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   112984篇
  免费   6894篇
  国内免费   905篇
林业   4509篇
农学   3705篇
基础科学   966篇
  14272篇
综合类   21073篇
农作物   4264篇
水产渔业   5259篇
畜牧兽医   58080篇
园艺   1521篇
植物保护   7134篇
  2021年   1014篇
  2020年   1130篇
  2019年   1348篇
  2018年   1926篇
  2017年   2185篇
  2016年   1941篇
  2015年   1794篇
  2014年   2097篇
  2013年   4457篇
  2012年   3857篇
  2011年   4589篇
  2010年   3116篇
  2009年   3021篇
  2008年   4232篇
  2007年   3985篇
  2006年   3654篇
  2005年   3421篇
  2004年   3084篇
  2003年   3177篇
  2002年   2919篇
  2001年   3462篇
  2000年   3520篇
  1999年   2673篇
  1998年   1110篇
  1997年   1056篇
  1995年   1078篇
  1994年   961篇
  1992年   2083篇
  1991年   2367篇
  1990年   2218篇
  1989年   2165篇
  1988年   2000篇
  1987年   2040篇
  1986年   2127篇
  1985年   1892篇
  1984年   1575篇
  1983年   1363篇
  1979年   1516篇
  1978年   1193篇
  1977年   985篇
  1976年   1028篇
  1975年   1084篇
  1974年   1388篇
  1973年   1386篇
  1972年   1331篇
  1971年   1166篇
  1970年   1165篇
  1969年   1139篇
  1968年   954篇
  1967年   1005篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
91.
The surgical repair of tracheal collapse in 25 dogs is described. The initial presenting signs included coughing, dyspnoea, gagging, retching, exercise intolerance, cyanosis and collapse. Diagnosis was based upon the clinical signs, plain radiography and tracheal endoscopy. The collapse was corrected by the application of a number of extraluminal polypropylene prosthetic rings applied to the affected trachea. Additionally, a left arytenoid lateralisation was also performed. The perioperative complication rate was approximately 4 per cent, while the success rate was 75 per cent. The technique reduces the likelihood of catastrophic postoperative complications associated with iatrogenic laryngeal paralysis which is a possible complication of placement of extraluminal tracheal support devices.  相似文献   
92.
Blood samples were collected from 91 rusa deer (Cervus timorensis russa), immediately after being shot. Serum mean biochemical values from shot deer are presented for blood urea nitrogen, creatinine, creatine kinase, alanine aminotransferase, aspartate aminotransferase, alkaline phosphatase, total bilirubin, total protein, albumin, calcium, and phosphorus. Mean total protein and albumin increased with age. There was an age-associated increase of gamma globulins. Mean creatine kinase activity and creatinine, albumin and phosphorus concentrations were higher in stags than in hinds. Pregnant hinds had lower mean creatine kinase activity and phosphorus and higher mean alanine aminotransferase and total protein than non-pregnant hinds. Mean calcium concentration increased when deer were agitated before bleeding.  相似文献   
93.
94.
Little is known about the effect of fertilization on the N uptake of sunflowers. A 42 factorial trial with 0, 60, 120 and 180 kg N ha−1 and 0, 15, 30 and 45 kg P ha−1 was conducted over three years. The N content and concentration of leaves, stems and capitula were determined at three growth stages. High N levels increased the N content and concentration of all plant parts at all growth stages sharply. High P levels increased the N content of all plant components through better growth. P has an inconsistent effect on N concentration but tended to decrease it. After flowering the crop assimilated 20 to 25 % of the total N. This implies that N applied can still be applied and utilized by the crop at a late stage. This should be substantiated by further research.  相似文献   
95.
Ultrasonography and MRI have become valuable tools for imaging of tendon injuries. The current study examines the histopathologic basis for the imaging abnormalities. Five injured equine forelimbs and two normal contralateral limbs were studied with high resolution real time ultrasound and MRI. Histologic sections were made and correlated with the diagnostic images. All lesions were readily seen by both modalities. Lesions characterized by hemorrhage, edma, and cellular infiltration were sonolucent on ultrasound and bright on MRI images. MRI returned to normal as fibrogenesis ensued. Ultrasound images remained abnormal until fibrillar reallgnment occured with completion of the healing process. High resolution real time ultrasound and MRI both accurately reflect the tissue abnormalities in acute tendon injuries. The injured horse does appear to be an effective model for correlative imaging studies of tendon injuries.  相似文献   
96.
Pharmacokinetic parameters for the beta 2-adrenergic agonist, cimaterol (CIM), were determined in growing Holstein steers. Compartmental analysis was used after measurement of CIM in body fluids by affinity chromatography and HPLC using UV detection. Recoveries from spiked plasma and urine standards were 70 +/- 1.2% and 68 +/- 1.1%, respectively. The minimum detection level in plasma was 1 ng/mL and the average CV was 5.1% for concentrations that ranged from 1 to 30 ng/mL. Four steers (276 +/- 24 kg) received 15 mg of CIM by bolus intravenous injection. Plasma CIM levels declined in a biphasic manner with half-lives of 2.5 min for the distribution phase and 54 min for the elimination phase. A two-compartment open model was used to describe the disappearance of CIM and the following pharmacokinetic parameters were obtained: central compartment volume (Vc) = .76 L/kg, apparent volume of distribution (Vd) = 4.1 L/kg, and transfer rate constants from the central to peripheral compartment (k12) = .177/min, from the peripheral to central compartment (k21) = .054/min and elimination from the central compartment (kel) = .074/min. After 8 h, total urinary CIM accounted for only 18.3% of the administered dose. Results suggest that circulating concentrations of CIM in growing steers are influenced by its accumulation in an unidentified peripheral pool and its conversion into unknown metabolite(s) before elimination.  相似文献   
97.
The conventional culture method was compared to coagglutination for detection of Actinobacillus (Haemophilus) pleuropneumoniae in 425 sets of pig lungs. Sera from the same animals were evaluated for antibodies to A. pleuropneumoniae by the complement fixation (CF) test. All samples were collected at 2 packing plants in Iowa. In 2 nonvaccinated herds with no history of respiratory disease, the difference between standard culture results and coagglutination was highly significant (P less than 0.001). None of the 57 pigs in this group were positive for A. pleuropneumoniae by conventional culture, but 7 were positive by the coagglutination test. There were 15 animals with CF titers between 1:8 and 1:32. Animals from 6 herds vaccinated for A. pleuropneumoniae and without recent respiratory problems were evaluated. One out of 118 animals tested was positive for A. pleuropneumoniae by standard culture as compared to 9 positive by coagglutination. The difference in positive results between culture and coagglutination was highly significant (P less than 0.001). Twenty-eight animals had CF titers to A. pleuropneumoniae (1:4 to greater than or equal to 1:128). Two hundred fifty lungs and sera samples were collected from 7 herds which had recently experienced varying degrees of respiratory disease. Thirty-nine lungs were positive for A. pleuropneumoniae by culture and 182 were positive by coagglutination. The number of positives detected by coagglutination was significantly different (P less than 0.001) from the number positive by culture. There were 172 animals with antibody titers ranging from suspect to greater than or equal to 1:128. There were significantly fewer positive animals detected by standard culture than with the CF test (P less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
98.
The release of tumor necrosis factor-alpha (TNF-) from cultured bovine alveolar macrophages (BAM) was evaluated following stimulation of BAM with bovine herpesvirus-1 (BHV-1), parainfluenza-3 (PI-3) virus, bovine respiratory syncytial virus (BRSV), Escherichia coli 0111:B4 endotoxin, Pasteurella haemolytica type 1 endotoxin, Pasteurella multocida endotoxin, and virus/endotoxin combinations. A cytotoxic assay system using Georgia bovine kidney cells as targets was used to measure TNF- activity. The cytotoxic activity was neutralized by an anti-human TNF- monoclonal antibody.

Stimulation of BAM with 1 median tissue culture infectious dose (TCID50) of live or ultraviolet (UV)-inactivated PI-3 virus/cell resulted in release of TNF- in significantly (P<0.05) higher amounts than sham-induced BAM. The quantities of TNF- released after live or UV-inactivated BHV-1 or BRSV induction were not significantly higher than sham-induced BAM. E. coli 0111:B4, P. haemolytica type 1 and P. multocida endotoxins stimulated TNF- release in a dose-dependent manner. Sequential exposure of BAM to 1 TCID50 per cell of either live BHV-1, PI-3 virus or BRSV and then 5 μg ml−1 of either E. coli 0111:B4, P. haemolytica type 1 or P. multocida endotoxin caused a significant (P<0.05) reduction in detectable TNF- in seven of nine virus/endotoxin combinations tested, when compared with 5 μg ml−1 of endotoxin alone. Parainfluenza-3 virus/endotoxin combinations stimulated higher TNF- release when compared with other virus/endotoxin combinations. Five out of six test animals had serum-neutralizing antibodies to PI-3 virus, one out of six had serum-neutralizing antibodies to BHV-1, and two out of six had serum-neutralizing antibodies to BRSV, suggesting a possible relationship between serum neutralizing antibodies and TNF- release from in vitro cultivated BAM.  相似文献   

99.
100.
Eastern equine encephalitis (EEE) was diagnosed in a flock of emus in southeastern Louisiana. The outbreak involved juvenile and adult breeders ranging in age from 20 to 36 months, with an attack rate of 76% and a case fatality rate of 87%. The diagnosis was confirmed by isolation and characterization of the viral agent, and by detection of EEE antibody in two recovered emus. High mortality was preceded by marked depression, hemorrhagic diarrhea, and emesis of blood-stained ingesta. On postmortem examination, hemorrhagic enteritis and multiple petechia of viscera were observed. Microscopic changes included severe necrosis of hepatocytes, intestinal mucosa, and necrotizing vasculitis of the spleen and lamina propria of the intestine. No nervous system lesions were observed. This outbreak occurred concurrently with EEE in horses and was attributed to unseasonably heavy rainfall with an abundance of arthropod vectors and proximity to free-living reservoir host species.  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