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   89957篇
  免费   5073篇
  国内免费   321篇
林业   4352篇
农学   3278篇
基础科学   628篇
  12065篇
综合类   11747篇
农作物   3355篇
水产渔业   4632篇
畜牧兽医   48641篇
园艺   1115篇
植物保护   5538篇
  2020年   791篇
  2019年   907篇
  2018年   1579篇
  2017年   1684篇
  2016年   1578篇
  2015年   1360篇
  2014年   1642篇
  2013年   3821篇
  2012年   2834篇
  2011年   3401篇
  2010年   2278篇
  2009年   2264篇
  2008年   3189篇
  2007年   3078篇
  2006年   2927篇
  2005年   2608篇
  2004年   2486篇
  2003年   2484篇
  2002年   2295篇
  2001年   3238篇
  2000年   3126篇
  1999年   2418篇
  1998年   986篇
  1997年   951篇
  1996年   883篇
  1995年   992篇
  1994年   856篇
  1993年   803篇
  1992年   1778篇
  1991年   2007篇
  1990年   1990篇
  1989年   1913篇
  1988年   1768篇
  1987年   1739篇
  1986年   1707篇
  1985年   1573篇
  1984年   1310篇
  1983年   1068篇
  1979年   1262篇
  1978年   995篇
  1976年   844篇
  1975年   860篇
  1974年   1031篇
  1973年   1025篇
  1972年   1035篇
  1971年   901篇
  1970年   923篇
  1969年   896篇
  1968年   792篇
  1967年   818篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
A growth trial was carried out to test the effect of organic, trivalent chromium and L-carnitine on the body composition of growing rats. At the same time, an evaluation of different measurement methods (weight of epididymal fat pad, adipocyte morphometry, total body electrical conductivity) was performed. Outbred Wistar rats of 30 days of age were fed diets of different (0, 10 and 20%) protein level. The diets were supplemented with 4 mg/kg Cr as chromium nicotinate, and 100 mg/kg L-carnitine. The experimental feeding lasted 15 days, after a 5-day-long adjustment period. It was found that Cr addition increased feed intake. Both treatments caused changes in body composition, increasing fat and protein deposition. Organic chromium had no effect at either protein level, while L-carnitine improved the protein retention only at an optimum (20%) protein supply. No statistically significant correlation was found between total body electrical conductivity (TOBEC) and body composition, which could be attributed to the great individual differences. A close correlation was found among total body fat percentage, weight of epididymal fat pad and the adipocyte surface. The data suggest that there is an interaction between dietary protein supply and the effect of repartitioning agents.  相似文献   
993.
Pavetamine, the active principle of plants causing gousiekte in ruminants, was found in this study to be an inhibitor of protein synthesis in the rat heart. Sprague-Dawley rats were injected intra-peritoneally with 8-10 mg/kg pavetamine and the levels of protein synthesis in different organs determined utilizing L-[4-3H]phenylalanine incorporation. In contrast to the more than 23% inhibition found in heart tissue at 4, 24 and 48 h after administration of pavetamine, the effect on the kidney, liver, spleen, intestine and skeletal muscle was minimal or returned to pretreatment levels within 48 h. These results may offer an explanation for the clinical signs observed in ruminants with gousiekte, where the heart only is affected.  相似文献   
994.
The effects of feeding two fibre-rich diets with contrasting solubility and a concentrated low dietary fibre on the rate of gastric emptying were examined in six gastric cannulated pregnant sows. Additionally, it was examined whether any effect could be related to the physico-chemical properties of digesta, i.e. viscosity and/or water binding capacity. The sows were fed each diet for one week in a 3 x 3 Latin Square design and the samples were taken in a randomised order 0.5, 1, 2, 3, 5 and 15.5 h after the morning meal. The stomach contents were evacuated through the gastric cannula once daily. The evacuated gastric digesta was quantified and a representative sample was taken to determine its viscosity, water binding capacity and its content of dry matter, dietary components and solid (Cr2O3) and liquid (polyethylene glycol) phase markers. The flow of liquid digesta was calculated as the difference between digesta and dry matter. Increasing the content of dietary fibre in the diet led to higher recovery of liquid digesta but did not have any significant effect on the gastric emptying of dry matter and dietary components. The effect of dietary fibre could not be attributed to the viscosity of the liquid phase of digesta but might be related to the ability of the increased gastric dietary fibre content to hold water. The stomach selectively retained the insoluble dietary fibre components most noticeably seen with the bran-supplemented diet where the concentration of insoluble NSP in digesta increased significantly from 2 hours and onward.  相似文献   
995.
996.
Objective— To determine the incidence of femoral medullary infarction after modifying the depth of femoral reaming and filing when performing total hip replacement (THR) using the Zurich Cementless Total Hip Replacement system (ZCTHR).
Study Design— Case series.
Animals— Dogs (n=31) that had ZCTHR (34).
Methods— Thirty-one dogs (34 THR) had ZCTHR (May 2003–September 2006) and with >1 year radiographic (craniocaudal, mediolateral views) follow-up after THR were evaluated for the presence of femoral medullary infarcts. Incidence was compared with a previous study performed before the technique modification.
Results— Femoral medullary infarction occurred in 1 femur (2.9%; dog <18 months at THR) compared with 19.5% before the technique change, a significant decrease ( P <.001).
Conclusion— Limiting the depth of reaming and filing of the medullary canal resulted in a significant decrease in the incidence of femoral medullary infarction.
Clinical Relevance— Depth of reaming and filing the medullary canal should be limited when performing THR using the ZCTHR.  相似文献   
997.
This epidemiological survey on canine babesiosis was carried out in three distinct rural regions (Lavras, Belo Horizonte and Nanuque) of the State of Minas Gerais, Brazil. Ticks and blood samples were collected during a dry season (Lavras, n = 92; Belo Horizonte, n = 50; Nanuque, n = 102) and the subsequent rainy season (Lavras, n = 71; Belo Horizonte, n = 28; Nanuque, n = 66) from dogs living on farms. Plasma samples were analyzed by the indirect fluorescent antibody test for detection of anti-Babesia canis vogeli antibodies. DNA was extracted from blood of serologically positive dogs and molecular characterization of Babesia species was performed. Rhipicephalus sanguineus, Amblyomma cajennense and Boophilus microplus were the tick species identified in all regions. In Lavras, in addition to those tick species, A. tigrinum and A. ovale were also identified. The most prevalent tick species was A. cajennense (35.3%), followed by R. sanguineus (19%) and B. microplus (4.0%). Dogs living in Nanuque region were more heavily infested with ticks than dogs living in Belo Horizonte and Lavras regions. The overall frequency of anti-B. c. vogeli antibodies in the canine population in rural areas of Minas Gerais was 28.7%, with prevalence rates of 49.0% in Nanuque, 34.0% in Belo Horizonte and 3.3% in Lavras. The age of the animals and tick infestation were associated with seroprevalence of B. c. vogeli. The sequence analysis showed that B. c. vogeli was the only Babesia species present in all three regions. This study showed different rates of prevalence and incidence of canine babesiosis among the three rural regions sampled in Minas Gerais State. The results point to the importance of canine babesiosis in rural areas and to the need for further studies related to its transmission and maintenance in nature.  相似文献   
998.
Serosurvey of five viruses in chickens on smallholdings in Bangladesh   总被引:1,自引:0,他引:1  
A serologic survey was undertaken in chickens in smallholdings in Bangladesh for avian influenza A virus (AIV), egg drop syndrome '76 virus (EDS'76V), infectious bronchitis virus (IBV), Newcastle disease virus (NDV) and reovirus (RV) in three phases: January 2002-May 2003, September 2003-August 2004, and August 2005-March 2006. Four hundred thirty-six sera collected in the 2nd phase, 295 in the first phase, 755 in the 1st plus 2nd phases and 295 in the 1st phase were investigated for AIV, EDS'76V, IBV and RV, respectively, using enzyme linked immunosorbent assays. All 854 sera collected in the three phases were screened for NDV using hemagglutination inhibition test. In chickens 20% were seropositive to AIV, 3% to EDS'76V, 74% to IBV, 88% to NDV, and 47% to RV. The seroprevalence in flocks was 23% to AIV, 6% to EDS'76V, 79% to IBV, 89% to NDV and 56% to RV. Twenty-five percent chickens had > or = 10log(2)HI titers to NDV.  相似文献   
999.
Definitive diagnosis of vesicular or vesicular-like lesions in livestock animals presents challenges both for veterinary clinicians and diagnostic laboratories. It is often impossible to diagnose the causative disease agent on a clinical basis alone and difficult to collect ample vesicular epithelium samples. Due to restrictions of time and sample size, once laboratory tests have ruled out foot-and-mouth disease, vesicular stomatitis and swine vesicular disease a definitive diagnosis may remain elusive. With the ability to test a small quantity of sample for a large number of pathogens simultaneously, DNA microarrays represent a potential solution to this problem. This study describes the application of a long oligonucleotide microarray assay to the identification of viruses known to cause vesicular or vesicular-like lesions in livestock animals. Eighteen virus isolates from cell culture were successfully identified to genus level, including representatives of each foot-and-mouth disease virus serotype, two species of vesicular stomatitis virus (VSV), swine vesicular disease virus, vesicular exanthema of swine virus (VESV), bovine herpesvirus 1, orf virus, pseudocowpox virus, bluetongue virus serotype 1 and bovine viral diarrhoea virus 1. VSV and VESV were also identified in vesicular epithelium samples, with varying levels of sensitivity. The results indicate that with further development this microarray assay could be a valuable tool for the diagnosis of vesicular and vesicular-like diseases.  相似文献   
1000.
An experiment was conducted to analytically define several novel fish substrates and determine the effects of feeding diets containing these substrates on total tract nutrient digestibilities and on immune status of senior dogs. The control diet contained poultry by-product meal while test diets contained 20% milt meal (MM), pink salmon hydrolysate (PSH) and white fish meal (WFM) added at the expense of poultry by-product meal. Concentrations of lymphocytes positive for CD3, CD4, CD8 and CD21 cell-surface markers and immunoglobulin concentrations were measured. Gene expression of cytokines tumour necrosis factor (TNF)-, interleukin (IL)-6, interferon (IFN)-, IL-10 and transforming growth factor (TGF)-β was determined by quantitative real-time polymerase chain reaction. Major compositional differences were noted among fish substrates but apparent nutrient digestibility coefficients and immune indices were not affected by treatment. Fish protein substrates were found to be effective substitutes for poultry by-product meal, providing diets of high nutritive value for senior dogs.  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