全文获取类型
收费全文 | 38666篇 |
免费 | 2257篇 |
国内免费 | 285篇 |
专业分类
林业 | 1606篇 |
农学 | 1493篇 |
基础科学 | 337篇 |
5608篇 | |
综合类 | 5680篇 |
农作物 | 1506篇 |
水产渔业 | 1829篇 |
畜牧兽医 | 19955篇 |
园艺 | 463篇 |
植物保护 | 2731篇 |
出版年
2021年 | 371篇 |
2020年 | 431篇 |
2019年 | 474篇 |
2018年 | 794篇 |
2017年 | 843篇 |
2016年 | 763篇 |
2015年 | 637篇 |
2014年 | 790篇 |
2013年 | 1865篇 |
2012年 | 1333篇 |
2011年 | 1558篇 |
2010年 | 1067篇 |
2009年 | 1105篇 |
2008年 | 1494篇 |
2007年 | 1352篇 |
2006年 | 1270篇 |
2005年 | 1097篇 |
2004年 | 1040篇 |
2003年 | 1040篇 |
2002年 | 927篇 |
2001年 | 1258篇 |
2000年 | 1241篇 |
1999年 | 939篇 |
1998年 | 391篇 |
1997年 | 426篇 |
1996年 | 347篇 |
1995年 | 395篇 |
1994年 | 348篇 |
1992年 | 661篇 |
1991年 | 788篇 |
1990年 | 690篇 |
1989年 | 731篇 |
1988年 | 665篇 |
1987年 | 626篇 |
1986年 | 669篇 |
1985年 | 592篇 |
1984年 | 469篇 |
1983年 | 418篇 |
1979年 | 560篇 |
1978年 | 436篇 |
1977年 | 361篇 |
1976年 | 367篇 |
1975年 | 398篇 |
1974年 | 460篇 |
1973年 | 475篇 |
1972年 | 447篇 |
1970年 | 368篇 |
1969年 | 403篇 |
1968年 | 360篇 |
1967年 | 361篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
981.
982.
983.
Distribution and excretion of two chlorinated biphenyl isomers: 4-chlorobiphenyl and decachlorobiphenyl in lactating bovine. 总被引:1,自引:0,他引:1 下载免费PDF全文
One lactating cow was given a 4-chlorobiphenyl (50 mgm/kgm) as a single oral dose and was killed seven days postadministration. The presence of this compound was not detected in tissues, urine and milk. However, 28.9% of the total dose given was recovered in feces during the period of seven days postadministration. Another lactating cow received decachlorobiphenyl orally as single dose (50 mgm/kgm) and was also killed seven days postadministration. This totally chlorinated biphenyl was recovered at low concentrations from all tissues analyzed. The cumulative excretion into milk and feces accounted for 0.025 and 25.02% respectively. Decachlorobiphenyl was detected in urine. 相似文献
984.
1. Incorporating 0, 3, 6 or 9% rapeseed meal in the diet of brown‐egg laying birds for 28 d resulted in the production of 0, 1.2, 19.3 and 20.9% tainted eggs respectively, the first tainted eggs being laid on the fifth day.
2. During the second and third weeks the incidence of tainted eggs exceeded 20% but fell to 11.4% during the final week.
3. Omission of the rapeseed meal from the diets halted the production of tainted eggs.
4. Neither egg production nor the health of the birds was adversely affected by the treatments.
5. The taint was described as “ fishy ” or “ crabby ” and was distinctive, but the source was not identified. 相似文献
985.
986.
987.
Energetic status and body size are inconstant bodily states often considered to have feedback effects on behaviour. Feedbacks can be negative, like starvation‐threshold feedbacks (lower state values = increased risk taking due to higher energy need, and vice versa), or positive, like state‐dependent safety feedbacks (higher state values = lower vulnerability when expressing risky behaviours, and vice versa). Few studies have investigated feedback effects during the early‐life survival bottleneck of territorial species, when rapid growth is particularly important to maintain competitive ability. We investigated effects of body size and energetic status on the emergence latency into an unknown environment in highly territorial brown trout fry. We manipulated energetic status using high and low food rations in different combinations over two consecutive periods. In contrast to results from other small‐sized fishes, no general behavioural effects of energetic state were detected, but larger fish had on average shorter emergence latency. Starvation‐threshold feedbacks were not supported, but the size effect favours the presence of a state‐dependent safety feedback. When individuals were split into clusters characterised by long (>34 s) and short (<34 s) emergence latency, a state‐dependent safety feedback, based on energetic status, was found in the short‐latency cluster. Positive state‐dependent feedbacks may reflect the high level of competition among wild trout fry during the first summer, particularly among the more active individuals (i.e., from the short latency cluster). High state levels could be utilised to gain advantages over lesser competitors (e.g., to establish territories), particularly if the overall risk of starvation is higher than the risk of predation. 相似文献
988.
Riana Gardiner Glen Bain Rowena Hamer Menna E. Jones Christopher N. Johnson 《Landscape Ecology》2018,33(11):1837-1849
Context
The classical theory of island biogeography explains loss of species in fragmented landscapes as an effect of remnant patch size and isolation. Recently this has been challenged by the habitat amount and habitat continuum hypotheses, according to which persistence in modified landscapes is related to total habitat amount rather than habitat configuration or the ability of species to use all habitats to varying degrees. Distinguishing between these theories is essential for effective conservation planning in modified landscapes.Objective
Identify which factors of habitat type, amount and configuration predict the persistence of a keystone woodland specialist, the eastern bettong Bettongia gaimardi, in a fragmented landscape.Method
In the Midlands region of Tasmania we carried out camera surveys at 62 sites in summer and winter. We included habitat and landscape features to model whether habitat amount or patch size and isolation influenced the presence of the eastern bettong, and to measure effects of habitat quality.Results
Habitat amount within a 1 km buffer was a better predictor of occupancy than patch size and isolation. Occupancy was also affected by habitat quality, indicated by density of regenerating stems.Conclusion
Our results support the habitat amount hypothesis as a better predictor of presence. For a species that is able to cross the matrix between remnant patches and utilise multiple patches, the island biogeography concept does not explain habitat use in fragmented landscapes. Our results emphasize the value of small remnant patches for conservation of the eastern bettong, provided those patches are in good condition.989.
Five potato cultivars of autumn crop, dipped in corn oil, were observed for 90 days at room temperature under natural light. Chlorophyll formation took place in minute quantities in treated tubers of ‘Désirée’ and ‘Prima’, and a little more in ‘Primura’, ‘Montana’ and ‘Multa’, whereas all control tubers became completely green after 90 days. Sugars were significantly lower and starch higher in treated tubers than in controls. No sprout-development was observed in treated tubers of all cultivars, but 50–70% control tubers had sprouted during this period. Peeling-losses were minimum in treated tubers and were 67.5–85.4% in controls. The oil-dipped potato tubers of all cultivars could safely be stored for 60 days without quality deterioration under ordinary conditions of temperature and light. 相似文献
990.
Lasmar Chaim J. Queiroz Antônio C. M. Rosa Clarissa Carvalho Nathália S. Schmidt Fernando A. Solar Ricardo R. C. Paolucci Lucas N. Cuissi Rafael G. Ribas Carla R. 《Landscape Ecology》2021,36(9):2583-2597
Landscape Ecology - Harmful effects of habitat loss and fragmentation can be detected across multiple spatial scales, yet most studies that aim to characterize these effects take place at a single... 相似文献