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   101864篇
  免费   5686篇
  国内免费   317篇
林业   4518篇
农学   3628篇
基础科学   759篇
  13164篇
综合类   16188篇
农作物   3868篇
水产渔业   5074篇
畜牧兽医   52435篇
园艺   1292篇
植物保护   6941篇
  2019年   959篇
  2018年   1668篇
  2017年   1798篇
  2016年   1635篇
  2015年   1396篇
  2014年   1774篇
  2013年   4133篇
  2012年   3062篇
  2011年   3659篇
  2010年   2531篇
  2009年   2605篇
  2008年   3716篇
  2007年   3381篇
  2006年   3288篇
  2005年   2954篇
  2004年   3001篇
  2003年   2968篇
  2002年   2745篇
  2001年   3365篇
  2000年   3330篇
  1999年   2636篇
  1998年   1211篇
  1997年   1154篇
  1996年   1060篇
  1995年   1185篇
  1994年   1122篇
  1993年   986篇
  1992年   2059篇
  1991年   2184篇
  1990年   2216篇
  1989年   2101篇
  1988年   1954篇
  1987年   1842篇
  1986年   1935篇
  1985年   1781篇
  1984年   1504篇
  1983年   1355篇
  1979年   1476篇
  1978年   1175篇
  1977年   1024篇
  1976年   991篇
  1975年   1048篇
  1974年   1165篇
  1973年   1239篇
  1972年   1183篇
  1971年   1038篇
  1970年   1059篇
  1969年   1088篇
  1967年   957篇
  1966年   914篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
A nine-year-old, neutered female Shetland sheepdog was presented with crusted, ulcerative skin lesions affecting the footpads, commissures of the lips and the lateral canthi of the eyes. Histopathological examination of skin biopsies revealed changes consistent with superficial necrolytic dermatitis and biochemical analysis demonstrated elevated liver enzymes. Abdominal radiography revealed a small liver which, on ultrasonography, appeared diffusely mottled and showed changes suggestive of periportal fibrosis. On exploratory laparotomy, the pancreas appeared normal, but the liver was small and had multiple nodules throughout the parenchyma. This appearance was confirmed as cirrhosis on histopathological examination. The dog was placed on a hepatic support diet and treated with colchicine, essential fatty acid supplementation and raw egg yolks. After four weeks, the skin lesions had resolved and the dog remained free of clinical signs over a 22-month follow-up period.  相似文献   
942.
A technique was developed for closure of gastrotomy and enterotomy incisions using disposable skin staples. The technique was used successfully in three dogs with gastrointestinal linear foreign bodies that required a gastrotomy and one or more enterotomies. The method allows for secure closure of gastrointestinal incisions and minimizes the surgical time in patients requiring multiple gastrointestinal incisions.  相似文献   
943.
Thurston RJ  Korn N 《Avian diseases》2000,44(4):1007-1011
A case is described wherein a turkey male (tom) possessed two vent areas with duplicated cloacae served by one large intestine. Both cloacae were functional in that feces were excreted and semen could be collected from each. The left vas deferens and ureter emptied into the left cloaca, and the right vas deferens and ureter emptied into the right cloaca. This allowed semen from each testis/duct to be collected separately from the corresponding cloaca. Thus, a unique opportunity was presented to collect semen separately from the left and right testis/duct system for semen analysis and fertility determination. Sperm concentration and percentage of dead sperm were not significantly different when semen from left vs. right reproductive tract were compared. The concentrations of spermiophages in semen from both reproductive tracts fell into the range reported for normal semen (0-8 x 10(5)/ml); however, semen from the right side had consistently higher spermiophage concentrations than that from the left side. On the basis of observations from one male made possible by an anatomic anomaly, it appears that the fecundities of the left and right testis/duct systems of the turkey male do not significantly differ and that recruitment of spermiophages into one tract (because of immunologic challenge, etc.) does not necessarily mean that the opposite testis/ducts will respond similarly.  相似文献   
944.
Genetic typing of classical swine fever virus   总被引:18,自引:0,他引:18  
Three regions of the classical swine fever virus (CSFV) genome that have been widely sequenced were compared with respect to their ability to discriminate between isolates and to segregate viruses into genetic groups. Sequence data-sets were assembled for 55 CSFVs comprising 150 nucleotides of the 5' non-translated region, 190 nucleotides of the E2 envelope glycoprotein gene and 409 nucleotides of the NS5B polymerase gene. Phylogenetic analysis of each data-set revealed similar groups and subgroups. For closely related viruses, the more variable or larger data-sets gave better discrimination, and the most reliable classification was obtained with sequence data from the NS5B region. No evidence was found for intertypic recombination between CSFVs. A larger data-set was also analysed comprising 190 nucleotides of E2 sequence from 100 CSFVs from different parts of the world, in order to assess the extent and global distribution of CSFV diversity. Additional groups of CSFV are evident from Asia and the nomenclature of Lowings et al. (1996) [Lowings, P., Ibata, G., Needham, J., Paton, D., 1996. J. Gen. Virol. 77, 1311-1321] needs to be updated to accommodate these. A tentative assignment, adapting rather than overturning the previous nomenclature divides CSF viruses into three groups with three or four subgroups: 1.1, 1.2, 1.3; 2.1, 2.2, 2.3; 3.1, 3.2, 3.3, 3.4. The expanding data-base of CSFV sequences should improve the prospects of disease tracing in the future, and provide a basis for a standardised approach to ensure that results from different laboratories are comparable.  相似文献   
945.
This study revealed the dose dependency of prednisolone tertiary-butylacetate (PTBA) treatment on the establishment of Echinococcus multilocularis in the small intestine of Mongolian gerbil (Meriones unguiculatus) and that some of the physiological parameters of host were correlated with the doses of PTBA and establishment of the worm. Twenty Mongolian gerbils were divided into 5 groups, according to the doses of PTBA; 0 mg, 0.5 mg, 2 mg, 5 mg and 10 mg per head. All animals were injected intraperitoneally with PTBA every other day from 6 days before to 6 days after infection. Doses of PTBA and the number of worms recovered at 7 days post-infection showed a positive correlation (r = 0.929, P < 0.0001). The increase of total protein (TP) and the decrease of the percentage of lymphocytes in the peripheral leukocytes were dependent on doses of PTBA (TP: r = 0.811, P < 0.0001, percentage of lymphocyte: r = -0.92, P < 0.0001). The TP and the percentage of lymphocyte also correlated with the number of worms recovered (TP: r = 0.617, P = 0.0049; percentage of lymphocyte: r = -0.800, P < 0.0001).  相似文献   
946.
Mycobacterium avium ss paratuberculosis causes a chronic progressive enteritis in cattle and other ruminants referred to as Johne's disease. It also has been suggested by some as possibly being associated with Crohn's disease in humans. In a previous study we observed that incubation of bovine monocytes with recombinant bovine growth hormone (bGH) altered the ingestion and intracellular growth of M. avium ss paratuberculosis in vitro. This led us to investigate whether bGH also has a direct effect on M. avium ss paratuberculosis. We observed that addition of bGH (5 microg/ml) had a direct inhibitory effect on the growth of M. avium ss paratuberculosis in Middlebrook 7H9 broth. In contrast, the growth of Mycobacterium smegmatis was unaffected, even at a bGH concentration of 50 microg/ml. Using 125I-bGH we observed high affinity binding (Kd = 1.32 nM) of bGH to M. avium ss paratuberculosis, with an estimated 204 binding sites per bacillus. To the best of our knowledge, this is the first report of a mammalian hormone binding to this important enteric pathogen.  相似文献   
947.
The aim of this study was to investigate the effects of a porcine reproductive and respiratory syndrome virus (PRRSV) infection on the development of the immune response after pseudorabies virus (PRV) vaccination in pigs. Pigs were intranasally inoculated with the European PRRSV strain, Lelystad virus ter Huurne, and were vaccinated intramuscularly with PRV 2 weeks later (LV-PRV group). Control pigs were vaccinated with PRV only (PRV group). Eight weeks after PRV vaccination, pigs from both groups were challenged intranasally with wild-type PRV. We measured the lymphoproliferative, and the cytolytic responses to PRV of peripheral blood mononuclear cells (PBMC), isolated from blood samples. In addition, serum samples were examined for antibodies against PRV and LV. One week after PRV vaccination, PBMC proliferated abundantly to PRV in both groups. However, in the LV-PRV group the lymphoproliferative response declined after 1 week, whereas, in the PRV group, the lymphoproliferative response was high for 3 weeks and declined thereafter (P<0.05). After challenge, the lymphoproliferative response was 1 week earlier and was consistently and significantly higher in the PRV group than in the LV-PRV group. The PRV-specific killing was higher at 3 weeks after PRV vaccination and 5 weeks after PRV challenge 19+/-3 and 24+/-6%, respectively, in the PRV group, compared to 7+/-4 and 6+/-9%, respectively, in the LV-PRV group (P<0.05). However, later after vaccination and challenge the cytolytic response was identical in both groups. The antibody titre against PRV developed equally in both groups. After challenge, no PRV virus was isolated from both groups. From these results we conclude that, although PRRSV infection did cause changes in the time course of the T-lymphocyte response after PRV vaccination, PRRSV infection did not inhibit the development of vaccine-induced protection after PRV.  相似文献   
948.
OBJECTIVE: To identify risk factors associated with respiratory tract disease in horses during 3 epidemics caused by influenza virus infections. DESIGN: Cross-sectional and prospective longitudinal observational studies. ANIMALS: 1,163 horses stabled at a Thoroughbred racetrack. PROCEDURES: Investigations were conducted during a 3-year period. An epidemic of respiratory tract disease caused by influenza virus infections was identified in each year. Routine observations and physical examinations were used to classify horses' disease status. Data were analyzed to identify factors associated with development of disease. RESULTS: Results were quite similar among the epidemics. Concentrations of serum antibodies against influenza virus and age were strongly associated with risk of disease; young horses and those with low antibody concentrations had the highest risk of disease. Calculation of population attributable fractions suggested that respiratory tract disease would have been prevented in 25% of affected horses of all horses had high serum antibody concentrations prior to exposure. However, recent history of vaccination was not associated with reduction in disease risk. Exercise ponies had greater risk of disease than racehorses, which was likely attributable to frequent horse-to-horse contact. CONCLUSIONS AND CLINICAL RELEVANCE: Particular attention should be paid to young horses, those with low serum antibody concentrations, and horses that have frequent contact with other horses when designing and implementing control programs for respiratory tract disease caused by influenza virus infections. It appears that control programs should not rely on the efficacy of commercial vaccines to substantially reduce the risk of disease caused by influenza virus infections.  相似文献   
949.
Between 1977 and 1992, 15 adult dogs were presented to the Universities of Bristol, Glasgow, Liverpool and London with fractures of the radial carpal bone which had occurred without a known episode of violent trauma. Eleven of the dogs were boxers and the other breeds affected were a Pharaoh hound, rottweiler, springer spaniel and a greyhound. The condition was bilateral in three cases. Eleven dogs were treated surgically by cast immobilisation (four cases), fragment removal (one case), lag screw fixation (three cases), pan-carpal arthrodesis (one case) and lag screw reduction followed by pan-carpal arthrodesis (two cases). The remaining four dogs were treated conservatively with rest and tactical use of non-steroidal anti-inflammatory drugs. Follow-up periods ranged from two months to 11 years. Varying degrees of lameness, which tended to be exacerbated by exercise, persisted in all cases. The radial carpal bone appears to have at least three separate centres of ossification which eventually fuse; their planes of fusion correspond approximately to the two main fracture lines. These areas of fusion could be weak points within the radial carpal bone.  相似文献   
950.
The insecticidal effects of the phenylpyrazole, fipronil, and a pyrethroid, beta-cyfluthrin, on larvae of the blowfly Lucilia sericata were determined in laboratory assays. When first stage larvae of L. sericata were reared on homogenized pig liver which had been treated with known amounts of test compounds, both fipronil and beta-cyfluthrin induced significant levels of mortality compared to acetone and water controls. However, fipronil was approximately 10 times more toxic than beta-cyfluthrin to L. sericata larvae following ingestion. Beta-cyfluthrin had little effect on mortality until concentrations of approximately 0.5 ppm were reached. In contrast, fipronil effected L. sericata mortality at a concentration of 0.05 ppm and 100% mortality was reached by 0.5 ppm. The lethal concentration (LC50) value for beta-cyfluthrin was 1.56 ppm as compared to 0.14 ppm for fipronil. Following contact of first and third stage larvae with cloth impregnated with known amounts of test compound, the mortality profiles of fipronil and beta-cyfluthrin were similar. At short contact times, the LC50 values for fipronil were lower than those for beta-cyfluthrin. However, at the highest contact time evaluated for the first stage larvae, 300 s, there was a reversal in this trend. The results suggest that the phenylpyrazole fipronil may represent a new potential insecticide for development against blowfly strike of sheep.  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