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   1762篇
  免费   135篇
  国内免费   161篇
林业   153篇
农学   196篇
基础科学   90篇
  322篇
综合类   559篇
农作物   136篇
水产渔业   94篇
畜牧兽医   329篇
园艺   64篇
植物保护   115篇
  2024年   12篇
  2023年   31篇
  2022年   64篇
  2021年   85篇
  2020年   71篇
  2019年   73篇
  2018年   40篇
  2017年   61篇
  2016年   75篇
  2015年   96篇
  2014年   84篇
  2013年   105篇
  2012年   129篇
  2011年   130篇
  2010年   116篇
  2009年   103篇
  2008年   93篇
  2007年   118篇
  2006年   81篇
  2005年   68篇
  2004年   54篇
  2003年   72篇
  2002年   59篇
  2001年   52篇
  2000年   36篇
  1999年   31篇
  1998年   14篇
  1997年   19篇
  1996年   13篇
  1995年   7篇
  1994年   6篇
  1993年   13篇
  1992年   7篇
  1991年   6篇
  1990年   13篇
  1989年   6篇
  1988年   6篇
  1987年   4篇
  1984年   1篇
  1983年   1篇
  1980年   2篇
  1971年   1篇
排序方式: 共有2058条查询结果,搜索用时 0 毫秒
81.
A problem for dairy cows following milk stasis is to cope with a high risk of intramammary infection and there is a need to initiate an extensive renewal of secretory modules in mammary glands so that milk production in next lactation may be optimized. We recently reported that ultrasonicated Enterococcus faecium SF68 (SF68) is compatible with cow mammary glands and an enhancer of innate immunity during the immediate post‐milk stasis period. The current study further examines the concomitant effect of ultrasonicated SF68 on mammary tissue remodeling. Four Holstein cows each received intramammary infusions of regular antibiotic dry‐cow formula (positive control) and two different doses of SF68 in different quarters. Analyses of individual quarter secretion samples showed faster neutrophil infiltration, earlier modifications in protein composition, including caseins and lactoferrins, as well as more prompt elevation of the specific unit of 92‐kDa matrix metalloproteinase 9 (MMP9) in SF68‐infused quarters compared to the positive controls. Intramammary infusion of ultrasonicated SF68 seems able to accelerate the regression of mammary synthetic capacity and potentiate the breakdown of glandular extracellular matrix, indicating a more efficient mammary gland involution. Correlation analyses imply that the ability of ultrasonicated SF68 to induce faster neutrophil chemotaxis and the associated MMP9 release is partly responsible.  相似文献   
82.
本文研究狗脊、盾蕨、凤丫蕨的耐旱能力,为其园林应用提供参考依据。对三种蕨类植物材料在温室中采用自然干旱的方式进行干旱胁迫,每5天测定1次胁迫过程中生理生化指标,并测定其叶片保水力。结果表明:干旱胁迫下,狗脊的叶片保水力变化幅度最大,凤丫蕨其次,盾蕨最小;各植物电导率与对照组相比,均持续上升;叶绿素的含量均下降,凤丫蕨叶绿素含量变化幅度最大;叶片中的SOD活性、POD活性、MDA含量、可溶性蛋白含量变化趋势均为先升后降,凤丫蕨SOD活性、POD活性、MDA含量在前期迅速增长,狗脊、盾蕨缓慢上升。综合分析表明三种蕨类耐旱能力依次为狗脊盾蕨凤丫蕨。  相似文献   
83.
In this paper we analyze and compare natural regeneration in shrub-tree community in three areas below Pinus caribaea stands and a natural area (cerrado sensu stricto). We also analyze the influence of...  相似文献   
84.
A simplified water balance model in conjunc-tion with an evapotranspiration (ET) model and cumulative forest cover data were used to quantify the changes in annual water yield in response to reforestat...  相似文献   
85.
The effect of a combination of (-)-epigallocatechin-3-gallate (EGCG) with fish oil on intestinal tumorigenesis in Apc (Min/+) mice fed a high-fat diet was investigated in the present study. The combined treatment of EGCG and fish oil for 9 weeks reduced the tumor number by 53% as compared to controls while neither agent alone had a significant effect. Apoptosis was significantly increased in all treatment groups. beta-Catenin nuclear positivity in adenomas from the combination group was lower than control mice, implicating the modulation of Wnt signaling by the combination. Fish oil and the combination significantly reduced prostaglandin E 2 (PGE 2) levels in small intestinal tumors as compared to controls, suggesting modulation of aberrant arachidonic acid metabolism by fish oil. Akt phosphorylation in adenomas was significantly reduced in all treatment groups, which may have contributed to the observed increase in apoptosis. The results indicate that a combination of low doses of EGCG and fish oil can inhibit tumor multiplicity in Apc (Min/+) mice.  相似文献   
86.
In this study, we characterized the secreted proteins of Brucella abortus into the enriched media under the bacterial laboratory growth condition and investigated the pathogenic importance of culture supernatant (CS) proteins to B. abortus infection. CS proteins from stationary phase were concentrated and analyzed using 2D electrophoresis. In MALDI TOF/TOF analysis, more than 27 proteins including CuZn SOD, Dps, Tat, OMPs, Adh, LivF, Tuf, SucC, GroEL and DnaK were identified. Cytotoxic effects of CS proteins were found to increase in a dose-dependent manner in RAW 264.7 cells. Upon B. abortus challenge into phagocytes, however, CS proteins pre-treated cells exhibited lower bacterial uptake and intracellular replication compared to untreated cells. Immunization with CS proteins induced a strong humoral and cell mediated immune responses and exhibited significant higher degree of protection against virulence of B. abortus infection compared to mice immunized with Brucella broth protein (BBP). Taken together, these results indicate that B. abortus secreted a number of soluble immunogenic proteins under laboratory culture condition, which can promote antibody production resulted in enhancing host defense against to subsequently bacterial infection. Moreover, further analysis of CS proteins may help to understand the pathogenic mechanism of B. abortus infection and host–pathogen interaction.  相似文献   
87.
The pathogenic mechanisms of Brucellosis used to adapt to the harsh intracellular environment of the host cell are not fully understood. The present study investigated the in vitro and in vivo characteristics of B. abortus betaine aldehyde dehydrogenase (BetB) (Gene Bank ID: 006932) using a betB deletion mutant constructed from virulent B. abortus 544. In test under stress conditions, including osmotic- and acid stress-resistance, the betB mutant had a lower osmotic-resistance than B. abortus wild-type. In addition, the betB mutant showed higher internalization rates compared to the wild-type strain; however, it also displayed replication failures in HeLa cells and RAW 264.7 macrophages. During internalization, compared to the wild-type strain, the betB mutant was more adherent to the host surface and showed enhanced phosphorylation of protein kinases, two processes that promote phagocytic activity, in host cells. During intracellular trafficking, colocalization of B. abortus-containing phagosomes with LAMP-1 was elevated in betB mutant-infected cells compared to the wild-type cells. In mice, the betB mutant was predominantly cleared from spleens compared to the wild-type strain after 2 weeks post-infection, and the vaccination test with the live betB mutant showed effective protection against challenge infection with the virulent wild-type strain. These findings suggested that the B. abortus betB gene substantially affects the phagocytic pathway in human phagocytes and in host cells in mice. Furthermore, this study highlights the potential use of the B. abortus betB mutant as a live vaccine for the control of brucellosis.  相似文献   
88.
2013年5月22日,南通博物苑鸟类调查组在南黄海边的如东洋口发现一处黑嘴鸥(Larzus saundersi)繁殖地。该繁殖地位于如东大、小洋口之间新围垦的海堤内侧(E121°06’,N32°28’),面积约10000m2。黑嘴鸥营巢于碱蓬(Suaeda gauca)地上,与普通燕鸥(Sternahirundo)、白额燕鸥(Sternaalbifrons)、红脚鹬(Tringatotanus)、环颈鹆(Charadriusalexandrinus)等混群营巢。采用直接计数法统计繁殖区内所见黑嘴鸥的数量在500只以上。应迅速建立保护区以切实保护好这一濒危鸟类及其新繁殖地。  相似文献   
89.
采用剪标记法,现场模拟测定了钻井噪声与振动对鲤鱼生长的影响,研究结果表明,钻井噪声与振动对鲁鱼生长有显著影响,其临界等效噪声级和振动级约为83.9dB和89.7dB,影响域径约为9m,污染持续时间,体重和群居行为等生态因素,能显著改变钻井噪声与振动对鲤鱼生长的污染效应,由在污染消失后鲤鱼生长率能迅速恢复,这说明钻井噪声与振动鲤鱼的生长的污染效应,由于在污染消失后鲤鱼生长率能迅速恢复,这说明钻井噪声与振动对鲤鱼的影响机制可能是合理的。  相似文献   
90.
近年来,河蟹池塘养殖已积累了一些成熟的经验,但从目前河蟹池塘养殖技术水平来看,还存在一些问题。譬如,河蟹商品规格偏小;池塘的空间、水体及饵料资源未充分利用;病虫害防治方法不当;成活率不高等等。因此,需要我们水产工作者进一步去研究、探索。为使主养河蟹池塘获得优质高产高效,并促进池塘河蟹养殖业的可持续发展,我们于1999~2000年进行了《主养河蟹池塘套混养名优品种技术试验》,取得了显著的经济效益和社会效益。现将2000年的试验情况报告如下: 一、试验条件 1.试验地点位于上虞市96丘围涂地区,199…  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