全文获取类型
收费全文 | 18127篇 |
免费 | 113篇 |
专业分类
林业 | 3690篇 |
农学 | 1310篇 |
基础科学 | 155篇 |
2951篇 | |
综合类 | 1043篇 |
农作物 | 2134篇 |
水产渔业 | 1862篇 |
畜牧兽医 | 2004篇 |
园艺 | 1144篇 |
植物保护 | 1947篇 |
出版年
2023年 | 12篇 |
2021年 | 23篇 |
2020年 | 34篇 |
2019年 | 41篇 |
2018年 | 2774篇 |
2017年 | 2724篇 |
2016年 | 1219篇 |
2015年 | 85篇 |
2014年 | 44篇 |
2013年 | 78篇 |
2012年 | 861篇 |
2011年 | 2200篇 |
2010年 | 2143篇 |
2009年 | 1292篇 |
2008年 | 1388篇 |
2007年 | 1632篇 |
2006年 | 89篇 |
2005年 | 156篇 |
2004年 | 161篇 |
2003年 | 213篇 |
2002年 | 110篇 |
2001年 | 55篇 |
2000年 | 83篇 |
1999年 | 31篇 |
1998年 | 21篇 |
1997年 | 26篇 |
1996年 | 18篇 |
1993年 | 27篇 |
1992年 | 29篇 |
1991年 | 23篇 |
1990年 | 28篇 |
1989年 | 51篇 |
1988年 | 49篇 |
1987年 | 39篇 |
1986年 | 37篇 |
1985年 | 19篇 |
1984年 | 19篇 |
1983年 | 22篇 |
1982年 | 15篇 |
1979年 | 19篇 |
1978年 | 16篇 |
1977年 | 20篇 |
1976年 | 17篇 |
1974年 | 18篇 |
1973年 | 19篇 |
1972年 | 24篇 |
1971年 | 11篇 |
1970年 | 18篇 |
1969年 | 13篇 |
1968年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
994.
Anthony Winson 《Agriculture and Human Values》2008,25(4):499-511
To understand the phenomenon of the rapidly increasing prevalence of overweight and obese children and youth, it is especially
important to examine the school food environment, the role of structural factors in shaping this environment, and the resulting
nutrition and health outcomes. The paper examines research on school food environments in the US and Canada. It notes evidence
of widespread availability of poor nutrition products in both environments and delineates reasons for the situation, and examines
initiatives presently being undertaken in a number of jurisdictions in both countries to encourage healthy eating in schools.
Empirical data are presented from a pilot study of high schools in the Canadian province of Ontario. The study documents the
extent of student purchasing of nutrient-poor foods and beverages, and the structural factors internal and external to the
school that appear responsible for the availability of such products in food environments in this critical institutional sphere.
The paper also examines positive local initiatives in high schools that seek to encourage healthy eating in schools.
Anthony Winson PhD is a professor in the department of sociology and anthropology at the University of Guelph. He has written on agriculture, food and rural development, and restructuring issues related to Canada and the Third World for more than 20 years. Among his books are The Intimate Commodity: Food and the Development of the Agro-Industrial Complex in Canada (Garamond 1993) and, more recently, Contingent Work, Disrupted Lives (University of Toronto 2002, with Belinda Leach) which examines economic restructuring, the changing world of work, and the factors underlying sustainability in small manufacturing-dependent rural communities in several regions of Ontario. This book won the John Porter Book Prize of the Canadian Sociology and Anthropology Association for 2003. Dr. Winson’s current work is focused on the analysis of factors shaping the contemporary Canadian food environment and their role in producing what has been termed the “epidemic of obesity.” Particular attention is being paid to supermarkets and schools as part of broader-ranging research on the political economic context of the food environment. 相似文献
Anthony WinsonEmail: |
Anthony Winson PhD is a professor in the department of sociology and anthropology at the University of Guelph. He has written on agriculture, food and rural development, and restructuring issues related to Canada and the Third World for more than 20 years. Among his books are The Intimate Commodity: Food and the Development of the Agro-Industrial Complex in Canada (Garamond 1993) and, more recently, Contingent Work, Disrupted Lives (University of Toronto 2002, with Belinda Leach) which examines economic restructuring, the changing world of work, and the factors underlying sustainability in small manufacturing-dependent rural communities in several regions of Ontario. This book won the John Porter Book Prize of the Canadian Sociology and Anthropology Association for 2003. Dr. Winson’s current work is focused on the analysis of factors shaping the contemporary Canadian food environment and their role in producing what has been termed the “epidemic of obesity.” Particular attention is being paid to supermarkets and schools as part of broader-ranging research on the political economic context of the food environment. 相似文献
995.
D H Persing S R Telford P N Rys D E Dodge T J White S E Malawista A Spielman 《Science (New York, N.Y.)》1990,249(4975):1420-1423
In order to investigate the potential for Borrelia burgdorferi infection before the recognition of Lyme disease as a clinical entity, the polymerase chain reaction (PCR) was used to examine museum specimens of Ixodes dammini (deer ticks) for the presence of spirochete-specific DNA sequences. One hundred and thirty-six archival tick specimens were obtained representing various continental U.S. locations; DNA sequences characteristic of modern day isolates of B. burgdorferi were detected in 13 1940s specimens from Montauk Point and Hither Hills, Long Island, New York. Five archival specimens of Dermacentor variabilis (dog tick) from the same collection and 118 Ixodes specimens from other endemic and nonendemic sites were negative. These data suggest that the appearance of the Lyme disease spirochete in suitable arthropod vectors preceded, by at least a generation, the formal recognition of this disease as a clinical entity in the United States. 相似文献
996.
D F White 《Science (New York, N.Y.)》1968,159(811):218-219
Isolated wingless cabbage aphids, Brevicoryne brassicae (L.), produced only wingless young. Aphids isolated from birth all became wingless, whereas over 50 percent of grouped controls developed wings. Isolation caused hyperactivity of the corpus allatum; this hyperactivity may have caused the wingless form. 相似文献
997.
E S Vesell C M Lang W J White G T Passananti S L Tripp 《Science (New York, N.Y.)》1973,179(76):896-897
Reduction of aniline hydroxylase activity, ethylmorphine N-dementhylase activity, and cytochrome P-450 content occurred in hepatic microsomes of rats kept under dirty conditions, defined as accumulation for 1 week of urine and feces in pans under the wire mesh cages. In comparison with rats that had urine and feces removed twice daily from such pans, rats kept over Kimpak bedding or over Litter Green, changed twice daily, also showed reduced drug-metabolizing activity in hepatic microsomes, but to a lesser degree than the dirty rats. Placement of a filter top on cages for 1 week also decreased drug-metabolizing activity. These experiments suggest that the relative cleanliness of an animal's environment can influence hepatic microsomal drug metabolism. 相似文献
998.
Diana Stuart 《Agriculture and Human Values》2008,25(2):177-181
I explore the role of nature in the agrifood system and how attempts to fit food production into a large-scale manufacturing
model has lead to widespread outbreaks of food borne illness. I illustrate how industrial processing of leafy greens is related
to the outbreak of E. coli 0157:H7 associated with spinach in the fall of 2006. I also use this example to show how industry attempts to create the
illusion of control while failing to address weaknesses in current processing systems. The leafy greens industry has focused
efforts on sterilizing the growing environment and adopting new technologies, while neglecting to change the concentrated
structure of processing systems. Repeated breakdowns in these systems illustrate a widening fault line between attempted and
failed control of nature in industrial food production.
相似文献
Diana StuartEmail: |
999.
Shahzad Ali Amanullah Jan Jia Zhikuan Amir Sohail Cai Tie Wei Ting Zhang Peng Manzoor Irshad Ahmad Mati Ur Rahman Ren Xiaolong Liu Xiaoli Xu Yue Yue 《Russian Agricultural Sciences》2016,42(3-4):224-229
A field experiment was conducted on silty clay loam soil in the years 2011–2012. Two sesame (Sesamum indicum L.) cultivars (Local Black and Local White) were evaluated using various 3 different sowing dates (20th June, 10th and 30th July) and four agrotechnical level (0, 40, 80 and 120 kg N ha–1) at New Developmental Farm The University of Agriculture, Peshawar, Pakistan. The objective of this study was to evaluate the effects of sowing dates on growth, yield and oil fatty acid composition of two sesame cultivars grown under different nitrogen fertilization. Results showed that cv. Local Black was characterized by significantly higher content of oil (47%), seed yield (696 kg ha–1) and oil yield (335 kg ha–1) while cv. Local White had higher palmitic acid (8%) and linoleic acid (38.7%). Yield and its main components were positively affected by the earlier sowing date. With regard to fatty acid composition, a decrease in oleic and stearic acid and an increase in linoleic and palmitic acid were observed. At early sowing, oleic and palmitic acid decreased whereas linoleic and stearic acid increased. The decrease in the oleic/linoleic acid ratio observed at early sowing, suggests a possible role of temperature on the activity of oleate desaturase in the developing seeds. Intensive technology of cultivation (120 kg N ha–1), compared to the economical technology (40 kg N ha–1), significantly increased the seed yield of both sesame cultivars. This was due to higher number of branches, Capsules m–2, capsules plant–1, seeds capsule–1 and 1000 seed weight (g). The intensive technology of cultivation had a beneficial effect on the content of palmitic acid, linolenic acid and oleic acid in sesame seed. 相似文献
1000.
Wei Feng Wenying Shen Li He Jianzhao Duan Binbin Guo Yingxue Li Chenyang Wang Tiancai Guo 《Precision Agriculture》2016,17(5):608-627
In this study, we investigated the possibility of using ground-based remote sensing technology to estimate powdery mildew disease severity in winter wheat. Using artificially inoculated fields, potted plants, and disease nursery tests, we measured the powdery mildew canopy spectra of varieties of wheat at different levels of incidence and growth stages to investigate the disease severity. The results showed that the powdery mildew sensitive bands were between 580 and 710 nm. The best two-band vegetation index that correlated with wheat powdery mildew between 400 and 1000 nm wavelength were the normalized spectrum 570–590 and 536–566 nm bands for the ratio index, and 568–592 and 528–570 nm for the normalized difference index. The coefficients of determination (R 2) for both were almost the same. The optimum dual-green vegetation index was constructed based on a calculation of the ratio and normalized difference between the normalized spectrum within the two green bands. The coefficients of determination (R 2) of DGSR (584, 550) (dual-green simple ratio) and DGND (584, 550) (dual-green normalized difference) were both 0.845. The inverse models of disease severity performed well in the test process at the canopy scale, and indicated that, compared with the traditional vegetation indices of Lwidth, mND705, ND (SDr, SDb), SIPI, and GNDVI, the novel dual-green indices greatly improved the remote sensing detection of wheat powdery mildew disease. Following these results, combined disease severity and canopy spectra were shown to be of enormous value when applied to the accurate monitoring, prevention, and control of crop diseases. 相似文献