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   136篇
  免费   3篇
林业   10篇
农学   6篇
  31篇
综合类   19篇
农作物   2篇
水产渔业   3篇
畜牧兽医   53篇
园艺   10篇
植物保护   5篇
  2022年   3篇
  2021年   2篇
  2020年   2篇
  2019年   4篇
  2018年   3篇
  2017年   1篇
  2016年   4篇
  2015年   2篇
  2014年   1篇
  2013年   6篇
  2012年   4篇
  2011年   3篇
  2010年   5篇
  2009年   3篇
  2008年   8篇
  2007年   14篇
  2006年   4篇
  2005年   7篇
  2004年   2篇
  2002年   1篇
  2001年   6篇
  2000年   5篇
  1998年   2篇
  1997年   2篇
  1995年   3篇
  1994年   4篇
  1992年   1篇
  1990年   1篇
  1989年   1篇
  1988年   2篇
  1987年   4篇
  1986年   1篇
  1985年   5篇
  1984年   1篇
  1982年   2篇
  1981年   1篇
  1979年   4篇
  1978年   1篇
  1977年   1篇
  1976年   2篇
  1974年   3篇
  1971年   1篇
  1970年   1篇
  1968年   1篇
  1967年   2篇
  1966年   2篇
  1947年   1篇
排序方式: 共有139条查询结果,搜索用时 609 毫秒
41.
42.
Chongqing is among the heaviest polluted cities in China. Combustion of coal with relatively high sulfur content causes high sulfur emission and deposition in the area. Effects on soils and waters of the acid deposition in the Chongqing area have been studied in the field at a forested site outside the city. Deposition chemistry and fluxes, soil and soil water chemistry as well as surface water chemistry are presented for the period 1996–1998. There are some stress symptoms at the forest in the area and severe forest damage has been reported at Nanshan, closer to Chongqing center. Monitoring of the acidification situation in the area must be followed closely as impacts may be expected if the deposition is not reduced in the future. The deposition of sulfur, H+ as well as calcium at the site is high. Wet deposition of sulfur is estimated to 4.7 – 5.7 g S m?2 yr?1 during the three years sampled; dry deposition is probably of similar size. Annual volume-weighted pH in bulk deposition was 4.0 – 4.2 and the calcium wet deposition flux was 2.6 – 3.6 g Ca2+ m?2. There are considerable seasonal variations in the concentrations, related to the seasonal variations in precipitation amount (dry winter, wet summer). The soils at the site are acid with median base saturation of 12% and 8% in the topsoil and subsoil, respectively. In soil water, aluminum concentrations are typically in the range 3–8 mg L?1. However, due to the high base cation deposition, the Al/(Ca2++Mg2+) molar ratio is below unity in most samples, indicating little damage of forest due to aluminum in soil water.  相似文献   
43.
低湿地与土壤湿度的卫星遥感监测与制图   总被引:8,自引:0,他引:8       下载免费PDF全文
戴昌达  T.Vogt 《土壤学报》1995,32(4):377-382
本文介绍应用陆地卫生TM图象数据监测法国莱茵河谷地土壤湿度变化,绘制1:50,000低湿地分布图的技术方法。对夏、秋两个时相的TM图像进行K-L变换等应用处理,获得的第3主分量集中了土壤湿度差异的信息。把两个时相的第3主人量精确配准后,以Hyperboxes分类器进行湿度分类,分出水体、恒湿地、夏秋季潮湿地、冬春季潮湿地、恒干地等5个湿度类别。经实地检验准确可信,表明此方法适合于土壤湿度变化监测与  相似文献   
44.
Virtual patients have become an interesting alternative in medical education. Due to increasing demands regarding theoretical and clinical teaching and to improve an interdisciplinary approach, a new blended learning concept including virtual patients was developed and implemented in the veterinary curriculum of the Freie Universität Berlin. In the presented project, three virtual patients from the field of canine reproduction were developed. They focus on pregnancy diagnosis with suspected luteal insufficiency, pyometra and benign prostatic hyperplasia, respectively. The results of an evaluation by veterinary students of the 7th semester showed a high acceptance of virtual patients in a blended learning reproduction module in the interdisciplinary lectures. Students especially preferred videos, such as video lectures, hands-on videos and animations as well as a glossary for background information, to successfully and autonomously work on a virtual case. The content covered by the new modules that were developed in the context of this project is part of a spiral curriculum; they will be revised and enhanced during the clinical year.  相似文献   
45.
In August 2008, the first detection of the spotted wing drosophila, Drosophila suzukii, to the North America mainland in California caused great concern, as the fly was found infesting a variety of commercial fruits. Subsequent detections followed in Oregon, Washington, Florida and British Columbia in 2009; in Utah, North Carolina, South Carolina, Michigan, and Louisiana in 2010; and in Virginia, Montana, Wisconsin, Pennsylvania, New Jersey, Maryland and Mexico in 2011. In Europe, it has been detected in Italy and Spain in 2009 and in France in 2010. Economic costs to the grower from D. suzukii include the increased cost of production (increased labor and materials for chemical inputs, monitoring and other management tools) and crop loss. An effective response to the invasion of D. suzukii requires proper taxonomic identification at the initial phase, understanding basic biology and phenology, developing management tools, transferring information and technology quickly to user groups, and evaluating the impact of the research and extension program on an economic, social, and environmental level. As D. suzukii continues to expand its range, steps must be initiated in each new region to educate and inform the public as well as formulate management tactics suitable for the crops and growing conditions in each.  相似文献   
46.
Cutting scores for 3399 horses representing 327 different sires from the NCHA World Championship Futurities of 1975 through 1983 were used to calculate heritability and yearly change in scores. The analysis yielded a heritability estimate of 19%±5%. Appropriate selection schemes, then, should be successful in bringing about an improvement in cutting score. The analysis of yearly trend, however, indicates that average cutting scores at NCHA futurities are decreasing by 0.78 points per year.  相似文献   
47.
Congenital anury (taillessness) was observed in 2 Cairn Terriers. Fecal staining of the hind limbs was evident in both dogs. A mating of the affected dogs produced a litter of 2 apparently normal pups. The small litter size may have been attributable to lethal gene combinations similar to those described in mice. The radiographic appearance of the caudal and sacral vertebrae was atypical in both tailless dogs. Results of myelography and electrodiagnostic testing of anal sphincter function were normal in one of the dogs. One of the tailless dogs was euthanatized and was found to lack specific muscles and muscle parts responsible for normal defecation, which resulted in fecal staining of the hind limbs. Pedigree analysis revealed the tailless dogs to be closely related and slightly inbred, suggesting a hereditary etiologic factor for anury.  相似文献   
48.
Data on 4,411 male and 4,148 female progeny from the University of Missouri-Columbia swine research herd were analyzed to evaluate the associations of sire, breed, weight at birth, and sex of the pigs with the development of congenital splayleg. Statistically significant differences in frequency of splayleg were found among sires and among breeds indicating a genetic influence. Differences in frequencies among male vs female progeny also were found to be statistically significant. The overall frequency among male progeny was 1.74 times that observed among female progeny. Comparison of birth weights of splayleg vs normal pigs showed birth weights to be statistically significantly smaller among affected pigs.  相似文献   
49.
50.
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