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   3969篇
  免费   341篇
  国内免费   1篇
林业   352篇
农学   158篇
基础科学   31篇
  990篇
综合类   128篇
农作物   281篇
水产渔业   542篇
畜牧兽医   1412篇
园艺   100篇
植物保护   317篇
  2024年   12篇
  2023年   48篇
  2022年   80篇
  2021年   160篇
  2020年   163篇
  2019年   195篇
  2018年   205篇
  2017年   215篇
  2016年   222篇
  2015年   157篇
  2014年   210篇
  2013年   247篇
  2012年   307篇
  2011年   351篇
  2010年   173篇
  2009年   172篇
  2008年   226篇
  2007年   204篇
  2006年   185篇
  2005年   130篇
  2004年   126篇
  2003年   95篇
  2002年   105篇
  2001年   62篇
  2000年   52篇
  1999年   24篇
  1998年   8篇
  1997年   17篇
  1996年   10篇
  1995年   15篇
  1994年   7篇
  1993年   5篇
  1992年   13篇
  1991年   11篇
  1990年   9篇
  1989年   12篇
  1988年   4篇
  1985年   8篇
  1984年   3篇
  1983年   4篇
  1978年   3篇
  1977年   3篇
  1975年   3篇
  1972年   4篇
  1939年   2篇
  1934年   2篇
  1928年   4篇
  1926年   3篇
  1925年   2篇
  1924年   5篇
排序方式: 共有4311条查询结果,搜索用时 15 毫秒
951.
Antibiotics have traditionally been used for growth promotion in the pork industry; however, their use in animal feed has recently been limited because of human health concerns. The intestinal microbiota plays an important role in mediating many physiological functions such as digestion and animal growth. It was hypothesized that use of antibiotics as growth promotants and subsequent variations in intestinal microbiota induce significant changes in the intestinal glycoconjugate composition, which ultimately affects animal growth and disease susceptibility. The aim of this study was to characterize the lectin binding profiles of the ileum of weanling pigs in response to the absence of intestinal microbiota and to the use of the antibiotic chlortetracycline as growth promotant. Eighteen half-sib piglets obtained by cesarean section were divided into 3 treatment groups (n = 6) and maintained as control, antibiotic-fed, and gnotobiotic piglets until 5 wk of age. The glycoconjugate composition of the ileal tissues was examined by lectin histochemistry. Lycopersicon esculentum lectin, Jacalin, Pisum sativum agglutinin, Lens culinaris agglutinin (LCA), and Sambucus nigra lectin showed significant differences (P < 0.05) in binding intensities on the dome and villous epithelium between the treatment groups. Griffonia simplicifolia lectin I, Glycine maxi agglutinin, and Arachis hypogea agglutinin exhibited differences (P < 0.05) between treatment groups in lectin binding on goblet cells. Triticum vulgaris agglutinin, Pisum sativum agglutinin, and LCA showed significant differences (P < 0.05) in binding intensities on dome, corona, and follicular regions of the ileum among treatment groups of animals. Overall, ileal tissues from gnotobiotic piglets expressed significantly weaker (P < 0.05) lectin binding for many lectins compared with control and antibiotic groups. This suggests that the intestinal microbiota plays an important role in the expression of sugar moieties in the intestine. Lectins LCA, Phaseolus vulgaris Leucoagglutinin, and Maackia amurensis lectin II showed significant differences (P < 0.05) in lectin bindings between control and antibiotic-fed piglets. This indicates that chlortetracycline as a growth promotant induces biologically relevant changes in the lectin binding profile of the ileum. These findings will help in further understanding the role of the gut microbiota and the mechanisms of action of antibiotics as growth promotants in pigs.  相似文献   
952.
The aim of this study was to document the effects of mesotherapy in working dogs diagnosed with hip osteoarthritis (OA) and related pain. Ten police working dogs with hip OA and related pain were treated with a combination of lidocaine, piroxicam, and thiocolchicoside, injected in multiple intradermal points. Seven treatment sessions were conducted. The Canine Brief Pain Inventory (CBPI) and the Hudson Visual Analogue Scale (HVAS) were used in the assessment of response to treatment compared to evaluation before treatment (T0), after 15 d, 30 d, 60 d, 90 d, 120 d, 150 d, and 180 d after initial treatment. Results were compared using the Wilcoxon signed-rank test. Significant differences were experienced in CBPI scores comparing moments with T0: at 15 d (P = 0.03 for Pain Interference Score — PIS) and P = 0.02 for Pain Severity Score — PSS), 30 d (P < 0.05 for PIS and P < 0.05 for PSS), 60 d (P = 0.04 for PIS and P = 0.01 for PSS) and 180 d (P = 0.04 for PSS). Individual treatment results were considered successful in 40% of animals at 15 d and 30 d, 66.7% at 60 d, 44% at 90 d, 37.5% at 120 d, and 25% at 150 d. The HVAS scores showed no significant differences. Mesotherapy may be an option for the treatment of canine musculoskeletal-related pain. Further studies are required.  相似文献   
953.
The Haemophilus parasuis aroA gene encodes 5-enolpyruvylshikimate-3-phosphate synthase and participates in the aromatic amino acids and the folic acid universal metabolic pathway of bacteria. The application of aroA-based PCR-RFLP methodology yields a significant degree of diversity in H. parasuis and Actinobacillus species. PCR amplification of the aroA gene rendered a 1,067-bp fragment in all 15 H. parasuis serovars, and also in Actinobacillus pleuropneumoniae serotypes 1-12, Actinobacillus lignieresii, Actinobacillus equuli, Actinobacillus porcinus, Actinobacillus rossii, Actinobacillus suis, Actinobacillus ureae, Actinobacillus minor and Actinobacillus indolicus. Sau3AI and RsaI digestions of the aroA PCR products rendered seven different restriction fragment length polymorphism (RFLP) patterns: group I (H. parasuis serovars 1, 2, 4-6, and 8-15, A. porcinus and A. ureae), group II (H. parasuis serovars 3 and 7, and A. pleuropneumoniae serotypes 1, 4, 5, 9, 11 and 12), group III (A. lignieresii), group IV (A. pleuropneumoniae serotype 7), group V (A. pleuropneumoniae serotypes 2, 3, 6 and 8, A. equuli, A. rossii, A. minor and A. indolicus), group VI (A. suis) and group VII (A. pleuropneumoniae serotype 10). This is the first report describing the presence of aroA gene in H. parasuis, A. lignieresii, A. porcinus, A. rossii, A. suis, A. ureae, A. minor and A. indolicus and the data presented here demonstrates a significant degree of aroA genetic diversity in H. parasuis and species of the genus Actinobacillus.  相似文献   
954.
955.
Due to its remarkable sediment pollution caused by past industrial activities, the Mediterranean coastal area facing the town of Cogoleto (near Genoa) has been declared of “national interest”. Seven sediment cores were analysed to provide information on history and trend of heavy metal inputs, with a particular focus on Cr. Grain-size compositions and 210Pb chronologies account for a strong influence of terrigenous inputs near the coast and the mouth of the Lerone Creek (draining the industrial settlement), with sand contents and accumulation rates varying from 40.3% and 0.7 cm year?1 near shore to 2.5% and 0.1 cm year?1 offshore. Heavy metal concentrations are high, notably Cr and Ni, and to a lesser extent, Ag, Hg, Pb, Cu and Zn. Cr, that shows past values up to 3,642 µg g?1, presents only minor recent decreases. In the area, it has both natural and anthropogenic sources and their relative contributions were distinguished on the basis of Cr/Ni ratios. These are 1–1.5 in soils and rocks of the Lerone Creek catchment area but reach very high values (up to 10) in marine sediments due to the anthropogenic contribution. The anthropogenic influence is higher near the creek mouth, gradually decreases offshore and is negligible at the most distal site (ca. 2.9 km from the coast).  相似文献   
956.
The application of pig slurry to soil can result in the accumulation of soil Cu and Zn, alter soil Cu and Zn fractions, and result in soil, surface water, and subsurface water contamination. This study evaluated the accumulation and distribution of Cu and Zn in soil profiles from 10 areas treated with pig slurry for 3–30 yr. A microwatershed within the municipality of Braço do Norte in the state of Santa Catarina and in the southern region of Brazil was studied. Hapludalf soil samples were collected at depths of 0.0–0.05, 0.05–0.10, 0.10–0.20, 0.20–0.30, and 0.30–0.40 m from 10 areas where pig slurry had been applied for 3–30 yr. The soil samples were dried, prepared, and analyzed to determine their chemical characteristics, particularly to quantify their Cu and Zn fractions. Although long-term pig slurry application resulted in greater available Cu and Zn concentrations in the surface soil layer, the migration of available Cu in the soil profile only occurred in areas that were subjected to more than 17 yr of slurry application. The application of pig slurry increased the available Cu and Zn contents in the surface soil layer; however, the available Cu in the soil profile increased in the areas with more than 17 yr of slurry application. Cu accumulation mainly occurred in the organic and mineral soil fractions, and Zn accumulation mainly occurred in the mineral fraction. Overall, Cu posed a greater risk for surface water contamination than Zn, especially in areas with more than 17 yr of pig slurry application.  相似文献   
957.
本文以京411/红忙春21构建的重组自交系群体(RILs)为研究材料,通过对小麦抽穗期、盛花期和灌浆期旗叶叶绿素含量分布频率的分析,得出叶绿素含量在花后7d以前多呈正态分布,而灌浆中后期呈偏态分布或双峰分布,说明前者为受多基因控制的数量性状,而后者存在与其相关的主效基因。在多个生长发育时期中,旗叶叶绿素含量呈现先上升后下降的趋势,在盛花期达到最大值,花后24d,叶绿素含量则急剧下降,成熟时接近为0。在RILs群体中对小麦细胞分裂素氧化酶基因等位变异进行鉴定,得出TaCKOX4基因在群体中表现较好的多态性,其变异与小麦旗叶多个生长发育时期(如抽穗期,盛花期及灌浆期)叶绿素含量呈显著或极显著相关,且具有A带型家系的叶绿素含量显著高于B带型家系的叶绿素含量,说明TaCKOX4基因变异与小麦旗叶叶绿素含量关系密切。  相似文献   
958.
Combining deep tillage (DT) with lime application at greater depths may improve sugarcane yield. The objective of this study was to evaluate the impact of conventional tillage (CT) and DT systems and liming on sugarcane productivity and soil physical attributes. The experiment was conducted in a clayey-textured Rhodic Hapludox soil cultivated with sugarcane for two growing seasons (first and third ratoons) using a randomized block design with four treatments and four repetitions. The treatments consisted of DT without liming (DT0), DT with liming (DT2), CT without liming (CT0) and CT with liming (CT2). In addition to sugarcane stalk and sugar yields, macroporosity (MA), microporosity (MI), total soil porosity (TP), weighted average diameter (WAD), aggregate stability index (ASI), soil penetration resistance (SPR), pH, potential acidity (H + Al), total soil organic carbon (SOC), particulate organic carbon (POC), mineral-associated organic carbon (MOC), calcium (Ca) and magnesium (Mg) were analysed. DT2 promoted the best soil conservation effect on sugarcane. In general, DT, regardless of lime application, reduced SOC and POC in soil surface layer. Nevertheless, in the long term, localized lime addition resulted in significant reductions in compaction, reaching values below 2 MPa. The soil fertility improvement provided by DT2 promoted increased sucrose concentrations and stalks yield. Considering that there was an improvement in the physical quality and fertility of soil for better plant development, the deep tillage with localized lime addition can be considered an effective alternative for sugarcane cultivation.  相似文献   
959.
The present study investigated the effects of isopulegol, a monoterpene alcohol, in PTZ-induced convulsions and verified possible involved mechanisms. Saline, isopulegol or diazepam were intraperitonealy injected 30 min before PTZ. The latency for development of convulsions and mortality, as well as the mortality protection percentage was recorded. For investigating the involvement of GABAergic system, flumazenil was utilized. The activity of antioxidant enzyme catalase as well as the levels of reduced glutathione and lipid peroxidation were measured in brain hippocampus. Similarly to diazepam, isopulegol significantly prolonged the latency for convulsions and mortality of mice. All animals were protected against mortality at higher dose of isopulegol. Flumazenil pretreatment decreased the prolongation of seizure latency induced by both diazepam and isopulegol, although it was not able to reverse the latency and protection percent for mortality. Isopulegol also significantly prevented PTZ-induced increase in lipid peroxidation, preserved catalase activity in normal levels, and prevented the PTZ-induced loss of GSH in hippocampus of mice. These results suggest that the anticonvulsant and bioprotective effects of isopulegol against PTZ-induced convulsions are possibly related to positive modulation of benzodiazepine-sensitive GABAA receptors and to antioxidant properties.  相似文献   
960.
Heat treatment of Pinus pinaster and Eucalyptus globulus woods, two important species in Portugal, was performed in the absence of air by steaming, inside an autoclave, for 2–12 h at 190–210°C. Mass losses increased with treatment time and temperature reaching 7.3% for pine and 14.5% for eucalypt wood. The wood behaviour with moisture was improved. The equilibrium moisture content decreased by 46% for pine and 61% for eucalypt, the dimensional stability increased (maximum anti-shrinking efficiency in the radial direction of 57 and 90% for pine and eucalypt, respectively) and the surface wettability was lowered. In relation to mechanical properties, the modulus of elasticity was little affected (maximum decrease of 5% for pine and 15% for eucalypt) but the bending strength was reduced (by 40% at 8% mass loss for pine and 50% at 9% mass loss for eucalypt wood). The variation of properties was related to treatment intensity and mass loss but significant improvements could already be obtained for a 3–4% mass loss without impairing the mechanical resistance. The response of eucalypt was higher than that of pinewood. Heat treatment of eucalypt wood shows an interesting potential to improve the wood quality for solid timber products.  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