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   246692篇
  免费   14919篇
  国内免费   652篇
林业   23568篇
农学   13983篇
基础科学   2956篇
  42533篇
综合类   21506篇
农作物   17171篇
水产渔业   16559篇
畜牧兽医   94131篇
园艺   6299篇
植物保护   23557篇
  2021年   2472篇
  2020年   2874篇
  2019年   3655篇
  2018年   3958篇
  2017年   4367篇
  2016年   4777篇
  2015年   4299篇
  2014年   5652篇
  2013年   16261篇
  2012年   5983篇
  2011年   7830篇
  2010年   7228篇
  2009年   7826篇
  2008年   7271篇
  2007年   6293篇
  2006年   6894篇
  2005年   6222篇
  2004年   6049篇
  2003年   5897篇
  2002年   5178篇
  2001年   5583篇
  2000年   5220篇
  1999年   5011篇
  1998年   4101篇
  1997年   4178篇
  1996年   3881篇
  1995年   4477篇
  1994年   3864篇
  1993年   3554篇
  1992年   4090篇
  1991年   4304篇
  1990年   4027篇
  1989年   3997篇
  1988年   3539篇
  1987年   3568篇
  1986年   3537篇
  1985年   3842篇
  1984年   3621篇
  1983年   3375篇
  1982年   2834篇
  1981年   2718篇
  1980年   2705篇
  1979年   3172篇
  1978年   2826篇
  1977年   2687篇
  1976年   2502篇
  1975年   2347篇
  1974年   2510篇
  1973年   2512篇
  1972年   2226篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
911.
1. Eggs from sea level which are transferred to higher altitude for incubation require changes in incubator gas composition if they are to develop normally.

2. This not only requires compensation for the reduced O2 partial pressure but, and of equal importance, recognition of the increased diffusivity of gases at altitude.

3. Equations are developed for predicting the required changes in O2, CO2 and water vapour composition of incubator gas at any altitude.  相似文献   

912.
1. Starter diets containing either maize or salseed meal to replace the maize were fed to chicks over a period of 2 weeks.

2. Growth rate, food intake, food utilisation and proportion of dietary nitrogen retained were much poorer in chicks receiving salseed meal.

3. The chicks receiving salseed meal developed pathological lesions in liver and kidney.

4. The red blood cell count, white blood cell count, haemoglobin concentration and packed cell volume of the chicks receiving salseed meal were lower than those of chicks receiving maize.

5. The apparent metabolisable energy value of salseed meal was determined as 6.83 MJ/kg.  相似文献   

913.
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   
914.
915.
916.
917.
Fossil birds     
  相似文献   
918.
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   
919.
Strips of rumen wall from bovine fetuses were incubated in an organ bath with prostaglandin F2 alpha (0.13 to 33.76 g/ml). The highest reactivity with a submaximal dose (17.03 g/ml) was observed in the period between 3.0 and 7.9 months of fetal age. A smaller response, but higher than in 1.0 to 2.9 months old fetuses, was observed in the 8.0 to 8.9 months fetuses. The period of the highest reactivity to prostaglandin F2 alpha coincides with the age of onset of papillary morphogenesis and the period of highest reactivity to autonomic and putative transmitter drugs.  相似文献   
920.
A mycoplasma identified asMycoplasma mycoides subspmycoides LC type was isolated from an outbreak of caprine pleuropneumonia and serofibrinous peritonitis. This isolate was the first to be reported in Italy. Experimentally, a rapidly fatal condition followed its inoculation into goats and sheep but inoculation of calves did not lead to clinically apparent infection. Although the organism was recovered for up to 45 days following experimental inoculation of calves, pathological changes were limited to the drainage lymph nodes except in one case where there were histological changes in the lungs.  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