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   62536篇
  免费   2834篇
  国内免费   42篇
林业   2516篇
农学   1645篇
基础科学   429篇
  6150篇
综合类   14759篇
农作物   2461篇
水产渔业   2346篇
畜牧兽医   31047篇
园艺   643篇
植物保护   3416篇
  2017年   553篇
  2016年   536篇
  2014年   540篇
  2013年   1863篇
  2012年   1202篇
  2011年   1436篇
  2010年   913篇
  2009年   877篇
  2008年   1427篇
  2007年   1389篇
  2006年   1371篇
  2005年   1386篇
  2004年   1319篇
  2003年   1391篇
  2002年   1354篇
  2001年   1493篇
  2000年   1473篇
  1999年   1206篇
  1998年   531篇
  1997年   538篇
  1995年   596篇
  1994年   573篇
  1993年   561篇
  1992年   1309篇
  1991年   1395篇
  1990年   1449篇
  1989年   1488篇
  1988年   1399篇
  1987年   1356篇
  1986年   1400篇
  1985年   1373篇
  1984年   1150篇
  1983年   1005篇
  1982年   718篇
  1981年   688篇
  1980年   644篇
  1979年   1127篇
  1978年   916篇
  1977年   819篇
  1976年   768篇
  1975年   860篇
  1974年   1126篇
  1973年   1063篇
  1972年   1128篇
  1971年   1080篇
  1970年   1022篇
  1969年   874篇
  1968年   710篇
  1967年   844篇
  1966年   693篇
排序方式: 共有10000条查询结果,搜索用时 14 毫秒
991.
The tenacity of viruses in liquid manure of cattle was examined in a total of five samples inoculated with ECBO-virus (strain LCR-4) representing viruses without envelope and Aujeszky virus (field isolate) representing enveloped viruses. The titers were examined at regular intervals over a period of 26 weeks. On the day of inoculation each sample had a titer of 10(5) ID50/ml. After 16 weeks complete inactivation was observed in the Aujeszky virus sample stored at 20 degrees C. The Aujeszky virus sample wich was kept at 4 degrees C at 26 weeks had a titer of 10(1,75) ID50/ml. In the samples inoculated with ECBO virus after 26 weeks of inoculation a titer of 10(3) ID50/ml was found in the manure stored at 20 degrees C. No influence on the virus titers in the liquid manure samples was observed either from pH or the number of bacteria (3,4 x 10(7)-1.16 x 10(8)/ml during the examination period.  相似文献   
992.
The vascular patterns to pelvic limb muscles were studied in 6 dogs (12 limbs) to identify muscles most suitable for transposition in the treatment of large wounds. Gross dissection of injected specimens and angiography were used to identify the vascular pedicles. The vascular pedicles to several muscles were generally consistent, and any variations would not interfere with most muscle transfers. The cranial part of the sartorius, gracillis, semitendinosus, and rectus femoris muscles were identified as suitable candidates for transfer. The caudal part of the sartorius, cranial tibial, and long digital extensor muscles have segmentalized vascular patterns that would limit its arc of rotation.  相似文献   
993.
Our primary objective of this experiment was to evaluate potential genetic differences between two diverse Bos taurus breeds [Angus (AG) and Romosinuano (RO)] in response to an endotoxin challenge. Eighteen steers (n = 9 steers/breed; 299.4 ± 5.2 kg BW) were acclimated to environmentally controlled chambers maintained at thermoneutrality (19.7 °C) and then fitted with indwelling jugular catheters and rectal temperature (RT) recording devices 1 d before the endotoxin challenge. The next day, blood samples were collected at 30-min intervals from −2 to 8 h, and RT was measured continuously at 1-min intervals throughout the study. At time 0, all steers received an intravenous bolus injection of lipopolysaccharide (LPS; 2.5 μg/kg BW). Serum samples were stored at −80 °C until analyzed for cortisol, proinflammatory cytokines [tumor necrosis factor-alpha (TNF-α), interleukin-1 beta (IL-1β), IL-6, and interferon gamma (IFN-γ)], and acute phase proteins (serum amyloid A, acid soluble protein, ceruloplasmin, and α-acid glycoprotein). Rectal temperatures increased in both breeds within 1 h after LPS, with RO producing a greater increase in RT than AG steers (P < 0.001). Serum cortisol and TNF-α increased (P < 0.01) in both breeds within 1 h after the LPS challenge. For cortisol, an overall breed effect (P < 0.02) was detected, such that AG steers had a higher cortisol response than RO steers. A breed × time interaction (P < 0.01) was observed for TNF-α, such that the response was delayed and extended in the RO steers compared with the AG steers. At 2 and 2.5 h after LPS, TNF-α concentrations were greater (P < 0.03) in RO steers than in AG steers. For IL-1β, a breed × time interaction (P < 0.04) was also observed. At 3 h after LPS, IL-1β concentrations were greater (P < 0.01) in RO steers than in AG steers. Serum IL-6 and IFN-γ increased (P < 0.01) in a similar manner in both groups after the LPS challenge. These data show differences in the innate immune response between two diverse Bos taurus breeds which may provide insight about differences observed in productivity, heat tolerance, disease resistance, and longevity among cattle breeds.  相似文献   
994.
Glanzmann thrombasthenia (GT) is an inherited, intrinsic platelet defect characterized by a quantitative or qualitative change in the platelet glycoprotein complex IIb-IIIa (integrin alpha(IIb)beta3). The subunits are encoded by separate genes and both subunits must be expressed for a stable complex to form on the platelet surface; therefore, a defect in either gene can result in GT.  相似文献   
995.
An outbreak of upper respiratory tract disease was investigated in a group of 17 housed home-bred calves on a mixed dairy, beef and sheep farm in Devon. Conjunctival swabs were collected and tested for Chlamydophila spp. DNA using a PCR test that detects Chlamydophila abortus and Chlamydophila psittaci. Six of the calves tested gave a positive result. Further epidemiological observations and laboratory testing indicated that the adult dairy cows, from which the affected calves originated, were the most likely source of infection.  相似文献   
996.
As a direct consequence of rising drug resistance among common nematodes of grazing animals, efforts toward state-of-the-art vaccine development have clearly intensified in recent years, fuelled primarily by the advent of newer technologies in gene discovery, by advancements in antigen identification, characterisation and production. In this regard, it is appropriate to review progress that has been made in generating helminth vaccines and in particular, vaccines against common nematodes of production animals for consumption. In like manner, it is prudent to evaluate barriers that have hindered progress in the past and continue to present obstacles that must be solved when utilizing and depending on host immunity to attenuate parasitic infections.  相似文献   
997.
A female German Holstein calf was not able to stand up after birth. Resting the animal was lying in normal position and could lift its head. Sensory stimuli like auditory or tactile impulses induced myoclonic jerking of the whole body. Afterwards it calmed down quickly. The signs observed correspond to the clinical findings of congenital myoclonus in poll Hereford calves. The pathological examination revealed no indications for changes in organs. The inbreeding coefficient of the calf was 1.56 %. The present type of congenital myoclonus in the calf examined is likely to be genetically determined, even if the point mutation in exon 2 of the glycin receptor alpha 1 gene was not confirmed.  相似文献   
998.
An experiment was conducted to determine the effects of dietary concentrations of Co on vitamin B12 production and fermentation of mixed ruminal microbes grown in continuous culture fermentors. Four fermentors were fed 14 g of DM/d. The DM consisted of a corn and cottonseed hull-based diet with Co supplemented as CoCO3. Dietary treatments were 1) control (containing 0.05 mg of Co/kg of DM), 2) 0.05 mg of supplemental Co/kg of DM, 3) 0.10 mg of supplemental Co/kg of DM, and 4) 1.0 mg of supplemental Co/kg of DM. After a 3-d adjustment period, fermentors were sampled over a 3-d sampling period. This process was repeated 2 additional times for a total of 3 runs. Ruminal fluid vitamin B12 concentrations were affected by Co supplementation (P < 0.01), and there was a treatment x day interaction (P < 0.01). By sampling d 3, cultures fed the basal diet supplemented with 0.10 mg of Co/kg had greater (P < 0.05) vitamin B12 concentrations than those supplemented with 0.05 mg of Co/kg of DM, and increasing supplemental Co from 0.10 to 1.0 mg/kg of DM increased (P < 0.01) ruminal fluid vitamin B12 concentration. Ruminal fluid succinate also was affected (P < 0.10) by a treatment x day interaction. Cobalt supplementation to the control diet greatly decreased (P < 0.05) succinate in ruminal cultures on sampling d 3 but not on d 1 or 2. Molar proportions of acetate, propionate, and isobutyrate, and acetate:propionate were not affected by the addition of supplemental Co to the basal diet. However, molar proportions of butyrate, valerate, and isovalerate increased (P < 0.05) in response to supplemental Co. The majority of long-chain fatty acids observed in this study were not affected by Co supplementation. However, percentages of C18:0 fatty acids in ruminal cultures tended (P < 0.10) to be greater for Co-supplemented diets relative to the control. Methane, ammonia, and pH were not greatly affected by Co supplementation. The results indicate that a total (diet plus supplemental) Co concentration of 0.10 to 0.15 mg/kg of dietary DM resulted in adequate vitamin B12 production to meet the requirements of ruminal microorganisms fed a high-concentrate diet in continuous-flow fermentors.  相似文献   
999.
CASE DESCRIPTION-A 12-week-old female English Springer Spaniel was evaluated for lethargy, vomiting, and pyrexia 1 week after treatment of patent ductus arteriosus (PDA) via coil occlusion. CLINICAL FINDINGS-Test results were consistent with septicemia, and the assumption was made that the PDA occlusion coils were infected. Radiography revealed partial migration of the coil mass into the pulmonary artery and signs of congestive heart failure. TREATMENT AND OUTCOME-After successful treatment of the septicemia and heart failure, surgical removal of the coils and resection of the PDA were undertaken. Although the coil that embolized to the pulmonary vasculature was left in place, the dog's clinical signs resolved. CLINICAL RELEVANCE-This case highlights the fact that as PDA coil occlusion devices become more widely used in dogs, practitioners must be prepared to treat implant infections aggressively, with both medical and surgical interventions if necessary.  相似文献   
1000.
BACKGROUND: Identification and control of infections are important in the management of diabetic cats. Urinary tract infections have not been well characterized in diabetic cats. This retrospective study was performed to review and characterize urinary tract infections in diabetic cats. HYPOTHESIS: Urinary tract infections are common in diabetic cats. ANIMALS: A review was made of the medical records of 141 diabetic cats that had had urine obtained for culture by antepubic cystocentesis and that had not been treated with antibiotics, undergone urinary tract catheterization or urinary tract surgery within 2 weeks of urine collection or had urethral obstruction at the time of urine collection. METHODS: A review of medical records. RESULTS: Urinary tract infection was identified in 18 of 141 diabetic cats. Escherichia coli was the most common isolate (67%). Female cats were at increased risk (prevalence odds ratios [POR], 3.7; 95% confidence interval [CI], 1.3 to 10.2; P = .013). Clinical signs of lower urinary tract disease and findings on urine sediment examination were good predictors of positive urine cultures. CONCLUSIONS AND CLINICAL IMPORTANCE: Urinary tract infections are common in diabetic cats regardless of status of diabetic control, suggesting routine monitoring with urine sediment exams or urine culture is warranted.  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