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   45174篇
  免费   1866篇
  国内免费   4496篇
林业   3771篇
农学   4951篇
基础科学   3494篇
  5723篇
综合类   20199篇
农作物   2733篇
水产渔业   1756篇
畜牧兽医   5038篇
园艺   1417篇
植物保护   2454篇
  2024年   328篇
  2023年   829篇
  2022年   1532篇
  2021年   1658篇
  2020年   1666篇
  2019年   1972篇
  2018年   1291篇
  2017年   2065篇
  2016年   2359篇
  2015年   2163篇
  2014年   2607篇
  2013年   2678篇
  2012年   3847篇
  2011年   3675篇
  2010年   2922篇
  2009年   2702篇
  2008年   2333篇
  2007年   2612篇
  2006年   2150篇
  2005年   1702篇
  2004年   1252篇
  2003年   956篇
  2002年   775篇
  2001年   637篇
  2000年   581篇
  1999年   584篇
  1998年   497篇
  1997年   438篇
  1996年   376篇
  1995年   400篇
  1994年   307篇
  1993年   272篇
  1992年   300篇
  1991年   237篇
  1990年   225篇
  1989年   211篇
  1988年   145篇
  1987年   91篇
  1986年   53篇
  1985年   18篇
  1984年   8篇
  1983年   5篇
  1982年   9篇
  1981年   12篇
  1979年   7篇
  1978年   7篇
  1977年   4篇
  1962年   10篇
  1956年   5篇
  1955年   14篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
为探究高寒草甸不同退化程度生态系统多功能性的变化,本研究综合考虑了植物物种多样性和多种生态系统功能如生产力、养分循环和固存能力、土壤涵养水分能力等,运用降维因子分析的方法计算了生态系统多功能性,综合分析了植物物种多样性与生态系统功能和多功能性间的关系。结果表明:高寒草甸退化显著降低了Margalef指数、Shannon-weiner指数、Simpson指数和Pielou指数(P<0.05),各生态系统功能趋于恶化。高寒草甸退化导致生态系统多功能性的下降,综合统计量结果为轻度退化(21.9955)>中度退化(8.7295)>重度退化(—30.7245)。植物物种多样性与生态系统多功能性之间均存在正相关关系,其中Simpson指数与生态系统多功能性指数拟合的结果最好,说明高寒草甸生态系统多功能性受植物物种多样性的制约。本研究可为青藏高原草地退化及恢复的过程和机理研究提供理论依据。  相似文献   
992.
Ultrasonography of the liver of 181 Holstein-Friesian cows was performed and blood samples were collected for analysis. The hepatic ultrasonograms were evaluated and the echoes were analyzed digitally. After slaughter, liver specimens were taken and examined histopathologically. Of the 181 animals, 120 had a normal liver and 61 had hydropic degeneration of the liver, diagnosed through histopathologic examination. Diagnostic accuracy rates for hydropic degeneration were determined based on the following test positive conditions: a) for biochemical analysis—high levels of aspartate aminotransferase, alanine aminotransferase, total bilirubin and non-esterified fatty acids; b) for ultrasonography—presence of dark pattern and blurring of edges; and c) for digital analysis—low echo means at 1 cm and 3 cm from the hepatic surface. Digital analysis had the highest overall specificity, accuracy and positive predictive values for hydropic degeneration, followed by ultrasonography. The results suggest that ultrasonography and digital analysis of hepatic ultrasonograms can be used for diagnosis of hydropic degeneration of the liver in place of biochemical analysis.  相似文献   
993.
Tris(1,3-dichloro-2-propyl) phosphate (TDCIPP) is widely used as a flame retardant and is known to exhibit anti-androgenic effects in vitro and in vivo. To assess the reproductive toxicity potency of TDCIPP, we investigated the effects of 7 days of TDCIPP oral administration on epididymal sperm motion and concentration in adult male Wistar–Imamichi rats. Thirty-five days after the final administration, sperm parameters were evaluated by computer-assisted sperm analysis. Results showed that sperm swimming progression and vigor and sperm concentration in TDCIPP-treated rats were unexpectedly higher than those in control rats. TDCIPP did not significantly affect the percentage of motile sperms or sperm swimming pattern. These results contribute to the understanding of the biological effects of TDCIPP.  相似文献   
994.
995.
四个家鸡群体血液蛋白质多态性分析   总被引:2,自引:0,他引:2  
本实验采用醋酸纤维薄膜电泳和聚丙烯酰胺凝胶电泳方法测定了新兴黄鸡,矮脚鸡,竹丝鸡,粤黄鸡麻羽系血液蛋白质多态性,观察了以上群体多态性血液蛋白质基因频率的区别,根据基因频率计算出4个群体的平均杂合度,用主分量分析方法对它们之间的区别与联系进行了研究。主要分析表明竹丝鸡与新兴黄鸡的关系相对新兴鸡与矮脚鸡,竹丝鸡与矮脚鸡之间的关系更为疏远:矮脚鸡则与粤黄鸡麻羽系关系密切。  相似文献   
996.
青海草地建设投资与草地产出灰色关联分析   总被引:2,自引:0,他引:2  
用灰色关联分析方法,对青海省草地建设投资与草地产出作相关分析。结果表明:两者相关性较强,关联度系数γy018=0.7168(畜牧业产值)和γy028=0.7244(畜产品产量);在草地建设分项投资中,与草地产出关联度最大的为灭鼠投资,分别为γy015=0.9162(牧业产值)和γy025=0.8885(畜产品产量),其次顺序为种草投资(γy012=0.8266和γy022=0.8131),草地围栏  相似文献   
997.
The present study examined the presence of Babesia parasites in 104 domestic dogs in Nigeria. Sequentially, Babesia parasites infecting domestic dogs underwent genetic and phylogenetic analyses. The results of nested PCR based on the Piroplasmida 18S rRNA gene illustrated that 13.5% (14/104) of the samples were positive. The obtained positive samples determined the nucleotide sequences of the 18S rRNA genes. In the genetic and phylogenetic analyses, four of five nucleotide sequences were similar to Babesia canis rossi, and one sample exhibited a close similarity to a Babesia sp. isolated from a raccoon in Hokkaido, Japan. The present study revealed the widespread presence of B. canis rossi among domestic dogs in Nigeria.  相似文献   
998.
陕西省某山羊养殖场发生一起山羊鼻内肿瘤病,通过流行病学和临床症状调查、病理剖检、病理切片检查进行初步诊断。其次根据GenBank收录的山羊地方性鼻内肿瘤病毒基因组序列设计1对特异性引物,通过PCR方法从肿瘤组织中扩增获得目的片段并测序。序列比对分析表明,该目的基因和山羊地方性鼻内肿瘤病毒高度相似,最终确定该病为山羊地方性鼻内肿瘤病毒引起的山羊地方性鼻内肿瘤(ENT)。  相似文献   
999.
为探究从广东省清远的鹅体内采集的蛔虫(Ascaridia anseris)的种类,对临床采集的鹅蛔虫样品,在形态观察的基础上,用保守引物NC5和NC2扩增鹅蛔虫的ITS rDNA基因片段,将扩增产物经克隆测序后,获得大小为1 009bp的鹅蛔虫ITS rDNA基因序列(GenBank登录号为KC905082)。序列比对和系统进化分析表明,鹅蛔虫5.8SrRNA基因与GenBank收录的鸡蛔虫(登录号为AJ001508)同源性为96%,而与不同地理株的鸽蛔虫的同源性在91%~96%之间;禽蛔科的鸡蛔虫、鸽蛔虫和鹅蛔虫同一进化分支,鹅蛔虫、鸡蛔虫和鸽蛔虫处于各自的独立进化分支。上述证明,鹅蛔虫是不同于鸽蛔虫的一个独立有效种,在系统发生关系上与鸡蛔虫更亲近。  相似文献   
1000.

Objectives

To determine whether the Enterprise point-of-care blood analysis system (EPOC) produces results in agreement with two other blood gas analysers in regular clinical use (i-STAT and Radiometer ABL77) and to investigate the precision of the new machine when used with equine whole blood.

Study design

Prospective, randomized, non-blinded, comparative laboratory analyser study.

Animals

Horses admitted to a university teaching hospital requiring arterial or venous blood gas analysis as part of their routine clinical management.

Methods

One hundred equine blood samples were run immediately, consecutively and in randomized order on three blood gas analysers. Results of variables common to all three analysers were tested for agreement and compared with guidelines used in human medicine. These require 80% of results from the test analyser to fall within a defined range or percentage of results from the comparator devices to achieve acceptability. Additionally, 21 samples were run twice in quick succession on the EPOC analyser to investigate precision.

Results

Agreement targets were not met for haematocrit, haemoglobin and base excess for either i-STAT or ABL77 analysers. EPOC precision targets were not met for partial pressure of carbon dioxide, ionized calcium, haematocrit and haemoglobin. Overall comparative performance of the EPOC was good to excellent for pH, oxygen tension, potassium, bicarbonate and oxygen saturation of haemoglobin, but marginal to poor for other parameters.

Conclusions and clinical relevance

The EPOC may be useful in performing analysis of equine whole blood, but trend analysis of carbon dioxide tension, ionized calcium, haematocrit and haemoglobin should be interpreted with caution. The EPOC should not be used interchangeably with other blood gas analysers.  相似文献   
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