全文获取类型
收费全文 | 96746篇 |
免费 | 19481篇 |
国内免费 | 29741篇 |
专业分类
林业 | 10827篇 |
农学 | 11208篇 |
基础科学 | 10402篇 |
36907篇 | |
综合类 | 38525篇 |
农作物 | 7641篇 |
水产渔业 | 6959篇 |
畜牧兽医 | 12050篇 |
园艺 | 3508篇 |
植物保护 | 7941篇 |
出版年
2024年 | 959篇 |
2023年 | 1375篇 |
2022年 | 2221篇 |
2021年 | 2470篇 |
2020年 | 3867篇 |
2019年 | 7094篇 |
2018年 | 6560篇 |
2017年 | 7683篇 |
2016年 | 8294篇 |
2015年 | 8611篇 |
2014年 | 7720篇 |
2013年 | 8346篇 |
2012年 | 8600篇 |
2011年 | 7616篇 |
2010年 | 6497篇 |
2009年 | 5883篇 |
2008年 | 4792篇 |
2007年 | 4929篇 |
2006年 | 4467篇 |
2005年 | 3930篇 |
2004年 | 3149篇 |
2003年 | 2827篇 |
2002年 | 2531篇 |
2001年 | 2161篇 |
2000年 | 2009篇 |
1999年 | 2139篇 |
1998年 | 1914篇 |
1997年 | 1804篇 |
1996年 | 1655篇 |
1995年 | 1691篇 |
1994年 | 1649篇 |
1993年 | 1315篇 |
1992年 | 1323篇 |
1991年 | 1084篇 |
1990年 | 862篇 |
1989年 | 802篇 |
1988年 | 688篇 |
1987年 | 481篇 |
1986年 | 443篇 |
1985年 | 231篇 |
1984年 | 211篇 |
1983年 | 213篇 |
1982年 | 203篇 |
1981年 | 251篇 |
1980年 | 202篇 |
1979年 | 154篇 |
1978年 | 130篇 |
1975年 | 114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 31 毫秒
61.
Comparative effects of mu and kappa opiate agonists on the cecocolic motility in the pony.
下载免费PDF全文
![点击此处可从《Canadian journal of veterinary research》网站下载免费的PDF全文](/ch/ext_images/free.gif)
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
62.
对50例冠心病和50例随机做平板运动试验(TET)者作频域心电图(FCG)对比分析探讨FCG对冠心病的诊断价值。结果表明:单纯根据FCG异常诊断冠心病有较高假阳性(65.22%)。如将FCG的阳性指标作综合判断,虽然敏感性降低(88.00%降至70.00%)。假阳性都明显降低(65.22%降至26.00%)。认为TET的特异性较FCG高。若两者均为阴性,则冠心病可能性很小。 相似文献
63.
Landscape modeling requires the delineation of system boundaries and interior features. Quite often, these components are
complex and difficult to accurately represent. A rectangular grid is used to represent the study and adjacent non-study areas
in most cases. When the non-study area occupies a large portion of the grid, computer memory is wasted, and computational
time increases. An elliptical grid generator for non-orthogonal curvilinear coordinates is used to generate a boundary-fitted
grid for a landscape model. In a boundary-fitted grid coordinate system, one coordinate axis follows the landscape domain
boundary and is non-orthogonal to the second axis. The boundary-fitted grid uses elliptic partial differential equations to
distribute grid points inside the landscape domain. Although the boundary-fitted grid follows the domain boundary, the grid
pattern and point allocation remain structured. Thus, a landscape model can use a boundary-fitted grid without changing the
model’s data structure or the computational scheme. In this study, a boundary-fitted grid and a raster-based grid were applied
to the Everglades Landscape Fire Model. Use of the boundary-fitted grid decreased model simulation time by about one fifth
and computer storage by 58% relative to the raster-based grid. Also, the linear characteristics of interior geographical features
such as rivers and airboat trails were preserved by the boundary-fitted grid, but not by the raster-based grid. This preservation
provided a more reasonable base map for simulating ecological processes, such as fire across heterogenous landscapes. 相似文献
64.
65.
66.
67.
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
68.
Pharmacokinetic evaluation of ceftiofur in serum, tissue chamber fluid and bronchial secretions from healthy beef-bred calves.
下载免费PDF全文
![点击此处可从《Canadian journal of veterinary research》网站下载免费的PDF全文](/ch/ext_images/free.gif)
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
69.
越桔对长白山区酸性土壤的适应性 总被引:15,自引:1,他引:14
在长白山区3种类型的酸性土壤上栽培越桔,研究结果表明,`北空'、`美登'、`斯卫克'在暗棕色森林土上生长结果较好,树体营养关系合理;在草甸沼泽土上3个种生长结果均较差,`北空'在水湿地潜育土上生长旺盛,但产量低,综合分析认为,矮丛越桔类型品种对我国长白山区酸性土壤条件的适应性较强,半高丛越桔则适应性差,针对长白山不同类型的土壤,以美登最为适宜暗棕色森林土栽培,斯卫克次之;水湿地潜育土可栽培美登品种 相似文献
70.