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   33802篇
  免费   5024篇
  国内免费   2470篇
林业   2503篇
农学   2805篇
基础科学   1546篇
  5084篇
综合类   10037篇
农作物   2298篇
水产渔业   4022篇
畜牧兽医   9006篇
园艺   1423篇
植物保护   2572篇
  2024年   142篇
  2023年   409篇
  2022年   915篇
  2021年   1154篇
  2020年   1422篇
  2019年   1933篇
  2018年   1609篇
  2017年   2059篇
  2016年   1898篇
  2015年   2062篇
  2014年   2131篇
  2013年   2588篇
  2012年   2621篇
  2011年   2792篇
  2010年   2567篇
  2009年   2092篇
  2008年   2061篇
  2007年   1843篇
  2006年   1621篇
  2005年   1292篇
  2004年   970篇
  2003年   790篇
  2002年   930篇
  2001年   772篇
  2000年   664篇
  1999年   326篇
  1998年   189篇
  1997年   162篇
  1996年   98篇
  1995年   113篇
  1994年   114篇
  1993年   85篇
  1992年   80篇
  1991年   79篇
  1990年   65篇
  1989年   58篇
  1988年   53篇
  1987年   54篇
  1986年   22篇
  1985年   37篇
  1984年   28篇
  1983年   31篇
  1981年   25篇
  1979年   19篇
  1978年   27篇
  1977年   24篇
  1976年   29篇
  1974年   22篇
  1973年   23篇
  1972年   22篇
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
101.
采用22种限制性内切酶分析了三江白猪线粒体DNA的多态性。结果表明:在60头个体中检出30种限制性态型.归结为7种基因单倍型;其间的差异来自少数限制性位点的突变。单倍型间平均遗传距离为0.35%,群体遗传多态程度为0.044%。三江白猪遗传多样性非常贫乏,说明三江白猪与地方种猪起源于一个共同祖先,在品种形成早期受到创立者效应的制约。  相似文献   
102.
Captive juvenile African black-footed penguins (Spheniscus demersus) housed in an outdoor enclosure at the Baltimore Zoo have an average 50% mortality from avian malarial (Plasmodium sp.) infection each year without intense monitoring for disease and chemotherapeutic intervention. During the 1996 malaria transmission season, the safety and efficacy of an anti-circumsporozoite (CSP) DNA vaccine encoding the Plasmodium gallinaceum CSP protein against P. relictum were studied. The goal was to reduce clinical disease and death without initiating sterile immunity after release into an area with stable, endemic avian malaria. The birds were monitored for adverse clinical signs associated with vaccination, the stimulation of an anti-CSP antibody response, and protection afforded by the vaccine. The presence of P. relictum in trapped culicine mosquitoes within the penguin enclosure was monitored to assess parasite pressure. Among the vaccinated penguins, the parasitemia rate dropped from approximately 50% to approximately 17% despite intense parasite pressure, as determined by mosquito infection rate. During the year of the vaccine trial, no mortalities due to malaria occurred and no undesirable vaccination side effects occurred. This is the first trial of an antimalarial vaccine in a captive penguin colony.  相似文献   
103.
The aim of these experiments was to investigate the potential antiviral effect of Saccharomyces cerevisiaeβ‐glucan on the pneumonia induced by swine influenza virus (SIV). Forty colostrum‐deprived 5‐day‐old piglets were randomly divided into four groups of 10. The 20 pigs in groups 1 and 2 were administered Saccharomyces cerevisiaeβ‐glucan orally (50 mg/day/pig; En‐Bio Technology Co., Ltd) for 3 days before SIV infection and those in groups 3 and 4 were given culture medium/diluent alone. Groups 1 and 3 were inoculated intranasally with 3 ml of tissue culture fluid containing 2 × 106 tissue culture infective doses 50% (TCID50)/ml of SIV and those in groups 2 and 4 were exposed in the same manner to uninfected cell culture supernatant. The microscopic lung lesions induced by SIV infection (group 1 pigs) were significantly more severe than those induced by infection in animals pre‐administered β‐glucan (group 3) (P < 0.05). Significantly more SIV nucleic acid was detected in the lungs of pigs experimentally infected with SIV only (group 1) at 5, 7 and 10 days post‐inoculation (dpi) compared with lungs from pigs pre‐administered β‐glucan and infected with SIV (group 3) (P < 0.05). The concentrations of interferon‐γ (IFN‐γ) and nitric oxide (NO) in bronchoalveolar lavage fluid from pigs pre‐administered β‐glucan and infected with SIV (group 3) were significantly higher than for any other group at 7 and 10 dpi for IFN‐γ, and at 5, 7 and 10 dpi for NO (P < 0.05). Saccharomyces cerevisiaeβ‐glucan reduced the pulmonary lesion score and viral replication rate in SIV‐infected pigs. These findings support the potential application of β‐glucan as prophylactic/treatment agent in influenza virus infection.  相似文献   
104.
对捕食线虫性真菌——少孢节丛孢菌A1分离株(Arthrobotrys oligospora A1)的18S rDNA基因序列进行了研究。结果表明:其18S rDNA全基因序列为1769bp,在进化树上与同种国外分离株A.oligospora var oligospora 1最为接近,同源性为94.7%。这与二者都具有捕食性结构—菌网的特点相一致。证实捕食线虫性真菌的捕食性结构与捕食线虫性真菌种系发生有关。从少孢节丛孢菌3个分离菌株(Arthrobotrys oligospora A1、A.oligospora 1、A.oligospora2)的进化树及同源性来看。不同国家地区的丛孢菌属(Arthrobotrys)分离株确实存在差异。  相似文献   
105.
The original article to which this Correction refers was published in Pest Management Science 58 (7): 649–662 (2002).Copyright © 2004 Society of Chemical Industry  相似文献   
106.
本文通过2个试验对母猪在分娩圈和分娩栏中的卧向行为进行了观察,试验一中,选择10头大白(Yorkshire)经产母猪作为观察对象,试验二中分别选择10头大白(Yorkshire)经产母猪和10头长白(Landrace)经产母猪作为观察对象。观察采用瞬时记录方法,每周观察三次,隔日观察,每天上午、下午各观察一次,每次3h,每次观察间隔5min。观察中发现,母猪卧向以向外为主。分娩栏(四周是栏杆)中的母猪以选择卧向外(向北)为主,其次是卧向内(向南),卧向左和卧向右最少且差异不显著,上栏前和下栏后差异不显著。分娩圈(四周是墙壁)中母猪以选择卧向外为主,卧向内最少,卧向左和卧向右差异不显著。长白母猪比大白母猪选择卧向外的多,妊娠阶段比哺乳阶段选择卧向外的多。  相似文献   
107.
介绍构建野牦牛种质资源信息收集、整理及调查的方法;建立了野牦牛50头规模的可良性运作的遗传繁育基地,可年产2万支冷冻精液的动态保存库,以及0.1万~0.15万枚野牦牛冷冻胚胎动态保存库的条件与规模;创建了国内外相关野生动物保护组织、行业、协会等保护和利用野牦牛遗传资源的共享平台.  相似文献   
108.
构建表达eae和stx1/2B的融合基因,克隆eae基因的C端280个氨基酸残基(Int280)基因部分,以正确地阅读框定向插入到含有stx1/2B融合基因的质粒,构建重组质粒,将其转化于BL21(DE3),用IPTG进行诱导表达,经SDS—PAGE电泳检测,该融合蛋白获得了高效表达。薄层扫描分析表明:目的蛋白表达量占菌体总蛋白含量的50.67%。由于该融合蛋白由eae、stx1B、stx2B等三部分抗原组成,可刺激机体产生针对紧密素和StxB的抗体,在EHEC O157亚单位疫苗设计或单克隆抗体抗制备中具有重要价值。  相似文献   
109.
The study investigated the ultrastructural characteristics of tubular gland and duct cells, as well as luminal gland cells in the isthmus region of the oviduct of laying and natural moulting hens. Tubular glands in laying birds were composed of type 1 and 2 cells. Based on the preponderance of each cell type, in relation to the location of a developing egg in the oviduct of the domestic fowl, these gland cells may represent different functional states of the same cell. The findings of the study on natural moulting birds suggest that autophagy is a process confined to the early stages of degeneration, while necrosis occurs in the terminal stages.  相似文献   
110.
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