全文获取类型
收费全文 | 2829篇 |
免费 | 179篇 |
国内免费 | 2篇 |
专业分类
林业 | 220篇 |
农学 | 61篇 |
基础科学 | 16篇 |
683篇 | |
综合类 | 365篇 |
农作物 | 133篇 |
水产渔业 | 227篇 |
畜牧兽医 | 1046篇 |
园艺 | 87篇 |
植物保护 | 172篇 |
出版年
2024年 | 5篇 |
2023年 | 22篇 |
2022年 | 37篇 |
2021年 | 78篇 |
2020年 | 79篇 |
2019年 | 102篇 |
2018年 | 106篇 |
2017年 | 92篇 |
2016年 | 98篇 |
2015年 | 79篇 |
2014年 | 106篇 |
2013年 | 137篇 |
2012年 | 215篇 |
2011年 | 258篇 |
2010年 | 124篇 |
2009年 | 107篇 |
2008年 | 195篇 |
2007年 | 157篇 |
2006年 | 197篇 |
2005年 | 161篇 |
2004年 | 139篇 |
2003年 | 136篇 |
2002年 | 122篇 |
2001年 | 25篇 |
2000年 | 34篇 |
1999年 | 15篇 |
1998年 | 19篇 |
1997年 | 14篇 |
1996年 | 8篇 |
1995年 | 18篇 |
1994年 | 10篇 |
1993年 | 5篇 |
1992年 | 8篇 |
1991年 | 5篇 |
1990年 | 6篇 |
1987年 | 5篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 4篇 |
1980年 | 5篇 |
1979年 | 5篇 |
1978年 | 5篇 |
1977年 | 3篇 |
1975年 | 6篇 |
1974年 | 6篇 |
1973年 | 3篇 |
1957年 | 2篇 |
1954年 | 2篇 |
1928年 | 3篇 |
1924年 | 4篇 |
排序方式: 共有3010条查询结果,搜索用时 15 毫秒
181.
182.
Eric A. DeVuyst Scott W. PryorGreg Lardy Wallace EideRon Wiederholt 《Agricultural Systems》2011,104(8):609-614
The recent surge in US ethanol production has led to an increase in the amount of byproduct feed available to cattle producers. The impetus behind the increase in ethanol production is US policy to increase the production and use of renewable fuels. Manure from cattle production can also be used to produce a renewable fuel: methane. By co-locating a cattle feedlot with an existing ethanol plant, there may be synergies between ethanol, methane and cattle production. Byproduct from the ethanol plant can be fed to feeder cattle, manure from the cattle used in methane production, and the methane used as an energy source in the ethanol plant. Alternatively, methane can be used to generate electricity. We investigate the economic advisability of these systems. Using cost estimates for construction and operating a feedlot and anaerobic digester for methane production, we project revenues and costs over a 20-year time period. Our findings are consistent with studies that have considered just cattle production and methane production. The investment required to build a slatted-floor feedlot and concrete anaerobic digester cannot be justified under existing economic conditions. An increase in renewable fuel/electricity subsidies of at least $0.053 per kW h or $72 per 1000 m3 of methane are required to just break even when methane is used to produce electricity or is burned for thermal energy, respectively. 相似文献
183.
184.
185.
186.
187.
Phylogenetic MCMC algorithms are misleading on mixtures of trees 总被引:2,自引:0,他引:2
Markov chain Monte Carlo (MCMC) algorithms play a critical role in the Bayesian approach to phylogenetic inference. We present a theoretical analysis of the rate of convergence of many of the widely used Markov chains. For N characters generated from a uniform mixture of two trees, we prove that the Markov chains take an exponentially long (in N) number of iterations to converge to the posterior distribution. Nevertheless, the likelihood plots for sample runs of the Markov chains deceivingly suggest that the chains converge rapidly to a unique tree. Our results rely on novel mathematical understanding of the log-likelihood function on the space of phylogenetic trees. The practical implications of our work are that Bayesian MCMC methods can be misleading when the data are generated from a mixture of trees. Thus, in cases of data containing potentially conflicting phylogenetic signals, phylogenetic reconstruction should be performed separately on each signal. 相似文献
188.
Highly extensible Escherichia coli DNA molecules in planar extensional flow were visualized in dilute solution by fluorescence microscopy. For a narrow range of flow strengths, the molecules were found in either a coiled or highly extended conformation, depending on the deformation history of the polymer. This conformation hysteresis persists for many polymer relaxation times and is due to conformation-dependent hydrodynamic forces. Polymer conformational free-energy landscapes were calculated from computer simulations and show two free-energy minima for flow strengths near the coil-stretch transition. Hysteresis cycles may directly influence bulk-solution stresses and the development of stress-strain relations for dilute polymer flows. 相似文献
189.
Marta Crivos María Rosa Martínez María Lelia Pochettino Carolina Remorini Cynthia Saenz Anahí Sy 《Agriculture and Human Values》2004,21(2-3):111-125
Through the ethnographic record of the subsistence activities partially or completely performed in the domestic sphere in two Mbyá-Guaraní settlements in Misiones (Argentina), we outline factors important in describing the local natural environment. Data was collected through systematic observation and also through semi-structured interviews. Analysis indicates that the natural environment of the area is characterized by the indigenous community in several different ways. Thus, local people view the environment as made up of different “micro-environments,” and they consequently think of the elements that compose these “micro-environments” as having different, distinct characteristics. In the context of their daily activities, both these “micro-environments” and the elements that compose them are regarded as resources, in that they are viewed in terms of what is in them relevant to the subsistence of these groups in the rainforest. It is the intention of this study to initiate the systematic recording and processing of information on how these indigenous communities know and manage the natural resources available to them in their daily life. 相似文献
190.
R W Schevitz M A Navia D A Bantz G Cornick J J Rosa M D Rosa P B Sigler 《Science (New York, N.Y.)》1972,177(47):429-431
An isomorphous osmium derivative of crystalline yeast initiator transfer RNA has been prepared and interpreted to 6-angstrom resolution. The coordinates of the heavy atoms have been determined by Patterson and "direct" methods applied to the difference coefficients of the centric projections, followed by least-squares refinement. There is one dominant site per asymmetric unit, consistent with the finding by neutron-activation analysis that there is approximately one osmium atom per molecule of transfer RNA. The osmium derivative appears to be a normal substrate for enzymatic aminoacylation. 相似文献