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   12254篇
  免费   303篇
  国内免费   16篇
林业   930篇
农学   475篇
基础科学   63篇
  1770篇
综合类   700篇
农作物   1224篇
水产渔业   874篇
畜牧兽医   5513篇
园艺   270篇
植物保护   754篇
  2022年   102篇
  2021年   169篇
  2020年   112篇
  2019年   141篇
  2018年   274篇
  2017年   325篇
  2016年   321篇
  2015年   395篇
  2014年   416篇
  2013年   450篇
  2012年   798篇
  2011年   710篇
  2010年   340篇
  2009年   282篇
  2008年   380篇
  2007年   353篇
  2006年   315篇
  2005年   997篇
  2004年   861篇
  2003年   629篇
  2002年   261篇
  2001年   265篇
  2000年   134篇
  1999年   205篇
  1992年   133篇
  1991年   159篇
  1990年   190篇
  1989年   166篇
  1988年   178篇
  1987年   122篇
  1986年   150篇
  1985年   123篇
  1984年   95篇
  1983年   86篇
  1982年   35篇
  1979年   135篇
  1978年   107篇
  1977年   93篇
  1976年   77篇
  1975年   140篇
  1974年   134篇
  1973年   118篇
  1972年   143篇
  1971年   121篇
  1970年   91篇
  1969年   105篇
  1968年   117篇
  1967年   127篇
  1966年   102篇
  1965年   71篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Hormonal asynchronies during oestrus, related to the presence of suprabasal plasma-progesterone (P4) concentrations and a delayed ovulation, interfere with the fertility of repeat-breeder heifers (RBH). Since tubal dysfunction can occur in connection with hormonal asynchronies and constrained availability of fertile spermatozoa at the time of ovulation, the present study tested the hypothesis that frequent sperm deposition from onset of oestrus to ovulation may improve pregnancy rates in RBH. Five RBH and five virgin heifers (VH; controls) were repeatedly artificially inseminated (AI) at 6 h intervals from onset of oestrus to spontaneous ovulation. Hormone analyses revealed suprabasal P4 concentrations and a delay in the occurrence of the luteinising hormone (LH) surge, but a normal cortisol profile in RBH. Compared with controls, RBH presented longer interval from onset of oestrus to ovulation, and therefore, received more AIs. Pregnancy rates in RBH reached control levels (60%; NS), indicating that the hypothesis might be correct. Pregnancy rates in VH were below the expected range, presumably attributed to a deleterious influence of the frequent handling. The study suggests that pregnancy rates can be improved in RBH by frequent AI in relation to spontaneous ovulation. However, this practice of repeated manipulations, while seeming not to show adverse effects, lacks practicality for routine use.  相似文献   
62.
A malignant catarrhal fever (MCF)-like disease was induced experimentally in 3 sheep after aerosol inoculation with ovine herpesvirus-2 (OvHV-2). Each of 3 OvHV-2-negative sheep was nebulized with 2 ml of nasal secretions containing approximately 3.07 X 10(9) OvHV-2 DNA copies from a sheep experiencing an intensive viral-shedding episode. Ovine herpesvirus-2 DNA became detectable by polymerase chain reaction in the peripheral blood leukocytes of all 3 sheep within 3 days, and all 3 seroconverted between 6 and 8 days postinfection (PI). The sheep developed clinical signs, with copious mucopurulent nasal discharge and fever around 14 days PI. One of the 3 clinically affected sheep was euthanized at 18 days PI. Major lesions at necropsy were multifocal linear erosions and ulcers in mucosa of the cheeks, tongue, pharynx, and proximal esophagus and mild disseminated pneumonia. Microscopically, there was extensive moderate superficial histiocytic-lymphocytic rhinitis with epithelial dissociation and degeneration. Moderate multifocal histiocytic bronchointerstitial pneumonia was associated with loss of terminal bronchiolar epithelium. Lymphocytic vasculitis was present only in the lung. The remaining 2 sheep recovered clinically, approximately 25 days PI. The study revealed that clinical signs and lesions resembling MCF can develop when uninfected sheep are exposed to a high dose of aerosolized OvHV-2.  相似文献   
63.
Five animals in a colony of cynomolgus monkeys (Macaca fascicularis) died or were euthanatized because of alveolar echinococcosis, during a period of 5 years. The remainder of the colony was screened for possible infection with Echinococcus multilocularis, using serology and ultrasonography. A total of 46 animals out of a group of 55 were examined. The presence of anti-Em2 antibodies analyzed with enzyme-linked immunosorbent assay was demonstrated in 3 monkeys. In 2 of these 3 monkeys, multilocular structures compatible with metacestodal cysts in the liver were identified, using ultrasonography. The presence of alveolar echinococcosis was subsequently confirmed at postmortem examination in 1 animal. The other animals are still alive. Two other monkeys were negative in the serological examination but had cystic structures in the liver, which were identified as bile duct cysts at postmortem examination in 1 animal. The other monkey is still alive. These findings suggest that serology for antibodies against the Em2 antigen may represent a useful method in identifying animals that might be infected with E. multilocularis and are therefore at risk of developing fatal alveolar echinococcosis.  相似文献   
64.
根据栽培面积以及为农民提供的收入而言,红辣椒在韩国一直是仅次于水稻而居第二位的重要农作物。按人均计算,韩国人均年辣椒粉的消费量达2.5kg以上,可能是世界最高记录。辣椒在韩国主要用作泡菜(韩国称之为Kimchi)香辛料和菜肴中的  相似文献   
65.
The effects of exposure of human erythrocytes to different concentrations of 4-chloro-2-methylphenoxyacetic acid (MCPA) and its metabolite—2,4-dimethylphenol (2,4-DMP) were studied. The investigations concerned mainly the content of glutathione (GSH and GSSG), glutathione peroxidase (GSH-Px), glutathione transferase (GST), and the level of adenine energy charge (AEC). Reactive oxygen species (ROS) such as hydroxyl radical, superoxide anion, hydrogen peroxide, and nitric oxide are produced during normal processes in the cell. Under normal conditions, antioxidant systems of the cell minimize damage caused by ROS. When ROS generation increases to an extent that it overcomes the cellular antioxidant systems, the result is oxidative stress. We observed that MCPA and 2,4-DMP decreased the level of GSH in erythrocytes in comparison with control. MCPA did not affect glutathione peroxidase and glutathione transferase activity, while 2,4-DMP increased their activity. 2,4-DMP decreased the level of ATP and increased the content of ADP and AMP, leading to the fall of the level of AEC. MCPA and 2,4-DMP transform hemoglobin into methemoglobin, thus preventing oxygen transport. Comparison of the toxicity of MCPA and 2,4-DMP revealed that the most prominent changes occurred in human erythrocytes incubated with 2,4-DMP.  相似文献   
66.
萝卜离体再生的影响因素   总被引:5,自引:0,他引:5  
对影响萝卜离体再生的因素进行了研究,结果表明,不同基困型材料均以带柄子叶的再生频率最高;不同基因型材料所需的最适激素浓度和配比不同,在一定范围内提高6-BA浓度可促进不定芽的分化;苯基脲类细胞分裂素TDZ与嘌呤型细胞分裂素6-BA和KT相比,对促进萝卜不定芽分化的效果更好;AgNO_3可显著提高不定芽再生频率;NAA诱导生根的效果优于IAA。  相似文献   
67.
农杆菌介导的蝴蝶兰基因转化系统的建立   总被引:9,自引:1,他引:9  
柴明良  金斗焕 《园艺学报》2004,31(4):537-540
 针刺后的蝴蝶兰‘White Hikaru’的类原球茎(PLB),与含绿色荧光蛋白基因( )和潮霉素磷酸转移酶基因(^p£)的pCAMBIA 1300一SmGFP的根瘤农杆菌LBA4404 共培养,培育出了转基因的蝴蝶兰。经绿色荧光蛋白检测和Southern印迹,证实了再生植株中含cop基因和hpt基因。  相似文献   
68.
C difficile has emerged as an important cause of diarrheic disease in horses. C difficile diarrhea is usually diagnosed in mature horses, mostly when they are treated with antimicrobials and hospitalized. It is important for clinicians at veterinary hospitals to have knowledge about the organism and the infection. To prevent C difficile diarrhea, judicious use of antimicrobials is important, as is minimizing different stress factors at the animal hospital or clinic. Infected horses must be isolated. Routine examination for C difficile and toxin A or B is recommended in horses with antibiotic-associated diarrhea. When treating foals for R equi pneumonia, it is important to avoid accidental ingestion of erythromycin by the dams. To reduce the number of environmental spores, thorough cleaning and surface disinfection of the animal hospital and clinic are important. Routine handwashing should be performed by all staff.  相似文献   
69.
Ultrasonography is currently used in veterinary medicine to examine the anatomy and physiology of internal organs and to establish normal standards for exotic animal species. Specifically, the kidneys may be evaluated and measured in length, width, and height. The kidneys in 33 oncilla (Leopardus tigrinus), including 20 males and 13 females, were opportunistically located, characterized, and measured by ultrasound using a 7.5-MHz linear transducer at two different zoologic facilities in S?o Paulo, Brazil. Renal volume was calculated from these linear measurements. The mean linear measurements and volume of the right and left kidneys were not significantly different.  相似文献   
70.
Within the framework of the active surveillance for transmissible spongiform encephalopathies in sheep in Sweden, 4 cases of the atypical form of scrapie, Nor98, were identified during 2003. Nor98 is a recently recognized and poorly understood variant of scrapie, first described in Norway. The cases were positive by the rapid test (enzyme-linked immunosorbent assay). Immunohistochemical staining showed diffuse thin-granular staining of the cerebellar cortex. Western immunoblotting analysis of specimens of brain stem and cerebellum showed a light band of approximately 12 kDa. Typical scrapie was ruled out based on the confirmatory testing. The affected ewes were from 4 different flocks. They were between 7 and 9 years old. Two were of the ARQ/ARQ genotype, 1 ARR/ARQ, and 1 ARR/AHQ. Two ewes had shown ataxia, and the other 2 had no clinical signs. Whole-flock slaughter was applied, and testing of the flock mates did not reveal additional cases. Nor98 differs from typical scrapie in its epidemiology, frequency of genotypes of sheep affected, clinical signs, microscopic lesions, distribution of scrapie prion protein in the brain, and characteristics of the immunostaining and immunoblotting profiles.  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