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   70867篇
  免费   3965篇
  国内免费   34篇
林业   3224篇
农学   2529篇
基础科学   477篇
  8696篇
综合类   11415篇
农作物   2606篇
水产渔业   3640篇
畜牧兽医   36607篇
园艺   907篇
植物保护   4765篇
  2018年   1053篇
  2017年   1129篇
  2016年   1056篇
  2015年   907篇
  2014年   1136篇
  2013年   2628篇
  2012年   2058篇
  2011年   2461篇
  2010年   1683篇
  2009年   1751篇
  2008年   2592篇
  2007年   2362篇
  2006年   2326篇
  2005年   2124篇
  2004年   2193篇
  2003年   2165篇
  2002年   2036篇
  2001年   2366篇
  2000年   2375篇
  1999年   1925篇
  1998年   912篇
  1997年   821篇
  1996年   782篇
  1995年   861篇
  1994年   843篇
  1993年   767篇
  1992年   1523篇
  1991年   1542篇
  1990年   1662篇
  1989年   1507篇
  1988年   1450篇
  1987年   1340篇
  1986年   1384篇
  1985年   1336篇
  1984年   1128篇
  1983年   1025篇
  1982年   628篇
  1979年   1004篇
  1978年   795篇
  1977年   717篇
  1976年   676篇
  1975年   719篇
  1974年   766篇
  1973年   830篇
  1972年   803篇
  1971年   748篇
  1970年   729篇
  1969年   744篇
  1967年   658篇
  1966年   631篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
191.
Two prochiral sulphide drugs, fenbendazole (FBZ) and albendazole (ABZ) were administered orally to sheep. Blood samples were analysed for parent drug and S-oxidation metabolites and the chirality of the sulphoxide metabolites was determined. The plasma concentrations of the enantiomers of the sulphoxides were never a racemate. On the contrary, the ratios were greater than 1 as soon as the sulphoxide compounds could be detected in plasma. They subsequently increased linearly throughout the time course of the kinetics, reaching the level 86:14 after FBZ and 95:5 after ABZ treatment. The major enantiomer represented 74% and 86% of the total AUC of SO.FBZ and SO.ABZ, respectively.  相似文献   
192.
Between 1983 and 1989, we examined 1,201 aborted fetuses and dead lambs from the north central United States. Toxoplasmosis was diagnosed in 17.5%, campylobacteriosis in 9.9%, chlamydiosis in 4.7%, and miscellaneous infections in 15.1%. Inflammatory lesions suggestive of infectious causes were seen in 13%. Noninfectious causes were identified in 6.1%, and a diagnosis was not reached in 33.3%. An agglutination test was used to detect Toxoplasma gondii-specific antibodies in ovine fluids. Toxoplasma gondii antibodies were detected in 223 of 1,064 (20.9%) fluids from fetuses and dead lambs. Of 201 seropostive (greater than or equal to 16) fetuses and lambs, T gondii antibody titers (reciprocal) were 16 (21 fetuses and lambs), 32 (10 fetuses and lambs), 64 (2 fetuses and lambs), 128 (7 fetuses and lambs), 256 (9 fetuses and lambs), 512 (5 fetuses and lambs), 1,024 (15 fetuses and lambs), 2,048 (13 fetuses and lambs), 4,096 (13 fetuses and lambs), 8,196 (13 fetuses and lambs), 16,392 (19 fetuses and lambs), and greater than or equal to 32,784 (74 fetuses and lambs).  相似文献   
193.
194.
Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) was first isolated in swine alveolar macrophages (SAMs) and has subsequently been reported to replicate in other cell lines. Entry of the virus inside the cell takes place by receptor-mediated endocytosis. Following the entry of the virus into the cell, several not completely understood changes take place. PRRSV has been reported to be an apoptotic-inductor virus both in vivo and in vitro. Interestingly, it has been suggested that PRRSV-induced apoptosis occurs in cells other than those in which PRRSV replicates by a bystander mechanism. In this paper the ultrastructural pathogenesis of PRRSV will be reviewed.  相似文献   
195.
OBJECTIVE: To evaluate the use of arthroscopy as the primary method for removal of large patellar fracture fragments. DESIGN: Retrospective study. ANIMALS: 5 performance horses of various breeds with patellar fractures. PROCEDURE: Clinical signs of lameness, external evidence of injury, and radiography were used to diagnose and determine fracture orientation. Arthroscopy of the stifle joint was performed on the affected limb with the horse positioned in dorsal recumbency and under general anesthesia. Progress after surgery was determined by evaluating medical records and via telephone conversations with owners. RESULTS: 4 of 5 horses had fractures of the medial aspect of the patella and 1 horse had a fracture of the lateral aspect of the patella. There were no postoperative complications with the joint or the arthroscopic portal incisions. Recovery periods ranged from 3 to 5 months. All horses recovered completely from surgery, and performed at the same or higher level of competition as before arthroscopy. CONCLUSIONS AND CLINICAL RELEVANCE: Femoropatellar joint arthroscopy is a favorable means by which evaluation of the stifle joint and removal of large fracture fragments can be achieved with negligible postoperative complications.  相似文献   
196.
1. Perching behaviour of Ross broiler chickens was studied in order to increase the available space at the floor level. Pens (36 m2) were equipped with wooden perches mounted 20 and 33 cm above the litter (5 cm perch length per bird). 2. The influences of stocking density (22 vs 11 birds/m2, experiment 1) and of a 10 min artificial dusk (17 birds/m2, experiment 2) were tested in a 2x2 factorial design (n=4 by treatment) including control pens without perches. In experiment 3, perching behaviour between 2 different group sizes (1020 vs 4590 birds, 17 birds/m2, no replicates) was compared. 3. The percentage of perching birds (PPB) was assessed by scan sampling observations during the 20 to 22 h light period (8 observations per d, 5 d a week, from week 3). In experiment 2, infra-red video recording was performed at the time lights were dimmed and during the dark period. In experiment 1, carcase lesions were noted. In experiment 2, tibia breaking strength of frequently perching birds was compared with controls. Birds were weighed before slaughter and food conversion was calculated. 4. PPB increased with age and density (P<0.01) and was highest at the 22 birds/m2 density during week 6 (mean=10.6%, maximum=13.5%). PPB was not influenced by a 10-min artificial dusk. In experiment 3, PPB was slightly lower during weeks 5 and 6 in the large group size compared to the small group size (6.8% vs 7.9% respectively in week 6). 5. Prevalence of carcase lesions and tibia breaking strengths were unrelated to perch usage. Final body-weights were not influenced by the presence of perches except at the 22 birds/m2 density where males were slightly heavier in the control pens compared with males in pens equipped with perches (2096 g vs 2051 g, P=0.015). Food conversion was independent of the presence of perches.  相似文献   
197.
The aim of this trial was to evaluate the effects of polyunsaturated fatty acid (PUFA) supplementation in different-stages atopic dogs fed on a controlled diet. Twenty-two non-seasonal atopic dogs of different breeds and ages were included in the 2-month trial. All the patients were given an essential fatty acid (EFA) supplementation [17 mg/kg eicosapentaenoic acid (EPA) + 5 mg/kg docosahexaenoic acid (DHA) + 35 mg/kg gammalinolenic acid (GLA)], the global (diet + supplementation) omega-6 to omega-3 ratio was 5.5-1. Two groups of dogs were considered: group A 'pre-immunotherapy' (15 cases) included dogs with early stages atopy, which had not been submitted to any treatment yet; group B 'post-immunotherapy' (seven cases) included dogs with chronic atopy immunotherapy non-responsive. Clinical evaluations were performed at the beginning, on day 30 and at the end of the trial. Blood serum fatty acids profile was determined at the beginning and at the end of the study. Better clinical results were obtained in group A, a great difference was found between the two groups on pruritus score. Serum arachidonic acid (AA) was significantly lower at the end of the trial in group A while GLA was significantly higher in group B. We hypothesized that different-stages atopic dogs could have different response to EFA supplementation, maybe because of a different fatty acids metabolism. Early stages cases seem to be more responsive to EFA supplementation.  相似文献   
198.
Summary In this study we investigated the effect of the acetyl-L-carnitine (ALC) supplementation on the myenteric neurons of the jejunum of rats made diabetic at the age of 105 days by streptozotocin (35 mg/kg body weight). Four groups were used: non-diabetic (C), non-diabetic supplemented with ALC (CC), diabetic (D), diabetic supplemented with ALC (DC). After 15 weeks of diabetes induction the blood was collected by cardiac puncture to evaluate glycaemia and glycated haemoglobin. Next the animals were killed and the jejunum was collected and subjected to whole-mount preparation to evidence the myenteric neurons through the histochemical technique of the NADH-diaphorase. The neuronal counts were made in 80 microscopic fields, in tissue samples of five animals of each group. The profiles of the cell bodies of 1000 neurons per group were analysed. Diabetes induced a significant increase in the area of the cell body and decrease in the number of NADH-diaphorase positive myoenteric neurons. ALC suplementation to the diabetic group promoted smaller hypertrophic effects and less neuronal loss than in the myoenteric neurons of the diabetic rats, and in addition diminished the body weight decrease and reduced the fasting glycaemia.  相似文献   
199.
A study was conducted to determine the influence of sodium salicylate on the behaviour and the food and water consumption of broiler chickens after lipopolysaccharide (LPS) injection. An oral dose of 100 mg/kg sodium salicylate was given and an acute phase reaction in broiler chickens was provoked through the intravenous injection of Escherichia coli LPS. Water intake was higher in the LPS and salicylate-treated group than in the positive control group. The salicylate treatment, however, did not restore the food intake, or influence the behaviour of the chickens. These data show that sodium salicylate has a positive effect on the water intake after intravenous injection of LPS in chickens and suggests that there is a difference in mechanism of action of food and water consumption after LPS injection in chickens.  相似文献   
200.
Balloon valvuloplasty has become the accepted method of therapy for the majority of dogs with valvular pulmonic stenosis. Success rate of balloon valvuloplasty is excellent but the procedure can have a steep learning curve. An understanding of the equipment used and specifics of the procedure is important to maximize success and minimize risk. This paper will review criteria for choosing candidates for balloon valvuloplasty, discuss the equipment used, discuss the procedure in detail, and finally discuss assessment of success and possible complications.  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