全文获取类型
收费全文 | 106775篇 |
免费 | 19993篇 |
国内免费 | 30237篇 |
专业分类
林业 | 11981篇 |
农学 | 12828篇 |
基础科学 | 11209篇 |
37881篇 | |
综合类 | 42118篇 |
农作物 | 8233篇 |
水产渔业 | 7294篇 |
畜牧兽医 | 13252篇 |
园艺 | 3792篇 |
植物保护 | 8417篇 |
出版年
2024年 | 1113篇 |
2023年 | 1580篇 |
2022年 | 2809篇 |
2021年 | 3096篇 |
2020年 | 4468篇 |
2019年 | 7557篇 |
2018年 | 6917篇 |
2017年 | 8109篇 |
2016年 | 8525篇 |
2015年 | 9026篇 |
2014年 | 8145篇 |
2013年 | 8867篇 |
2012年 | 9209篇 |
2011年 | 8423篇 |
2010年 | 7138篇 |
2009年 | 6540篇 |
2008年 | 5282篇 |
2007年 | 5334篇 |
2006年 | 4905篇 |
2005年 | 4419篇 |
2004年 | 3318篇 |
2003年 | 2927篇 |
2002年 | 2528篇 |
2001年 | 2231篇 |
2000年 | 2052篇 |
1999年 | 2240篇 |
1998年 | 2067篇 |
1997年 | 1970篇 |
1996年 | 1741篇 |
1995年 | 1859篇 |
1994年 | 1699篇 |
1993年 | 1392篇 |
1992年 | 1389篇 |
1991年 | 1142篇 |
1990年 | 942篇 |
1989年 | 802篇 |
1988年 | 696篇 |
1987年 | 529篇 |
1986年 | 455篇 |
1985年 | 247篇 |
1984年 | 235篇 |
1983年 | 209篇 |
1982年 | 209篇 |
1981年 | 247篇 |
1980年 | 198篇 |
1979年 | 153篇 |
1978年 | 137篇 |
1975年 | 109篇 |
1965年 | 138篇 |
1964年 | 142篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Comparative effects of mu and kappa opiate agonists on the cecocolic motility in the pony. 下载免费PDF全文
The electrical and mechanical activity of the large intestine and its response to administration of opiate mu and kappa agonists were assessed from electrodes and inductograph coils chronically implanted on the cecocolic segment in six ponies given a diet of hay and concentrates. Before the drugs were given, migrating complexes propagating from the cecum into the colon occurred at the rate of 1.5 to 16/hour. During this propulsive activity, the cecocolic sphincter opened and closed allowing the outflow of cecal contents and preventing the backflow of colic contents. Each pony was used as its own control and was given fentanyl (0.01 and 0.05 mg/kg of body weight, IV) and U50488H (0.1 and 0.5 mg/kg, IV) at weekly intervals. The mu agonist fentanyl elicited a marked phase of inhibition of the propulsive activity and a closure of the cecocolic sphincter that lasted one to two hours depending on the dose. The kappa agonist U50488H induced an inhibition of the short spiking activity, i.e. of the resting muscle tone. It did not disturb the occurrence of migrating complexes nor that of the openings of the cecocolic sphincter. These kappa compounds may be drugs of choice to alleviate visceral pain in colic stases without inducing delay of transit unlike mu compounds. 相似文献
72.
灌区节水改造对塔里木河流域治理的作用 总被引:3,自引:0,他引:3
在对塔里木河“四源一干”及流域大型灌区存在主要问题分析的基础上,探讨了大型灌区建戌节水改造,对工业及城镇生活需水增长、灌溉及周边生态环境用水、流域综合治理和社会经济可持续发展的作用,深入阐明大型灌区续建配套与节水改造对区域水资源合理配置生态及环境发展的重大意义。 相似文献
73.
随着我国社会主义市场经济体制建设的逐步推进和市场经济的快速发展,种子经营管理也从体制到观念、方式等都发生着急剧的变革.财务管理作为种子经营管理的重要组成部分,其作用也越来越大. 相似文献
74.
双孢蘑菇覆土层有益微生物消长规律 总被引:1,自引:0,他引:1
将土样制备成稀释液,采用平板涂布培养法,分析双孢蘑菇覆土期、充分发菌期,原基形成期、采收前和采收后2d5个时期土样中几种有益微生物和消长规律。结果表明,原基表成前和采收后,采白色和采收后2d5下时期土样中几种有益微生物的消化规律。结果表明,原其开前和采收后,采白色孢杆菌(Bacillus)、固氮苗(Azotobacter)和假单胞杆菌(Pseudomonas)数量增加;链霉菌(Streptomyces)数量增减比较平缓,而土灰色芽孢杆菌的数量一直呈下降趋势。 相似文献
75.
76.
77.
78.
葡萄二倍体与四倍体品种间杂交胚珠的离体培养 总被引:12,自引:2,他引:10
为了克服葡萄二倍体品种与四倍体品种之间的交配障碍,进行了杂交胚珠的离体培养研究。发芽胚珠的取样时期集中于授粉后35~75d,其中多数以二倍体作母本的组合在授粉后55d取样的发芽率最高,以四倍体作母本的组合授粉后70d取样的发芽率最高。基本培养模式为:发育培养基-(剥胚)萌发培养基-生长培养基。用二倍体品种作母本时,所有组合均获得了培养苗,在1号培养基上培养的胚珠发芽率高于2号培养基。反交对培养基的要求有相反趋势,其中2个组合未得到培养苗。授粉后60d,直接剥出杂种胚进行培养,得到了最高发芽率。综合分析初步认为,用二倍体品种作母本比用四倍体品种作母本容易获得杂交后代。 相似文献
79.
80.
Pharmacokinetic evaluation of ceftiofur in serum, tissue chamber fluid and bronchial secretions from healthy beef-bred calves. 下载免费PDF全文
S L Halstead R D Walker J C Baker R E Holland G E Stein J G Hauptman 《Canadian journal of veterinary research》1992,56(4):269-274
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