首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   4090篇
  免费   199篇
  国内免费   13篇
林业   267篇
农学   149篇
基础科学   33篇
  792篇
综合类   651篇
农作物   237篇
水产渔业   387篇
畜牧兽医   1339篇
园艺   136篇
植物保护   311篇
  2023年   35篇
  2022年   26篇
  2021年   87篇
  2020年   93篇
  2019年   100篇
  2018年   87篇
  2017年   85篇
  2016年   93篇
  2015年   65篇
  2014年   101篇
  2013年   185篇
  2012年   239篇
  2011年   278篇
  2010年   180篇
  2009年   144篇
  2008年   231篇
  2007年   251篇
  2006年   224篇
  2005年   230篇
  2004年   223篇
  2003年   212篇
  2002年   197篇
  2001年   55篇
  2000年   51篇
  1999年   51篇
  1998年   35篇
  1997年   29篇
  1996年   39篇
  1995年   31篇
  1994年   33篇
  1993年   45篇
  1992年   29篇
  1991年   39篇
  1990年   29篇
  1989年   38篇
  1988年   31篇
  1987年   22篇
  1986年   29篇
  1985年   27篇
  1984年   34篇
  1983年   21篇
  1982年   13篇
  1981年   16篇
  1979年   14篇
  1978年   13篇
  1977年   11篇
  1976年   12篇
  1975年   16篇
  1974年   16篇
  1973年   16篇
排序方式: 共有4302条查询结果,搜索用时 15 毫秒
81.
82.
83.
Zhai  Changyuan  Long  John  Taylor  Randal  Weckler  Paul  Wang  Ning 《Precision Agriculture》2020,21(3):589-602
Precision Agriculture - With recent developments allowing increased planter speeds, improving spatial uniformity of crop emergence will continue to be an important focus of research. Vertical...  相似文献   
84.
A new cytotoxic thiodepsipeptide, verrucosamide (1), was isolated along with the known, related cyclic peptide thiocoraline, from the extract of a marine-derived actinomycete, a Verrucosispora sp., our strain CNX-026. The new peptide, which is composed of two rare seven-membered 1,4-thiazepane rings, was elucidated by a combination of spectral methods and the absolute configuration was determined by a single X-ray diffraction study. Verrucosamide (1) showed moderate cytotoxicity and selectivity in the NCI 60 cell line bioassay. The most susceptible cell lines were MDA-MB-468 breast carcinoma with an LD50 of 1.26 µM, and COLO 205 colon adenocarcinoma with an LD50 of 1.4 µM. Also isolated along with verrucosamide were three small 3-hydroxy(alkoxy)-quinaldic acid derivatives that appear to be products of the same biosynthetic pathway.  相似文献   
85.
Schinus terebinthifolia is a dioecious tree native to South America that has become an invasive weed in Florida, southern California, southern Arizona, Texas and Hawaii and has been naturalised in over 20 countries. Biological control is considered a viable long-term control option for S. terebinthifolia because release from natural enemies appears to be at least partly responsible for its success in Florida. We examined leaf phenology of S. terebinthifolia over a period of 15 months at five sites in central and southern Florida to provide information that may help in predicting the impacts of potential biocontrol agents for this weed. We documented leaf lifespan, the seasonality of leaf development and abscission and the survivorship of leaves that emerged during either spring, summer or autumn. Average leaf lifespan was >4.5 months at all sites, and leaf phenology followed the seasons closely. Although S. terebinthifolia possesses leaves throughout the year, leaf production was greatest from April to September, and most leaves were abscised in February and March. Spring- and summer-emerging leaves were also longer-lived than leaves produced during autumn. These results suggest that leaves of S. terebinthifolia would be most vulnerable to herbivory during the spring and summer months when newly growing leaf tissue is most plentiful. Biocontrol agents capable of damaging these tissues during spring/summer might be an effective means of controlling this invasive weed.  相似文献   
86.
Regional economic resilience can be defined as an economy’s ability to withstand and recover quickly from shocks. The ability to measure resilience is necessary to developing our understanding of what influences resilience. In this paper, we develop a new, two‐dimensional quantitative measure of resilience using observed differences between expected and actual employment in a region following a shock and distinguish the response to the shock from random variation. We demonstrate one application of this metric to US county‐level employment data to compare county responses to the 2007–2009 national recession and discuss how different regions of the United States responded to the shock of the Great Recession in terms of resilience.  相似文献   
87.
A GIS-based plant prediction model for wetland ecosystems   总被引:5,自引:0,他引:5  
An existing non-spatial model for the prediction of response of wetland plant species on ecological factors has been transformed into a GIS-based prediction model which produces spatial output at the landscape scale. The input, spatial patterns of the ecological factors, were constructed with geostatistical spatial interpolation (kriging). With this GIS-based model the spatial patterns of presence and absence of 78 wetland plant species are predicted for an area with wetlands in the Netherlands of approximately 500 square kilometers. The GIS-based model has been validated, and the estimated uncertainty of the input has been propagated through the model. At the species level the output shows spatially coherent and non-random patterns. The validation is affected by the propagation of input errors through the model. The number of valid predictions declines approximately 10–20% when 95% confidence intervals are used in the validation. This study shows that it is feasible to use a geostatistical interpolation method to construct spatial patterns of ecological factors on a landscape scale and to use these patterns as input for a GIS-based prediction model. The added uncertainty on the input values however, affects the number of valid predictions of the model.  相似文献   
88.
89.
90.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号