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   108篇
  免费   9篇
林业   4篇
农学   1篇
基础科学   2篇
  5篇
综合类   31篇
农作物   1篇
水产渔业   2篇
畜牧兽医   62篇
园艺   3篇
植物保护   6篇
  2022年   2篇
  2021年   1篇
  2020年   2篇
  2018年   3篇
  2017年   2篇
  2016年   1篇
  2015年   2篇
  2014年   1篇
  2013年   5篇
  2012年   5篇
  2011年   4篇
  2010年   3篇
  2009年   5篇
  2008年   3篇
  2007年   5篇
  2006年   1篇
  2005年   3篇
  2004年   4篇
  2003年   1篇
  2002年   1篇
  2001年   1篇
  2000年   1篇
  1999年   1篇
  1995年   1篇
  1994年   1篇
  1993年   1篇
  1992年   2篇
  1991年   4篇
  1990年   4篇
  1989年   3篇
  1988年   4篇
  1987年   3篇
  1986年   5篇
  1984年   6篇
  1983年   1篇
  1982年   1篇
  1981年   1篇
  1979年   1篇
  1978年   1篇
  1976年   2篇
  1975年   1篇
  1974年   1篇
  1973年   3篇
  1972年   2篇
  1971年   1篇
  1970年   1篇
  1968年   4篇
  1967年   5篇
  1893年   1篇
排序方式: 共有117条查询结果,搜索用时 12 毫秒
1.
Lipid nutrition     
Cows in early lactation or producing more than 80 lb of milk per day need supplemental fat and can benefit from it. Fat should be added to the diet over a period of several weeks to allow the cows to become accustomed to it. Feed intake should be monitored because additional fat may decrease feed intake and offset the benefit of the fat. Supplemental fat should not exceed 4 to 5% of the dry matter intake. The first 2% of added fat should be supplied by oilseeds under most circumstances. The next 1 or 2% can come from commodity fat if availability and handling ability permits its use. If the last increment of fat is needed, it should be supplied by specialty fats that have been processed to improve ruminal inertness. Extra calcium, magnesium, and ruminally undegraded protein should be added to the diet when fat is added. Fat is a more expensive source of energy than feed grains in most of the world and should not be used beyond needs.  相似文献   
2.
Following challenge with Trichosirongylus colubrifonizis, increased numbers of T-cells and immunoglobulin responses are seen in the intestine of sheep immunised by repeated infection with live worms. IL-6 mRNA expression in the small intestine from T. colubriformis-immunised and naive sheep was determined by in situ hybridisation, whereas CD4(+), IgA(+), IgG(+) cells in the gut were evaluated by immunohistochemistry. There was constitutive expression of IL-6 mRNA by cells in the naive gut, and the number of these cells was increased by parasite challenge. There were corresponding increases in numbers of CD4(+) and TCR gamma/delta(+) T-cells and IgG(+) B-cells. Our data are consistent with a role for IL-6, perhaps produced by CD4(+) and/or TCR gamma/delta(+) T-cells or B-cells, in B-cell terminal differentiation. Infiltration of B-cells, particularly IgG(+) B-cells, may reflect parasite immunity in the host.  相似文献   
3.
In an on‐farm study, 40 weaned piglets aged 3 weeks were vaccinated with Lawsonia intracellularis vaccine orally, IM or IP while a fourth group remained unvaccinated. All vaccinated animals showed increased serum levels of L. intracellularis‐specific IgG antibodies, but significantly elevated concentrations of specific IgG, IgA and cytokines were generated in ileal mucosal secretions from the orally and IP vaccinated pigs when examined at 17 days after vaccination.  相似文献   
4.
Low-intensity fires were important for maintaining the structure of Eastern deciduous forests (EDFs) for thousands of years before European settlement of North America, though fire suppression became a standard management practice in the 1930s. More recently, prescribed fires have been reintroduced to EDF habitats to aid in the restoration of native plant diversity, but invasions of non-native species such as Microstegium vimineum (Japanese stiltgrass) may increase prescribed fire intensity and suppress colonization of native species. As fire becomes a more common management tool in these habitats, it is vital to predict fire temperature effects on the native and non-native species present in the system. In this study, we found that prescribed fires in areas invaded by Microstegium can be 250-300 °C hotter than fires in nearby native-dominated areas. We then compared the effects of fire on germination rates of six native and three non-native EDF understory species representing the range of functional groups common in this habitat. We manipulated both fire intensity (temperature and length of exposure) and type of fire effect (direct flame and indirect furnace heat) to generate germination curves and make predictions about potential prescribed fire effects on populations of these species. There were very different responses among species to both direct (flame) and indirect (furnace) heating. Germination of three native species, Lycopus americana (American water horehound), Verbesina alternifolia (wingstem), and Vernonia gigantea (tall ironweed), showed signs of being stimulated by heating at low temperatures, while germination of all non-native species (M. vimineum, Elaeagnus umbellata, and Schedonorus phoenix) were inhibited at these lower intensities. High fire intensity (temperatures above 300 °C) effectively killed most species, though one native species, Senna hebacarpa (American senna) and one non-native species, E. umbellata (autumn olive), were capable of tolerating 500 °C temperatures. We conclude that high-intensity prescribed fires in habitats invaded by Microstegium may reduce seed germination of some non-native species, but may also inhibit the regeneration of native understory species.  相似文献   
5.
The New Horizons spacecraft observed Jupiter's icy satellites Europa and Ganymede during its flyby in February and March 2007 at visible and infrared wavelengths. Infrared spectral images map H2O ice absorption and hydrated contaminants, bolstering the case for an exogenous source of Europa's "non-ice" surface material and filling large gaps in compositional maps of Ganymede's Jupiter-facing hemisphere. Visual wavelength images of Europa extend knowledge of its global pattern of arcuate troughs and show that its surface scatters light more isotropically than other icy satellites.  相似文献   
6.
Quiescent T lymphocytes as an inducible virus reservoir in HIV-1 infection   总被引:73,自引:0,他引:73  
To better understand the basis for human immunodeficiency virus type 1 (HIV-1) persistence and latency, the form in which viral DNA exists in the peripheral T lymphocyte reservoir of infected individuals was investigated. In asymptomatic individuals, HIV-1 was harbored predominantly as full-length, unintegrated complementary DNA. These extrachromosomal DNA forms retained the ability to integrate upon T cell activation in vitro. In patients with acquired immunodeficiency syndrome (AIDS), there was an increase in integrated relative to extrachromosomal DNA forms. By analysis of DNA from patient lymphocyte subpopulations depleted of human lymphocyte antigen-Dr receptor-positive cells, quiescent T cells were identified as the source of extrachromosomal HIV-1 DNA. Thus quiescent T lymphocytes may be a major and inducible HIV-1 reservoir in infected individuals.  相似文献   
7.
The discovery of embryonic halos around uranium-rich sites that exhibit very high (238)U/(206)Pb ratios suggests that uranium introduction may have occurred far more recently than previously supposed. The discovery of (210)Po halos derived from uranium daughters, some elliptical in shape, further suggests that uranium-daughter infiltration occurred prior to coalification when the radionuclide transport rate was relatively high and the matrix still plastically deformable.  相似文献   
8.
Freeze-fracturing shows particles within membranes. In plasma membranes of most cells the particles are more strongly bound to the inner half. In unfixed endothelial cells, this polarity is reversed. Glutaraldehyde fixation results in conventional polarity. The reverse polarity may be related to a mechanism for preferential fusion of pinocytotic vesicles with the plasma membrane.  相似文献   
9.
Flint pebbles and nodules from the Upper Cretaceous chalks of Europe occur offshore and at many seaports along the Atlantic coast of North America, where they were brought as ship's ballast. Isolated pieces imported from Europe as gunflints also are present.  相似文献   
10.
Discussions of the evolution of intelligence have focused on monkeys and apes because of their close evolutionary relationship to humans. Other large-brained social animals, such as corvids, also understand their physical and social worlds. Here we review recent studies of tool manufacture, mental time travel, and social cognition in corvids, and suggest that complex cognition depends on a "tool kit" consisting of causal reasoning, flexibility, imagination, and prospection. Because corvids and apes share these cognitive tools, we argue that complex cognitive abilities evolved multiple times in distantly related species with vastly different brain structures in order to solve similar socioecological problems.  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