全文获取类型
收费全文 | 4906篇 |
免费 | 237篇 |
国内免费 | 210篇 |
专业分类
林业 | 739篇 |
农学 | 244篇 |
基础科学 | 279篇 |
437篇 | |
综合类 | 2290篇 |
农作物 | 258篇 |
水产渔业 | 316篇 |
畜牧兽医 | 507篇 |
园艺 | 177篇 |
植物保护 | 106篇 |
出版年
2024年 | 23篇 |
2023年 | 80篇 |
2022年 | 125篇 |
2021年 | 174篇 |
2020年 | 169篇 |
2019年 | 208篇 |
2018年 | 79篇 |
2017年 | 156篇 |
2016年 | 210篇 |
2015年 | 226篇 |
2014年 | 362篇 |
2013年 | 410篇 |
2012年 | 488篇 |
2011年 | 479篇 |
2010年 | 373篇 |
2009年 | 335篇 |
2008年 | 251篇 |
2007年 | 249篇 |
2006年 | 203篇 |
2005年 | 164篇 |
2004年 | 118篇 |
2003年 | 103篇 |
2002年 | 58篇 |
2001年 | 55篇 |
2000年 | 37篇 |
1999年 | 31篇 |
1998年 | 36篇 |
1997年 | 33篇 |
1996年 | 17篇 |
1995年 | 19篇 |
1994年 | 19篇 |
1993年 | 14篇 |
1992年 | 15篇 |
1991年 | 7篇 |
1990年 | 6篇 |
1989年 | 7篇 |
1988年 | 4篇 |
1987年 | 4篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 1篇 |
排序方式: 共有5353条查询结果,搜索用时 0 毫秒
81.
拖拉机变速器使用性能及其智能化对提高拖拉机机组作业效率和稳定性具有重要意义。目前,先进国家全动力换挡变速器技术十分成熟,而我国还处于理论研究阶段,变速器制造的自动化水平低,轮毂、齿轮的加工精度达不到设计要求;换挡控制策略只处于理论研究阶段,目前动力换挡变速器仍然采用手动控制,CAN总线技术应用水平低,其关键技术亟待自主研发。随着土地流转规模的增加,大型拖拉机的使用会越来越多,国家对农业装备研发投入的增加将促进我国拖拉机全动力换挡技术的开发和应用。 相似文献
82.
Improved approaches for irrigation scheduling require specific protocols for adaptation to different growing conditions. We assessed crop intercepted radiation as the main factor for decision on irrigation scheduling. Over two growing seasons (2007-2008), apple trees growing in a large weighing lysimeter were used to measure daily canopy transpiration (Td). Seasonal patterns of daily canopy intercepted photosynthetically active radiation (IPARd) and midday stem water potential were also measured. In 2007, irrigation was withheld in two different times to study Td responses to midday stem water potential. Before harvest, under full irrigation, Td increased linearly with IPARd (R2 = 0.81 in 2007 and 0.84 in 2008). With the two year data combined, R2 increased from 0.74 to 0.80 when VPD was considered as a second variable. When irrigation was withheld in 2007 the ratio between Td and IPARd, which is defined here as transpiratory radiation use efficiency (TRUE), decreased linearly (R2 = 0.49) as midday stem water potential decreased. Due to the highly significant effect of IPARd and VPD on Td, TRUE showed potential applications in estimating the amount of irrigation water. 相似文献
83.
84.
85.
86.
Energetic status and body size are inconstant bodily states often considered to have feedback effects on behaviour. Feedbacks can be negative, like starvation‐threshold feedbacks (lower state values = increased risk taking due to higher energy need, and vice versa), or positive, like state‐dependent safety feedbacks (higher state values = lower vulnerability when expressing risky behaviours, and vice versa). Few studies have investigated feedback effects during the early‐life survival bottleneck of territorial species, when rapid growth is particularly important to maintain competitive ability. We investigated effects of body size and energetic status on the emergence latency into an unknown environment in highly territorial brown trout fry. We manipulated energetic status using high and low food rations in different combinations over two consecutive periods. In contrast to results from other small‐sized fishes, no general behavioural effects of energetic state were detected, but larger fish had on average shorter emergence latency. Starvation‐threshold feedbacks were not supported, but the size effect favours the presence of a state‐dependent safety feedback. When individuals were split into clusters characterised by long (>34 s) and short (<34 s) emergence latency, a state‐dependent safety feedback, based on energetic status, was found in the short‐latency cluster. Positive state‐dependent feedbacks may reflect the high level of competition among wild trout fry during the first summer, particularly among the more active individuals (i.e., from the short latency cluster). High state levels could be utilised to gain advantages over lesser competitors (e.g., to establish territories), particularly if the overall risk of starvation is higher than the risk of predation. 相似文献
87.
88.
89.
90.