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   785篇
  免费   53篇
  国内免费   51篇
林业   94篇
农学   40篇
基础科学   9篇
  129篇
综合类   339篇
农作物   64篇
水产渔业   78篇
畜牧兽医   53篇
园艺   30篇
植物保护   53篇
  2023年   14篇
  2022年   24篇
  2021年   29篇
  2020年   34篇
  2019年   21篇
  2018年   19篇
  2017年   31篇
  2016年   43篇
  2015年   41篇
  2014年   42篇
  2013年   71篇
  2012年   57篇
  2011年   69篇
  2010年   50篇
  2009年   54篇
  2008年   38篇
  2007年   43篇
  2006年   41篇
  2005年   37篇
  2004年   17篇
  2003年   15篇
  2002年   18篇
  2001年   15篇
  2000年   20篇
  1999年   13篇
  1998年   12篇
  1997年   4篇
  1996年   3篇
  1995年   4篇
  1994年   4篇
  1993年   2篇
  1992年   3篇
  1990年   1篇
排序方式: 共有889条查询结果,搜索用时 15 毫秒
21.
采用多层次模糊综合评判法,综合分析了宁夏南部黄土丘陵区不同改良措施下的改良效果,结果表明:“88542”整地+播种综合评判值最高,达到5.9188,改良效果最好,施肥和封育的综合评判值其次,带状翻耕+播种的综合评判值最低,仅为2.7750,远低于对照。  相似文献   
22.
The Puente-Chino Hills, extending west into the highly urbanized Los Angeles Basin, represent one of the largest expanses of lowland habitats in the region. During spring and early summer of 1997 and 1998, birds and vegetation surveys were conducted to clarify the influence of geographical position in the distribution of birds in the hills. Using logistic regression, the inclusion of longitudinal position as a variable is shown to make a statistically significant contribution to bird species presence beyond that of habitat alone for 12 of the 49 most commonly detected species. Species more common than would be expected based on habitat in the east were typical of grassland and open habitats, whereas those more common in the west were characteristic of tall scrub or urban habitats. Thus, species’ distributions in the hills are likely influenced by landscape-scale vegetation patterns and by the aggregate amount of urbanized areas in the west. This emphasizes the importance of using geographical position as a variable when analyzing patterns in bird distribution and siting conservation areas.  相似文献   
23.
Abstract

This study examined the training needs of non-timber forest products' (NTFP) operators in Southern Nigeria. One hundred-fifty forest operators in Ondo State, Nigeria were sampled using structured questionnaire. Data collected was analyzed using percentages, mean, Chi-square test, and Pearson Product Moment Correlation. The results revealed that majority of the forest operators in Southern Nigeria are middle-aged women with low level of education. The need for training is high and significant (P < 0.05), and there is a positive relationship between training/ information received and the operators's productivity.  相似文献   
24.
中国农村存在巨大的区域差异,从社会结构上看,可以分为华南宗族团结型地区、中部原子化分散型地区和华北小亲族分裂型地区三大区域.通过数据挖掘和分析,发现我国出生性别比的分布极具规律性,显示出区域上的差异.结合中国南方方言形成及南方宗族型文化发育的共同历史移民根源,笔者将南方方言分布图与出生性别比地图及三大区域划分进行比较研究,具体分析了三者间高度关联性的共同历史移民根源.  相似文献   
25.
暴雨预报的力子天气学模式和应用实例(英文)   总被引:1,自引:0,他引:1  
[目的]对2010年湖南汛期13次暴雨过程进行力子天气学分析。[方法]采用力子天气学原理,通过对2010年出现在湖南的13次区域性暴雨天气过程进行合成分析。[结果]2010年湖南汛期的13次暴雨都与自组织的辐合水气流密切相关,暴雨是自组织辐合水汽流爆发后所产生的必然结果;自组织辐合水汽流的孕育区对应未来12-36小时出现暴雨的区域;自组织辐合水汽流孕育区一旦消失,则未来12-36小时不会有区域性暴雨出现;自组织辐合水汽流孕育区产生在大尺度南北力子偶的交汇域。[结论]南北力子偶是否存在可以作为汛期区域性暴雨的短期预报模式。  相似文献   
26.
Outbreaks of anemia (CCA) in channel catfish, Ictalurirs punctatus, in Mississippi occur predominantly during spring and fall. Between October, 1990 and May, 1991, twenty-seven cases of CCA were submitted to the Mississippi Cooperative Extension Service-College of Veterinary Medicine Fish Diagnostic Laboratories. Histologic lesions were identified in the head and trunk kidneys of all affected fish. The head kidney contained less than 25% immature hematopoietic tissue in 3 of 22 cases (14%). 25-50% immature hematopoietic tissue in 7 of 22 cases (32%), and greater than 50% immature hematopoietic tissue in 12 of 22 cases (54%). The trunk kidney contained less than 25% immature hematopoietic tissue in 5 of 27 cases (18%), 25-50% immature hematopoietic tissue in 15 of 27 cases (56%), and greater than 50% immature hematopoietic tissue in 7 of 27 cases (26%). The histopathologic findings noted in cases of CCA from Mississippi affect primarily the hematopietic tissues of the head and trunk kidneys.  相似文献   
27.
Sarafin (sarafloxacin hydrochloride), a new antibacterial, was evaluated in the field on a naturally induced infection of Edwardsiella ictaluri in channel catfish, -Ictalurus punctatus. Healthy channel catfish (mean weight = 50 g) were stocked into nine cages at 200 fish per cage in a pond with an undergoing E. ictaluri infection. Seven days after stocking, dead fish were observed in the cages with clinical signs of enteric septicemia of channel catfish (ESC). After E. ictaluri was confirmed through isolative biochemical tests, medicated feed was applied for five consecutive days. During this period, fish in three control cages received a commercial 32% protein floating feed, three other cages of fish served as positive controls and were fed Romet, and three cages received the test feed with Sarafin. Both medicated feeds reduced the increase in cumulative percent mortality. In the control cages, cumulative percent mortalities continued to increase throughout the study period. Average daily mortality rates were significantly lower following both treatments of medicated feed, and treatments receiving Sarafin showed the greatest reduction in average daily mortality rates. Average daily mortality rates in the control did not change after the medicated feeding period. Toward the end of the study, temperatures reached 30°C, above the active range of ESC infections, and all mortalities ceased.  相似文献   
28.
用灰色系统理论的斜率关联度分析法,求得影响黑杨派南方型无性系高生长的主要气候因子依次为:降水量、极低温、极高温、蒸发量、日照时数、积温、年均温(或相至湿度)、相以湿度(或年均温)、无霜期。  相似文献   
29.
黑杨派南方型无性系引种及选择的研究   总被引:1,自引:1,他引:0  
对黑杨派南方型8个无性系的引种试验进行了总结。主要考察了高,径,材积冠幅和抗病能力,应用方差分析和系统聚类分析,将参试无性系划为3个等级,确定了适宜江西省江,湖冲积滩地推广的优良无性系,处于第1级的S1-8和725,其材积生长量分别比1-72杨提高32.6%和30.2%,且有较好的抗病性能,处于第2级的50、710、55/56和S1-6,它们的材积生长量比I-72杨提高17.6% ̄21.7%。  相似文献   
30.
在南方利用塑料大棚进行一年两收葡萄栽培不仅可以提早上市,也更利于休闲观光采摘。为了提高土地的利用,增加土地单位面积的产出,开展一年两收葡萄套种蔬菜的栽培技术研究,详细介绍了南方塑料连栋大棚一年两收夏黑葡萄夏季套种菜心、冬季套种莴笋的栽培技术,包括连栋大棚的设计、品种的选择、修剪、肥水管理、病虫害防治等田间管理技术,促农增收。  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