全文获取类型
收费全文 | 148119篇 |
免费 | 8213篇 |
国内免费 | 133篇 |
专业分类
林业 | 5703篇 |
农学 | 4342篇 |
基础科学 | 901篇 |
16861篇 | |
综合类 | 28541篇 |
农作物 | 5970篇 |
水产渔业 | 6901篇 |
畜牧兽医 | 76432篇 |
园艺 | 1778篇 |
植物保护 | 9036篇 |
出版年
2019年 | 1349篇 |
2018年 | 1993篇 |
2017年 | 2271篇 |
2016年 | 2030篇 |
2015年 | 1785篇 |
2014年 | 2238篇 |
2013年 | 5440篇 |
2012年 | 4081篇 |
2011年 | 4954篇 |
2010年 | 3198篇 |
2009年 | 3268篇 |
2008年 | 4838篇 |
2007年 | 4602篇 |
2006年 | 4403篇 |
2005年 | 4106篇 |
2004年 | 3946篇 |
2003年 | 4009篇 |
2002年 | 3750篇 |
2001年 | 4567篇 |
2000年 | 4540篇 |
1999年 | 3635篇 |
1998年 | 1499篇 |
1997年 | 1440篇 |
1996年 | 1350篇 |
1995年 | 1570篇 |
1994年 | 1407篇 |
1993年 | 1440篇 |
1992年 | 2974篇 |
1991年 | 3162篇 |
1990年 | 3024篇 |
1989年 | 3061篇 |
1988年 | 2701篇 |
1987年 | 2835篇 |
1986年 | 2942篇 |
1985年 | 2848篇 |
1984年 | 2247篇 |
1983年 | 2031篇 |
1982年 | 1432篇 |
1979年 | 2071篇 |
1978年 | 1639篇 |
1977年 | 1407篇 |
1975年 | 1437篇 |
1974年 | 1831篇 |
1973年 | 1861篇 |
1972年 | 1843篇 |
1971年 | 1766篇 |
1970年 | 1701篇 |
1969年 | 1552篇 |
1968年 | 1299篇 |
1967年 | 1348篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
The effect of component number on Rhynchosporium secalis infection and yield in mixtures of winter barley cultivars 总被引:1,自引:1,他引:1
Mixtures of winter barley cultivars containing up to six components were grown over three years with and without fungicide treatment. Yield increases were recorded for mixtures compared with the mean of their monoculture components and there was a significant trend towards greater benefit from increased number of components. These benefits were partially attributable to a corresponding increase in control of Rhynchosporium secalis as component number increased. The potential for exploitation of mixtures in cereals for control of splash-dispersed pathogens is discussed. 相似文献
952.
Strains of Erwinia herbicola effective in the biocontrol of fire blight of hawthorn were used to investigate the possibility that the antagonistic activity is coded by plasmid-born genes. Agarose gel electrophoresis of isolated plasmids from four antagonistic Erw. herbicola strains showed a band of a supercoiled 12 kb plasmid in each strain, with a second band greater than 16.2 kb consistently seen in two strains. Erw. herbicola strains showed resistance to penicillin-G, which could be conferred on penicillin-G sensitive Escherichia coli TG1 by transformation with a pure Erw. herbicola plasmid preparation. Transformed strains of Esc. coli appeared to contain the Erw. herbicola 12 kb plasmid, but not the > 16.2 kb plasmid. In an agar plate assay, Esc. coli transformants produced an inhibition zone against Erw. amylovora similar to those produced by the original Erw. herbicola strains. In two biocontrol assays, the transformed Esc. coli strains had a suppressive effect on disease development on infected pear fruit slices and hawthorn blossoms. 相似文献
953.
I. C. RUMBOS 《EPPO Bulletin》1997,27(4):463-468
Eutypa canker and dieback of almonds was first recorded in commercial plantations in Greece in mid-1980 and confirmed as due to the ascomycetous fungus Eutypa lata. The disease manifests itself by the formation of cankers at the point of the junction of the branches and the trunk and is associated with pruning wounds made by the growers in order to confront the'non-infectious bud failure'disorder or to remove unwanted limbs close to the crotch. Pathogenicity tests made on young trees of cv. Texas with different isolates originated from grapevine, almond, apricot and pear showed significant differences in virulence. Pathogenicity tests carried out on 20 different almond cultivars gave a significant indication of the existence of tolerant and susceptible cultivars. 相似文献
954.
Dose response of weeds to methyl iodide and methyl bromide 总被引:1,自引:0,他引:1
W. M. ZHANG M. E. McGIFFEN JR J. O. BECKER H. D. OHR J. J. SIMS R. L. KALLENBACH 《Weed Research》1997,37(3):181-189
Labonuory bioassay and field experiments were conducted to characterize the dose response of weeds to methyl iodide and methyl bromide as soil fumigants. The patterns in potency of both fumigants and in sensitivity of diffcretit weed species to the fumigants were distinguished with the use of logistic dose-response models. Similar to its response to methyl hromide fumigation. Amaranthus retrofleus L. was the most sensitive to methyl iodide fumigation. Cyperus rotundtis L. was the least sensitive to methyl iodide fumigation, whereas Portuloca oleracea L. was the least sensitive to methyl hromide. Lolium multiflorum Lam. Abutilon theophrasti Medik. Chenopodium album L. P. ateracea . Brassica kaber (D.C.) L.C. Wheeler and Cyperus escuden-tus L. were similar in sensitivity to methyl iodide. Methyl iodide was as potent as methyl bromide for A. retroflexus but more potent than methyl bromide for L. multiflorum , A. theophrasti. C. album. P. oleracea. B. kaber, C. esculentus and C. rotundus. The dose response for weeds in the field was similar to that obtained in laboratory bioassays. Under fieid conditions. 280 kg ha-1 methyl iodide killed all species tested except Solanum nigrum L Methyl iodide appears to be a suitable replacement for meihyl bromide because it can be used in situations simitar to methyl bromide fumigation, has superior efficacy against a broad spectrum of pests and has a low potential for degrading the earth's ozone lavers. 相似文献
955.
The influence of cropping systems constituted by the combination of three input levels (low, intermediate and high) and four 2-year rotations between a spring summer crop and winter wheat ( Triticum aestivum L.) on the density and composition of the winter wheat weed flora was evaluated from 1989 to 1992. Spring summer crops were maize ( Zea mays L.). soyabean ( Glycine max (L.) Merr.). sugar beet ( Beta vulgaris L.) and sunflower ( Helianthus and L.) Total weed density at the end of the wheat tillering stage ranged from 2.5 to 128.7 plants m-2 and generally decreased with cropping intensity, although to a differing extent, depending upon rotation. Differences among weed communities of the 12 cropping systems were mostly dependent upon input level. The effect of the preceding crop on the weed flora composition of winter wheat was evident only with reduced herbicide use (low-input systems). Weed species could be ascribed to four categories: (a) species sporadically associated with cropping systems (most of the weeds); (b) species associated with a specific input level (e.g. Papaver rhoeas L. to low and intermediate input levels and Veronica persica Poiret to the high input ievel): (c) species associated with a specific rotation (e.g. Anm majus L. to sugar beet winter wheat): and (d) species associated with specitie plots that become important every other year (e.g. Convolvulus arvensis L. and Stellaria media (L.) Vill.). 相似文献
956.
Seeds of oilseed rape ( Brassica napus L., cv. Liglandor) were induced into secondary dormancy by imbibing them for 2-4 weeks in an osmotic solution at -1500 kPa in darkness. Treated seeds were then transferred into pots or Petri dishes, where they were given adequate water for germinatton. These pots and Petri dishes were exposed to various temperature regimes during the course of the following 6–14 months. During that time, seed samples were taken at 2- to 6- weekly intervals to monitor survival of seeds and to study their dormancy. The ntimber of seeds that had not germinated in pots or Petri dishes remained constant in the majority of treatments. but it declined when seeds were exposed to pronounced daily temperature alternations. This decline was due to germination and not to death of seeds from other causes. Germination tests on subsamples of seeds showed that they were light sensitive, more seeds germinating in the light than in the dark. The seeds'germination behaviour differed between samphng dates. This variation in germination behaviour appeared not to display the classical dormancy:non-dormancy cycle. Part of the variation seemed to be due to the changes in temperature that the seeds experienced when taken from pots and set up in the germination tests. Seeds exposed to a constant temperature for 1 year appeared lo become more dormant with time. The relevance of this work in relation to the persistence of volunteer oilseed rape is discussed. 相似文献
957.
958.
959.
Landscape metrics with ecotones: pattern under uncertainty 总被引:1,自引:0,他引:1
960.
Characterizing historical and modern fire regimes in Michigan (USA): A landscape ecosystem approach 总被引:4,自引:0,他引:4
Cleland David T. Crow Thomas R. Saunders Sari C. Dickmann Donald I. Maclean Ann L. Jordan James K. Watson Richard L. Sloan Alyssa M. Brosofske Kimberley D. 《Landscape Ecology》2004,19(3):311-325
We studied the relationships of landscape ecosystems to historical and contemporary fire regimes across 4.3 million hectares in northern lower Michigan (USA). Changes in fire regimes were documented by comparing historical fire rotations in different landscape ecosystems to those occurring between 1985 and 2000. Previously published data and a synthesis of the literature were used to identify six forest-replacement fire regime categories with fire rotations ranging from very short (<100 years) to very long (>1,000 years). We derived spatially-explicit estimates of the susceptibility of landscape ecosystems to fire disturbance using Landtype Association maps as initial units of investigation. Each Landtype Association polygon was assigned to a fire regime category based on associations of ecological factors known to influence fire regimes. Spatial statistics were used to interpolate fire points recorded by the General Land Office. Historical fire rotations were determined by calculating the area burned for each category of fire regime and dividing this area by fifteen (years) to estimate area burned per annum. Modern fire rotations were estimated using data on fire location and size obtained from federal and state agencies. Landtype Associations networked into fire regime categories exhibited differences in both historical and modern fire rotations. Historical rotations varied by 23-fold across all fire rotation categories, and modern forest fire rotations by 13-fold. Modern fire rotations were an order of magnitude longer than historical rotations. The magnitude of these changes has important implications for forest health and understanding of ecological processes in most of the fire rotation categories that we identified.This revised version was published online in May 2005 with corrections to the Cover Date. 相似文献