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   273038篇
  免费   15466篇
  国内免费   763篇
林业   24285篇
农学   14222篇
基础科学   3012篇
  45031篇
综合类   29372篇
农作物   17591篇
水产渔业   17415篇
畜牧兽医   107495篇
园艺   6418篇
植物保护   24426篇
  2021年   2637篇
  2020年   3038篇
  2019年   3810篇
  2018年   4261篇
  2017年   4666篇
  2016年   5012篇
  2015年   4491篇
  2014年   5864篇
  2013年   16593篇
  2012年   6907篇
  2011年   8884篇
  2010年   7642篇
  2009年   8098篇
  2008年   8125篇
  2007年   7116篇
  2006年   7666篇
  2005年   6887篇
  2004年   6757篇
  2003年   6444篇
  2002年   5810篇
  2001年   6480篇
  2000年   6229篇
  1999年   5633篇
  1998年   4177篇
  1997年   4235篇
  1996年   3982篇
  1995年   4648篇
  1994年   4063篇
  1993年   3742篇
  1992年   4808篇
  1991年   5046篇
  1990年   4737篇
  1989年   4692篇
  1988年   4190篇
  1987年   4312篇
  1986年   4358篇
  1985年   4571篇
  1984年   4108篇
  1983年   3902篇
  1982年   3090篇
  1981年   2976篇
  1980年   2929篇
  1979年   3592篇
  1978年   3192篇
  1977年   2824篇
  1976年   2700篇
  1974年   2911篇
  1973年   2961篇
  1972年   2689篇
  1971年   2651篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
Following the discovery of the 11-year solar cycle signal in earth rotation, linear techniques were employed to investigate the amplitude and phase of the difference between ephemeris time and universal time (DeltaT) as a function of time. The amplitude is nonstationary. This difference was related to Delta(LOD), the difference between the length of day and its nominal value. The 11-year term in Delta(LOD) was 0.8 millisecond at the close of the 18th century and decreased below noise level from 1840 to 1860. From 1875 to 1925, Delta(LOD) was about 0.16 millisecond, and it decreased to about 0.08 millisecond by the 1950's. Except for anomalous behavior from 1797 to 1838, DeltaT lags sunspot numbers by 3.0 +/- 0.4 years. Since DeltaT lags Delta(LOD) by 2.7 years, the result is that Delta(LOD) is approximately in phase with sunspot numbers.  相似文献   
993.
994.
995.
Burk RL  Stuiver M 《Science (New York, N.Y.)》1981,211(4489):1417-1419
Values of the oxygen isotope ratios (delta(18)O) in tree-ring cellulose closely reflect the delta(18)O values in atmospheric precipitation and hence mean annual temperature. The change in delta(18)O in cellulose is 0.41 per mil per degree Celsius for selected near-coastal stations. The values of delta(18)O in precipitation and cellulose also change with altitude, as demonstrated for Mount Rainier, Washington. A temperature lapse rate of 5.2 degrees +/- 0.5 degrees C per 1000 meters calculated from cellulose delta(18)O values agrees with the accepted mean annual lapse rate of 5 degrees C per 1000 meters for this region. Cellulose delta(18)O values and delta(18)O values of carbon dioxide equilibrated with leaf water differ by a fixed 16 per mil.  相似文献   
996.
997.
Rapid increases in the price of imported crude oil have accelerated a shift in the raw material base for chemical feedstocks from natural gas to crude oil to coal. Widespread use of ethylene as a feedstock has depended on the availability at attractive prices of natural gas and petroleum. As the resource base shifts from natural gas and petroleum to coal, ethylene is being replaced by synthesis gas (a mixture of hydrogen and carbon monoxide of varying composition), which can be manufactured directly from any of these carbonaceous sources. This trend is expected to accelerate in the 1980's. Organics likely to be produced from synthesis gas include ethanol, ethylene glycol, and vinyl acetate.  相似文献   
998.
Pesticides have not been a satisfactory solution to the control of insect pests. Pheromones show promise as a component of integrated pest management.  相似文献   
999.
In this investigation electrical conductivity (EC) and somatic cell concentration (SCC) were compared in 3 herds for their ability to correctly identify the infection status of quarters. For EC, thresholds of 6.0 and 6.8 mS/cm were used and comparisons were made between quarters within each cow. The method of comparison between quarters was the same as that described by the manufacturers of the AHI Mastitis Detector. For SCC a threshold of 500 x 10(3) cells/ml was used. For both EC and SCC considerable variation was found between herds for sensitivity (the proportion of quarters infected with a major pathogen and detected as abnormal), and specificity (the proportion of quarters free from infection and detected as normal). The mean sensitivity and specificity of EC for the three herds was 49% and 79% respectively, whereas for SCC the means were 71% and 81% respectively. The variation in sensitivity and specificity of EC between herds was attributed to differences in the distribution of EC for quarters of similar infection status. It was concluded that these differences in herd EC precluded the use of pre-determined EC thresholds which were applicable for detecting mastitis in all herds.  相似文献   
1000.
Summary Serological investigation by the indirect haemagglutination test forToxoplasma antibodies was carried out in a flock of dairy goats. Antibody titres ranging from 1:4 to 1:1,024 were found in 34% of 371 apparently healthy animals. High antibody titres of 1:256 suggestive of recent infection were noted in 18 (4.9%) animals including 13 nannies. The occurrence of a number of non-brucella abortions associated with high serological titres and the isolation of the parasite from the brain tissues of an aborted foetus suggestedToxoplasma as a probable aetiologic factor. Sera having significant levels of haemagglutinating antibodies (1:128) were also found positive to the card agglutination test. Epidemiological aspects and the public health potential of subclinical toxoplasmosis in dairy goats are discussed.
Infecciones Y Abortos En Cabras Lecheras Debido AToxoplasma
Resumen Se realizaron investigaciones serológicas de toxoplasmosis mediante la prueba indirecta de hemaglutinación en un rebaño de cabras lecheras. Se encontraron títulos de 1:4 a 1:1024 en 34% de 371 animales aparentemente sanos. Títulos de 1:256 sugestivos de infecciones recientes, se encontraron en 18 (4.9%) animales, incluyendo 13 hembras paridas. La ocurrencia de un significativo número de abortos no asociados con brucela, títulos serológicos altos de toxoplasmosis y el aislamiento del parásito del cerebro de fetos abortados, sugiere que eltoxoplasma es el factor etiológico más importante. Se discuten aspectos epidemiológicos de importantcia para la salud pública, relacionados con el potencial de la toxoplasmosis (subclínica) en cabras lecheras.

Toxoplasmose Et Avortement Chez Les Chevres Latiteres
Résumé Des investigations sérologiques par la méthode de l'agglutination indirecte pour rechercher des anticorps toxoplasmiques ont été faites sur un troupeau de chèvres laitières. Des anticorps ont été mis en évidence chez 34 p.100 des 371 animaux apparemment sains à des titres allant de 1:4 à 1:1024. Des titres élevés d'anticorps atteignant 1:256 ont été relevés chez 18 (soit 4,9 p.100) des animaux comprenant 13 chèvres, ce qui fait penser à une infection récente.L'existence de nombre d'avortements non-brucelliques associés avec des titres sérologiques élevés et l'isolement du parasite des tittus cérébraux d'un foetus avorté suggerent que les Toxoplasmes en ont probablement été le facteur étiologique. Des sérums ayant des taux significativement élevés d'anticorps agglutinants (1:128) ont également été reconnus comme positifs par le test d'agglutination en EAT. Les aspects épidémiologiques et le danger potentiel pour l'homme de ces cas subcliniques de toxoplasmose chez les chèvres laitières sont discutés.
  相似文献   
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