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   72829篇
  免费   3488篇
  国内免费   202篇
林业   3978篇
农学   2286篇
基础科学   1028篇
  9071篇
综合类   10273篇
农作物   2465篇
水产渔业   4639篇
畜牧兽医   37016篇
园艺   2047篇
植物保护   3716篇
  2023年   825篇
  2022年   745篇
  2021年   1157篇
  2020年   1403篇
  2019年   788篇
  2018年   1970篇
  2017年   2294篇
  2016年   1880篇
  2015年   1466篇
  2014年   1976篇
  2013年   2865篇
  2012年   3630篇
  2011年   4729篇
  2010年   2738篇
  2009年   2561篇
  2008年   4363篇
  2007年   4266篇
  2006年   4398篇
  2005年   4118篇
  2004年   4003篇
  2003年   3810篇
  2002年   3844篇
  2001年   3488篇
  2000年   3404篇
  1999年   2732篇
  1998年   903篇
  1997年   645篇
  1996年   601篇
  1995年   406篇
  1994年   426篇
  1993年   372篇
  1992年   277篇
  1991年   318篇
  1990年   251篇
  1989年   303篇
  1988年   215篇
  1987年   191篇
  1986年   153篇
  1985年   233篇
  1984年   248篇
  1983年   152篇
  1982年   160篇
  1981年   115篇
  1980年   130篇
  1979年   128篇
  1978年   130篇
  1977年   115篇
  1976年   95篇
  1975年   89篇
  1973年   127篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
Complex vertebral malformation (CVM), a familial syndrome of Holstein calves, has been reported in aborted fetuses and in prematurely born, stillborn, and neonatal calves. Affected calves have anomalies in the vertebral column, including hemivertebrae, fused and misshapen vertebrae and ribs, scoliosis, and vertebral synostosis. Concurrent low body weight, symmetrical arthrogryposis, and cardiac anomalies have been documented in affected calves. The syndrome was identified and characterized in Holstein cattle in Denmark; however, a global distribution of this genetic disorder is likely based on identification of common ancestral sires widely used for artificial insemination. This is the first documented case of CVM in a Holstein calf in the USA.  相似文献   
142.
Twenty of 25 horses in a well-managed Missouri boarding stable were diagnosed with gingivitis/stomatitis. Gross examination of the affected horses revealed varying degrees of gingivitis ranging from mild periodontal swelling to marked swelling and erythema with ulceration and hemorrhage. Fine hair-like material was embedded within the intensely affected areas. Gingival biopsies from 4 affected horses contained pyogranulomatous inflammation with, in some cases, numerous eosinophils and several grass awns in cross and longitudinal section. Numerous foxtail seed heads were identified in hay samples. Examination of the records revealed that all of the affected horses had been fed the suspect hay, with the exception of 1 horse. Although not deliberately fed the suspect hay, this horse did have access to the hay when turned out into the exercise paddock. The lesions resolved following a change in hay source.  相似文献   
143.
Reptiles can harbor pathogenic microorganisms asymptomatically and serve as potential reservoirs of infection for humans, domestic animals, and other reptiles. Infectious diseases are also problematic for free-ranging reptile populations and are an important consideration in reptile reintroduction and translocation projects. There have been limited serologic studies of free-ranging reptiles for evidence of exposure to potential pathogens. In the present study, serum or plasma samples from five male and five female free-ranging Venezuelan anacondas (Eunectes murinus) were screened for antibodies to eastern, western, and Venezuelan equine encephalitis viruses, vesicular stomatitis virus, ophidian paramyxovirus, 19 Leptospira interrogans serovars, and Cryptosporidium serpentes. Antibodies to these agents were not detected, or antibody titers were low and possibly nonspecific. These results for the limited number of anacondas surveyed suggest that they do not serve as significant reservoirs for these infectious agents at this location.  相似文献   
144.
Two black rhinoceroses (Diceros bicornis michaeli) developed clinical leptospirosis without hemolytic crises. The first rhinoceros presented with peracute depression, anorexia, rear leg trembling, dysuria, glucosuria, gastrointestinal discomfort, and decreased fecal output and died within 12 hr. Necropsy and histopathology revealed lesions within multiple organs. Leptospirosis was diagnosed postmortem based on positive fluorescent antibody staining of liver. The second rhinoceros presented 2 mo later with similar signs. It survived with treatment and was diagnosed with leptospirosis based on serology using microscopic agglutination testing and detection of urinary antigen using a fluorescent antibody technique. Leptospira kirschneri serovar grippotyphosa was postulated as the etiologic agent, with transmission probably occurring through wallow contamination by wild raccoons (Procyon lotor).  相似文献   
145.
146.
The deaths of two Asian elephants (Elephas maximus) in August 1996 led the United States Department of Agriculture to require the testing and treatment of elephants for tuberculosis. From August 1996 to September 1999. Mycobacterium tuberculosis infection was confirmed by culture in 12 of 118 elephants in six herds. Eight diagnoses were made antemortem on the basis of isolation of M. tuberculosis by culture of trunk wash samples; the remainder (including the initial two) were diagnosed postmortem. We present the case histories, epidemiologic characteristics, diagnostic test results, and therapeutic plans from these six herds. The intradermal tuberculin test, enzyme-linked immunosorbent assay serology, the blood tuberculosis test, and nucleic acid amplification and culture are compared as methods to diagnose M. tuberculosis infection in elephants.  相似文献   
147.
The trade-off between summer water conservation from alfalfa production and the effects on plant growth, agricultural crop value, and grower returns in the low desert area (Imperial, Palo Verde, Parker and Yuma Valleys of California and Arizona) was analysed using an experimental plot in the Palo Verde Valley. The purpose was to provide a basis for evaluating water conservation decisions and policies. Four summer dry-down periods (withdrawing water for a predetermined period) of 0 days (control), 35 days, 70 days, and 105 days were analysed. Our results show that whereas the various dry-down periods would provide water conservation ranging from 254 to 944 million m3, the region would experience agricultural income decline by US $16 to US $73 million. For growers, benefits of dry-down would be effective only if the water price exceeds US $0.045 m−3 in California and US $0.036 m−3 in Arizona. Also dry-down would have possible adverse effects in the following areas including declines in plant stand density, long-term productivity, service industry sales, employment and biological benefits of soil fertility and organic matter that is attributed by the alfalfa plant in which case agriculture's and growers monetary losses could increase.  相似文献   
148.
A 3-year-old neutered female mixed breed dog was examined because of severe, generalized seizure activity, tetraparesis, and encephalopathic signs. Cerebrospinal fluid (CSF) evaluation was unremarkable except for a mild increase in protein. Serum and CSF titers for infectious diseases were negative. Magnetic resonance (MR) imaging examination of the brain was performed and lesions were found within the cerebral gray matter of the temporal and parietal lobes. The lesions had increased signal intensity on T1, T2, and proton density-weighted images. There was mild inhomogeneous enhancement following intravenous contrast medium administration. Neurologic status improved and the seizures were well controlled, but the dog never regained normal mentation and euthanasia was performed 10 weeks after initial evaluation. At necropsy, severe cerebral cortical necrosis was found in the regions corresponding to the lesions seen on MR imaging examination. Large numbers of fat-containing macrophages (gitter cells) were found within these areas, and are thought to be responsible for the characteristic hyperintensity seen on the MR images.  相似文献   
149.
The objective of the present study was to examine the interaction between graded levels of leucine and dietary crude protein. Dose–response curves were generated using four 3 × 3 Latin squares (two dogs/square). Each square represented one of two concentrations of crude protein (140 or 280 g/kg diet) and one of two combinations of three concentrations of leucine (5.0, 7.0 and 9.0 g/kg diet or 9.0, 11 and 13 g/kg diet). An additional experiment was performed by feeding crude protein at 210 g/kg diet with either 7.0 or 11 g leucine/kg diet. Weight gain, food intake, nitrogen retention, plasma albumin and plasma amino acids were measured. The requirement was determined to be the minimum leucine concentration required to maximize weight gain and nitrogen retention. For 8–14-week-old male Beagle dogs, 140 g crude protein/kg diet in a diet containing 18 kJ metabolizable energy/g does not appear to support maximal growth. The leucine requirement was not affected by doubling the dietary crude protein level from 140 to 280 g/kg diet. From these results, the leucine requirement of 8–14-week-old Beagle dogs appears to be 11 g leucine/kg diet independent of the level of dietary crude protein, whereas dogs over 14 weeks require only 7 g leucine/kg diet for maximal nitrogen retention.  相似文献   
150.
OBJECTIVE: To compare the efficacy of a Salmonella bacterin and a modified live Salmonella ser. Choleraesuis vaccine on a commercial dairy. ANIMALS: 450 cows in late gestation and 80 calves. PROCEDURE: Group-1 cows (n = 150) were vaccinated once with a modified live S. Choleraesuis (serogroup C1) strain 54 (SC54) vaccine, group-2 cows (150) were vaccinated on enrollment and 30 days later with a Salmonella ser. Montevideo (serogroup C1) bacterin, and group-3 cows (150) served as unvaccinated controls. One gallon of colostrum harvested from the first 80 cows to calve was fed to each calf. Outcome assessments included fecal shedding of Salmonella spp for the first 10 days after parturition (cows) or birth (calves), milk production, involuntary culling rate, mastitis incidence, antimicrobial use, and mortality rate. RESULTS: Salmonellae were isolated from 306 of 309 (99%) cows and 64 of 74 (86.5%) calves. Shedding frequency was less in SC54-vaccinated cows and calves that received colostrum from those cows, compared with the other groups, and vaccination was specifically associated with less shedding of serogroup C1 salmonellae. Production data were similar among groups. CONCLUSIONS AND CLINICAL RELEVANCE: Vaccination of pregnant cows with an autogenous Salmonella bacterin had no effect on fecal shedding of salmonellae, whereas vaccination with a modified live S. Choleraesuis vaccine reduced the frequency of fecal shedding of serogroup C1 salmonellae during the peripartum period. A commercial S. Choleraesuis vaccine licensed for use in swine may be more efficacious than autogenous Salmonella bacterins on dairies infected with serogroup C1 salmonellae.  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