全文获取类型
收费全文 | 208454篇 |
免费 | 12003篇 |
国内免费 | 1079篇 |
专业分类
林业 | 9026篇 |
农学 | 6803篇 |
基础科学 | 1787篇 |
25824篇 | |
综合类 | 38347篇 |
农作物 | 8023篇 |
水产渔业 | 10343篇 |
畜牧兽医 | 103918篇 |
园艺 | 3039篇 |
植物保护 | 14426篇 |
出版年
2019年 | 2074篇 |
2018年 | 3107篇 |
2017年 | 3515篇 |
2016年 | 3189篇 |
2015年 | 2954篇 |
2014年 | 3431篇 |
2013年 | 7966篇 |
2012年 | 6560篇 |
2011年 | 7819篇 |
2010年 | 5121篇 |
2009年 | 4903篇 |
2008年 | 7350篇 |
2007年 | 7020篇 |
2006年 | 6517篇 |
2005年 | 6159篇 |
2004年 | 5805篇 |
2003年 | 5696篇 |
2002年 | 5510篇 |
2001年 | 6171篇 |
2000年 | 6067篇 |
1999年 | 4919篇 |
1998年 | 2037篇 |
1997年 | 1998篇 |
1996年 | 1854篇 |
1995年 | 2267篇 |
1994年 | 2031篇 |
1993年 | 1961篇 |
1992年 | 4002篇 |
1991年 | 4292篇 |
1990年 | 4326篇 |
1989年 | 4248篇 |
1988年 | 3893篇 |
1987年 | 4027篇 |
1986年 | 4156篇 |
1985年 | 3893篇 |
1984年 | 3327篇 |
1983年 | 2934篇 |
1982年 | 2000篇 |
1981年 | 1889篇 |
1979年 | 2963篇 |
1978年 | 2436篇 |
1977年 | 2061篇 |
1976年 | 1929篇 |
1975年 | 2028篇 |
1974年 | 2701篇 |
1973年 | 2710篇 |
1972年 | 2651篇 |
1971年 | 2431篇 |
1970年 | 2272篇 |
1969年 | 2116篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
11.
Mark D. Rawling Nicola Pontefract Ana Rodiles Ilektra Anagnostara Eric Leclercq Marion Schiavone Mathieu Castex Daniel L. Merrifield 《Journal of the World Aquaculture Society》2019,50(6):1108-1122
Fish were fed a single‐strain yeast fraction (SsYF; 2 g/kg) or a multistrain yeast fraction (MsYF; 0.8 g/kg) for 10 weeks. The results demonstrated significant (p ≤ 0.03) elevations in weight gain, specific growth rate, protein efficiency ratio, and feed conversion ratio in fish fed the yeast fraction‐supplemented diets. In the distal intestine, a significant elevation in microvilli density was observed after 5 and 10 weeks of dietary supplementation with MsYF and SsYF, respectively, compared to control fed fish (p < 0.001). A significant elevation (p = 0.02) in the perimeter ratio was observed in fish fed diets supplemented with the yeast fractions. After 10 weeks of feeding on the experimental diets, Rt‐qPCR demonstrated a significant downregulation (p < 0.05) in the stress response genes, heat‐shock protein 70 (hsp70) and proliferating cell nuclear antigen (pcna), in fish fed diets supplemented with the yeast fractions. Significant (p < 0.05) elevations in interleukin 1‐beta (il1β) and interleukin‐10 (il10) gene expression were observed in fish fed diets supplemented with the MsYF compared to the other dietary groups. These findings suggest that feeding an MsYF specifically at a lower incorporation rate < 1 g/kg, compared to a commercial SsYF at 2 g/kg, is effective in improving the intestinal health status and growth performance of European seabass. 相似文献
12.
全面客观评价生态资源承载能力,进而为区域可持续发展提供理论依据。以甘肃省迭部县为例,首先,采用压力-状态-响应(P-S-R)模型构建生态承载力评估指标体系,运用层次分析法(Analytic Hierarchy Process,AHP)确定各指标权重,评估生态环境承载力现状;其次,在生态功能区层面对迭部县水源涵养功能及生物多样性功能进行评价;最后,对生态承载力状况进行成因解析。研究结果表明:(1)迭部县整体生态承载力得分为0.239,处于“较强”承载力状态,构成整体生态承载力的生态弹性力、资源和环境承载力和人类社会影响力得分分别为0.222、0.062和0.063。(2)2016年,迭部县单位面积水源涵养量为111.76 mm,水源涵养功能处于“高”状态,其中,水源涵养功能较高地区主要集中在以草地和针阔混交林为主的区域。(3)自然栖息地面积占迭部县总面积的83.67%,其自然栖息质量指数(Natural Habitant Quality Index,NHQI)处于“高”状态。(4)从自然和经济2个因素进行分析,发现迭部县生态环境条件较好,但林草地面积已呈现出逐渐减少状态,因此需要通过合理科学的开发引导,发展文化旅游事业,推动迭部县的可持续发展。 相似文献
13.
14.
Sepsis of the calcaneal bursae (CB) presents significant treatment challenges with limited clinical data available in the literature. The objective of this retrospective cohort study was to assess the clinical outcomes associated with CB lavage using either a through-and-through needle or bursoscopic technique. Clinical records of 29 horses treated for septic calcaneal bursitis using either technique between 2005 and 2019 were reviewed. Fisher’s exact test was used to assess statistical significance between first surgical technique and success at first surgery (i.e. not requiring >1 lavage), survival to discharge and return to work (RTW). Bursoscopy was performed in 13/29 (44.8%) cases, and needle lavage in 16/29 (55.2%). In the needle group, 12 (75%) horses were discharged following the first surgery. Four had repeat interventions; two (12.5%) had needle lavage and two (12.5%) had bursoscopy. Of the two horses to have repeat needle lavage, one was subjected to euthanasia and one discharged, and of the two that underwent bursoscopy, one was discharged and one received a third bursoscopy prior to discharge. In the bursoscopy group, seven (53.8%) were discharged and three (23.1%) were subjected to euthanasia following the first surgery. Three (23.1%) received a second bursoscopic lavage with one discharged, one subjected to euthanasia and one having a third bursoscopic lavage prior to discharge. Overall, 18/24 (75%) followed up cases RTW, 10 (55.5%) from the needle group, eight (44.4%) the bursoscopy group. No statistically significant differences between first surgical technique used and success at first surgery (no subsequent lavage(s) required), survival to discharge or return to work were detected. The main limitations of this study are that it is a retrospective study, has a small population with limited statistical power and potential selection bias. No statistically significant differences existed between the outcomes of the two techniques, contrary to the belief that bursoscopic lavage is superior. Larger, multicentred studies, with greater statistical power are required to further assess this relationship. 相似文献
16.
Seasonal Changes in the Milt Quality of Waigieu Seaperch,Psammoperca waigiensis: Implications for Artificial Propagation
下载免费PDF全文
![点击此处可从《Journal of the World Aquaculture Society》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Milt quality of Waigieu seaperch was sampled at the beginning, the middle, and the end of the spawning season to assess if the sampling season has an effect on milt quality parameters. The milt volume and total sperm production were higher at the middle of the spawning season while the sperm concentration in milt was significantly higher at the beginning and the end of the spawning season. Sperm morphology and the major parameters of seminal plasma (pH, total protein, Mg2+, or Ca2+ concentrations) did not differ throughout the spawning season. The concentrations of Na+ and the osmolality increased throughout the spawning season. The percentage of motile cells did not differ during the spawning season while the duration of sperm motility (swimming duration) and velocity (swimming speed) were significantly different during the spawning season. The fertilization rate (75.7 ± 6.8%) and hatching rate (56.5 ± 3.1%) at the middle of the spawning season were higher than at the beginning and end periods. Results indicated that milt quality parameters and fertility success in the middle of the spawning season were higher compared to earlier and later sampling dates. Thus, we recommend collecting milt during this time for maximal fertilization and hatching success. 相似文献
17.
18.
Spatiotemporal patterns of mountain whitefish (Prosopium williamsoni) in response to a restoration of longitudinal connectivity
下载免费PDF全文
![点击此处可从《Ecology of Freshwater Fish》网站下载免费的PDF全文](/ch/ext_images/free.gif)
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
19.
Soil protection for a sustainable future: options for a soil monitoring network for Ireland
下载免费PDF全文
![点击此处可从《Soil Use and Management》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The increased recognition of the importance of soil is reflected in the UN Post‐2015 Development Agenda with sustainable development goals that directly and indirectly relate to soil quality and protection. Despite a lack of legally binding legislation for soil protection, the European Commission remains committed to the objective of soil protection. However, the achievement of a legally binding framework for soil protection relies on the implementation of a soil monitoring network (SMN) that can detect changes to soil quality over time. As beneficiaries do not pay for the provision of soil information, the options for soil monitoring are limited. The use of existing data sets should be considered first. Using Ireland as an example, this research explored the opportunities for a SMN for Ireland considering three existing national data sets. The options for a SMN are considered in terms of their spatial and stratified distribution, the parameters to be measured and an economic analysis of the options proposed. This research finds that for Ireland, either a 10 or a 16 km2 grid interval stratified by land use and drainage class offers the best potential in relation to the spatial distribution of existing data sets to reflect local data at a national level. With existing data, the stratified SIS data using the 16 km2 grid offers the best value for money, with baseline costs for analysis, excluding field costs, of between €706 481 and €2.8 million. Acknowledging the impossibility of measuring all parameters with ideal frequency, this study proposes a two‐tier system for optimized monitoring frequency. Parameters must anticipate future policy requirements. Finally, the implementation of a SMN must be accompanied by standardized methods, defined thresholds and action mandates to maintain soil quality within allowable limits. 相似文献
20.