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   62970篇
  免费   3414篇
  国内免费   95篇
林业   4486篇
农学   1892篇
基础科学   285篇
  8071篇
综合类   11224篇
农作物   2329篇
水产渔业   2918篇
畜牧兽医   30464篇
园艺   891篇
植物保护   3919篇
  2020年   520篇
  2019年   577篇
  2018年   1096篇
  2017年   1110篇
  2016年   1059篇
  2015年   923篇
  2014年   1075篇
  2013年   2479篇
  2012年   2150篇
  2011年   2330篇
  2010年   1447篇
  2009年   1448篇
  2008年   2165篇
  2007年   2255篇
  2006年   2108篇
  2005年   1935篇
  2004年   1807篇
  2003年   1786篇
  2002年   1678篇
  2001年   1804篇
  2000年   1750篇
  1999年   1403篇
  1998年   522篇
  1997年   538篇
  1996年   543篇
  1995年   633篇
  1994年   539篇
  1993年   534篇
  1992年   1025篇
  1991年   1026篇
  1990年   1029篇
  1989年   1053篇
  1988年   992篇
  1987年   1001篇
  1986年   1009篇
  1985年   999篇
  1984年   825篇
  1983年   728篇
  1979年   755篇
  1978年   616篇
  1977年   531篇
  1975年   576篇
  1974年   778篇
  1973年   706篇
  1972年   750篇
  1971年   755篇
  1970年   676篇
  1969年   675篇
  1968年   521篇
  1967年   572篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
Feline chronic gingivitis/stomatitis (FCGS) is a painful inflammatory disease in cats. Extraction of teeth, including all premolars and molars, has been shown to be the therapy of choice in cats not responding sufficiently to home care (e.g. tooth brushing) and/or medical treatment (corticosteroids and/or antibiotics). In this study, we hypothesize that a cat food with an omega‐6 polyunsaturated fatty acid (ω6 PUFA) to ω3 PUFA ratio of 10:1 reduces inflammation of the FCGS and accelerates soft tissue wound healing of the gingiva after dental extractions, compared to a cat food with a ω6:ω3 PUFA ratio of 40:1. The cats were fed diets with chicken fat and fish oil as sources of fatty acids. In one diet, part of the fish oil was replaced by safflower oil, resulting in two diets with ω6:ω3 PUFA ratios of 10:1 and 40:1. This double‐blinded study in two groups of seven cats revealed that dietary fatty acids influence the composition of plasma cholesteryl esters and plasma levels of inflammatory cytokines. The diet with the 10:1 ratio lowered PGD2, PGE2 and LTB4 plasma levels significantly, compared to the diet with the 40:1 ratio (p = 0.05, p = 0.04, and p = 0.02 respectively). However, feeding diets with dietary ω6:ω3 PUFA ratios of 10:1 and 40:1, given to cats with FCGS for 4 weeks after extraction of all premolars and molars, did not alter the degree of inflammation or wound healing.  相似文献   
942.
A glasshouse study was undertaken to investigate the effects of the copper fungicide Kocide 101 and its residues in soil on the growth, nodulation and nitrogen fixation of beans (Phaseolus vulgaris L.). The soil used was a sandy clay loam classified as Typic Rhodustalf. The bean variety SUA 90 was used as test crop. The bean rhizobia strains CIAT 899, PV, and a local isolate were used. Kocide 101 applied at the recommended rate (equivalent to 1.7 mg kg‐1 soil) had no significant negative effects on the growth, nodulation or nitrogen fixation of bean plants. Higher levels of Kocide 101 significantly (P < 0.05) reduced plant growth, nodulation and nitrogen fixation. The bean plants inoculated with the “local isolate”; rhizobia had the highest dry matter weights, nodule numbers and nodule dry weights, and also had more N fixation. They were followed by those inoculated with the PV, strain and, lastly, those inoculated with CIAT 899. The growth and nodulation of bean plants were still curtailed by the Kocide 101 residues four months after the fungicide was first applied to the soil. Therefore, occurrence of high levels of Kocide 101 in soils can have long‐term effects on the performance of the bean‐rhizobia symbiosis.  相似文献   
943.
944.
Lipid metabolism in dogs can be divided into exogenous and endogenous pathways and exhibits some unique characteristics compared to other species. Hyperlipidemia is common in dogs, and can be either primary or secondary to other diseases. Secondary hyperlipidemia is the most common form and can be a result of endocrine disorders, pancreatitis, cholestasis, protein-losing nephropathy, obesity, and high fat diets. Primary hyperlipidemia is less common and usually associated with certain breeds. Hypertriglyceridemia of Miniature Schnauzers is the most common type of primary hyperlipidemia in dogs in the United States, and appears to have a genetic basis although its etiology remains unknown. Possible complications of canine hyperlipidemia include pancreatitis, liver disease, atherosclerosis, ocular disease, and seizures. Management is achieved by administration of low fat diets with or without the administration of lipid-lowering agents such as omega-3 fatty acids, gemfibrozil, and niacin.  相似文献   
945.
Circulating antibodies to larval Argas polonicus antigen detected in the blood of pigeons by means of ELISA reach their highest level 3-6 days post-tick attachment. During 6-8 days post infestation when most larvae detach from their host, there is an abrupt drop of the antibody level in blood followed by second peak at day 10-15. During the secondary and subsequent infestations the dynamics of the antibody production is analogous, but the maximum absorbance values found are higher with each following infestation. This is in direct correlation with the growth of immune resistance of hosts. The transfer of immunoglobulins of resistant pigeons produces in naive hosts a partial resistance in a statistically significant (P less than 0.01) reduction of the number of engorged larvae, in the shortening of larval feeding period and in the decrease of their mean weight after feeding. However this resistance was significantly (P less than 0.01) less expressed than in naturally resistant pigeons during secondary infestation. The protracted effect on the duration of premoulting period and the percentage of moulted larvae manifested in larvae after secondary infestation was not apparent in molecular weight of approximately 19, 21, 23, 27, 45 and 165 kilodaltons, were recognized by serum of resistant pigeons.  相似文献   
946.
In recent years, spotting of ray florets of gerbera flowers has become an important problem. This type of small necrotic lesions may occur before, but especially shortly after harvesting the flowers.Botrytis cinerea was easily isolated from such lesions. Inoculation withB. cinerea only gave typical necrotic lesions, when dry conidia were dusted on the flowers with a short period of high rh after inoculation. At 18–25 °C a high rh for at least 5 hours was necessary. Rotting of ray florets and receptacles byB. cinerea occurred when inoculated flowers were kept wet for a few days. Spots consist of one to several necrotic, usually epidermal cells. A single conidium could give rise to a necrotic lesion after germination. Germination of conidia and lesion formation occurred between 4 and 25 °C; at 30 °C, germination and lesion formation did not occur. Between 18 and 25 °C, many lesions became visible within 1 day after inoculation; at 4 °C it took 2 to 3 days before lesions could be seen. If kept dry, conidia ofB. cinerea remained ungerminated on ray florets of gerbera flowers and could be removed from the ray florets. Within 1 day at high rh, germination occurred and lesions were produced. Conidia ofB. cinerea, stored dry, were able to survive much longer than the lifetime of a gerbera flower. Even after storage at room temperature for up to 14 months, some conidia were able to germinate in vitro and on ray florets and induce the formation of lesions. Addition of gerbera pollen diffusate stimulated germination and lesion formation.  相似文献   
947.
The word stability is ubiquitous in colloid science but means different things to different people. In a suspension concentrate it is possible to identify three different physical processes which may lead to instability in a particulate system, namely a change in particle size due to Ostwald ripening, particle aggregation and particle sedimentation. Although these processes may occur simultaneously they are here treated separately in order to promote understanding of the physical forces involved.  相似文献   
948.
Synthesis of a range of 1-, 2-and 3-phenyl, benzyl and phenethyl substituted cyclopentanols and cyclopentylmethyl alcohols and related unsaturated compounds is reported. Results of bioassays of their esters with known pyrethroidal acids against two species of insect lead to recognition of some structure-activity relationships in this series. The distance and relative orientation between the alcoholic and aryl functions appears crucial, but in other respects the nature of the spacer group between them is less important. The most effective alcoholic components are based on cyclopentenes.  相似文献   
949.
The effectiveness of two poison bait formulations was examined in trials against infestations of Mus domesticus in farm buildings. Treatment was extensive on each of four farms, covering all buildings and lasting 1 year. In the four trials, bait containing calciferol and warfarin in combination, or difenacoum was used. Mouse infestation level was monitored, using the mark-recapture trapping technique, before poison-baiting was begun and every 6 weeks thereafter. Poison bait consumption was high initially and mouse numbers were substantially reduced on all four farms within 6 weeks. Subsequently, the difenacoum treatments were the most effective in keeping infestation levels extremely low. Mice still resident when the treatments were terminated were removed by live and kill traps. Laboratory feeding tests were conducted on the live-caught survivors; all animals died after consuming poison bait of the kind used in the trials. Additional live-trapping, 6 months and 1 year later, produced evidence of recolonization and of renewed population growth. It is concluded that sustained control effort is needed to prevent the build-up of M. domesticus numbers in farm buildings.  相似文献   
950.
A technique for virus elimination by meristem culture was developed for a range ofAlstroemeria cultivars. Meristems were excised from the rhizome tips and placed on a medium with indole-3-butyric acid, the required concentration of which was cultivar dependent: Six to eight weeks after dissection a shoot had formed which was transferred to a medium without growth regulators. On filter paper bridges, in a liquid medium, root formation was better than on a solid medium. In many cases a new rhizome developed. If not, the plantlet eventually died. Transfer into soil was more successful with the plantlets rooted in liquid medium than with those rooted in solid medium. Virus elimination was cultivar dependent, but in most cultivars plants resulting negatively in serological tests could be obtained. After repeated testing and selection for horticultural properties these plants may be used to start high quality mother plots.Samenvatting Een techniek werd ontwikkeld om door middel van meristeemcultuur virus te elimineren uit een aantalAlstroemeria cultivars. De meristemen werden uit de toppen van de rhizomen geprepareerd en op een voedingsbodem met IBA als auxine geplaatst. De IBA-concentratie nodig voor scheutvorming was afhankelijk van de cultivar. Na een periode van zes tot acht weken had het meristeem zich ontwikkeld tot een scheutje, dat vervolgens werd overgebracht op een voedingsbodem zonder groeistoffen. In ongeveer twee maanden vormden zich dan wortels. Deze wortelvorming was beter op een vloeibare voedingsbodem met papieren bruggetjes dan op een vaste voedingsbodem van agar. In veel, maar niet in alle gevallen, vormde zich ook een nieuw rhizoom. Indien geen rhizoom werd gevormd stierf de plant. Gewortelde plantjes groeiden beter in grond indien de beworteling op papieren bruggetjes had plaatsgevonden. Het succes van het elimineren van hetAlstroemeria-mozaïek virus hing af van de cultivar. Na herhaalde toetsing kunnen de negatief reagerende planten worden gebruikt voor de opbouw van een partij gezonde moederplanten. Op deze manier kan de kwaliteit van het uitgangsmateriaal vanAlstroemeria worden verbeterd.  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