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   439篇
  免费   13篇
林业   1篇
农学   10篇
基础科学   8篇
  17篇
综合类   208篇
农作物   9篇
水产渔业   11篇
畜牧兽医   167篇
园艺   6篇
植物保护   15篇
  2021年   4篇
  2019年   6篇
  2018年   3篇
  2017年   7篇
  2015年   5篇
  2014年   4篇
  2013年   8篇
  2012年   10篇
  2011年   15篇
  2010年   5篇
  2009年   3篇
  2008年   15篇
  2007年   15篇
  2006年   13篇
  2005年   11篇
  2004年   6篇
  2003年   8篇
  2002年   5篇
  2001年   11篇
  2000年   6篇
  1999年   8篇
  1998年   7篇
  1997年   6篇
  1996年   7篇
  1995年   4篇
  1994年   6篇
  1992年   10篇
  1991年   6篇
  1990年   5篇
  1989年   4篇
  1988年   3篇
  1987年   4篇
  1986年   4篇
  1984年   5篇
  1983年   5篇
  1982年   6篇
  1981年   2篇
  1980年   2篇
  1979年   2篇
  1978年   8篇
  1977年   23篇
  1976年   25篇
  1975年   17篇
  1974年   26篇
  1973年   22篇
  1972年   30篇
  1971年   27篇
  1970年   7篇
  1963年   2篇
  1955年   2篇
排序方式: 共有452条查询结果,搜索用时 421 毫秒
131.
132.
This study investigated possible associations between soil types, land use and badger numbers in an area of the Irish Midlands where badger removal had been conducted during 1989–1994. For this purpose, the area was divided into approximately 2500 geo-referenced square grids of 0.5°km per side. For the outcomes (setts per grid, badgers per grid and tuberculous badgers per grid), Poisson models of land use, of soil type, and a combination of these two were developed. Influential grids were removed and the models adjusted for over-dispersion in the badger outcomes. Mineral-based soils, dry and very-dry peat soils supported increased numbers of setts and badgers. High-quality pasture was the major land use (pastures often are found on mineral-based soils) and supported increased numbers of setts, badgers, and tuberculous badgers. “Natural” areas also supported more setts and broad-leaf forested areas were associated with increased tuberculous badger numbers. Discontinuous urban areas tended to decrease sett numbers per grid. Hedgerow length was not an important predictor given the information on soil type and land use.

Spatial correlations existed for badger setts in 1°km grids, for badger numbers in 1.5°km grids, and for tuberculous badgers in 2°km grids. The latter two grids have approximately the same area as the territory size used by a social group of badgers. There were no spatial correlations at the smallest (0.5 km) grid size.  相似文献   

133.
To determine breed differences in ovarian function and endocrine secretion, daily rectal ultrasonography was conducted on multiparous lactating Angus (temperate Bos taurus; n = 12), Brahman (tropical Bos indicus; n = 12), and Senepol (tropical Bos taurus; n = 12) cows during an estrous cycle in summer. Blood was collected daily to quantify plasma concentrations of FSH, LH, progesterone, estradiol, GH, insulin-like growth factor (IGF)-I, IGF-II, IGF binding proteins (IGFBP), insulin, glucose, and plasma urea nitrogen (PUN). Numbers of small (2 to 5 mm), medium (6 to 8 mm), and large follicles (> or = 9 mm) were greater (P < .05) in Brahman than in Angus and(or) Senepol cows. Length of the estrous cycle (SEM = .6 d) was similar (P > .10) among Senepol (20.4 d), Angus (19.5 d), and Brahman (19.7 d) cows. Senepol cows had greater (P < .05) diameters of the corpus luteum (CL) and a delayed regression of the CL as compared with Angus cows. The secondary surge of FSH (between d 1 and 2; d 0 = estrus) was greater in Angus than Brahman or Senepol cows (breed x day, P < .05). Between d 2 and 14 of the estrous cycle, concentrations of progesterone, LH, IGF-II, and binding activities of IGFBP-3, IGFBP-2, and the 27- to 29-kDa IGFBP in plasma did not differ (P > .10) among breeds. Concentrations of GH, IGF-I, insulin, and PUN were greater (P < .001) and binding activities of the 22-kDa and 20-kDa IGFBP tended (P < .10) to be greater in plasma of Brahman than in Angus or Senepol cows. Plasma glucose concentrations were greater (P < .05) in Senepol than in Brahman or Angus cows. In conclusion, Brahman (Bos indicus) and Senepol cows (tropical Bos taurus) had greater numbers of follicles in all size categories and greater diameter of CL than Angus (temperate Bos taurus) cows. These ovarian differences may be due to changes in the pattern of secretion of FSH, insulin, IGF-I, and GH but not LH, IGF-II, or IGFBP-2 or -3.  相似文献   
134.
This exceptionally reactive compound 1,4-benzoquinone tetracarboxylic acid dianhydride is one of the strongest vr-acids yet described.  相似文献   
135.
Clonal cell lines derived from specific types of central neurons can be used to identify and characterize properties specific to those neurons. With somatic cell fusion techniques, nine clonal hybrid cell lines have been developed from the septal region of the mouse basal forebrain. Two lines express characteristics typical of cholinergic neurons--choline acetyltransferase activity and immunoreactivity, neurite formation with neurofilament protein immunoreactivity, and aggregation in rotation-mediated cell culture. These cell lines may be useful for studying the trophic interactions that support the development and maintenance of central cholinergic connections.  相似文献   
136.
137.
138.
139.
Hammond N 《Science (New York, N.Y.)》1972,178(4065):1092-1093
Obsidian from two sources in highland Guatemala has been found at 23 sites of the Classic Mayan civilization, mainly in the nonvolcanic lowlands to the north. The distribution, together with trade routes suggested by topography and documentary sources, suggests efficient waterborne transport and competition between sources for the lowland market.  相似文献   
140.
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