全文获取类型
| 收费全文 | 5263篇 |
| 免费 | 227篇 |
| 国内免费 | 8篇 |
专业分类
| 林业 | 313篇 |
| 农学 | 159篇 |
| 基础科学 | 46篇 |
| 798篇 | |
| 综合类 | 836篇 |
| 农作物 | 246篇 |
| 水产渔业 | 438篇 |
| 畜牧兽医 | 2078篇 |
| 园艺 | 152篇 |
| 植物保护 | 432篇 |
出版年
| 2023年 | 31篇 |
| 2022年 | 28篇 |
| 2021年 | 99篇 |
| 2020年 | 118篇 |
| 2019年 | 128篇 |
| 2018年 | 89篇 |
| 2017年 | 105篇 |
| 2016年 | 97篇 |
| 2015年 | 106篇 |
| 2014年 | 122篇 |
| 2013年 | 247篇 |
| 2012年 | 289篇 |
| 2011年 | 350篇 |
| 2010年 | 199篇 |
| 2009年 | 153篇 |
| 2008年 | 345篇 |
| 2007年 | 291篇 |
| 2006年 | 296篇 |
| 2005年 | 301篇 |
| 2004年 | 273篇 |
| 2003年 | 273篇 |
| 2002年 | 263篇 |
| 2001年 | 58篇 |
| 2000年 | 67篇 |
| 1999年 | 74篇 |
| 1998年 | 54篇 |
| 1997年 | 45篇 |
| 1996年 | 44篇 |
| 1995年 | 40篇 |
| 1994年 | 50篇 |
| 1993年 | 55篇 |
| 1992年 | 45篇 |
| 1991年 | 40篇 |
| 1990年 | 41篇 |
| 1989年 | 35篇 |
| 1988年 | 36篇 |
| 1987年 | 37篇 |
| 1986年 | 28篇 |
| 1985年 | 36篇 |
| 1984年 | 40篇 |
| 1983年 | 39篇 |
| 1982年 | 20篇 |
| 1981年 | 33篇 |
| 1980年 | 24篇 |
| 1979年 | 26篇 |
| 1978年 | 25篇 |
| 1977年 | 19篇 |
| 1975年 | 18篇 |
| 1974年 | 25篇 |
| 1973年 | 24篇 |
排序方式： 共有5498条查询结果，搜索用时 734 毫秒
31.
32.
Tania S. Peña James R. Watson Laura I. González-Guzmán Timothy H. Keitt 《Landscape Ecology》2017,32(8):1643-1656
Context
Many nearshore species are distributed in habitat patches connected only through larval dispersal. Genetic research has shown some spatial structure of such metapopulations and modeling studies have shed light onto possible patterns of connectivity and barriers. However, little is known about human impact on their spatial structure and patterns of connectivity.Objectives
We examine the effects of fishing on the spatial and temporal dynamics of metapopulations of sedentary marine species (red sea urchin and red abalone) interconnected by larval dispersal.Methods
We constructed a metapopulation model to simulate abalone and sea urchin metapopulations experiencing increasing levels of fishing mortality. We performed the modularity analysis on the yearly larval connectivity matrices produced by these simulations, and analyzed the changes of modularity and the formation of modules over time as indicators of spatial structure.Results
The analysis revealed a strong modular spatial structure for abalone and a weak spatial signature for sea urchin. In abalone, under exploitation, modularity takes step-wise drops on the path to extinction, and modules breakdown into smaller fragments followed by module and later metapopulation collapse. In contrast, sea urchin showed high modularity variation, indicating high- and low-mixing years, but an abrupt collapse of the metapopulation under strong exploitation.Conclusions
The results identify a disruption in larval connectivity and a pattern of collapse in highly modular nearshore metapopulations. These results highlight the ability of modularity to detect spatial structure in marine metapopulations, which varies among species, and to show early changes in the spatial structure of exploited metapopulations.33.
Comparing mating designs to restore seed production of interspecific hybrids between Trifolium repens (white clover) and Trifolium uniflorum 
下载免费PDF全文
下载免费PDF全文 Muhammad Naeem Isabelle Marjorie Verry Peter D. Kemp James P. Millner Warren Mervyn Williams 《Plant Breeding》2017,136(3):420-426
Interspecific hybrids between Trifolium uniflorum and cultivated white clover (Trifolium repens) have highly useful characteristics for temperate pastoral systems derived from both parent species. However, the early hybrids (F1 and BC1) also have unacceptably poor seed production for commerce. This study analysed the basis for the poor seed production and investigated breeding strategies for overcoming the problem. The BC1F1 generation produced lower‐than‐expected numbers of heads per plant and seeds per floret. Backcrossing of selected hybrids to white clover corrected these deficiencies and created new variation. Seed numbers were also returned to near target levels by recurrent selection within the BC1 generation. Thus, it was possible to retain a theoretical average of 25% of T. uniflorum genome and still achieve high seed production per plant. The BC1F2 and BC2F1 generations produced high seed numbers per plant, along with reasonable variation. Both of these second‐generation hybrid forms have high reproductive potential and should be the focus for the selection of the desired combinations of agronomic and seed production traits. 相似文献
34.
35.
Evaluation of Heterotrophic Algae Meal as a Diet Ingredient for Channel Catfish,Ictalurus punctatus 下载免费PDF全文
下载免费PDF全文 Zachary A. Kupchinsky Shawn D. Coyle Leigh A. Bright James H. Tidwell 《Journal of the World Aquaculture Society》2015,46(4):445-452
Most microalgae evaluated in aquaculture diets have been produced autotrophically. In order to produce a cost‐efficient biomass at greater magnitudes for biofuel feedstock, heterotrophic production may be warranted. However, the chemical/nutritional attributes of these microalgae could differ from those grown autotrophically. An 8‐wk feeding trial was conducted to evaluate Chlorella spp. algae meal (AM) that had been cultured heterotrophically. The oil (lipid) was extracted to simulate biofuel production. Juvenile channel catfish, Ictalurus punctatus (5.7 ± 1.4 g; 8.9 ± 0.8 cm), were stocked at 10 fish/tank into fifteen 37.7‐L aquaria in a closed recirculating system and fed one of the five experimental diets to apparent satiation twice daily. Diets contained either 0 (control, CTL), 10, 20, or 40% AM and an additional diet containing 40% AM was supplemented with 2% lysine (40% AM+LYS). After 8 wk, there were no statistically significant differences in terms of survival, dressout percentages, whole‐body proximate composition, or fatty acid composition of the fillets among fish fed the diets containing varying levels of AM without added lysine. Feed consumption and weight gain for fish fed the 10, 40, and 40% AM+LYS diets were significantly greater than those fed the CTL diet. Feed conversion ratio was significantly lower for fish fed the 40% AM+LYS diet compared to those fed all other diets, which did not differ significantly from each other. These data indicate that channel catfish readily accept and can efficiently utilize heterotrophically produced AM at levels up to at least 40% of the total diet and that AM may enhance diet palatability. 相似文献
36.
37.
38.
39.
Lowenberg-DeBoer James Huang Iona Yuelu Grigoriadis Vasileios Blackmore Simon 《Precision Agriculture》2020,21(2):278-299
Precision Agriculture - This study reviewed research published after 1990 on the economics of agricultural mechatronic automation and robotics, and identified research gaps. A systematic search was... 相似文献
40.
James L. Smith 《Asia Pacific viewpoint》2017,58(1):99-111
This article is a discussion of the “discourse on the unthinkable” surrounding potential future democratic engagements with rivers as non‐human persons or natural objects. In the context of the Asia–Pacific region, this article suggests that the developments in material philosophy entitled “new materialism” are essential tools in the reconceptualisation of rivers as democratic entities but that local socio‐historical conditions must also be taken into the account. In order to make its case, the article not only surveys the context for considering rivers as non‐human persons in a juridical context but also discusses the new material context that assists modern democracies in the renegotiation of the demos that forms the body politic of democracy – often in the face of neoliberal exploitation and a legacy of extremes in instrumentalism. The article argues that the incorporation of water in the democratic project of enfranchisement is an essential exercise born of many Western beliefs and ideals but articulated uniquely at a regional and national level. 相似文献