全文获取类型
收费全文 | 339121篇 |
免费 | 19231篇 |
国内免费 | 3859篇 |
专业分类
林业 | 26789篇 |
农学 | 15883篇 |
基础科学 | 4641篇 |
50117篇 | |
综合类 | 53308篇 |
农作物 | 19715篇 |
水产渔业 | 19441篇 |
畜牧兽医 | 136241篇 |
园艺 | 8722篇 |
植物保护 | 27354篇 |
出版年
2021年 | 4148篇 |
2020年 | 4361篇 |
2019年 | 5191篇 |
2018年 | 5551篇 |
2017年 | 6434篇 |
2016年 | 6180篇 |
2015年 | 6051篇 |
2014年 | 7498篇 |
2013年 | 18459篇 |
2012年 | 10562篇 |
2011年 | 13077篇 |
2010年 | 10717篇 |
2009年 | 11018篇 |
2008年 | 12008篇 |
2007年 | 10929篇 |
2006年 | 10695篇 |
2005年 | 9815篇 |
2004年 | 8933篇 |
2003年 | 8504篇 |
2002年 | 7709篇 |
2001年 | 8467篇 |
2000年 | 8185篇 |
1999年 | 7006篇 |
1998年 | 4383篇 |
1997年 | 4434篇 |
1996年 | 4109篇 |
1995年 | 4823篇 |
1994年 | 4181篇 |
1993年 | 3869篇 |
1992年 | 5477篇 |
1991年 | 5744篇 |
1990年 | 5483篇 |
1989年 | 5478篇 |
1988年 | 4895篇 |
1987年 | 4891篇 |
1986年 | 4965篇 |
1985年 | 5101篇 |
1984年 | 4557篇 |
1983年 | 4149篇 |
1982年 | 3281篇 |
1981年 | 3079篇 |
1980年 | 3047篇 |
1979年 | 4117篇 |
1978年 | 3513篇 |
1977年 | 3237篇 |
1976年 | 3078篇 |
1975年 | 3068篇 |
1974年 | 3369篇 |
1973年 | 3416篇 |
1972年 | 3073篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
31.
Effects of Feeding Frequency on Apparent Energy and Nutrient Digestibility/Availability of Channel Catfish,Ictalurus punctatus,Reared at Optimal and Suboptimal Temperatures
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)
This study examined the effects of feeding frequency (daily versus every other day [EOD]) on nutrient digestibility/availability of channel catfish, Ictalurus punctatus, reared at optimal (30 C) and suboptimal (24 C) temperatures. A 28% protein practical diet was used as the test diet, and chromic oxide was used at 0.75% of the diet as a maker. Twenty channel catfish of a mean weight of 141 g/fish were stocked into 12 flow‐through aquaria (110 L). Fish were fed daily or EOD to apparent satiation for 7 d before fecal samples were collected by the dissection method. Fish fed EOD to apparent satiation consumed more diet on days fed than those fed daily, and fish reared at 30 C consumed more diet than those reared at 24 C. Neither feeding frequency nor water temperature significantly affected apparent digestibility coefficients for energy and protein and apparent availability coefficients for total amino acids. Results show no advantage by feeding EOD over feeding daily on nutrient digestibility and diet utilization efficiency. 相似文献
32.
33.
以草莓"红颜"为试材,以PVC(聚氯乙烯)膜为对照,研究了PE(聚乙烯)膜、PHM(多层PE系)膜、纳米银膜、PVDC(聚偏二氯乙烯)膜等5种保鲜处理在(4.0±0.6)℃贮藏条件下对草莓果实失重率、硬度和可溶性固形物、维生素C、可溶性糖含量及感官品质的影响。结果表明:相较于PVC保鲜膜,PVDC膜与PHM膜能够较好的保持草莓果实的贮藏品质。其中PVDC膜包装可以有效地减少果实水分散失,保持草莓的硬度以及果实可溶性固形物含量;PHM膜包装可以较好的保持草莓果实可溶性固形物、维生素C、可溶性糖含量,维持果实的营养成分。 相似文献
34.
Evaluating the Effects of Dietary Prebiotic Mixture of Mannan Oligosaccharide and Poly‐β‐Hydroxybutyrate on the Growth Performance,Immunity, and Survival of Rainbow Trout,Oncorhynchus mykiss (Walbaum 1792), Fingerlings
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)
Ebrahim H. Najdegerami Amir Tokmachi Farideh Bakhshi 《Journal of the World Aquaculture Society》2017,48(3):415-425
This study investigated the effects of dietary poly‐β‐hydroxybutyrate (PHB) and mannan oligosaccharide (MOS) on the growth performance, digestive enzyme activities, and disease resistance of rainbow trout fingerlings. A total of 420 fingerlings, distributed equally into four different groups, each with three replicates, were fed a basal diet (control), 2% PHB (PHB), 0.2% MOS (MOS), or a diet supplemented with a mixture of 2% PHB and 0.2% MOS (PHB+MOS) for 60 d. Results showed that dietary PHB and/or MOS did not cause significant improvement in the growth performance of the fingerlings. The gut pH was markedly reduced; however, the activities of the digestive enzymes (except for pepsin and amylase by dietary PHB) were not significantly improved by the dietary supplements. Results also showed that dietary PHB and/or MOS markedly increased the resistance of the fingerlings toward Yersinia ruckeri infection, while the lowest total antibody was observed in fish fed control and MOS, indicating that MOS exerts its protective effects by modulating other mechanisms. Overall, this study provided a first screening effort to determine the effects of PHB and/or MOS as dietary supplements in rainbow trout fingerlings to develop an optimal prebiotic mix for improving the growth and health status of rainbow trout. 相似文献
35.
36.
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
37.
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
38.
39.
以盆花月季'仙境'为试材,采用根部浇灌方式,设置0(P0)、10 (P10)、20(P20)、30(P30)g·柱-14个磷供应水平,研究了磷水平对盆花月季'仙境'根系生长、不同器官氮磷养分吸收、叶片SPAD值以及开花品质(花朵数量、花径、花朵干质量等)的影响,以期为月季养分管理提供参考依据.结果 表明:磷不同供应水平明显改变了'仙境'的根系形态,P20和P30处理较P0和P10处理,总根长增加了81%~111%,总根表面积扩大了133%~196%,细根总长度增加了95%~119%.植株不同器官(新生叶片、新生枝条和根系)生物量和氮、磷养分积累量表现出与根系生长类似的趋势.与P0和P10处理相比,P20和P30处理单株花朵数量增加128%~206%,叶片SPAD值增加16%~21%.与对照P0相比,P10、P20和P30花径分别增加12%、19%和24%.植株总根长与总氮和总磷吸收量表现出显著的正相关关系.综上所述,盆花月季'仙境'对不同磷供应水平表现出高度的根形态可塑性,适宜的磷供应水平促进根系生长和养分吸收,提高开花数量和品质. 相似文献
40.