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   660篇
  免费   31篇
  国内免费   1篇
林业   55篇
农学   26篇
基础科学   8篇
  108篇
综合类   95篇
农作物   36篇
水产渔业   54篇
畜牧兽医   234篇
园艺   19篇
植物保护   57篇
  2023年   8篇
  2022年   5篇
  2021年   16篇
  2020年   19篇
  2019年   27篇
  2018年   12篇
  2017年   31篇
  2016年   21篇
  2015年   17篇
  2014年   26篇
  2013年   42篇
  2012年   37篇
  2011年   60篇
  2010年   43篇
  2009年   12篇
  2008年   35篇
  2007年   41篇
  2006年   26篇
  2005年   26篇
  2004年   28篇
  2003年   28篇
  2002年   25篇
  2001年   4篇
  2000年   10篇
  1999年   6篇
  1998年   2篇
  1997年   8篇
  1995年   6篇
  1994年   5篇
  1993年   4篇
  1992年   4篇
  1989年   2篇
  1988年   4篇
  1987年   2篇
  1986年   2篇
  1985年   4篇
  1984年   4篇
  1983年   4篇
  1980年   2篇
  1979年   2篇
  1974年   5篇
  1973年   4篇
  1972年   4篇
  1970年   4篇
  1968年   2篇
  1967年   1篇
  1941年   1篇
  1931年   2篇
  1919年   1篇
  1911年   2篇
排序方式: 共有692条查询结果,搜索用时 15 毫秒
41.
We determined the prevalence of tuberculosis and brucellosis reactors in the dairy herds in the Mbarara district of Uganda in 2002. This is one of the most important dairy-production areas of the country and includes both pastoral and agro-pastoral zones. A total of 340 (of 11,995) randomly selected herds were tested for tuberculosis, using the intradermal tuberculosis-skin test and 315 (of 10,562) herds tested for brucellosis using the serum Rose Bengal test.

The herd prevalence for tuberculosis reactors was 74.1% (95% confidence intervals 69, 78), the individual-animal prevalence was of 6.0% (5.6, 6.5) and within-herd range was 1–50% (up to 100% if suspicious reactors were included).

The herd prevalence for brucellosis was 55.6% (50, 61.2) individual-animal prevalence 15.8% (14.8, 16.7) and within-herd range 1–90%.

The reactor prevalence increased with the age of the animals for both tuberculosis and brucellosis.

Tuberculosis reactor prevalences were higher in animals from the agro-pastoral zone. However, the individual-animal and herd prevalences of brucellosis seroprevalences were higher in the pastoral zone.  相似文献   

42.

• Introduction   

Various processes contribute to shaping the local assemblage of species in hyperdiverse tropical forest. The relative contribution of environmental factors and dispersal limitation in determining the spatial distribution of saplings at local scale is unclear.  相似文献   
43.
As a consequence of the recent introduction of the pine wood nematode Bursaphelenchus xylophilus in Portugal, nematodes of the genus Bursaphelenchus were looked for in various French pine forests, in trees attacked by Monochamus galloprovincialis, the vector insect of B. xylophilus, and in the insects themselves. Trap trees were felled in 12 localities distributed all over the country. Nematodes were extracted from transversal stem discs; insects emerging from the trap trees were studied. B. hellenicus, B. leoni, B. mucronatus and B. sexdentati were isolated, but not B. xylophilus. The presence of B. mucronatus and the absence of B. xylophilus were confirmed by molecular markers. B. mucronatus was isolated from several regions with an average prevalence of 19%. The infestation of M. galloprovincialis by B. mucronatus reached 26.7%. The wide distribution of B. mucronatus in France could have an effect on the extension of B. xylophilus in a case of an introduction.  相似文献   
44.
  • ? A discriminant study based on samples taken from high and low yielding oleoresin trees of two Greek populations, Chalkidiki and Euboia, was carried out. Oleoresin of Pinus halepensis Mill. was characterised by GC/MS analysis.
  • ? The objectives of this study were: (i) to identify in detail the composition of the oleoresin of P. halepensis and in particular of high yielding trees (plus trees) (ii) to investigate a potential relationship between the oleoresin compounds and the oleoresin yield and (iii) to investigate any correlations among the compounds.
  • ? About forty monoterpenes, sesquiterpenes and diterpenes were identified as main compounds representing 97.40% of the analyzed oleoresin. α-Pinene, methyl abietate, abietic acid, palustric acid, isopimaric acid and neoabietic acid were the major compounds. Efficient discrimination was achieved between the two populations and between the two groups of trees (high and low yielding). In both cases, the differentiation was due to the quantitative variability of certain compounds. High positive correlations were found among certain compounds.
  • ? The results suggest that the oleoresin profile is a useful tool for the discrimination of trees according to their provenance or their oleoresin yield.
  •   相似文献   
    45.
    Agroforestry Systems - Complex agroforestry systems are suggested as a possible solution to reduce the effects of deforestation in the tropics while enhancing the livelihoods of local human...  相似文献   
    46.
    New Forests - Between the nineteenth and early twentieth centuries, large-scale afforestation projects were undertaken in the Swiss Prealps. The Swiss stone pine (Pinus cembra L.), an emblematic...  相似文献   
    47.
    Plasma treatments for black spruce wood (Picea mariana (Mill.) B.S.P.), a widespread forest species from Canada, were carried out in order to waterproof the exposed surfaces. Experiments were performed using inductively coupled argon plasma with carbon tetrafluoride as the gaseous precursor for plasma-enhanced chemical vapor deposition of functional fluoropolymer coatings on wood. Analysis of the wettability through water contact angle measurements showed water-repellent characteristics, with static contact angles up to 130° depending on plasma exposure time, CF4 concentration in the Ar/CF4 plasma, and plasma source-to-substrate distance. X-ray photoelectron spectroscopy investigations of plasma-treated wood surfaces confirmed the growth of a thin, fluorocarbon layer with fluorine atomic concentrations close to 50 % on highly hydrophobic wood surfaces. Estimation of the thickness of the coatings by stylus profilometry revealed that a minimum layer thickness of about 80 nm is required to obtain water repellant wood surfaces with minimum water uptake. This complete set of data indicates that fluorocarbon-containing plasmas represent a very promising approach for improving the durability of wood products in wet and humid conditions.  相似文献   
    48.
    As a result of diverse management policies, densities of red and roe deer have varied considerably for the last few decades in Europe. Their selective browsing on woody species can modify the interactions between competing species and thus change overall patterns of plant diversity in forests. However, no classification of woody species according to browsing by deer yet exists in Europe. The selectivity index, which measures the balance between availability and consumption of plant species by herbivores, could be used for such a classification. The aim of this study was, first, to rank woody species according to this selectivity index, then to test whether their rank changed when browsing pressure varied in space or time. We also explored morphological and ecological life traits (Ellenberg indicator values) that might explain differences in selectivity.In 2006, we re-sampled a network of 217 coupled floristic and browsing survey plots, which had been previously described twice (1976, 1981). The plots were located in the forest of Arc-en-Barrois (France), a large woodland area divided into a North and a South Forest which have had different histories of browsing pressure.For the three samples and the two forests, we calculated a selectivity index for 19 woody species. Dogwood (Cornus sp.) and field rose (Rosa arvensis) appeared to be the most selected species whereas beech (Fagus sylvatica), common mezereon (Daphne mezereum) and wild-service tree (Sorbus torminalis) were always avoided. The rank order of species according to their selectivity index remained stable over the three sample years and between the two forests (Spearman's rank correlations: ρ ≈ 0.8 spatially, and range from 0.3 to 0.8 temporally). This indicates that variations in browsing pressure did not strongly modify deer selectivity patterns. Selectivity was positively correlated with wood density and negatively correlated with maximum height of the species, but unrelated to other plant traits. Finally, selectivity can be seen as an intrinsic characteristic of those woody species, and could be used as an additional life trait when analyzing vegetation communities.  相似文献   
    49.
    The Aeta Magbukún of Mariveles are one of the least known and researched Indigenous peoples remaining on the fringe of the bay, and within the remaining forests in Bataan province on Luzon Island in the Philippines. This work describes the unique cultural systems and language of the Aeta Magbukún tribe in Biaan, Mariveles, and both the traditional forest resource use and the evolving new subsistence practices developed to adapt to the encroachment of non-Indigenous peoples onto ancestral lands. The Aeta's forest resource use practices are discussed from a sustainable Indigenous development context within unique socioeconomic, cultural, and environmental circumstances in Bataan.  相似文献   
    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