (significant on the 0.01 probability level in the case of 50 and 40 mm escape windows) suggested that the relative fishing power of the pots increased with the implementation of an escape window. This result could be explained as a “saturation effect”, i.e. the probability of cod entering the pot is likely to be negatively dependent on cod density in the pot. The high species selectivity and the low catch rate of undersized fish have consolidated the floating pots position as a highly benign fishing method and the foremost alternative gear for the future coastal cod fishery in the Baltic Sea. 相似文献