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   185268篇
  免费   9648篇
  国内免费   7088篇
林业   13006篇
农学   14368篇
基础科学   6711篇
  24754篇
综合类   40595篇
农作物   7605篇
水产渔业   7573篇
畜牧兽医   72280篇
园艺   3404篇
植物保护   11708篇
  2022年   1757篇
  2021年   3064篇
  2020年   2975篇
  2019年   2889篇
  2018年   3030篇
  2017年   3884篇
  2016年   3712篇
  2015年   4164篇
  2014年   4347篇
  2013年   7718篇
  2012年   7091篇
  2011年   8087篇
  2010年   6425篇
  2009年   6269篇
  2008年   7234篇
  2007年   7520篇
  2006年   7131篇
  2005年   6511篇
  2004年   4675篇
  2003年   4434篇
  2002年   3995篇
  2001年   5000篇
  2000年   5329篇
  1999年   4778篇
  1998年   2827篇
  1997年   2607篇
  1996年   2468篇
  1995年   2641篇
  1994年   2337篇
  1993年   2252篇
  1992年   3453篇
  1991年   3413篇
  1990年   3138篇
  1989年   3001篇
  1988年   2814篇
  1987年   2840篇
  1986年   2736篇
  1985年   2581篇
  1984年   2067篇
  1983年   1837篇
  1979年   1870篇
  1978年   1533篇
  1977年   1321篇
  1975年   1367篇
  1974年   1806篇
  1973年   1764篇
  1972年   1777篇
  1971年   1682篇
  1970年   1549篇
  1969年   1448篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
The contributions presented at the EIFAC Symposium on Fisheries and Society in 2000 are reviewed. There have been considerable advances in many sectors of inland fisheries over the past decade. Nowadays the value of inland fisheries to society is generally better understood and increasingly recognized. However, it was recognized that the allocation of fishery resources and their management could still be made more effective through more extensive application of the recent approaches and techniques discussed during the Symposium and reviewed in the paper.  相似文献   
142.
Abstract. The ability of two nitrogen cycle models, of contrasting complexity, to predict N mineralization from a range of grassland soils in the UK, was evaluated. These were NCYCLE, a simple mass balance model of the N cycle in UK grasslands, and CENTURY, a more complex model simulating long-term C, N, P & S dynamics in grassland ecosystems. The models were tested using field measurements of net N mineralization from a range of grassland soils (differing in soil type, history & management practice), obtained over a 2 year period using a soil core incubation technique. This method was considered to measure the total net release of mineral N from the soil organic matter over a specified time, including N which may have been recycled several times. NCYCLE consistently under-estimated mineralization rates at all sites. By contrast, there was some correlation between CENTURY predictions of net N mineralization and field measurements. This may have reflected the different abilities of the two models to simulate N recycling. Neither model, however, was able to predict adequately the effect of cultivation and reseeding on net N mineralization.  相似文献   
143.
144.
Two serological tests (indirect immunofluorescence and enzyme-linked immunosorbent assay) were developed for the detection of fetal antibody to Chlamydia psittaci. Fetal blood and thoracic fluid from 126 field cases of suspected ovine chlamydial abortion were examined using both tests. Placenta and fetal tissues (lung, liver, and kidney) from the same animals were also examined by the following conventional diagnostic methods: isolation in McCoy cells, detection of chlamydial lipopolysaccharide (LPS), modified Ziehl-Nielsen staining, and direct fluorescent antibody staining of chlamydia in frozen cryostat sections. Seventy cases were positive by fetal serology, and of these, 68 were also positive by isolation and/or LPS detection. The remaining 56 cases had negative fetal serology, and of these, 39 were positive by isolation and/or LPS detection. Results indicate that fetal serology, although less sensitive than either isolation in McCoy cells or detection of chlamydial LPS antigen, may be of particular use when placenta is not available.  相似文献   
145.
Hege  Kippenes  DVM  Patrick R.  Gavin  DVM  PhD  Susan L.  Kraft  DVM  PhD  Ronald D.  Sande  DVM  MS  PhD  Russell L.  Tucker  DVM 《Veterinary radiology & ultrasound》2001,42(2):130-133
The pituitary gland was measured from transverse magnetic resonance T1-weighted images after Gadolinium administration in 96 dogs weighing from 13 to 45 kg. The measurements were done by hand with calipers. The mean (+/- standard deviation) pituitary gland height was 5.1 mm (+/-0.9 mm). The mean width was 6.4 mm (+/- 1.1 mm). The correlation coefficient between pituitary and brain measurements, between pituitary measurement and body weight, and brain measurements and body weight was 0.0 to 0.3. A hyperintense region was present on T1-weighted images in the center of the pituitary gland in 64% of the dogs. At necropsy the pituitary glands were grossly and histologically normal. No pituitary gland measurements were performed at necropsy.  相似文献   
146.
147.
新疆南疆是一个有着悠久农业历史、发达的传统农业的地区,传统农具的使用历史超过了4000年,且非常适合于当地的农业生产条件。本文基于作者的调查和历史文献,介绍南疆地区的传统农具的类型、功能及使用方法。  相似文献   
148.
149.
150.
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