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   1170篇
  免费   80篇
林业   82篇
农学   29篇
基础科学   1篇
  157篇
综合类   105篇
农作物   70篇
水产渔业   152篇
畜牧兽医   523篇
园艺   27篇
植物保护   104篇
  2023年   22篇
  2022年   45篇
  2021年   71篇
  2020年   67篇
  2019年   68篇
  2018年   54篇
  2017年   52篇
  2016年   52篇
  2015年   32篇
  2014年   54篇
  2013年   57篇
  2012年   71篇
  2011年   72篇
  2010年   42篇
  2009年   31篇
  2008年   54篇
  2007年   59篇
  2006年   32篇
  2005年   33篇
  2004年   32篇
  2003年   25篇
  2002年   20篇
  2001年   8篇
  2000年   21篇
  1999年   20篇
  1998年   5篇
  1997年   5篇
  1995年   4篇
  1991年   3篇
  1990年   5篇
  1989年   5篇
  1988年   6篇
  1987年   3篇
  1986年   5篇
  1985年   8篇
  1984年   3篇
  1983年   6篇
  1982年   6篇
  1980年   3篇
  1979年   3篇
  1971年   4篇
  1970年   6篇
  1969年   6篇
  1967年   5篇
  1902年   3篇
  1900年   5篇
  1898年   2篇
  1897年   2篇
  1896年   3篇
  1887年   2篇
排序方式: 共有1250条查询结果,搜索用时 859 毫秒
991.
Fisheries and aquaculture production, imports, exports and equitability of distribution determine the supply of aquatic food to people. Aquatic food security is achieved when a food supply is sufficient, safe, sustainable, shockproof and sound: sufficient, to meet needs and preferences of people; safe, to provide nutritional benefit while posing minimal health risks; sustainable, to provide food now and for future generations; shock‐proof, to provide resilience to shocks in production systems and supply chains; and sound, to meet legal and ethical standards for welfare of animals, people and environment. Here, we present an integrated assessment of these elements of the aquatic food system in the United Kingdom, a system linked to dynamic global networks of producers, processors and markets. Our assessment addresses sufficiency of supply from aquaculture, fisheries and trade; safety of supply given biological, chemical and radiation hazards; social, economic and environmental sustainability of production systems and supply chains; system resilience to social, economic and environmental shocks; welfare of fish, people and environment; and the authenticity of food. Conventionally, these aspects of the food system are not assessed collectively, so information supporting our assessment is widely dispersed. Our assessment reveals trade‐offs and challenges in the food system that are easily overlooked in sectoral analyses of fisheries, aquaculture, health, medicine, human and fish welfare, safety and environment. We highlight potential benefits of an integrated, systematic and ongoing process to assess security of the aquatic food system and to predict impacts of social, economic and environmental change on food supply and demand.  相似文献   
992.
Landscape Ecology - Spatial patterns of fire severity are influenced by fire-vegetation patch dynamics and topography. Since the late nineteenth century, fire exclusion has increased fuels and...  相似文献   
993.
The severe yellowing disease (amarelão) on melon plants is a serious problem in Brazil, although the causative agent remained unknown for a long time. Recently, recombinant isolates of cucurbit aphid-borne yellows virus (CABYV) were reported as the possible causative agents of this disease on melon plants. Although aphids are known to be the vectors of the common type of CABYV isolates, almost no aphid colony was observed in the major melon fields in Brazil with high incidence of the severe yellowing disease. In contrast, whiteflies are often abundant. Based on this observation, the hypothesis of the transmission of recombinant CABYV by whiteflies was evaluated. After thorough transmission experiments, we found that this recombinant CABYV isolate was transmitted by the whitefly Bemisia tabaci MEAM1, but not by Aphis gossipii. Furthermore, the host response by whitefly-based inoculation in cucurbits and other indicator plants showed differences in host range when compared to the common type of CABYV. Due to its transmissibility by the whitefly and the distant relationship of the P3/P5 protein to CABYV, the name “cucurbit whitefly-borne yellows virus” is proposed for this recombinant CABYV. This is the second report of polerovirus transmission by the whitefly B. tabaci, following the report of pepper whitefly-borne vein yellows virus.  相似文献   
994.
995.
996.
997.
Prototheca zopfii has been considered one of the most important causes of environmental mastitis in Brazil. These algae are refractory to conventional therapy and cause great damage to the mammary gland. The present study evaluated the in vitro algaecide effect of sodium hypochlorite and iodine based antiseptics on 27 P. zopfii strains isolated from the milk of cattle. Low concentrations of sodium hypochlorite (0.0390625-0.15625%) and iodine (0.15625-0.625%) were effective against the isolates. These antiseptics may be recommended for hygiene routines, pre and postdipping and cauterization of bovine mammary glands infected by P. zopfii.  相似文献   
998.
Leaves of Ilex paraguariensis are used to prepare a tea known as maté which is a common beverage in several South American countries. The ethanol extract was fractionated to identify the compounds responsible for the anti-adipogenic activity in 3T3-L1 cells. Extracts of both fresh and dried maté leaves were subjected to column chromatography using molecular permeation to obtain the saponin (20?% yields) and the polyphenol extracts (40?% yields) from the fresh and dried leaves. The phenolic content was determined using high-performance liquid chromatography analysis and the Folin-Ciocalteau method. Also, maté extracts (50?μg/ml to 1,000?μg/ml) did not display citotoxicity using MTT. The polyphenol extract from the dried leaves was the most effective (50?μg/ml) in the inhibition of triglyceride accumulation in 3T3-L1 adipocytes, and rutin (100?μg/ml) likely accounted for a large portion of this activity. Additionally, maté extracts had a modulatory effect on the expression of genes related to the adipogenesis as PPARγ2, leptin, TNF-α and C/EBPα.  相似文献   
999.
The seed coat is the first host or non-host tissue contacted by bruchids, suggesting its participation in the evolutionary adaptation of bruchids to favor legume seeds. In the present work, we studied the influence of seed coat on the ability of Callosobruchus maculatus (Coleoptera: Bruchidae F.) larvae to penetrate, develop in and survive on seeds. Our results showed that the oviposition, larval eclosion and adult emergence of C. maculatus were drastically reduced in some seeds and that the time necessary for the surviving larvae to perforate the seed coat increased by up to 100% in these seeds. The surviving larvae that crossed the Phaseolus vulgaris (Leguminosae B.) seed coat reached only 55.6% of the mass of a normal larva. The seed coat of some seeds was very toxic to insect larval development. Despite individual variations, seed coats were generally highly restrictive to the development and survival of the bruchid. The study of the seed coat efficiency as a protection tissue against penetration of insects can provide an important tool for new strategies for crop protection. The strengthening of the seed coat defense mechanisms may represent an efficient strategy because the seed chemical defense barrier would be moved to the outer structures and damage to the embryo would be minimized or avoided.  相似文献   
1000.
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