全文获取类型
收费全文 | 17912篇 |
免费 | 1083篇 |
国内免费 | 1333篇 |
专业分类
林业 | 2554篇 |
农学 | 1357篇 |
基础科学 | 1163篇 |
2222篇 | |
综合类 | 8928篇 |
农作物 | 813篇 |
水产渔业 | 608篇 |
畜牧兽医 | 1456篇 |
园艺 | 407篇 |
植物保护 | 820篇 |
出版年
2024年 | 135篇 |
2023年 | 312篇 |
2022年 | 625篇 |
2021年 | 650篇 |
2020年 | 651篇 |
2019年 | 763篇 |
2018年 | 395篇 |
2017年 | 684篇 |
2016年 | 862篇 |
2015年 | 823篇 |
2014年 | 1059篇 |
2013年 | 1108篇 |
2012年 | 1508篇 |
2011年 | 1512篇 |
2010年 | 1252篇 |
2009年 | 1222篇 |
2008年 | 1010篇 |
2007年 | 1179篇 |
2006年 | 897篇 |
2005年 | 714篇 |
2004年 | 522篇 |
2003年 | 449篇 |
2002年 | 350篇 |
2001年 | 322篇 |
2000年 | 192篇 |
1999年 | 193篇 |
1998年 | 160篇 |
1997年 | 121篇 |
1996年 | 125篇 |
1995年 | 107篇 |
1994年 | 81篇 |
1993年 | 82篇 |
1992年 | 70篇 |
1991年 | 60篇 |
1990年 | 51篇 |
1989年 | 27篇 |
1988年 | 25篇 |
1987年 | 12篇 |
1986年 | 4篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1978年 | 2篇 |
1963年 | 1篇 |
1962年 | 1篇 |
1956年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 187 毫秒
11.
12.
Eradication of lice in cattle. 总被引:8,自引:0,他引:8
The purpose of this field study was to develop and evaluate eradication as a strategy to control lice in cattle. Thirty-three herds of cattle were selected and observed during a period of two and a half years. Before eradication, biting lice (Damalinia bovis) were present in 94% of the herds and 27% of the animals. Sucking lice (Linognathus vituli) were present in 42% of the herds and 5% of the animals. These levels were very similar to those reported from other countries in Northern Europe. The eradication strategy was successful in 28 of 33 herds, but lice were still present in 5 herds 3 to 6 months after treatment. Biting lice were present in all these 5 herds, sucking lice were present in 3 herds. During the next 12 months, nine of the 28 herds were reinfected with lice. Six herds were reinfected with just biting lice, 2 herds with just sucking lice and one herd was reinfected with both. There was no significant difference between the 2 louse species regarding the risk of unsuccessful eradication or reinfection. The only significant risk factor for reinfection was either purchase of livestock or use of common pasture, combined with failure in pre-treatment of newly introduced animals. 相似文献
13.
Kunio Shirakihara Masahiko Yoshida Machiko Nishino Yoshimi Takao Kouichi Sawada 《Fisheries Science》2001,67(3):430-435
SUMMARY: The vertical distribution of dwarf ayu Plecoglossus altivelis altivelis in the pelagic waters of Lake Biwa was evaluated from June to September in 1995–97 from eight acoustic surveys using a quantitative echosounder. In each survey, echoes from a depth range of 3 m to the sea bottom were collected at a station every 2.7 s for 24 h together with measurements of vertical profiles of water temperature and chlorophyll-a. The ayu's echoes were identified using an underwater video camera. The ayu were observed near the maximum chlorophyll-a depth and above the thermocline. Their density was highest at depths of 4–11 m with 2–4 individuals/m 3 and was almost zero below 20 m. Echo signs were recorded as having a frequent duration of more than 1 h at night, whereas were of a shorter duration in the day. The fish stay in the epilimnion during the day without any clear vertical migration, but are distributed more uniformly at night. The advantages of remaining in the epilimnion are discussed in terms of food availability and predator avoidance. 相似文献
14.
老年旅游市场开发研究 总被引:3,自引:1,他引:2
介绍了老年旅游的概念,分析了老年人的消费心理与老年旅游市场中存在的问题,并提出了开发老年旅游市场的策略。 相似文献
15.
通过学习借鉴先进企业的先进服务营销理念,从而更新了图书馆工作人员的传统服务理念,提高了图书馆的服务质量和服务方式;提出了图书馆的服务创新策略,推动了图书馆服务工作的发展。 相似文献
16.
17.
18.
祁连山水源涵养林综合效能的计量研究 总被引:13,自引:1,他引:12
以马克思级差地租理论,建立了祁连山水源涵养林综合效能的评价模型:E=E_d+E_e+E_s;以E与生产投资I的比值为计量水源林的效能指数T,T=E/I。并以祁连山寺大隆森林生态定位研究站实测资料和社会调查数据为计量评价参数,确定其经济评价指标体系和计量系统。结果表明:祁连山水源涵养林综合效能指标为11.93;其中生态效能和社会效能分别是直接经济效能的4.51倍和6.05倍。说明祁连山水源涵养林现行的保护、发展和综合利用的经营方针是正确的。建议河西各受益部门应按水资源效能总收入的0.5%缴作水源林再生产费,以确保国民经济建设的长远发展。 相似文献
19.
介绍了不确定度理论及其评定方法,结合车辆碰撞事故的具体特点,给出了基于能量守恒的车速计算表达式.通过选择适当的输入参数作为不确定因子,分别计算各分量等效速度标准不确定度.考虑不确定度累积和传播,得到车速输出结果的合成标准不确定度和展伸不确定度,以此检验事故再现结果的准确程度.通过具体事故案例的分析计算和计算机检验,验证该方法的适用性. 相似文献
20.