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   617篇
  免费   23篇
林业   13篇
农学   4篇
基础科学   6篇
  61篇
综合类   75篇
农作物   55篇
水产渔业   22篇
畜牧兽医   353篇
园艺   25篇
植物保护   26篇
  2020年   6篇
  2019年   8篇
  2018年   9篇
  2017年   8篇
  2016年   10篇
  2015年   9篇
  2013年   21篇
  2012年   14篇
  2011年   21篇
  2010年   15篇
  2009年   14篇
  2008年   22篇
  2007年   25篇
  2006年   23篇
  2005年   20篇
  2004年   28篇
  2003年   27篇
  2002年   23篇
  2001年   17篇
  2000年   30篇
  1999年   23篇
  1998年   11篇
  1997年   9篇
  1996年   11篇
  1995年   8篇
  1994年   5篇
  1993年   7篇
  1992年   7篇
  1991年   6篇
  1990年   9篇
  1989年   17篇
  1988年   8篇
  1987年   6篇
  1986年   8篇
  1985年   7篇
  1984年   5篇
  1981年   8篇
  1980年   5篇
  1979年   4篇
  1978年   6篇
  1975年   4篇
  1973年   8篇
  1971年   4篇
  1966年   6篇
  1947年   4篇
  1945年   6篇
  1944年   4篇
  1943年   9篇
  1935年   4篇
  1934年   4篇
排序方式: 共有640条查询结果,搜索用时 203 毫秒
51.
Several lupines (Lupinus spp.) present on western U.S. rangelands contain alkaloids that are teratogenic to livestock and cause congenital birth defects in calves (crooked calf disease). Periodically, large losses of calves due to lupine-induced "crooked calf disease" occur in northern Oregon and eastern Washington state. Five lupine populations from this area representing three species (L. leucophyllus, L. sulfureus, and L. sericeus) were evaluated taxonomically and by gas chromatography/mass spectrometry, and the major alkaloids in each lupine species were identified. The teratogenic alkaloid anagyrine was present in both of the lupine species responsible for the high outbreaks in east-central Washington and northeastern Oregon. However, the alkaloid profiles of the two lupines identified as L. leucophyllus were dissimilar, as were the alkaloid profiles of the two lupines identified as L. sulfureus. Botanical classification is not sufficient to determine potential teratogenicity, and it must be followed by chemical characterization to determine risk to livestock.  相似文献   
52.
Pure cultures and mixtures of equal numbers of plants of Irish. New Zealand and S23 perennial ryegrass were grown in artificially constructed swards under field conditions. They were cut every time they reached grazing height for three successive years. Counts of plant numbers and tiller numbers per plant made each November show that S23 suppressed both plant number and tiller number per plant of Irish ryegrass and to a lesser extent of New Zealand perennial ryegrass. Its own plant number and tiller number per plant were improved in these combinations. New Zealand perennial ryegrass suppressed plant and tiller numbers of Irish perennial ryegrass.
Varying the method of cutting to match the growth of each variety resulted in improved plant survival and tiller number per unit area of Irish perennial ryegrass when cut according to the growth of S23, i.e. late in the spring. The importance of the effect of experimental technique is discussed.  相似文献   
53.
54.
55.
56.
57.
Abtract Epidemiology models, modified to include landscape pattern, are used to examine the relative importance of landscape geometry and disturbance dynamics in determining the spatial extent of a disturbance, such as a fire. The models indicate that, except for very small values for the probability of spread, a disturbance tends to propagate to all susceptible sites that can be reached. Therefore, spatial pattern, rather than disturbance dynamic, will ordinarily determine the total extent of a single disturbance event. The models also indicate that a single disturbance will seldom become endemic,i.e., always present on the landscape. However, increasing disturbance frequency can lead to a landscape in which the proportion of susceptible, disturbed, and recovering sites are relatively constant. Research supported by Ecological Research Division, Office of Health and Environmental Research, U.S. Department of Energy under contract No. DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. Contribution No. 10 to the Sevilleta LTER program. Environmental Sciences Division Publication No. 3812, ORNL.  相似文献   
58.
Analysis of extracts of Glen Ample raspberries (Rubus idaeus L.) by gradient, reverse phase HPLC with diode array and tandem mass spectrometry identified eleven anthocyanins, including cyanidin-3-sophoroside, cyanidin-3-(2(G)-glucosylrutinoside), cyanidin-3-glucoside, cyanidin-3-rutinoside, pelargonidin-3-sophoroside, pelargonidin-3-(2(G)-glucosylrutinoside), and pelargonidin-3-glucoside. Significant quantities of an ellagitannin, sanguiin H-6, with an M(r) of 1870 were detected along with lower levels of a second ellagitannin, lambertianin C, which has an M(r) of 2804. Other phenolic compounds that were detected included trace levels of ellagic acid and its sugar conjugates along with one kaempferol- and four quercetin-based flavonol conjugates. Fractionation by preparative HPLC revealed that sanguiin H-6 was a major contributor to the antioxidant capacity of raspberries together with vitamin C and the anthocyanins. Vasodilation activity was restricted to fractions containing lambertianin C and sanguiin H-6.  相似文献   
59.
Element mass balance estimates for South Carolina Coastal Plain watersheds indicate that fertilizers and liming materials are the major sources for inputs of Ca, Mg, K, Cl, and HCO3 whereas precipitation is the major input for Na and SO4. Stream flow is the chief mode of output for all of these elements. A balance between input and output is evident only for Cl. Retentions of 50% or more are shown by Ca, Mg, K, HCO3 and SO4 whereas Na shows an apparent net loss. The retention of Ca, Mg and HCO3 suggests that less than 25% of the dolomitic liming materials applied to the landscape actually dissolve and that the carbonate chemistry of Lower Coastal Plain streams is therefore probably largely controlled by seepage of groundwaters from underlying calcareous aquifers. The retention of K and the loss of Na may be due to cation exchange reactions on soil clays whereas the apparent retention of SO4 is probably due to reduction to H2S in floodplain environments and soil adsorption.  相似文献   
60.
It has been known that lipoxygenase (LOX) isozymes exhibit differences in product formation, but most product information to date is for LOX 1 among soybean (Glycine max) LOX isozymes. In this study, LOXs 2 and 3 were purified and used to generate hydroperoxide (HPOD) products in an in vitro system using linoleic acid as a substrate in the presence of either air or O2. The products were analyzed to determine their stereoisomeric characteristics. The control (no enzyme) showed only low levels of hydroperoxide production and no stereoisomeric specificity. LOX 2 shows high specificity in product formation, producing roughly 4 times more 13-HPOD than 9-HPOD, nearly all of which was 13-S(Z,E)-HPOD. LOX 3 produced more 9-HPOD than 13-HPOD at approximately a 2:1 ratio. No single stereoisomer was predominant among LOX 3 products. These results demonstrate that different isozymes of LOX have characteristic product profiles in in vitro reactions.  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