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   342185篇
  免费   18235篇
  国内免费   674篇
林业   25092篇
农学   14376篇
基础科学   3007篇
  48952篇
综合类   49776篇
农作物   17600篇
水产渔业   19202篇
畜牧兽医   149975篇
园艺   6646篇
植物保护   26468篇
  2019年   3787篇
  2018年   4715篇
  2017年   5097篇
  2016年   5302篇
  2015年   4636篇
  2014年   6055篇
  2013年   16723篇
  2012年   8833篇
  2011年   11308篇
  2010年   8542篇
  2009年   8877篇
  2008年   10568篇
  2007年   9989篇
  2006年   10072篇
  2005年   9275篇
  2004年   9096篇
  2003年   8921篇
  2002年   8151篇
  2001年   9491篇
  2000年   9196篇
  1999年   7916篇
  1998年   4506篇
  1997年   4532篇
  1996年   4225篇
  1995年   4871篇
  1994年   4286篇
  1993年   4027篇
  1992年   6373篇
  1991年   6576篇
  1990年   6468篇
  1989年   6507篇
  1988年   5876篇
  1987年   5965篇
  1986年   5887篇
  1985年   5920篇
  1984年   5123篇
  1983年   4774篇
  1982年   3609篇
  1981年   3411篇
  1980年   3296篇
  1979年   4559篇
  1978年   3875篇
  1977年   3499篇
  1976年   3313篇
  1975年   3317篇
  1974年   3842篇
  1973年   3950篇
  1972年   3698篇
  1971年   3545篇
  1970年   3451篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The activity of rat liver glutathione-insulin transhydrogenase (GIT) was measured by HPLC. The degradation of fluorescein isothiocyanate-I (FITC-I)-labeled insulin is separated into several peaks, which are bound different amount of FITC-I. We selected mono-fluorescein-thiocarbamylated insulin to estimate the decrease of insulin content and it became possible to assay GIT activity. This novel method was time-saving and simple, and this system could utilize instead of previous method.  相似文献   
992.
RNA was extracted from single or small groups of ovine ovarian follicles after treatment of ewes with FSH and/or LH. The content of mRNA for the alpha-inhibin and beta A-inhibin subunits was analyzed by hybridization with specific cDNA probes. All ewes were treated with progestin vaginal pessaries to suppress spontaneous preovulatory follicle maturation and ewes were given three intramuscular injections of gonadotropins at 8-hr intervals starting 24 hr prior to collection of ovaries. In experiment I, both Schering-FSH and NIDDK-oFSH-17 (oFSH) significantly increased alpha- and beta A-inhibin mRNA per ewe in 2-5 mm follicles and tended to increase alpha- and beta A-inhibin mRNA in large (greater than 5 mm) follicles. In experiment II, oFSH and NIDDK-oLH-25 (oLH) were administered in a 2X2 factorial arrangement. Separate administration of oFSH or oLH increased (P less than .05) the alpha-inhibin mRNA concentration in large follicles. alpha-inhibin mRNA concentration in 4-5 mm follicles was also increased by oFSH but was decreased by oLH. Concomitant treatment with oFSH and oLH did not change alpha-inhibin mRNA concentrations from those measured in oFSH treated ewes. In experiment II, beta A mRNA concentrations followed a pattern similar to that of alpha A mRNA, but the differences were not statistically significant. We conclude that, in the ewe, exogenous FSH increases the concentration of inhibin mRNA in the whole follicle. The ability of exogenous oLH to alter expression of the inhibin subunit genes may depend upon the stage of follicle maturation.  相似文献   
993.
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   
994.
The contamination of poultry in the Netherlands with Salmonella enteritidis was tested. For this, different methods (detection of S. enteritidis in faecal samples of 25 g; detection of S. enteritidis in cloacal swabs; detection of S. enteritidis by serological testing of antibodies in serum) were compared for their efficiency to detect S. enteritidis in flocks of poultry. Testing of faecal samples clearly yielded the best results. This method was used in a transmission study, in which 14 flocks descending from a contaminated primary mother flock were screened for the presence of S. enteritidis. The method was also used for screening 49 flocks of laying hens and 52 flocks of broiler chickens throughout the Netherlands. From the transmission study it became clear that S. enteritidis, phage type 2 (Dutch phage set) was isolated both from the mother flock and from five of the descendent flocks. Screening of poultry flocks for the presence of salmonella revealed that salmonella was present in 47% of the layer flocks and in 94% of the broiler flocks. S. enteritidis was isolated from 15% of the flocks screened.  相似文献   
995.
Feline lameness.     
Feline lameness is an increasingly recognized clinical problem. Today's veterinary practitioner must be comfortable with his/her ability to diagnose the various conditions responsible for lameness in the cat and be able to discuss the significance of their findings with the client. Disease of the feline musculoskeletal system can be grouped into broad categories, including: trauma, arthritis, infectious causes, developmental disorders, neoplasia, and muscular problems. Specific diseases from each of these categories will be discussed, particularly those that differ in their presentation or clinical behavior from that same disease in the canine patient.  相似文献   
996.
One-month-old Syrian hamsters of the APA and Std: golden strains were inoculated intraperitoneally with 10(5) PFU/head of the D variant of encephalomyocarditis (EMC) virus and examined virologically and pathologically up to 7 days after inoculation. APA hamsters developed apparent hyperglycemia due to pancreatic islet cell damage while Std:golden hamsters did not. Hamsters of both strains showed clear histopathologic changes in the testis with prominent viral replication as well as in the brain, heart and exocrine pancreas. The susceptibility to EMC virus-infection was higher in males than in females and in APA than in Std: golden hamsters.  相似文献   
997.
998.
999.
1000.
The association between ocular squamous cell tumors and various measures of solar radiation was evaluated for 2,775 contemporaneous Hereford cattle. The animals were from 34 herds located in 21 states in the United States and in one Canadian province. Examinations and photographs were made of the eyes of all cows and heifers in a herd; 33 herds were examined between June and August 1957 and 1 herd was examined in 1958. Solar radiation was measured by altitude and mean annual hours of sunshine applying to an individual herd, and global radiation, a cloudiness index, and elevation of nearby National Weather Service stations. All measures of solar radiation provided evidence of a significant association between increasing risks of developing cancer eye and increasing levels of radiation; adjustments were made for possible confounding effects of age and corneoscleral pigmentation. Associations were evident whether affliction was defined as the occurrence of any type of tumor (ie, plaque, papilloma, or carcinoma), or as the occurrence of only papilloma or carcinoma. Average ages of affected cattle also tended to be lower at high radiation levels than at low levels, further supporting the hypothesis that solar radiation has an important role in the development of this disease. In view of the heritability of eye pigmentation and its inhibitory effect on lesions, cancer eye may be controlled by selective breeding for increased pigmentation in and around the eye to afford protection against the harmful effects of incident UV radiation.  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