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   132461篇
  免费   7269篇
  国内免费   93篇
林业   5381篇
农学   4269篇
基础科学   831篇
  17035篇
综合类   21483篇
农作物   5393篇
水产渔业   6688篇
畜牧兽医   67961篇
园艺   1800篇
植物保护   8982篇
  2020年   1177篇
  2019年   1375篇
  2018年   2031篇
  2017年   2330篇
  2016年   2095篇
  2015年   1742篇
  2014年   2224篇
  2013年   5290篇
  2012年   4114篇
  2011年   5093篇
  2010年   3245篇
  2009年   3389篇
  2008年   4971篇
  2007年   4666篇
  2006年   4354篇
  2005年   4088篇
  2004年   3850篇
  2003年   3973篇
  2002年   3542篇
  2001年   4150篇
  2000年   4252篇
  1999年   3372篇
  1998年   1474篇
  1997年   1387篇
  1996年   1288篇
  1995年   1529篇
  1994年   1358篇
  1993年   1231篇
  1992年   2584篇
  1991年   2700篇
  1990年   2591篇
  1989年   2611篇
  1988年   2368篇
  1987年   2348篇
  1986年   2427篇
  1985年   2340篇
  1984年   1881篇
  1983年   1677篇
  1982年   1149篇
  1979年   1748篇
  1978年   1397篇
  1977年   1151篇
  1976年   1212篇
  1975年   1292篇
  1974年   1500篇
  1973年   1514篇
  1972年   1405篇
  1971年   1304篇
  1970年   1251篇
  1969年   1283篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Leaf samples of Lablab purpureus collected from two agroecological zones of Nigeria—the northern guinea savanna zone (NGSZ) and the derived savanna zone (DSZ)—were infected with viruses when serologically indexed against available antisera. Approximately 31.1 and 81.1% of the leaf samples collected from the NGSZ and DSZ, respectively, were infected. Seven viruses were found: Bean common mosaic virus (BCMV), Cowpea aphid-borne mosaic virus (CABMV), Cucumber mosaic virus (CMV), Cowpea mottle virus (CPMoV), Cowpea severe mosaic virus (CPSMV), Southern bean mosaic virus (SBMV) and Tobacco mosaic virus (TMV) were detected from samples collected from NGSZ, while CMV, CPMoV, Cowpea mosaic virus (CPMV) and CPSMV were detected from samples from DSZ.  相似文献   
992.
Three dogs presented for evaluation of acute onset tachypnoea and dyspnoea following episodes of vomiting and/or regurgitation. Thoracic radiographs were suggestive of a gastro‐oesophageal intussusception in all three dogs; one dog also showed evidence of aspiration pneumonia. All three dogs underwent surgical correction with a bilateral incisional gastropexy. All dogs recovered from anaesthesia uneventfully and were discharged from the hospital 3 days after presentation. Persistent megaoesophagus was evident in all three dogs, and they are being chronically managed with a strict feeding regime and pro‐motility agents.  相似文献   
993.
Improved cold tolerance during the juvenile phase is a major breeding goal to develop new sorghum cultivars suitable as an alternative energy crop in temperate regions. The objectives of this study were to identify marker‐trait associations for cold tolerance in a sorghum diversity panel fingerprinted with 2620 single nucleotide polymorphism (SNP) markers and to detect quantitative trait loci (QTL) in two F2:3 populations. Traits of interest were dry matter growth rate (DMGR), leaf appearance rate (LAR), chlorophyll content (SPAD) and chlorophyll fluorescence (Fv′/Fm′ and ФPSII) in relation to temperature. The association panel comprised 194 genotypes, while the F2:3 populations consisted of 80 and 92 genotypes. All populations were tested under controlled conditions in a minimum of four temperature regimes ranging from 9.4°C to 20.8°C. QTL were identified for means across environments and regression parameters describing temperature effects. Several marker‐trait associations for mean (m) DMGR, base temperature (Tb) of SPAD and ФPSII and temperature effect on LAR were validated by QTL detected in population 1 or 2. Promising QTL regions were found on chromosomes SBI‐01, SBI‐02, SBI‐03, SBI‐04, SBI‐06 and SBI‐09, among them genomic regions where candidate genes involved in the C‐repeat binding pathway or encoding cold‐shock proteins are located.  相似文献   
994.
A longitudinal field trial was conducted to determine the effectiveness of strategic anthelmintic treatments in the control of gastrointestinal (GI) nematodes and Fasciola gigantica in cattle. A total of 167 cattle (6–18 months) from three large-scale dairy farms, four traditional farms and nine small-scale dairy farms were randomly selected. The selected animals on each farm were ear tagged and allocated into three groups based on live weight and treated as follows: Group T4 was treated with albendazole 10% drench at 10 mg/kg four times a year (mid rainy, end of the rain, mid dry and late dry/early rainy season). Group T2 was treated with albendazole 10% drench at 10 mg/kg two times a year (mid rainy and late dry/early rainy season). Group UT remained as untreated control. Faecal, blood and pasture samples were taken every month for 13 months. In addition, individual body weight (BWT) was measured on every sampling date. Results showed that two and four strategic treatments significantly reduced faecal egg counts (FEC) by 49.5% and 62.3% respectively compared to untreated control animals (P < 0.001). Two and four strategic treatments per year significantly reduced the proportion of animals passing Fasciola eggs in faeces by 30.6% and 51.7% (P < 0.001), respectively. Animals treated two and four times a year significantly outgained untreated animals by 14.8 kg and 17.7 kg respectively at the end of the trial (P < 0.05). The management system had a significant effect on packed cell volume and the proportion of animals passing Fasciola eggs in faeces (P < 0.05). The programme of two strategic treatments per year was only effective in controlling GI nematodes. It is concluded that a programme of four strategic treatments per year was effective in controlling GI nematodes and F. gigantica and improved weight gain.  相似文献   
995.
Results from the aeroshell-mounted neutral mass spectrometer on Viking I indicate that the upper atmosphere of Mars is composed mainly of CO(2) with trace quantities of N(2), Ar, O, O(2), and CO. The mixing ratios by volume relative to CO(2) for N(2), Ar, and O(2) are about 0.06, 0.015, and 0.003, respectively, at an altitude near 135 kilometers. Molecular oxygen (O(2)(+)) is a major component of the ionosphere according to results from the retarding potential analyzer. The atmosphere between 140 and 200 kilometers has an average temperature of about 180 degrees +/- 20 degrees K. Atmospheric pressure at the landing site for Viking 1 was 7.3 millibars at an air temperature of 241 degrees K. The descent data are consistent with the view that CO(2) should be the major constituent of the lower martian atmosphere.  相似文献   
996.
The spatial variability of soil aggregate stability and its relationship to runoff and soil erosion were examined in a catena of soils and vegetation in a semiarid environment at the Rambla Honda field site (Tabernas, Almería, SE Spain) to evaluate the validity of structural stability as a soil erosion indicator in sandy loam range soils. The influence of soil properties and topography on the variability of aggregate stability was also examined. Methods include: 1) aggregate stability assessment at 12 sites (3 repetitions per site) on the hillslope by two methods: a) aggregate size distribution by dry sieving b) water drop test; 2) soil organic carbon content; 3) particle size distribution determination; 4) terrain attributes derived from a digital elevation model (1-m resolution); 5) monitoring runoff and erosion for nearly 3 years in eight (10 × 2 m) plots distributed over the hillslope. Results: 41% of the average soil mass is formed by > 2-mm aggregates. However, wet aggregate stability is poor, with a mean (of a total of 1440 aggregates) of only 26 drop impacts necessary to break up a wet aggregate (pF = 1). Significant relationships were found in the number of water drops required for aggregate breakdown and runoff and erosion rates. However, no significant relationships between the mean weight diameter of aggregates under dry conditions and runoff or erosion rates were observed. The relationships of aggregates with other soil properties, hillslope position and proximity to plants are also analysed. The most significant correlation found was between the number of drop impacts and soil organic matter content. The stability of topsoil aggregates seems to be a valuable indicator of field-assessed runoff and inter-rill erosion of sandy loam range soils under semiarid conditions.  相似文献   
997.
All the poultry in each of four distinct Moroccan villages were vaccinated against Newcastle disease using Hitchner B1 and inactivated vaccines. Poultry in a fifth village were monitored as controls. Mortality in the poultry was followed for 20 weeks after the first vaccination and blood samples were taken every 4 weeks from chickens for estimation of antibodies against Newcastle disease virus. Sixty-three percent of the chicken population and 60% of the turkey population in the control village died during the 20 weeks of observation. Necropsied birds showed lesions consistent with Newcastle disease. Mortality did not exceed 22% in the vaccinated villages.  相似文献   
998.
Effects of protein supplementation and of nematode control on production responses in young grazing sheep and on nematode population dynamics were assessed. Young Merino wether sheep (n = 270) were allocated to one of three supplementation (Su) treatments and one of three regimes of nematode control (drench, Dr) in a factorial design. Each of the nine treatments was replicated three times in a randomised complete block. Animals received no supplement (SuO) or were given supplements at a level equivalent to 200 g day−1, fed three times a week. The supplements contained 25% lucerne meal with either 75% sunflower meal (Su1) or 75% formaldehyde-treated sunflower meal (Su2). Nematode control regimes were: not drenched unless survival of individual animals was threatened (DrO); drenched according to a strategic drenching programme (Dr1); treatment with a controlled-release albendazole capsule (Dr2). Infections with Haemonchus contortus were suppressed by use of closantel. Due to drought conditions and lack of pasture growth, lucerne hay was fed to all sheep from Week 14 onwards (350 g per head per day, fed twice a week). Supplementation reduced the need for ‘survival drenching’ in Dr0 sheep considerably. Live-weight gain was increased significantly by supplementation with Su1 or Su2 during the 36-week experiment. Undrenched animals given Su1 or Su2 tended to grow faster than unsupplemented animals in the Dr1 and Dr2 groups. Greasy wool production and fibre diameter were increased by supplementation and anthelmintic treatment. Faecal worm egg counts (FEC) in undrenched sheep were significantly lower for Su1 and Su2 sheep than for Su0 sheep. There were no effects of supplementation on FEC in Dr1 or Dr2 sheep. Worm burden was generally unaffected by supplementation, while the drenching treatments Dr1 and Dr2 reduced worm burden at Week 16 and to a lesser extent at Week 27. Trichostrongylus colubriformis was the predominant nematode species, with smaller numbers of Nematodirus spp., Trichostrongylus vitrinus, Trichostrongylus axei and Ostertagia circumcincta also present. It was concluded that supplementary feeding with protein meal substantially reduced production losses attributable to nematode infections in young grazing sheep. This appears to be due to an enhanced resilience of the host, rather than any major changes in development of protective immunity.  相似文献   
999.
  1. The pharmacokinetics of enrofloxacin and marbofloxacin was studied in Japanese quails and common pheasants. Healthy mature birds from both species and both genders were treated intravenously and orally with enrofloxacin (10 mg/kg) and marbofloxacin (5 mg/kg).

  2. After intravenous administration enrofloxacin was extensively metabolised to ciprofloxacin. Metabolites of marbofloxacin were not detected. Values of volume of distribution were respectively 4.63 l/kg and 3.67 l/kg for enrofloxacin and 1.56 l/kg and 1.43 l/kg for marbofloxacin. In quails, total body clearance values were higher than those in pheasants and other avian species.

  3. After oral application enrofloxacin was rapidly absorbed in quails, more rapidly than marbofloxacin. Pheasants absorbed both antimicrobials at a lower rate. Higher bioavailability was observed for marbofloxacin (118%). Relatively low bioavailability was established in quails for enrofloxacin (26.4%), accompanied by extensive conversion to ciprofloxacin.

  4. Generally, quails absorbed and eliminated both fluoroquinolones more rapidly than pheasants; the latter showed pharmacokinetics similar to poultry. Because of favourable pharmacokinetic properties, marbofloxacin should be preferred for oral administration in Japanese quails and pheasants for treatment of infections caused by equally susceptible pathogens.

  相似文献   
1000.
A 47-yr-old female Asian elephant (Elephas maximus) developed a corneal stromal abscess in her right eye. The elephant was trained to open her eye for topical ophthalmic therapy, and was treated six times daily with antibiotics and an antifungal solution for almost 2 mo. Nonsteroidal anti-inflammatory drugs were used to control pain, and atropine was applied topically to dilate the pupil and provide additional comfort. Vascularization of the abscess began shortly after initiating therapy, and complete resolution was obtained by 7 wk.  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