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   60306篇
  免费   3417篇
  国内免费   36篇
林业   2425篇
农学   1851篇
基础科学   301篇
  6616篇
综合类   10220篇
农作物   2087篇
水产渔业   2798篇
畜牧兽医   33012篇
园艺   705篇
植物保护   3744篇
  2018年   895篇
  2017年   912篇
  2016年   808篇
  2015年   773篇
  2014年   839篇
  2013年   2575篇
  2012年   1670篇
  2011年   2019篇
  2010年   1269篇
  2009年   1250篇
  2008年   1879篇
  2007年   1846篇
  2006年   1727篇
  2005年   1580篇
  2004年   1501篇
  2003年   1543篇
  2002年   1470篇
  2001年   1809篇
  2000年   1783篇
  1999年   1462篇
  1998年   620篇
  1997年   606篇
  1996年   546篇
  1995年   641篇
  1994年   600篇
  1993年   571篇
  1992年   1221篇
  1991年   1369篇
  1990年   1258篇
  1989年   1265篇
  1988年   1164篇
  1987年   1137篇
  1986年   1194篇
  1985年   1194篇
  1984年   960篇
  1983年   900篇
  1982年   582篇
  1979年   891篇
  1978年   701篇
  1977年   694篇
  1976年   711篇
  1975年   714篇
  1974年   866篇
  1973年   842篇
  1972年   812篇
  1971年   727篇
  1970年   769篇
  1969年   733篇
  1968年   650篇
  1967年   656篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Desert sheep experimentally or naturally infected with Fasciola gigantica were used to study the influence of infection on the activities of some drug-metabolizing enzymes found in the liver. The enzymes investigated were aminopyrine N-demethylase, aniline 4-hydroxylase and UDP-glucuronyltransferase. The experimental infection was confirmed histologically by detection of Fasciola eggs in faeces and by measuring the activities of sorbitol dehydrogenase (SD), glutamate dehydrogenase (GD) and aspartate aminotransferase (AST) in plasma during the course of the disease. Liver specimens from naturally infected sheep were obtained from the slaughter house. The activities of aminopyrine N-demethylase and aniline 4-hydroxylase were significantly decreased in sheep either naturally infected or during the acute stage of experimental fascioliasis (killed 5 weeks post-infection). The activity of UDP-glucuronyltransferase was decreased in naturally infected sheep and those killed 9 or 13 weeks post-experimental infection.  相似文献   
992.
Four mature pony geldings weighing an average of 134 kg and fitted with ileal cannulas were used in a 4×4 Latin square experiment to determine the digestibility of soybean meal (SBM) protein in different segments of the equine digestive tract at various levels of protein intake. A complete basal corn-based diet was supplemented with SBM to formulate four diets with increasing crude protein. The diets, labeled A (basal), B, C and D, contained 4.9%, 9.5%, 14% and 16.5% crude protein (as fed), and provided nitrogen, per feeding, at approximately 44.8, 84.3, 123.9 and 146.3 mg/kg of body weight, respectively. Chromic oxide was fed to measure ileal flow and fecal excretion. Digestion and absorption of nitrogen was determined from changes in nitrogen: chromium ratios, and true digestion of nitrogen was computed by regression analysis. True total tract digestion of nitrogen was 95.7%. True digestibility of nitrogen in the small intestine over the range of linearity was 72.2%, while true digestibility of nitrogen reaching the large intestine was 89.8%. These data indicate that the protein in SBM was almost completely digested in the equine digestive tract. Furthermore, approximately 75% of the digestible protein was digested prececally when nitrogen intake was less than approximately 125 mg/kg of body weight per feeding.  相似文献   
993.
Biocontrol capacity of two plant growth-promoting rhizobacteria (PGPR) strains, against blast disease in rice paddy fields in Southern Spain was studied in three cropping seasons. Both strains (Pseudomonas fluorescens Aur 6 and Chryseobacterium balustinum Aur 9) had already shown biocontrol capacity against pathogens, ability to induce systemic resistance against leaf pathogens and against salt stress in different plant species. Bacterial treatments were carried out on seeds and/or on leaves. Strains were inoculated individually and in combination. Protection against natural disease incidence was evaluated, and rice production and quality measured in 2005 and 2006 trials. In 2004, natural disease incidence was low (between 0.1% and 0.35% of damaged leaf surface) due to environmental conditions; under these conditions, both strains significantly protected plants against rice blast. In 2005, disease incidence was higher than in 2004, reaching higher values of affected leaf surface in controls. In these conditions, each strain individually protected rice against rice blast, although the combination of both strains was the most effective treatment. All three treatments (Aur 6, Aur 9 and Aur 6 + Aur 9) reached 50% protection in panicles, with Aur 9 being the most effective. In 2006, the most effective treatment was the combination of both strains on leaves in three physiological stages, suggesting a biocontrol mediated protection. On the other hand, when bacteria were applied to seeds, disease incidence decreased up to 50%, suggesting induction of systemic resistance. Finally, a direct relation between protection mediated by the PGPR and the increase in rice productivity (mT/ha) and quality (weight of 1000 seeds and number of intact grains after milling) was found.  相似文献   
994.
A geometrical model of weft knitted spacer structures made with mono-filament yarn has been analysed to understand the spacer yarn path. Theoretical models have been created to predict the porosity and the radius of the capillaries of a knitted spacer structure depending on their geometrical parameters, such as course spacing, wale spacing, stitch length, fabric thickness, count of yarn and fibre density. Polyester knitted spacer fabrics were produced with different parameters; their porosity was determined by measuring the weight and compared with the theoretical porosity. The validity of the model was confirmed by experimental results. The porosity of knitted spacer structures made out of mono-filament yarn can be maintained above a certain level by adjusting the fabric parameters such as fabric thickness, course spacing and wale spacing.  相似文献   
995.
996.
997.
The growth rate of grass carp fed with hybrid napier was found to be nearly three times the growth attained with Hydrilla and about five times the growth attained with Ceratophyllum. The gross conversion ratio on fresh weight basis with hybrid napier was on average 26.99 : 1, while with Hydrilla and Ceratophyllum it was 93.98 : 1 and 128.43 : 1 respectively. The average net production of grass carp fed on hybrid napier worked out to 1579.3 kg/ha in 182 days.  相似文献   
998.
A 548 bp partial cDNA fragment of 17β-hydroxysteroid dehydrogenase (17β-HSD1) was obtained by RT-PCR from the ovary of Nile Tilapia. The expression of 17β-HSD1 was high from 0 to 11 days after spawning, but there was a sharp decline at the day of spawning (day 14) indicating its involvement in ovarian cycle.  相似文献   
999.
This paper provides data concerning biochemical composition of milt of two sturgeon species, Siberian sturgeon bred in aquaculture facility of Inland Fisheries Institute in North Poland and sterlet (from two different populations from Danube and Odra). Milt plasma of Siberian sturgeon and sterlet, when compared to teleost fish, is characterized by much lower osmolality (up 70 to 96 mOsm kg−1) and protein concentration (0.24–0.58 g l−1). In spite of the presence of an acrosome and acrosin the antiproteinase activity of seminal plasma was low (12.79–25.40 U l−1). Activities of arylsulfatase and β-N-acetylglucosaminidase were found in spermatozoa. This agrees with the presence of an acrosome in sturgeons sperm. Similarly to mammals, these enzymes are also present in milt plasma. We determined a range of enzymatic activities from the minimal (seminal plasma) to the maximal damage (supernatants obtained after freezing-thawing without cryoprotectant). Activities of arylsulfatase, β-N-acetylglucosaminidase, lactic dehydrogenase and acid phosphatase were released from spermatozoa after freezing-thawing. For this reason they are good potential candidates as a markers of cryoinjury to sperm acrosome and midpiece. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
1000.
The dietary supplementation of prebiotics, probiotics and symbiotic in hybrid surubins (a Pseudoplatystoma corruscans and P. fasciatum cross) was evaluated for the effects on their autochthonous intestinal microbiota and on haematological and immunological parameters. A total of 160 fish were divided into four treatment groups with four replicates each. The treatment groups were fed with the following diets for 15 days: control diet without supplementation; 0.5% inulin (prebiotic) supplementation; Weissella cibaria (CPQBA 001‐10 DRM 02) (7.87 ± 0.2 log CFU g?1) supplementation; or 0.5% inulin and W. cibaria supplementation (symbiotic group). The midgut intestines of the fish with the symbiotic diet supplementation had higher concentrations of lactic acid bacteria (7.07 ± 1.11 log CFU g?1) and low levels of Vibrio spp (1.90 ± 0.60 log CFU g?1) and Pseudomonas spp (2.23 ± 1.48 log CFU g?1). In addition, increased erythrocytes and reduced circulating neutrophils were observed in this group. No differences in blood glucose, serum protein or lysozyme levels were detected between treatment groups. However, a higher concentration of total immunoglobulin was observed in fish fed with the probiotic and symbiotic diets. The addition of 0.5% inulin (prebiotic) thus W. cibaria (probiotic) to the diet of Pseudoplatystoma hybrid surubins reduce the number of pathogenic bacteria and stimulate the beneficial intestinal microbiota and may possibly alter their immune defence system.  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