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   241974篇
  免费   14856篇
  国内免费   645篇
林业   23416篇
农学   13970篇
基础科学   2960篇
  42101篇
综合类   19983篇
农作物   17065篇
水产渔业   16496篇
畜牧兽医   91812篇
园艺   6258篇
植物保护   23414篇
  2021年   2428篇
  2020年   2842篇
  2019年   3633篇
  2018年   3946篇
  2017年   4272篇
  2016年   4740篇
  2015年   4278篇
  2014年   5628篇
  2013年   16198篇
  2012年   5960篇
  2011年   7774篇
  2010年   7202篇
  2009年   7825篇
  2008年   7269篇
  2007年   6275篇
  2006年   6996篇
  2005年   6247篇
  2004年   6091篇
  2003年   5817篇
  2002年   5190篇
  2001年   5622篇
  2000年   5358篇
  1999年   5067篇
  1998年   4116篇
  1997年   4152篇
  1996年   3897篇
  1995年   4452篇
  1994年   3888篇
  1993年   3570篇
  1992年   4154篇
  1991年   4233篇
  1990年   4049篇
  1989年   3891篇
  1988年   3480篇
  1987年   3450篇
  1986年   3413篇
  1985年   3728篇
  1984年   3500篇
  1983年   3324篇
  1982年   2764篇
  1981年   2704篇
  1980年   2694篇
  1979年   3039篇
  1978年   2746篇
  1977年   2593篇
  1976年   2432篇
  1975年   2242篇
  1974年   2308篇
  1973年   2330篇
  1972年   2040篇
排序方式: 共有10000条查询结果,搜索用时 718 毫秒
51.
Methods of augmenting bovine mononuclear cell responsiveness during physiological transitions of the udder may enhance resistance of the mammary gland to intramammary infections. Interleukin-2 is required for proliferation of T-lymphocytes and may contribute to B-lymphocyte proliferation. Recombinant bovine interleukin-2 (rBoIL-2) was evaluated as a potential immunoenhancer of bovine mammary gland mononuclear cells. Bovine mononuclear cells were isolated from five primiparous Holstein cows at 14-18 and 28-32 days of involution and at 7-13 days prior to parturition. Bovine blood and mammary gland mononuclear cells were highly responsive to rBoIL-2. Response of mammary gland mononuclear cells to rBoIL-2 was comparable with response of blood mononuclear cells. These data suggest that rBoIL-2 may be an effective immunoenhancer of bovine mononuclear cells during the non-lactating and prepartum periods.  相似文献   
52.
A disease causing the decline of papaya (Carica papaya) plants was noticed in the Jordan Valley in 1982. The disease caused severe yellowing of the upper leaves and dieback of the apex. The disease was not transmissible mechanically and continued efforts to associate viruses and viroids have so far been unsuccessful. Epidemiological observations suggested that the disease is airborne and probably caused by a mollicute. In order to test this hypothesis, plots were covered by an insect-proof net and with white nets providing 15, 30 and 50% shade. The 30%, 50% and insect-proof nettings provided complete protection, and the 15% netting reduced disease incidence to <2%, compared with>37% in the uncovered control plot. Spraying plants at weekly intervals with a commercial whitewash solution was also found to be effective in reducing disease incidence. The advantages of using a range of netting field treatments as a simple means for obtaining information on the epidemiological nature of a new disease are described.  相似文献   
53.
The clinical course of a feline leukaemia virus (FeLV)-negative and feline immunodeficiency virus (FIV)-positive cat affected with a large granular lymphocyte lymphoma is presented. Cyto-logical examination showed neoplastic cells in the pleural effusion and in two abdominal masses. Bone marrow and peripheral blood were moderately involved and chemotherapy was used to control the tumour. Cytochemistry, immunohis-tochemistry and ultrastructural studies were applied to define the cellular lineage; cytochemistry suggested a T-cell lineage.  相似文献   
54.
We have developed an antibody detection enzyme-linked immunosorbent assay (ELISA) for the identification of animals infected by feline immunodeficiency virus (FIV). The ELISA solid-phase antigen consists of recombinant FIV gag proteins expressed in bacteria. The proteins are purified from bacterial lysates as insoluble inclusion bodies. In the case of bacterially expressed p24gag, it is shown that all of the linear, sequential epitopes presented by viral p24 during infection are retained. Purified preparations can be substituted for solid-phase whole virus in the IDEXX PetChektm immunoassay. The antibody ELISA duplicates the sensitivity and specificity of the whole virus based PetChek plate assay.  相似文献   
55.
A systematic immunohistochemical study of the ultimobranchial tubule (UBT) has been carried out in 45 Wistar rats of different ages (0, 5, 10, 15, 20, 25, 30, 60 and 120 days). The existence of calcitonin immunoreactive cells in the UBT wall has been demonstrated in a 5-days old rat. In addition, immunohistochemical studies for thyroglobulin revealed positive staining in follicular cells connected to the UBT and, occasionally, in isolated cells lying within solid clusters from the UBT. These last results together with the continued and repeated existence of numerous mitosis and PAS (+) microfollicles, apparently rising from the UBT, support the hypothesis that the ultimobranchial body (UBB) may contribute partially to the formation of a part of the follicular component.  相似文献   
56.
Abstract. The use of an indirect enzyme-linked immunosorbent assay ( elisa ) for the detection of channel catfish antibody to Edwardsiella ictaluri is described. Changes in agglutination titre in fish immunized with Edwardsiella ictaluri heat killed whole bacterins or lipopolysaccharides were reflected by corresponding changes in elisa readings. Relatively high correlations were observed among elisa OD readings, computed elisa titres and corresponding agglutination titres.  相似文献   
57.
A sensitive and precise immunoassay for equine serum amyloid A protein (SAA) was established and used to determine, for the first time, the circulating concentration of this protein in health and disease. As in other species, equine SAA was present only at trace levels in healthy animals but behaved as an extremely sensitive and rapidly responding acute phase reactant following most forms of tissue injury, infection and inflammation, objectively reflecting the extent and activity of disease. Measurements of SAA should make a significant contribution to diagnosis and management of viral and bacterial infection in horses, and routine serial assays could provide an objective criterion for monitoring prospectively the general health of horses in training and racing.  相似文献   
58.
59.
White leghorn hens were experimentally infested with northern fowl mites (Ornithonyssus sylviarum) and antibody responses to mite immunogens were monitored over 12 weeks. Mite burdens increased during the early phase of infestation and declined over the latter weeks of the study. Antigen was prepared from homogenized whole mites, which were then sonicated and extracted with non-ionic detergent. Antigen extract was fractionated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and antibody-reactive polypeptides were identified by immunoblotting. At the start of infestation, hens had natural, pre-existing antibodies that reacted with several mite-extract components. Individual hens had different natural antibody reactivities; however, all birds had immunoglobulins reactive with extract polypeptides of 117,000, 77,000 and 36,000 molecular weight. A variety of mite extract components reacted with hen antibodies generated in response to experimental infestation. The number of antibody-reactive mite polypeptides increased through week 8 of infestation and then decreased by week 12. Fifteen polypeptides of northern fowl mite extract were reactive with antibodies developed by the majority of infested birds. These commonly reactive polypeptides had molecular weights ranging from 40,000 to 160,000. Glycoconjugates of fractionated mite extract were identified by blotting with lectins that have different carbohydrate binding specificities. Also identified were lectins that bound extract components with the same molecular weights as those moieties complexed by immunoglobulins of infested birds.  相似文献   
60.
The effect of food deprivation on ova transport, hormonal profiles and metabolic changes was studied in 20 crossbred multiparous sows during their second oestrus after weaning. To determine the time of ovulation, transrectal ultrasonographic examination was performed. The sows were divided into 2 groups, one control group (C-group), which was fed according to Swedish standards, and one experimental group (E-group). The E-group sows were deprived of food from the first morning meal after ovulation until slaughter. Blood samples were collected every second hour from about 12 h before expected ovulation in the second oestrus after weaning until slaughter and were analysed for progesterone, prostaglandin F2 alpha-metabolite, insulin, glucose, free fatty acids and triglycerides. All sows were slaughtered approximately 48 h after ovulation and the genital tract was recovered. The isthmic part of the oviduct was divided into 3 equally long segments and flushed separately with phosphate buffered saline (PBS). Uterine horns were also flushed with PBS. A significantly greater number of ova were found in the first and second part of the isthmus in the E-group (p = 0.05) while in the C-group most of the ova were found in the third part of the isthmus or the uterus (p = 0.01). The level of prostaglandin F2 alpha-metabolite was significantly higher in the E-group compared with the C-group. The concentration of progesterone increased in both groups after ovulation but there were no significant differences between the groups. The other blood parameters showed that the food-deprived sows were in a catabolic state. The 48 h period of fasting results, directly or indirectly in an delayed ova transport, which may be due to a delayed relaxation in the smooth circular muscle layer of the isthmus.  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