全文获取类型
收费全文 | 1051篇 |
免费 | 50篇 |
专业分类
林业 | 124篇 |
农学 | 24篇 |
基础科学 | 2篇 |
188篇 | |
综合类 | 124篇 |
农作物 | 42篇 |
水产渔业 | 50篇 |
畜牧兽医 | 484篇 |
园艺 | 11篇 |
植物保护 | 52篇 |
出版年
2021年 | 14篇 |
2020年 | 19篇 |
2019年 | 12篇 |
2018年 | 17篇 |
2017年 | 19篇 |
2016年 | 20篇 |
2015年 | 11篇 |
2014年 | 11篇 |
2013年 | 40篇 |
2012年 | 35篇 |
2011年 | 35篇 |
2010年 | 32篇 |
2009年 | 28篇 |
2008年 | 44篇 |
2007年 | 38篇 |
2006年 | 34篇 |
2005年 | 34篇 |
2004年 | 40篇 |
2003年 | 39篇 |
2002年 | 16篇 |
2001年 | 18篇 |
2000年 | 19篇 |
1999年 | 22篇 |
1998年 | 12篇 |
1994年 | 11篇 |
1992年 | 20篇 |
1991年 | 30篇 |
1990年 | 15篇 |
1989年 | 29篇 |
1988年 | 10篇 |
1987年 | 21篇 |
1986年 | 22篇 |
1985年 | 20篇 |
1984年 | 12篇 |
1982年 | 9篇 |
1980年 | 9篇 |
1979年 | 15篇 |
1978年 | 13篇 |
1977年 | 8篇 |
1976年 | 9篇 |
1975年 | 8篇 |
1974年 | 15篇 |
1973年 | 11篇 |
1972年 | 14篇 |
1970年 | 20篇 |
1969年 | 11篇 |
1968年 | 14篇 |
1967年 | 21篇 |
1966年 | 9篇 |
1936年 | 7篇 |
排序方式: 共有1101条查询结果,搜索用时 15 毫秒
201.
Organic farming has gained in importance in Germany during recent years. Therefore an increasing demand exists for varieties
with specific adaptation to this farming system. In the present study we therefore conducted comparative field experiments
with modern maize breeding materials under organic versus conventional farming conditions (ORG and CON, respectively) to estimate
quantitative genetic parameters needed for developing optimal breeding strategies and to investigate the perspectives of selection
for specific adaptation to ORG. Starting from two broad samples of elite germplasm, consisting of 178 flint and dent lines,
respectively, fractions of 11 flint and 11 dent lines were selected based on their testcross performance under ORG. A corresponding
set of lines was selected under CON. Testcross performance was evaluated in three regions of Germany and selection of superior
lines was practiced across two stages in 2004 and 2005, respectively. The specifically selected lines were crossed in a factorial
manner for production of experimental inter-pool single-cross hybrids which were field-tested under ORG and CON in two regions
in 2006. Average grain yields were about 16% lower under ORG than under CON. Variance components and entry-mean heritability
coefficients under ORG largely resembled those obtained under CON. Phenotypic correlations between ORG and CON were moderate
for grain yield and strong for grain dry matter content. No consistent estimates were obtained for the corresponding genotypic
correlation for grain yield. At the first stage of testcross selection no evidence of specific adaptation to ORG or CON was
observed whereas the factorial crosses tested in 2006 displayed distinct, yet non-significant, advantages when evaluated under
the respective target farming system. A small top fraction of hybrids showed outstanding performance under both ORG and CON.
The chances of detecting such broadly adapted genotypes are increased if ORG test sites are included in the regular testing
system. 相似文献
202.
Herb-layer diversity in deciduous forests: Raised by tree richness or beaten by beech? 总被引:3,自引:0,他引:3
Andreas Mlder Markus Bernhardt-Rmermann Wolfgang Schmidt 《Forest Ecology and Management》2008,256(3):272
Where natural production capacity permits, modern silvicultural management in Central Europe frequently aims at the development of mixed broadleaved stands, instead of pure European beech (Fagus sylvatica) stands. It is crucial to study the effects of these tree-layer diversity variations on herb-layer vegetation, since herb-layer vegetation contributes significantly to ecosystem functioning in forests. In Hainich National Park (Thuringia, Germany), we conducted observational research in deciduous stands to investigate whether herb-layer diversity was related to canopy-layer diversity, and to ascertain possible causal mechanisms. We found that herb-layer vegetation of deciduous forest stands rich in canopy species appeared to be more diverse than herb-layer vegetation of beech-dominated stands. We surmise that herbaceous understorey diversity was indirectly influenced by canopy tree species through the medium of the altered environmental factors soil pH and litter layer thickness. Apparently, lower beech proportion had a more profound effect than the number of secondary tree species. There were no correlations between herb-layer diversity and light transmissibility of the canopy layer, indicating that the light factor was not crucial for herb-layer diversity. At least for the Hainich research sites, our results indicated that small-scale light and soil heterogeneity is insignificant for herb-layer diversity. We found several herb-layer species whose occurrence was particularly correlated with tree-layer diversity and environmental factors. Remarkably, all species positively correlated with soil pH were important for the phytosociological classification of the research sites. Beech-dominated research sites showed high tree-layer volumes, whereas research sites with high tree-layer diversity tended to feature lower tree-layer volumes. These findings could be the result of differing former silvicultural systems and varying soil clay contents affecting tree species composition. In contrast, herb-layer biomass was positively correlated with tree-layer diversity. Herb-layer productivity might be promoted in more diverse research sites by increased nutrient supply and base saturation. It is also possible that greater beech proportion interfered with herb-layer productivity. However, herb-layer biomass was also positively correlated with herb-layer diversity. Hence, our study hints that positive diversity-functioning relationships might occur in the herb-layer of the deciduous forest under investigation. 相似文献
203.
Soil disturbance by heavy machinery usually causes a decrease in porosity and an increase in soil strength, which may limit access to nutrients and compromise seedling survival and growth. This study used a soil strength and a greenhouse experiment to assess the impact of compaction on lodgepole pine (Pinus contorta Dougl. Ex. Loud. var. latifolia Engelm.) seedling growth and the degree to which soil water influences the effects of compaction. A silt loam soil was collected from a forest landing in the central interior of British Columbia (BC) in the Sub-Boreal Spruce Biogeoclimatic zone. The silt loam soil was used in a soil strength experiment where soil with four water content levels (0.10, 0.18, 0.27, and 0.36 cm3 cm−3) was packed into 0.21 cm3 cores with three levels of compaction (74, 79, and 84% of maximum bulk density (MBD)). Soil strength was strongly affected by compaction and water content. In the greenhouse experiment, three water content levels (0.10–0.15, 0.20–0.30, and 0.30–0.35 cm3 cm−3) and three levels of compaction (67, 72, and 76% of MBD) were applied to soil in pots and 1-year old lodgepole pine seedlings were grown in the pots. Soil strength was highest (1275 kPa) for the high compaction and dry water content treatment in the greenhouse experiment. Though the soil strength for this treatment did not exceed 2500 kPa, the effect of compaction on growth was noticeable, with a decrease in diameter growth, total shoot mass, and new root mass as compaction increased at the dry water content. At dry water content and high compaction, foliar nutrient concentrations were greatest. Generally, water content had a greater impact on seedling growth than did compaction, at the levels of compaction used in this study. This study indicates that if there is a critical value for mechanical impedance of the conifer roots, it likely occurs below 2500 kPa. Our results are consistent with the explanation that soil strength incrementally affects root growth below 2500 kPa for this soil type. Expensive rehabilitation techniques may not be needed on lightly disturbed soils similar to that used in this study if soil water content is high enough throughout the conifer growing season to alleviate the effects of compaction on soil strength. 相似文献
204.
Constituents of Eulophia petersii. 总被引:1,自引:0,他引:1
The isolation of five known phenanthrenes and a mixture phytosterols from roots of Eulophia petersii is reported. 相似文献
205.
206.
G. D. Schmidt 《Forstwissenschaftliches Centralblatt》1951,70(11):641-665
Ohne ZusammenfassungMit 4 Abbildungen 相似文献
207.
Dr. H. Schmidt 《Journal of pest science》1944,20(1-3):10-12
Ohne Zusammenfassung
Mit 3 Abbildungen 相似文献
208.
J. H. Heckman H. G. Zuckerman C. H. Metzger W. B. Ward John S. Gardner Frank W. Hussey Ralph W. Donaldson A. G. Tolaas K. H. Fernow E. V. Hardenburg W. G. Been Robert Schmidt E. M. Gillig E. B. Tussing K. W. Lauer K. H. Klages Brooks D. Drain H. L. Bailey J. G. Milward 《American Journal of Potato Research》1935,12(7):196-204
209.
J. H. Heckman H. G. Zuckerman C. H. Metzger B. A. Brown E. R. Bennett W. B. Ward C. L. Fitch O. H. Elmer Jhon S. Gardner Julan C. Miller Frank W. Hussey E. L. Newdick Ralph W. Donaldson Lionel Harris H. O. Werner Wm. H. Martin E. V. Hardenburg T. E. Odland Robert Schmidt Geo E. Prince Brooks D. Drain Harold L. Bailey G. S. Ralston Chase D. Gaines J. G. Milward John Tucker 《American Journal of Potato Research》1935,12(6):158-170
210.