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   59688篇
  免费   3350篇
  国内免费   295篇
林业   2353篇
农学   2059篇
基础科学   459篇
  8434篇
综合类   8679篇
农作物   2294篇
水产渔业   3006篇
畜牧兽医   31315篇
园艺   705篇
植物保护   4029篇
  2020年   541篇
  2019年   659篇
  2018年   1100篇
  2017年   1224篇
  2016年   1103篇
  2015年   913篇
  2014年   1166篇
  2013年   2627篇
  2012年   1957篇
  2011年   2318篇
  2010年   1605篇
  2009年   1639篇
  2008年   2275篇
  2007年   2088篇
  2006年   1944篇
  2005年   1735篇
  2004年   1602篇
  2003年   1660篇
  2002年   1445篇
  2001年   1959篇
  2000年   1932篇
  1999年   1484篇
  1998年   627篇
  1997年   662篇
  1995年   621篇
  1994年   548篇
  1992年   1051篇
  1991年   1241篇
  1990年   1061篇
  1989年   1079篇
  1988年   1023篇
  1987年   990篇
  1986年   999篇
  1985年   944篇
  1984年   773篇
  1983年   675篇
  1979年   878篇
  1978年   691篇
  1977年   603篇
  1976年   609篇
  1975年   665篇
  1974年   750篇
  1973年   732篇
  1972年   708篇
  1971年   560篇
  1970年   575篇
  1969年   654篇
  1968年   597篇
  1967年   638篇
  1966年   606篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A West Nile virus (WNV) strain belonging to lineage 2 was for the first time detected in two pools of Culex pipiens collected in the province of Udine and in tissues of a wild collared dove (Streptopelia decaocto) found dead in the province of Treviso, in North East of Italy. It was molecularly identified by group and WNV lineage specific RT-PCRs and characterized by partial sequencing of the NS3 and NS5 genes. When compared with the sequences of same fragments of NS3 and NS5 of the WNV lineage 2 strain isolated from birds of prey in Hungary (2004), the phylogenetic analysis of these sequences revealed 100% and 99% similarity, respectively. As the Hungarian strain, the NS3 selected sequence differed from the 2010 Greek isolate by one amino-acid located at 249 site which is the site involved in genetic modulation of WNV pathogenicity. The Italian and Hungarian strains have histidine rather than proline at this site. The presence of a lineage 2 strain in regions where the lineage 1 strain is still circulating, creates a new scenario with unpredictable consequences. In this situation comprehensive investigations on the occurrence, ecology, and epidemiology of these different WNV strains circulating in Italy become the highest priority.  相似文献   
992.
993.
Salmonella and Yersinia are important enteropathogens in poultry and can affect birds of all ages, including embryos. These food-borne zoonotic enteropathogens are of great economic and medical concern worldwide and are intensely studied in poultry. Information regarding the prevalence of these bacteria in wild birds is scarce and biased toward avian species ecologically linked to humans, which have often been incriminated as both reservoirs and disseminators of these enteropathogens. The prevalence of Salmonella and Yersinia recovered from both the feces and eggs in a population of female pied flycatchers (Ficedula hypoleuca) breeding in nest-boxes in central Spain was evaluated. Salmonella arizonae was recovered from the feces of one female but was not recovered from eggs. Yersinia was not detected in either the feces or eggs. These results may suggest that Salmonella and Yersinia may be uncommon in this population studied and may indicate that these birds are unlikely reservoirs of Salmonella and Yersinia.  相似文献   
994.
The morphology of the tongue of the adult barking deer, Muntiacus muntjak, was examined by light and scanning electron microscopy. The result showed that the tongue of the barking deer was elongated with a rounded apex. Four types of lingual papillae were observed: filiform, fungiform, vallate and large conical papillae. The filiform papillae represented the most numerous types of lingual papillae. The fungiform papillae were distributed among the filiform papillae on the rostral and the body portions of the tongue. Ten to thirteen vallate papillae were distributed on both sides of the lingual prominence among the large conical papillae. Histologically, both the fungiform and vallate papillae contain taste buds in the epithelial layer. The distribution and types of lingual papillae found in the barking deer are similar to those in the other species that belong to the family Cervidae.  相似文献   
995.
This retrospective study evaluated the ocular side effects of cancer-bearing dogs and cats treated with external–beam Cobalt-60 (Co-60) radiation in which one or both orbit(s) were included in the radiation field. A total of 37 dogs and 12 cats presented to the Ontario Veterinary College during the 10-year study period (1999–2009) were evaluated. The radiation protocols ranged from a maximum of 60 Gray (Gy) in 24 fractions for curative intent to a minimum of 8 Gy in 1 fraction for palliative treatment. The main ocular side effect reported in both dogs and cats was conjunctivitis (79% and 55%, respectively). Other common ocular side effects included eyelid lesions in dogs (44%), ulcerative keratitis in cats (36%), and keratoconjunctivitis sicca in both dogs and cats (44% and 27%, respectively). The high incidence of ocular side effects in both patient populations indicates a need for regular ophthalmic examinations as a component of routine follow-up for radiation therapy involving the orbit. Radiation damage to ocular tissues is also reviewed.  相似文献   
996.
If real prices for fish remain at the levels they had attained by 1999, in the year 2050 demand for fish and shellfish as food could be of the order of 270 million tons (live weight equivalent) per year. If producers were able to supply these quantities consumption would rise by 176% over this 50-year period. To meet the demand supply would have to expand at the rate of 2.1% annually; but, a review of the pattern of population growth--and of historical patterns of increases in per capita consumption of fish--shows that annual growth in the volume of fish demanded is likely to be largest in the coming two decades, and then to taper off. Will producers be able to deliver? It is clear that wild marine stocks at present harvested by capture fishermen cannot support fisheries that would yield much more than 100 million tons per year and of this amount a significant proportion will continue to be used for fish meal and oil production. The question therefore narrows down to: can aquaculture, or non-traditional marine species, supply the required amounts? The historical context of supply is considerably different from that which has prevailed during the past 30 years. At that time the growing demand in OECD countries was met partly through imports of fish produced in the seas and lakes of developing countries. During coming decades the increased demand in developing countries must be met essentially through their own resources. In fact, in poor countries it seems unlikely that supply will respond to demand unless they experience economic growth.  相似文献   
997.
998.
999.
1000.
Typing of R factors by genetic properties was done with Salmonella typhimurium and Escherichia coli isolated from calves on a feedlot where epizootics of clinical or subclinical calf salmonellosis had repeatedly occurred during 5 years. Forty-nine R factors from S typhimurium were fi- (no fertility inhibition) and spp- (no restriction against phage lambda vir). Twenty-three (46.9%) of them belonged to compatibility group Ialpha and the remainder were nontypable. Fourteen R factors from E coli belonged to different genetic types: fi+ (11=78.6%) and fi- (3=21.4%); spp+ (1=7.1%) and spp- (13=92.9%); compatibility groups FII (5=35.7%), N (1=7.1%), and nontypable (8=57.2%). In contrast to the R factors of S typhimurium, 9 (64.3%) of the 14 R factors of E coli carried resistance against aminobenzyl penicillin with or without kanamycin resistance. The compatibility groups of R factors of S typhimurium seemed to be useful as a subsidiary epizootiologic marker in this feedlot.  相似文献   
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