全文获取类型
收费全文 | 35593篇 |
免费 | 1530篇 |
国内免费 | 16篇 |
专业分类
林业 | 1539篇 |
农学 | 1096篇 |
基础科学 | 224篇 |
5850篇 | |
综合类 | 4624篇 |
农作物 | 1372篇 |
水产渔业 | 1505篇 |
畜牧兽医 | 18259篇 |
园艺 | 494篇 |
植物保护 | 2176篇 |
出版年
2021年 | 309篇 |
2020年 | 373篇 |
2019年 | 465篇 |
2018年 | 698篇 |
2017年 | 784篇 |
2016年 | 690篇 |
2015年 | 454篇 |
2014年 | 696篇 |
2013年 | 1920篇 |
2012年 | 1165篇 |
2011年 | 1490篇 |
2010年 | 1050篇 |
2009年 | 961篇 |
2008年 | 1412篇 |
2007年 | 1383篇 |
2006年 | 1233篇 |
2005年 | 1059篇 |
2004年 | 1028篇 |
2003年 | 939篇 |
2002年 | 886篇 |
2001年 | 820篇 |
2000年 | 811篇 |
1999年 | 628篇 |
1998年 | 309篇 |
1997年 | 275篇 |
1995年 | 311篇 |
1992年 | 434篇 |
1991年 | 451篇 |
1990年 | 469篇 |
1989年 | 479篇 |
1988年 | 381篇 |
1987年 | 379篇 |
1986年 | 403篇 |
1985年 | 375篇 |
1984年 | 353篇 |
1983年 | 337篇 |
1979年 | 494篇 |
1978年 | 346篇 |
1977年 | 387篇 |
1976年 | 479篇 |
1975年 | 494篇 |
1974年 | 538篇 |
1973年 | 496篇 |
1972年 | 452篇 |
1971年 | 360篇 |
1970年 | 381篇 |
1969年 | 438篇 |
1968年 | 451篇 |
1967年 | 454篇 |
1966年 | 447篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Calibration and validation of optical chlorophyll‐measuring devices for use in predicting crude protein concentration in tropical grass herbage 下载免费PDF全文
M. P. Hughes C. H. O. Lallo V. Mlambo P. G. A. Jennings 《Grass and Forage Science》2017,72(1):118-136
The objective of this experiment was to evaluate the Fieldscout CM 1000 NDVI and Yara N‐Tester as easy‐to‐use and cost‐effective tools for predicting foliar chlorophylls (a, b and total) and crude protein (CP) concentrations in herbage from three tropical grass species. Optical chlorophyll measurements were taken at three stages (4, 8 and 12 weeks) of regrowth maturity in Guinea grass (Panicum maximum) and Mulato II (Brachiaria hybrid) and at 6 and 12 weeks maturity in Paspalum spp (Paspalum atratum). Grass samples were harvested subsequent to optical measurements for laboratory analysis to determine CP and solvent‐extractable chlorophylls (a, b and total) concentrations. Optical chlorophyll measurements and CP concentrations were highly correlated (Yara N‐Tester: r2 = 0·77–0·89; Fieldscout CM 1000 NDVI: r2 = 0·52–0·84). Crude protein prediction models from the Yara N‐Tester and Fieldscout CM 1000 NDVI accounted for 70–89% and 44–73% CP variability, respectively, in Mulato II and Guinea grass. The Yara N‐tester produced more accurate and reliable CP estimates based on very high concordance correlation coefficient [CCC (0·73–0·91)] and low rMSPE, mean and regression bias. It is concluded that the Yara N‐Tester produces more accurate and reliable CP estimates of tropical pastures. 相似文献
2.
Johannes Trini Hans Peter Maurer Sigrid Weissmann Tobias Würschum 《Plant Breeding》2020,139(5):906-915
Accurate hybrid prediction and knowledge about the relative contribution of general (GCA) and specific combining ability (SCA) are of utmost importance for efficient hybrid breeding. We therefore evaluated 91 triticale single-cross hybrids in field trials at seven environments for plant height, heading time, fresh biomass, dry matter content and dry biomass. Fresh and dry biomass showed the highest proportion (23%) of variance due to SCA. Prediction accuracies based on GCA were slightly higher than based on mid-parent values. Utilizing parental kinship information yielded the highest prediction accuracies when both parental lines have been tested in other hybrid combinations, but still moderate-to-low prediction accuracies for two untested parents. Thus, hybrid prediction for biomass traits in triticale is currently promising based on mid-parent values as emphasized by our simulation study, but can be expected to shift to GCA-based prediction with an increasing importance of GCA due to selection in hybrid breeding. Moreover, the performance of potential hybrids between newly developed lines can be predicted with moderate accuracy using genomic relationship information. 相似文献
3.
Elena V. Girich Anton N. Yurchenko Olga F. Smetanina Phan Thi Hoai Trinh Ngo Thi Duy Ngoc Mikhail V. Pivkin Roman S. Popov Evgeny A. Pislyagin Ekaterina S. Menchinskaya Ekaterina A. Chingizova Shamil S. Afiyatullov Ekaterina A. Yurchenko 《Marine drugs》2020,18(12)
Low molecular weight secondary metabolites of marine fungi Aspergillus flocculosus, Aspergillus terreus and Penicillium sp. from Van Phong and Nha Trang Bays (Vietnam) were studied and a number of polyketides, bis-indole quinones and terpenoids were isolated. The structures of the isolated compounds were determined by 1D and 2D NMR and HR-ESI-MS techniques. Stereochemistry of some compounds was established based on ECD data. A chemical structure of asterriquinone F (6) was thoroughly described for the first time. Anthraquinone (13) was firstly obtained from a natural source. Neuroprotective influences of the isolated compounds against 6-OHDA, paraquat and rotenone toxicity were investigated. 4-Hydroxyscytalone (1), 4-hydroxy-6-dehydroxyscytalone (2) and demethylcitreoviranol (3) have shown significant increasing of paraquat- and rotenone-treated Neuro-2a cell viability and anti-ROS activity. 相似文献
4.
5.
6.
Landscape Ecology - The increasing availability of lidar data and structure from motion processing techniques is moving pattern metric research toward the development of three-dimensional (3D)... 相似文献
7.
8.
Spatiotemporal patterns of mountain whitefish (Prosopium williamsoni) in response to a restoration of longitudinal connectivity 下载免费PDF全文
Peter M. Kiffney Ben Cram Paul L. Faulds Karl Burton Michele Koehler Thomas P. Quinn 《Ecology of Freshwater Fish》2018,27(4):1037-1053
To examine the role of longitudinal connectivity on the spatial and temporal dynamics of mountain whitefish (Prosopium williamsoni), we quantified movement and population dynamics following installation of the Landsburg Dam fishway, Cedar River, WA, USA. Mountain whitefish is widely distributed, poorly studied and not the focus of restoration. Before the fishway, mountain whitefish were not observed above the dam. Here, we focus on snorkel counts collected at reach and mesohabitat (e.g. pools) scales over 11 summers on the 20‐km above‐dam segment following restoration. A camera within the ladder provided number, size and movement timing, thereby informing on behaviour and recolonisation. Segment‐scale abundance increased following fish passage reaching an asymptote in 7 years, and mountain whitefish were detected throughout the main stem in 10 years. Annual movement through the ladder increased over time and was positively correlated with instream abundance and discharge, but negatively correlated with water temperature. About 60% of fish movements occurred in spring and early summer, potentially for foraging opportunities. Reach‐scale abundance peaked between 7 and 10 km from the dam; deep, cool (~10.6 to 11.6°C) conditions characterised these reaches. At the mesohabitat scale, mountain whitefish detection increased with depth and velocity after accounting for distance from the dam. Our results show how restoring longitudinal connectivity allowed this nontarget species to colonise newly available habitat. Their response supports the critical roles of longitudinal connectivity and environmental conditions, that manifest at different spatial scales, in dictating how freshwater fish respond to habitat disturbance. 相似文献
9.
Spyra Marcin Kleemann Janina Cetin Nuket Ipek Vázquez Navarrete Cesar Jesús Albert Christian Palacios-Agundez Igone Ametzaga-Arregi Ibone La Rosa Daniele Rozas-Vásquez Daniel Adem Esmail Blal Picchi Paolo Geneletti Davide König Hannes J. Koo HongMi Kopperoinen Leena Fürst Christine 《Landscape Ecology》2019,34(7):1715-1735
Landscape Ecology - Several case studies investigated the role of ecosystem services in participatory planning processes. However, no systematic study exists that cuts across a large number of... 相似文献
10.
M O Furr M V Crisman J Robertson O Barta W S Swecker 《Journal of the American Veterinary Medical Association》1992,201(2):307-309
Immune system dysfunction and immunoglobulin deficiency was diagnosed in a 2-year-old horse with disseminated lymphosarcoma. Prolonged (35 days) parenteral nutrition was delivered to support the horse during a period in which immune function studies could be performed. Correction of nutritional compromise by use of parenteral nutrition did not correct the immunoglobulin deficiency, and results of lymphocyte phenotype testing did not indicate abnormal proportions of leukocytes. Lymphoblast transformation studies were suggestive of a circulating immunosuppressive factor in the horse's serum. Normal cell function was detected when the cells were stimulated in precolostral equine serum. 相似文献