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   63919篇
  免费   26898篇
  国内免费   18150篇
林业   7494篇
农学   7434篇
基础科学   7555篇
  28820篇
综合类   29495篇
农作物   5713篇
水产渔业   5315篇
畜牧兽医   8512篇
园艺   2808篇
植物保护   5821篇
  2024年   1513篇
  2023年   1188篇
  2022年   1710篇
  2021年   1636篇
  2020年   2863篇
  2019年   5516篇
  2018年   5144篇
  2017年   5960篇
  2016年   6519篇
  2015年   6708篇
  2014年   5972篇
  2013年   6441篇
  2012年   6555篇
  2011年   5735篇
  2010年   4868篇
  2009年   4411篇
  2008年   3559篇
  2007年   3524篇
  2006年   3156篇
  2005年   2764篇
  2004年   2398篇
  2003年   2110篇
  2002年   1888篇
  2001年   1574篇
  2000年   1412篇
  1999年   1260篇
  1998年   1170篇
  1997年   1142篇
  1996年   992篇
  1995年   1074篇
  1994年   1034篇
  1993年   823篇
  1992年   838篇
  1991年   682篇
  1990年   519篇
  1989年   516篇
  1988年   445篇
  1987年   308篇
  1986年   325篇
  1985年   174篇
  1984年   156篇
  1983年   146篇
  1982年   151篇
  1981年   188篇
  1980年   146篇
  1979年   122篇
  1978年   108篇
  1972年   82篇
  1965年   113篇
  1964年   111篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
A Prüfer-coded genetic algorithm based on the decimal number of nodes is proposed and it is used to solve collaborative optimization planning of access solution of distributed generation(DG)and structure of distribution network.Using graph theory to generate theoretically feasible topology structureand the access nodes and the installed capacity of distributed generation are coded by the Prüfer number.The coding method makes the distribution network operation structure and the access solution of DG combine into the evolution problem of the same chromosomal gene.The length of the chromosome coding of this method is shorter than binary encoding.This solution takes full advantage of the Prüfer-coded to improve computational efficiency and convergence rateand makes some restrictions and improvements in some key parts of the algorithm to solve the problem illegal solution.Finallyfeasibility and superiority of the algorithm is validated by a case study.  相似文献   
52.
53.
Levels of alkaline phosphatase and osteocalcin or bone Gla-protein, a new marker of bone metabolism, were analyzed in blood samples of 50 clinically normal female Standardbred horses between four months and twenty years of age. Samples were collected in the morning before exercise. Serum osteocalcin was measured by radioimmunoassay using bovine antibodies. There was a significant inverse correlation between alkaline phosphatase, osteocalcin and the age of the animals up to 48 months. The decrease in osteocalcin levels in serum was very marked during the first 30 months of life. The mean osteocalcin concentration was respectively 47.3, 35.7 and 6.7 ng/mL for animals less than one year, between 1.5 and 2.5 years of age and older than 3.5 years. Alkaline phosphatase serum activity was higher in foals less than one year of age (means = 856 U/L) than in the two older groups (meansII = 339, meansIII = 351 U/L). We believe that osteocalcin is a useful parameter for the evaluation of bone metabolism in growing animals and in adults and is probably more specific than alkaline phosphatase.  相似文献   
54.
A case-control study is reported of 57 holdings (holdings in which salmon are raised in sea-water) with infectious salmon anaemia (ISA) and 61 holdings without ISA, together with a follow-up study of the control holdings, during 1988–1990. Generally the identified risk factors implied poor biosecurity since passive transmission (proximity to other ISA holdings) and active transmission (management practices which increased exposure to foreign biological material) were found to be major risk factors. The findings indicated that ISA was spread by infected live salmon or infected biological material i.e. animal waste or discharge from normal operations and slaughter. The findings also indicated that decreased host resistance could be important, but these results could be also be explained by a misclassification bias. Moreover, the findings indicate that a prompt eradication policy and improved biosecurity would be important means of controlling ISA.  相似文献   
55.
56.
利用玉米优良自交系农系531和X178杂交构建的200份RIL群体,基于GBS技术获得SNP标记构建高密度的重组bin遗传连锁图谱,定位控制玉米子粒容重相关QTL。结果表明,构建的物理图谱和遗传图谱的总长度分别为2 017.03 Mb和2 568.99 cM,相邻两个bin标记之间的平均物理距离和平均遗传距离分别为0.27 Mb和0.35 cM。运用所构建的遗传连锁图谱对RIL群体获得的所有目标性状进行连锁作图,两年共定位到4个与子粒容重相关的QTL位点,分别位于chr1、chr7和chr8上;穗部性状穗长、穗粗、穗行数、行粒数和出籽率两年分别共定位到了6、5、5、1、2个QTL位点,位点分布于chr1、chr2、chr3、chr4、chr5、chr7和chr8上。  相似文献   
57.
为保证地下沼气池结构的安全性,采用Rankine土压力理论比较了3种常见土质的静止、主动和被动土压力,并确定了地下钢筋混凝土圆形沼气池池壁土压力取值方法;根据线弹性有限元分析方法,研究了平板、球壳和圆锥台3种不同形状的沼气池池盖受力性能。结果表明:地下沼气池池壁土压力宜取其静止土压力计算;环梁的设置可以使中部开圆形洞口的圆锥台形池盖的最大应力减小约40%,宜为其设置环梁。  相似文献   
58.
通过选择性培养基的筛选,利用菜豆萎蔫毒素基因序列扩增引物和菜豆萎蔫毒素基因缺失菌株独有的ORF6序列的扩增引物,采用双重PCR技术实现了菜豆晕疫病菌的快速检测。  相似文献   
59.
中国花生网斑病研究   总被引:12,自引:0,他引:12  
花生网斑病是我国花生上新发生的一种叶部病害,近年来蔓延迅速,为害严重,经1983-1992年历时10年的系统研究,摸清了该病症状、发生和为害规律、分布特点;明确该病的发生与主要发病因子的关联性;于国内首次明确了该病原菌的分类地位;摸清了病害发生程度与花生荚果产量损失率之间的函数关系,找出了受害较敏感的花生物候期,建立了防治经济阈值模型;筛选出目前最好的杀菌剂、生物制剂和物理保护剂以及较抗病的花生品  相似文献   
60.
Six cases of ocular lymphocystis, a virus disease, are described. Lymphocystis is generally known as a benigh, unique, giant cell disease of fishes causing nodules on the skin and fins. It has been studied extensively because of the virus-host cell relationship that results in extreme size and lack of quick cellular destruction or stimulation to neoplasia. Lymphocystis cells were found behind or in one or both eyes and were also found on the cornea or adjacent skin surfaces. A retrobulbar mass produced extreme exophthalmos. Uveal (choroid and iris) masses were present in most cases. Optic nerve involvement was also seen. It is probable that the virus reached the eye by the blood with the resulting masses forming in situ rather than by direct extension from skin lesions.  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