全文获取类型
收费全文 | 39510篇 |
免费 | 2420篇 |
国内免费 | 4171篇 |
专业分类
林业 | 3174篇 |
农学 | 3385篇 |
基础科学 | 4453篇 |
7493篇 | |
综合类 | 15767篇 |
农作物 | 2750篇 |
水产渔业 | 1303篇 |
畜牧兽医 | 3989篇 |
园艺 | 1519篇 |
植物保护 | 2268篇 |
出版年
2024年 | 320篇 |
2023年 | 1004篇 |
2022年 | 1548篇 |
2021年 | 1583篇 |
2020年 | 1628篇 |
2019年 | 1864篇 |
2018年 | 1390篇 |
2017年 | 1723篇 |
2016年 | 2258篇 |
2015年 | 2313篇 |
2014年 | 2515篇 |
2013年 | 2689篇 |
2012年 | 3092篇 |
2011年 | 2783篇 |
2010年 | 2236篇 |
2009年 | 2142篇 |
2008年 | 1871篇 |
2007年 | 2053篇 |
2006年 | 1896篇 |
2005年 | 1521篇 |
2004年 | 1093篇 |
2003年 | 911篇 |
2002年 | 737篇 |
2001年 | 615篇 |
2000年 | 588篇 |
1999年 | 477篇 |
1998年 | 459篇 |
1997年 | 402篇 |
1996年 | 347篇 |
1995年 | 355篇 |
1994年 | 297篇 |
1993年 | 251篇 |
1992年 | 237篇 |
1991年 | 207篇 |
1990年 | 204篇 |
1989年 | 150篇 |
1988年 | 110篇 |
1987年 | 53篇 |
1986年 | 21篇 |
1985年 | 35篇 |
1984年 | 30篇 |
1983年 | 13篇 |
1982年 | 9篇 |
1981年 | 20篇 |
1980年 | 16篇 |
1979年 | 8篇 |
1978年 | 8篇 |
1977年 | 6篇 |
1976年 | 3篇 |
1975年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
虽然双层经营体制已经得到确立,但至今其经营客体或对象在实践中仍然是不清楚的,而且就统分结合两方面来看,统的职能也严重不足,致使家庭经营的弊端依然存在,双层经营的功能难以充分发挥.为此.笔者提出,“两田制”是完善双层经营体制的有效形式。它有利于实现农业的适度规模经营,可大幅度地提高农业劳动生产率和农产品商品率,还形成了农村劳动力合理流动的机制,有利于农村经济的全面发展和农业机械化的推广。所以,笔者主张,“两田制”应谈有计划地加以推广. 相似文献
102.
103.
Abstract. Current tillage erosion models account for the influence of tillage direction in the magnitude of the soil (tillage) transport coefficient. It is argued here that this is counter-intuitive and causes significant problems in modelling tillage erosion in areas of complex terrain. This article examines whether a re-modelling of tillage erosion is possible that separates tillage direction (an interaction with the landform) from the soil transport coefficient (a measure of tillage intensity representing the combination of implement erosivity and soil erodibility). Experimental data for mouldboard ploughing upslope, downslope and cross-slope at Coombe Barton Farm, Devon are examined. Integration of data for all directions into a single relationship, which relates translocation in the direction of tillage to slope in the direction of tillage and translocation perpendicular to tillage to slope perpendicular to tillage, is not possible using previously published methods of analysis. However, when total translocation distance is regressed against the tangent of the slope at 45° to the tillage direction (bisecting the tillage direction and the direction of overturning) it is found that a single relationship can be used to describe tillage in all three directions. Therefore, this relationship is used to determine a single value of the soil transport coefficient ( k fTa ) for constant soil and implement conditions but different tillage directions. This redefinition of tillage is important both for true estimation of tillage erosion severity, the adirectional coefficient being 40% larger than the directional coefficient, and for modelling of tillage erosion in complex terrain. These improvements are vital when tillage erosion simulation is used to direct soil conservation strategies. 相似文献
104.
CTGF, a member of the CCN family of immediate early genes, is a recently discovered profibrotic growth factor, which is involved in many pathophysiologic procedures. CTGF acts as a downstream effector of TGF-β acting on interstitial cells to enhance the progression of fibrotic renal diseases. It has been shown that CTGF gene expression can be induced or blocked by some kinds of cytokine and drugs. It is an interesting candidate target for future intervention strategies of renal interstitial fibrosis. 相似文献
106.
针对单机环境下暖通CAD软件重复开发量大、开发周期长等现状,提出了基于互联网环境的三层体系结构及其开发的通用环境,即整个暖通CAD系统被分为不同逻辑块,开发和管理的工作向服务器方转移,从而使分布处理数据成为可能,基于组件的软件开发是互联网环境中应用软件的主要开发方式,以构架为组装蓝图,以可重用组件为组装预制块,支持组装式软件重用,从而克服人力和物力的浪费,进而为了重用暖通CAD已开发出的功能模块,给出组件逆向工程的开发方法、组装流程和组件管理过程。 相似文献
107.
哈达门国家森林公园游客量动态预测 总被引:3,自引:0,他引:3
根据哈达门国家森林公园的票房数据,应用灰色系统理论建立旅游量灰色动态模型。基此,对哈园游客量进行了中短期预测,其预测检验精度为93.72。旨在对该园建设、管理等项决策提供科学依据。 相似文献
108.
对哈尔滨市工农兵钢箱索梁桥的模型进行了试验和分析。结果表明,该桥设计采用的理论与试验结果基本相符,为桥梁的修复设计提供了可靠依据。 相似文献
109.
L.S. Faudskar DVM M. R. Raffe DVM MS D. A. Randall BS D. R. Bing BS RRT 《Journal of Veterinary Emergency and Critical Care》1997,7(1):49-58
A laboratory evaluation was performed to evaluate the performance characteristics of a new veterinary ventilator. The ventilator studied was configured according to manufacturer's directions and attached to a test lung via a pneumotachograph and differential pressure transducer interfaced to a pulmonary mechanics analyzer system. Constant resistance (R=10 cm H2 O/L/sec) and compliance (C=3 ml/cm H2 O) factors were maintained for all trials. The ventilator operated at the manufacturer's preprogrammed parameters. In the first trial, body weight was the only variable. In the second trial, an endotracheal tube was placed in series between the ventilator's breathing circuit and the pneumotachograph. Body weights from 1–20 kgs were evaluated. Mean values for respiratory rate (RR), minute ventilation (VE), inspiratory time (Ti), peak inspiratory pressure (PIP), and peak inspiratory flow (Fpki) displayed on the ventilator control panel; tidal volume (VT), calculated from the displayed minute volume, and identical parameters measured by the pulmonary mechanics system at each body weight, were compared using a two factor analysis of variance. Significant differences (P< 0.05) were found between mean displayed and measured values for RR, PIP, and Fpki. 相似文献
110.