首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   23425篇
  免费   1305篇
  国内免费   2213篇
林业   1563篇
农学   1093篇
基础科学   1274篇
  2590篇
综合类   11017篇
农作物   1526篇
水产渔业   1085篇
畜牧兽医   4143篇
园艺   1539篇
植物保护   1113篇
  2024年   199篇
  2023年   487篇
  2022年   1032篇
  2021年   1063篇
  2020年   956篇
  2019年   948篇
  2018年   679篇
  2017年   1083篇
  2016年   771篇
  2015年   1170篇
  2014年   1102篇
  2013年   1491篇
  2012年   1978篇
  2011年   2102篇
  2010年   2028篇
  2009年   1798篇
  2008年   1755篇
  2007年   1618篇
  2006年   1291篇
  2005年   1055篇
  2004年   660篇
  2003年   374篇
  2002年   403篇
  2001年   344篇
  2000年   349篇
  1999年   107篇
  1998年   14篇
  1997年   5篇
  1996年   8篇
  1995年   6篇
  1994年   6篇
  1993年   13篇
  1992年   10篇
  1991年   5篇
  1990年   3篇
  1989年   2篇
  1987年   4篇
  1986年   1篇
  1981年   6篇
  1965年   1篇
  1962年   5篇
  1956年   4篇
  1955年   7篇
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
81.
This experiment was conducted to study the effect of selenium and vitamin E supplement on semen quality, antioxidant enzyme activities and heat shock protein expression of goat in Hainan high temperature season.16 adult Hainan Black goat with good health and approximate weight were randomly divided into 4 groups, fed with basal diet(control group), basal diet+0.5 mg/kg Se(Se group), basal diet+100 mg/kg VE(VE group), and basal diet+0.5 mg/kg Se+100 mg/kg VE(Se+VE group), respectively.The experimental period was 93 d.Semen samples were collected in the last week of the experiment on two consecutive days.The semen quality, antioxidant enzyme activities and heat shock protein expression were analyzed.The results showed that compared with control group, the ejaculate volume was not significantly affected by Se or VE supplement(P>0.05).Sperm density and sperm motility were increased significantly by Se and VE supplement(P<0.05), and the abnormal rate was decreased extremely significantly(P<0.01).The goats fed with Se and VE also had higher activities of GSH-Px(P<0.01), SOD(P<0.05), CAT(P<0.05) and T-AOC(P<0.01), and lower MDA concentration in seminal plasma(P<0.05).The relative expression levels of HSP70 and HSP90 mRNA in supplement groups were decreased extremely significantly(P<0.01).However, there were some certain differences between the Se and VE supplement groups on semen quality and heat shock protein expression.In conclusion, the supplementation of Se and VE could help to improve goat semen quality by increasing the sperm density, sperm motility, the antioxidant enzyme activities, and decreasing the abnormal rate in hot season of Hainan.Finally, Se and VE supplement had good effects on relieving the environment heat stress.  相似文献   
82.
国家种质多年生牧草圃资源整理整合及共享   总被引:6,自引:2,他引:4  
简要介绍了国家多年生牧草圃概况及资源保存状况,阐述了多年生牧草圃资源整理、整合及数字化表达现状,提出了资源共享利用的框架;指出了在牧草资源整理整合中存在的问题及对策.  相似文献   
83.
According to the requirements of communication power supply, such as high reliability, wide application and high availability, the authors raise a new remote control design for a multi-output power supply for communication equipment based on TCPIP protocol and embedded technology. Then the hardware design is given for the system is discussed based on Ethernet professional chip PRTL8019AS and related circuits. The firmware software design for the salve-computer based TCPIP technology, and the soft design of communieation and control for the master-computer are proposed based on Winsoek and MFC technology in detail A test machine is built to do the testing, the test results prove that the test machine works stably and well, satisfies the design rules and also provides a feasible method for the remote supervise in communication power supplier whose requirement includes remote supervise, no-people on duty and so on.  相似文献   
84.
In order to help tourists choose satisfying scenic spots, content hotels, convenient lines and convenient vehicles, vehicle monitoring control and scheduling management system in a scenic area is presented, which dispatches vehicle by appointments and the statistic of tourists at a sight spot and recommends the tour line by broadcasting in vehicle. Vehicle monitoring control and scheduling management system in a scenic area, a integrative vehicle management system, is an integration of GPS, GIS, trunked radio system, a management system of booking room and a tourist statistic system at a sight spot.  相似文献   
85.
In order to upgrade traditional industry and realize the automation of furnace condition judgement and standardization of operation, an expert system for various furnace conditions and breakdown judgement and operation guidance on blast furnace are developed based on the theory of artificial intelligence and knowledge engineering. The system is fulfilled through C++Builder and combined with the actual production picture. The system has efficient and convenient man-machine interface by using menu operation method. The testing and operation of actual production data in simulation system indicate that the system is exact and reliable and can provide both the warning of various furnace breakdown and corresponding operation guidance.  相似文献   
86.
The techniques of sharing a channel in HFC are original with regard to those of Ethernet and Ring. In this paper, the analysis is stressed on the method of the upstream bandwidth allocation, the collision resolving, and the relationship among them, beginning with MAP management message. The MAP is a complex frame, in which the messages of ranging opportunity needed by the CM at the access beginning, bandwidth allocation contention, bandwidth allocation and collision detection are included. The access way for a user is explained after the frame format and functions of a MAP are illustrated. Finally, upstream bandwidth allocation, the cause of collision, algorithm and the process to solve the collision and so on are studied.  相似文献   
87.
This paper analyzes the current situation of university students' credit evaluation and set up an evaluation index system of university students' credit to compensate a loan.Based on this index system,this paper sets up a matter-element evaluation model on the individual credit of university students loan by means of extensible engineering method.The grade of individual credit and the index for evaluating the level of individual credit are made as matter-element,meanwhile,the classical field,the segment field,the weight coefficient are obtained.Dependence function is established based on the grade of individual credit and then the grade is judged by calculating the synthetic dependence degree.Finally,this paper presents the evaluation method by giving an example.This method is simple in calculation,reasonable in evaluation with high applicability.  相似文献   
88.
89.
It is difficult to calculate the penetration rate (ROP) of percussion drilling. In the early research, the empirical or semiempirical formulae of the ROP are derived from the laboratory or field data collected by the researchers. However, the ROP value of the formulae is not as accurate as that in the field. Benefiting from the dynamic model, the geometric structure model and the theory of the roller cone bit, the mathematical model of the hammer bit, the mathematical model of the true bottom hole and the mathematical model between the tooth and the rock of the hammer bit are established. Hence the ROP of the air percussion drilling can be predicted by simulation. Field tests show the models are reliable and the results are coincident with that in the field. The research can help to redesign the hammer bit and optimize the drilling parameters.  相似文献   
90.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号