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   255604篇
  免费   15352篇
  国内免费   655篇
林业   23512篇
农学   14028篇
基础科学   2959篇
  42988篇
综合类   25792篇
农作物   17154篇
水产渔业   16822篇
畜牧兽医   97993篇
园艺   6366篇
植物保护   23997篇
  2021年   2487篇
  2020年   2884篇
  2019年   3691篇
  2018年   3987篇
  2017年   4381篇
  2016年   4817篇
  2015年   4320篇
  2014年   5698篇
  2013年   16280篇
  2012年   6415篇
  2011年   8346篇
  2010年   7345篇
  2009年   7924篇
  2008年   7712篇
  2007年   6779篇
  2006年   7425篇
  2005年   6652篇
  2004年   6490篇
  2003年   6269篇
  2002年   5594篇
  2001年   6157篇
  2000年   5757篇
  1999年   5422篇
  1998年   4159篇
  1997年   4197篇
  1996年   3931篇
  1995年   4514篇
  1994年   3898篇
  1993年   3641篇
  1992年   4357篇
  1991年   4420篇
  1990年   4154篇
  1989年   4205篇
  1988年   3664篇
  1987年   3766篇
  1986年   3724篇
  1985年   4004篇
  1984年   3685篇
  1983年   3488篇
  1982年   2888篇
  1981年   2818篇
  1980年   2771篇
  1979年   3099篇
  1978年   2823篇
  1977年   2634篇
  1976年   2464篇
  1975年   2309篇
  1974年   2497篇
  1973年   2474篇
  1972年   2210篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
201.
202.
203.
Lactate Kinetics in veterinary Critical Care: A Review   总被引:3,自引:0,他引:3  
Elevation in blood lactate concentration, with or without accompanying metabolic acidosis, is a hallmark finding in patients with circulatory compromise, and is also consistently noted in other conditions affecting critically ill or injured individuals. Little is reported in a veterinary literature regarding lactate measurement in the emergency and critical care setting, despite impressive reports of the clinical usefulness of lactate measurement in people. The purpose of this article is to review lactate kinetics and the clinical utility of lactate measurement. Limitations to lactate evaluation will also be discussed.  相似文献   
204.
Results are given of a survey on the occurrence of pathotypes (races) of Synchytrium endobioticum (potato wart) in both parts of re-unified Germany. An assortment of six differential cultivars appears to be sufficient to distinguish between the seven presently important German wart pathotypes. A total of ten pathotypes was recorded. Two West and two East German pathotypes seem to be identical, based on the results obtained with presently available cultivars. In contrast to pathotype 1 (common race) which prior to 1945 was distributed over the whole German territory, the 'new' pathotypes seem to prefer the central and south German mountain areas.  相似文献   
205.
合成了14个含有 N-莰-2-基的磷酰胺和硫代磷酰胺类化合物,用红外、核磁及元素分析确定其结构,并探讨了~(31)Pδ值的规律性变化。初步生物试验表明,磷酰胺类化合物对水稻发芽具有明显的抑制作用,其中化合物Ⅱ1的活性大于对照药剂抗倒胺但低于多效唑。  相似文献   
206.
207.
Ceftiofur is a new broad spectrum cephalosporin marketed for the treatment of acute bovine respiratory disease. In this investigation ceftiofur was administered by intramuscular injection, at 24 h intervals, to healthy beef-bred calves for four days at dosages of 2.2 and 4.4 mg/kg of body weight, with 4 wk intervals between dosing regimens. Serum, tissue chamber fluid (TCF), and bronchial secretion (BS) concentrations of ceftiofur were measured by microbiological assay after the first and fourth dose of each dosing regimen. Peak serum concentrations (Cmax) of 8.8 micrograms/mL and 17.3 micrograms/mL were obtained approximately 2 h (Tmax), the time of mean peak concentration) after single injections of 2.2 mg/kg and 4.4 mg/kg, respectively. The Cmax was increased approximately twofold following multiple doses of 2.2 mg/kg (Cmax = 13.1 micrograms/mL) and 4.4 mg/kg (Cmax = 24.1 micrograms/mL). Ceftiofur accumulated slowly into TCF and peak concentrations were found to be approximately 14% of those observed in serum after the first dose and approximately 24% after multiple dosing. Concentrations of ceftiofur in BS were obtained rapidly with peak concentrations reaching 45% of the serum Cmax after the first dose. After multiple dosing the Cmax for BS was approximately 25% of the serum Cmax. This study found that both the 2.2 mg/kg and 4.4 mg/kg dosing regimens resulted in continuous serum, TCF and BS concentrations of ceftiofur that exceeded the minimal concentration required to inhibit the bacteria most frequently isolated from calves with acute bovine respiratory disease.  相似文献   
208.
209.
210.
A study of the basis of virulence variation of bovine rotaviruses.   总被引:3,自引:0,他引:3  
Rotaviruses are enteric pathogens of cattle but sub-clinical infections are common. Virulence variation has been identified with bovine rotaviruses and some rotaviruses replicated without clinical signs in non-immune calves. The rotavirus genome is composed of eleven segments of double-stranded RNA and the fourth largest segment codes for a non-glycosylated surface protein, VP4, which has been linked with virulence. In the present study the biological basis of rotavirus virulence variation was studied in vivo and compared with the known properties of the fourth gene. Calves were inoculated orally with a virulent rotavirus or a rotavirus of low virulence which multiplied but failed to cause diarrhoea. They were taken for necropsy at intervals of 2 days after inoculation. Clinical signs, virus in faeces and the percentage of infected small intestinal epithelium were determined. Damage to the small intestine was assessed by measurement of villus heights and crypt-cell production rates. Virulence was associated with a greater level of colonization of the small intestinal epithelium, greater enterocyte damage and preferential infection of the upper small intestine. The fourth gene determines the ability of rotaviruses to spread in vitro and the finding that virulence was associated with greater colonization in vivo raises the possibility that this gene may have an important role in rotavirus virulence.  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