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   65719篇
  免费   2762篇
  国内免费   34篇
林业   5344篇
农学   2951篇
基础科学   384篇
  8616篇
综合类   8951篇
农作物   3613篇
水产渔业   4533篇
畜牧兽医   27754篇
园艺   1628篇
植物保护   4741篇
  2019年   428篇
  2018年   3457篇
  2017年   3466篇
  2016年   1911篇
  2015年   680篇
  2014年   795篇
  2013年   1827篇
  2012年   2166篇
  2011年   3821篇
  2010年   3233篇
  2009年   2328篇
  2008年   2998篇
  2007年   3182篇
  2006年   1569篇
  2005年   1534篇
  2004年   1531篇
  2003年   1556篇
  2002年   1270篇
  2001年   1935篇
  2000年   2043篇
  1999年   1507篇
  1998年   577篇
  1997年   501篇
  1996年   453篇
  1995年   617篇
  1994年   526篇
  1993年   474篇
  1992年   1166篇
  1991年   1220篇
  1990年   1184篇
  1989年   1114篇
  1988年   1072篇
  1987年   1092篇
  1986年   1045篇
  1985年   984篇
  1984年   807篇
  1983年   686篇
  1982年   422篇
  1981年   385篇
  1979年   640篇
  1978年   501篇
  1977年   449篇
  1976年   409篇
  1975年   451篇
  1974年   525篇
  1973年   509篇
  1972年   533篇
  1971年   452篇
  1970年   425篇
  1969年   467篇
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
151.
A newly recognised disease, which we have given the provisional name of familial stomatocytosis-hypertrophic gastritis (FSHG), is described in two families of dogs of the Drentse partrijshond breed. The affected dogs consisted of 3 females and 5 males, 3 to 19 (mean 9.5) months of age at admission. The main clinical problems were diarrhoea, icterus, and ataxia and paresis of the pelvic limbs. Laboratory evaluation revealed abnormal red cell shape (stomatocytosis), increased osmotic fragility, haemolytic anaemia, and increased liver enzymes and serum bilirubin. Gastroscopic and histopathologic examination of the gastric mucosa revealed hypertrophic gastritis resembling Ménétrier's disease in man. Histologic findings in the liver were suggestive of progressive liver disease. Cysts were found in the kidneys of the five oldest patients. Electroneurography in 2 dogs revealed polyneuropathy. In the parents of 2 patients (sister and brother), there were no clinical or laboratory abnormalities. An autosomal recessive hereditary defect of lipid metabolism is suspected.  相似文献   
152.
Deficiencies of vitamins A, D, K, E and thiamin can cause severe limitations in beef production. In particular, vitamin A and E can be common causes of lost profit, secondary to limitations of reproductive and growth potential. Prolonged dry periods will reduce available A and E in pasture forage, as can ensiling and prolonged storage of harvested feedstuffs. Polioencephalomalacia is a thiamin responsive disorder, associated with high concentrate feeding and lush pastures. Antimetabolites, such as amprolium, will cause thiamine deficiency when fed in excess. Recent information has shown improved performance with supplemental beta carotene and niacin. The positive responses in reproductive performance, noted with cattle fed supplemental beta carotene, was independent of vitamin A. Supplementation of vitamins above National Research Council recommendations can be justified. However, proper evaluation of feed and animal status, and documentation of a response to supplementation is necessary before diagnosing deficiencies of specific nutrients.  相似文献   
153.
Cellular alterations in level of expression of mRNA encoding for prostaglandin endoperoxide synthase were quantified within ovarian tissues of sheep obtained before, during and after induction of the preovulatory surge of LH and ovulation with LHRH. This was accomplished by isotopic in situ hybridization using a selective cRNA probe to ovine prostaglandin endoperoxide synthase mRNA. A significant elevation in mRNA was detected within the theca interna of the preovulatory follicle at 8, 16 and 24 hr following administration of LHRH. Very close to the time of ovulation (ie., at 24 hr post-LHRH) a marked rise in mRNA was observed in association with epithelial cells covering the apical surface of the follicle. Ovarian cyclooxygenase metabolites of arachidonic acid produced during the ovulatory process in the ewe originate within the thecal layer and germinal epithelium of the follicle destined to ovulate.  相似文献   
154.
155.
156.
Following diagnosis of scrapie in a clinically suspect Suffolk sheep, 7 clinically normal flockmates were purchased by the Pennsylvania Department of Agriculture to determine their scrapie status using an immunohistochemical procedure. Two of the 7 euthanized healthy sheep had positive immunohistochemical staining of the prion protein of scrapie (PrP-Sc) in their brains, nictitating membranes, and tonsils. The PrP-Sc was localized in the areas of the brain where, histopathologically, there was neurodegeneration and astrocytosis. The PrP-Sc occurred within germinal centers of the affected nictitating membranes and tonsils and was located in the cytoplasm of the dendrite-like cells, lymphoid cells, and macrophages. These results confirm that immunohistochemical examination of the nictitating membrane can be used as a screen for the presence of scrapie infection in clinically normal sheep at a capable veterinary diagnostic laboratory. In sheep with a PrP-Sc-positive nictitating membrane, the diagnosis of scrapie should be confirmed by histopathology and immunohistochemical examination of the brain following necropsy. Following full validation, immunohistochemistry assays for detection of PrP-Sc in nictitating membrane lymphoid tissues can improve the effectiveness of the scrapie control and eradication program by allowing diagnosis of the disease in sheep before the appearance of clinical signs.  相似文献   
157.
Humoral immune responses of black-footed penguins (Spheniscus demersus) to DNA-mediated immunization with a beta-galactosidase reporter gene expression plasmid were evaluated. Six male and 6 female adult penguins received either test plasmid, pCMV-beta, containing the beta-galactosidase gene or control plasmid, pCI, lacking a gene for expression. Three birds from each group were used previously in a diluent control group and given one injection of sterile saline. All samples were screened for anti-beta-galactosidase antibodies by indirect enzyme-linked immunosorbent assay with anti-chicken immunoglobulin G as secondary antibody. Antibodies to beta-galactosidase were detected in the sera of pCMV-beta-inoculated penguins, with a peak response on day 21. Antibody titers of the test plasmid group versus both control groups on days 21, 28, and 42 differed significantly. These results demonstrate that black-footed penguins can be safely transfected with the gene encoding beta-galactosidase and will mount a humoral response against the in vivo-expressed protein. Knowledge from this initial study can be applied to the development of DNA-mediated vaccines against specific infectious diseases of penguins.  相似文献   
158.
159.
A study consisting of two trials was conducted to determine the effects of monensin on the apparent absorption and retention of magnesium (Mg), phosphorus (P), calcium (Ca) and zinc (Zn) and to determine mineral changes in tissue and ruminal fluid. Eight lambs (39 kg) were used in trial 1, and 10 lambs (37 kg) were used in trial 2. Animals were blocked by weight and fed a high concentrate diet with or without 20 mg/kg monensin. Trials began with a dietary adjustment period lasting 18 d in trial 1 and 21 d in trial 2. Animals were then placed in metabolism stalls for a 10-d stall adjustment period followed by a 12-d collection period. Collections to determine mineral balance were made during the first 10 d of the collection period. Blood and ruminal fluid samples were taken on d 11 of the collection period. Lambs were slaughtered on d 12 of the collection period and tissue samples were collected. Monensin supplementation increased (P less than .05) Mg retention 42.0%. Urinary Ca excretion decreased (P less than .05) 60.0% when monensin was fed. Monensin supplementation decreased (P less than .05) liver Ca and bone Ca, 45.5 and 2.9%, respectively. Apparent P digestibility increased (P less than .05) 40.0% and P retention increased (P less than .10) 26.8% due to monensin supplementation. Both apparent absorption and retention of Zn increased (P less than .01) 50.0 and 45.0%, respectively, with monensin supplementation. Ruminal fluid Zn concentrations decreased (P less than .05) 33.0% with the addition of monensin.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
160.
Eight calves, weighing 50-150 kg, were given intramuscularly 5 ml of ampicillin (ABPC) aqueous suspensions (200 mg potency/ml) in their right and left gluteal and femoral regions. All calves were sacrificed one hour later to confirm the location of injected drug. The drug was found in a muscle layer when injected with a needle 15 mm long to the following positions, 1. the midpoint between the central position of the gluteal region (CG) and the tuber coxae (M-CTc), 2. the midpoint between CG and the tuber ossis ischii (M-CTo), 3. the central position of M. semimembranaceus in the femoral region (CF). Seven calves, weighing 130-150 kg, were given intramuscularly 5 ml of ABPC suspensions at M-CTo and CF and sacrificed one hour (4 calves) and 3 days (3 calves) later. ABPC diffused along the long axis of the muscle fibers but not to the radial direction. ABPC was detected only in the injected muscle layer even after 3 days indicating that the drug did not diffuse to the neighboring muscles. In the injected muscle layer, concentration of ABPC was remarkably different from part to part. From these results, sampling of the injected muscle for the drug residue study was proposed as follows: 1. isolate about 100 g of muscle just under the stick point marked on the skin considering the direction of drug diffusion, and 2. isolate separately about 200 g of the surrounding muscle to confirm if the sampling is appropriate.  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