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   21994篇
  免费   7371篇
  国内免费   640篇
林业   2552篇
农学   3040篇
基础科学   1584篇
  3210篇
综合类   9946篇
农作物   1822篇
水产渔业   1244篇
畜牧兽医   3930篇
园艺   1287篇
植物保护   1390篇
  2024年   10篇
  2023年   330篇
  2022年   812篇
  2021年   987篇
  2020年   1008篇
  2019年   550篇
  2018年   588篇
  2017年   998篇
  2016年   752篇
  2015年   1078篇
  2014年   1250篇
  2013年   1504篇
  2012年   1901篇
  2011年   1993篇
  2010年   1943篇
  2009年   1695篇
  2008年   1671篇
  2007年   1550篇
  2006年   1356篇
  2005年   1243篇
  2004年   788篇
  2003年   832篇
  2002年   1268篇
  2001年   1051篇
  2000年   727篇
  1999年   466篇
  1998年   275篇
  1997年   216篇
  1996年   186篇
  1995年   150篇
  1994年   124篇
  1993年   119篇
  1992年   138篇
  1991年   119篇
  1990年   89篇
  1989年   77篇
  1988年   48篇
  1987年   37篇
  1986年   20篇
  1985年   6篇
  1984年   9篇
  1983年   4篇
  1982年   5篇
  1981年   8篇
  1980年   4篇
  1979年   3篇
  1978年   2篇
  1962年   6篇
  1956年   2篇
  1955年   4篇
排序方式: 共有10000条查询结果,搜索用时 23 毫秒
1.
Betanodaviruses are the causative agents of viral nervous necrosis (VNN), a serious disease of cultured marine fish worldwide. Virus-like particles (VLPs) are one of the good novel vaccine candidates to control this disease. Until now, betanodavirus vaccine studies mainly focused on the humoral immune response and mortality after virus challenge. However, little is known about the activation of genes responsible for cellular and innate immunity by vaccines. In the present study, VLPs of orange-spotted grouper nervous necrosis virus (OGNNV) were produced in prokaryotes and their ability to enter Asian sea bass cells was the same as native virus, suggesting that they possess a similar structure to OGNNV. VLPs immunogenicity was then determined by intramuscularly vaccinating Epinephelus coioides at different concentrations (1.5 or 15 μg g?1 fish body weight, FBW) and immunizing frequencies (administration once, twice and thrice). A single vaccination with the dosage of 1.5 μg g?1 FBW is enough to provoke high titer antibodies (average 3 fold higher than that of negative control) with strong neutralizing antibody titer as early as 1 week post immunization. Furthermore, quantitative PCR analysis revealed that eleven genes associated with humoral, cellular and innate immunities were up-regulated in the liver, spleen and head kidney at 12 h post immunization, correlating with the early antibody response. In conclusion, we demonstrated that VLP vaccination induced humoral immune responses and activated genes associated with cellular and innate immunity against betanodavirus infection in orange-spotted grouper.  相似文献   
2.
南疆玉米和不同糖分甜高粱的青贮品质分析   总被引:4,自引:0,他引:4  
 为筛选适宜南疆地区的青贮甜高粱品种,采用罐装青贮的方法,以青贮玉米为对照,研究了糖分含量存在差异的6个品种甜高粱青贮的感观性状及其营养成分。结果显示,1)新高粱2号、考利和丽欧3个甜高粱品种青贮成功率均达到了100%,pH 值小于4.12,感官评价为1级优等。甜饲1号和玉米青贮成功率为80%,pH 值分别为4.27和4.18,感官定级为2级尚好。大力士青贮成功率为40%,pH 值为4.97,感官定级为3级中等。X096青贮 成功率为20%,pH5.31,定级为4级腐败。丽欧、考利、新高粱2号及玉米青贮的乳酸含量较高,丁酸含量较低;而X096、大力士青贮丁酸含量偏高,乳酸含量则较低。2)大力士甜高粱青贮水分含量最高(P <0.05),X096甜高粱青贮饲料水分含量最低(P <0.05)。甜高粱丽欧、考利和玉米青贮饲料的粗蛋白含量均超过了7%,显著高于其他各青贮样本(P <0.05),X096青贮饲料粗蛋白质含量最低(P <0.05)。粗脂肪含量以考利甜高粱青贮饲料最高,为3.30%,显著高于玉米、大力士和X096 青贮饲料(P <0.05)。考利和丽欧甜高粱青贮饲料水溶性碳水化合物(watersolublecarbohydrate,WSC)含量均超过了4.5%,显著高于其他各青贮饲料WSC 含量(P <0.05)。青贮玉米WSC 含量高于X096和甜饲1 号。3)粗纤维、中性洗涤纤维及酸性洗涤纤维含量,以X096 甜高粱青贮最高(P <0.05),丽欧、考利、甜饲1号和玉米青贮基本在同一水平(P >0.05)。4)各品种甜高粱及玉米青贮的灰分、钙、磷含量无显著性差异(P >0.05)。可见,在南疆可选择新高粱2号、考利、丽欧等甜高粱品种替代玉米来制作青贮饲料;选择大力士制作青贮,应采取延迟收割或晾晒等方法,以降低青贮前水分偏高的问题。  相似文献   
3.
江苏省杂草稻Rc基因的单体型分析   总被引:1,自引:0,他引:1  
  相似文献   
4.
刘杨  王勇  康静芹  张良 《湖北农业科学》2014,(7):1634-1636,1689
微生物的发酵过程十分复杂,培养基配方的组成以及培养条件的变化都会对菌体的代谢造成很大的影响。为了使抗生素的产量达到最大化,试验运用了Plackett-Burman试验设计、最陡爬坡试验、BoxBehnken试验设计对放线菌(Actinomycetes)G5进行了发酵条件优化。利用Plackett-Burman试验设计对影响放线菌G5发酵的诸多相关因素进行评估并筛选出3个主要因素,即碳源含量、培养温度和培养时间;之后根据3个主要因素的作用大小和方向进行了最陡爬坡试验;最后运用Box-Behnken试验设计确定最优值,得出其最优培养条件为淀粉含量21.66 g/L、培养温度27.70℃、培养时间7.75 d。在此条件下,抗生素对苹果轮纹病菌的抑菌圈直径为23.00 mm,与模型预测值基本一致。  相似文献   
5.
伊犁州引种杨树品种中龄期生长特性研究   总被引:1,自引:0,他引:1  
李宏  李丕军  雷双喜  郭光华  陈勇 《湖北农业科学》2012,51(12):2529-2532,2541
对伊犁哈萨克自治州27个引种杨树品种八年生植株的树高、胸径进行了方差分析,同时以每公顷材积年生长量为指标进行了聚类分析.结果显示,各杨树品种在胸径、树高生长方面存在显著的差异,表明各杨树品种对当地的环境适应性是不同的,其中年均胸径生长最快的品种是Ⅰ-467,年均树高生长最快的品种是Ⅰ-262,并且树高生长与胸径生长之间存在一定的正相关性.材积聚类分析筛选出表现最好的杨树品种有9个,表现较好的有3个,表现一般的有11个,表现较差的有4个;其中适应性最强、生长最快的杨树品种是大叶钻天杨.这个结果可为今后在伊犁哈萨克自治州进行杨树品种推广及平原造林提供依据.  相似文献   
6.
以柞蚕5龄幼虫为材料添食柞蚕微孢子虫(Nosema pernyi,Np),通过测定与分析不同性别5龄幼虫血淋巴蛋白质含量和组成变化的差异,为探究柞蚕对柞蚕微孢子虫感染的免疫应答提供依据。感染柞蚕微孢子虫的不同性别柞蚕5龄幼虫的血淋巴蛋白质含量随蚕体发育进程出现不同的变化:雌性个体与雄性个体分别在添食Np后24 h与48 h,血淋巴蛋白质含量极显著增加(P<0.01),且持续至添食后96 h,其中雌性个体的血淋巴蛋白质含量又极显著高于雄性个体(P<0.01);在添食Np后120 h,雌、雄个体的血淋巴蛋白质含量与对照组无显著性差异(P>0.05)。SDS-PAGE分析表明,添食Np后的柞蚕5龄幼虫血淋巴蛋白质组成与对照组基本一致,均显示出20条分子质量在20.1~97.2 kD的蛋白条带,其中雌、雄个体在添食后96 h,大小约44 kD的蛋白条带明显加深,大小约28 kD的蛋白条带雌性个体在添食后144 h、雄性个体在添食后120 h明显加深,且雄性个体在添食后120~168 h大小约42 kD的蛋白条带明显加深,但在添食后192 h与对照组无明显差异。研究结果说明Np侵染后,柞蚕5龄幼虫血淋巴蛋白质的含量及组成均产生了一定的变化,并且雌雄个体间的变化存在差异。  相似文献   
7.
Wang R  Wang F  Zhao J  Qi M  Ning C  Zhang L  Xiao L 《Veterinary parasitology》2012,187(3-4):534-537
The prevalence of Cryptosporidium spp. was investigated in scale quail (Coturnix coturnix japonica) farms in Henan Province, China between September 2006 and August 2007. One thousand eight hundred and eighteen fecal samples from 47 quail farms in five areas were collected for the examination of Cryptosporidium oocysts. The overall prevalence of Cryptosporidium was 13.1% (95% CI 13.1±1.6%) (29 of 47 farms), with 72-100-day-old quails having the highest prevalence (23.6%, 95% CI 23.6±2.6%) (χ(2)=64.91; ρ<0.01). The highest prevalence was observed in autumn (21.8%, 95% CI 21.8±3.1%) and the lowest in winter (χ(2)=74.83; ρ<0.01). Two hundred and thirty-nine Cryptosporidium-positive samples were analyzed by PCR-restriction fragment length polymorphism (RFLP) analysis of the small subunit (SSU) rRNA gene, and 42 were further analyzed by DNA sequencing of the PCR products. Two Cryptosporidium species were identified, Cryptosporidium baileyi in 237 birds on 29 farms, and potentially zoonotic Cryptosporidium meleagridis in only two birds on two farms. These findings may suggest that quails are not a major source of zoonotic Cryptosporidium in the study area.  相似文献   
8.
Wei Z  Fu Q  Liu X  Xiao P  Lu Z  Chen Y 《Veterinary microbiology》2012,159(3-4):519-525
Streptococcus equi ssp. zooepidemicus (Streptococcus zooepidemicus, SEZ) is responsible for a wide variety of infections in many species. Attempts to control the infection caused by this agent are hampered by a lack of effective vaccines and useful diagnostic kits. Surface proteins of bacterial species are usually involved in interaction with host and hopefully act as biomarkers for serodiagnosis and subunit vaccine components. In this study, the surface proteins of SEZ C55138 strain were systematically identified by surface shaving with trypsin and a total of 20 surface associated proteins were found. Further analysis of five selected novel proteins (SzM, FBP, SAP, CSP and 5'-Nu) revealed that they all expressed in vivo and their recombinant derived proteins could be reactive with convalescent sera. These identified immunogenic surface proteins have potential as SEZ vaccine candidates and diagnostic markers.  相似文献   
9.
Ding L  Xu X  Huang Y  Li Z  Zhang K  Chen G  Yu G  Wang Z  Li W  Tong D 《Veterinary microbiology》2012,158(1-2):12-22
Transmissible gastroenteritis virus (TGEV) has been reported to induce apoptosis in swine testis (ST) cells. However, the mechanisms underlying TGEV-induced apoptosis are still unclear. In this study we observed that TGEV infection induced apoptosis in porcine kidney (PK-15) cells in a time- and dose-dependent manner. TGEV infection up-regulated FasL, activated FasL-mediated apoptotic pathway, leading to activation of caspase-8 and cleavage of Bid. In addition, TGEV infection down-regulated Bcl-2, up-regulated Bax expression, promoted translocation of Bax to mitochondria, activated mitochondria-mediated apoptotic pathway, which in turn caused the release of cytochrome c and the activation of caspase-9. Both extrinsic and intrinsic pathways activated downstream effector caspase-3, followed by the cleavage of PARP, resulting in cell apoptosis. Moreover, TGEV infection did not induce significant DNA fragmentation in ammonium chloride (NH(4)Cl) pretreated PK-15 cells or cells infected with UV-inactivated TGEV. In turn, block of caspases activation also did not affect TGEV replication. Taken together, this study demonstrates that TGEV-induced apoptosis is dependent on viral replication in PK-15 cells and occurs through activation of FasL- and mitochondria-mediated apoptotic pathways.  相似文献   
10.
坝上地区紫花苜蓿氮、磷、钾肥料效应与推荐施肥量   总被引:6,自引:0,他引:6  
采用"3414"试验设计,进行田间试验,研究了坝上地区紫花苜蓿氮、磷、钾肥料效应及推荐施肥量。结果表明:在偏干旱条件下,施氮提高紫花苜蓿产量8%~25%,施磷增产率在122%~172%之间,施钾降低紫花苜蓿产量0~12%;与无肥处理相比,缺磷处理减产38.24%,其余处理增产26%~68%,高磷处理增产率最高(67.94%);氮钾互作效应降低紫花苜蓿产草量,而磷钾和磷氮互作效应为提高;缺氮、缺磷和缺钾处理的相对产量分别为75.02%、36.78%和91.60%,土壤速效氮、速效磷和速效钾的丰缺状况依次为中等、极缺和丰富。结合肥料效应函数法和养分丰缺指标法,得出坝上地区旱作条件下紫花苜蓿推荐年施氮(N)、磷(P2O5)和钾(K2O)量分别为80、50和60kg/hm2。  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