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   127355篇
  免费   6890篇
  国内免费   129篇
林业   5295篇
农学   3623篇
基础科学   720篇
  14074篇
综合类   23490篇
农作物   4752篇
水产渔业   5711篇
畜牧兽医   67696篇
园艺   1476篇
植物保护   7537篇
  2018年   1805篇
  2017年   1918篇
  2016年   1675篇
  2015年   1526篇
  2014年   1830篇
  2013年   4750篇
  2012年   3418篇
  2011年   4180篇
  2010年   2630篇
  2009年   2720篇
  2008年   4061篇
  2007年   3882篇
  2006年   3652篇
  2005年   3356篇
  2004年   3254篇
  2003年   3367篇
  2002年   3123篇
  2001年   3733篇
  2000年   3790篇
  1999年   3010篇
  1998年   1227篇
  1997年   1194篇
  1995年   1335篇
  1994年   1173篇
  1993年   1211篇
  1992年   2507篇
  1991年   2837篇
  1990年   2649篇
  1989年   2657篇
  1988年   2412篇
  1987年   2435篇
  1986年   2536篇
  1985年   2508篇
  1984年   1960篇
  1983年   1778篇
  1982年   1190篇
  1979年   1898篇
  1978年   1492篇
  1977年   1360篇
  1976年   1266篇
  1975年   1375篇
  1974年   1751篇
  1973年   1736篇
  1972年   1736篇
  1971年   1606篇
  1970年   1629篇
  1969年   1502篇
  1968年   1355篇
  1967年   1347篇
  1966年   1187篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Gray-scale ultrasonography was utilized in addition to radiography in the diagnosis of reproductive disease in 18 bitches. In 72% of the cases, ultrasonography was considered diagnostic because it revealed information on organ architecture, relationships of radiographically silhouetting soft tissue structures, and fetal viability that was unobtainable by radiography alone. In the remainder of the cases, ultrasonography was contributory to the diagnostic process by supporting the clinical and radiographic diagnoses. The benefits of ultrasonography are discussed, as is the ultrasonographic appearance of a variety of reproductive tract diseases.  相似文献   
992.
Performance records were analyzed for 1,869 purebred Duroc and Yorkshire gilts tested in littermate groups of two to four pigs in 703 pens. Traits studied were average daily gain during a standard test period from 56 d of age to 90.7 kg (ADG), average daily feed consumption during the test period (ADF), average backfat thickness (ABF) measured from polaroid photos of ultrasonic scans at 90.7 kg and efficiency of feed conversion for the whole test period (feed conversion efficiency, FCE). The main objectives were: 1) to estimate pertinent genetic and phenotypic parameters and 2) to discuss applications of the findings to the swine industry in the United States. A balanced 16% crude protein diet in pelleted form was provided ad libitum during the entire test period. Nested analyses of variance were computed with both the individual and pen mean records. Sire components of variance and covariance, adjusted for level of inbreeding, were used to estimate heritabilities and genetic and phenotypic variances, covariances and correlations. Heritabilities estimated from individual records were .098 for ADG and .423 for ABF. Estimates from pen means were .105 for ADF and .061 for FCE. Genetic correlations of FCE with ADG, ADF and ABF were estimated as -.520, -.520 and .694, respectively, while phenotypic correlations for the same traits were -.240, .570 and .212, respectively, all from pen means. Genetic and phenotypic correlations of ADG with ABF from individual records were .176 and .254, respectively. Implications for swine testing programs were discussed, including a proposal that would include FCE indirectly in an index with ADG and ABF, based on genetic covariances of FCE with ADG and ABF, thereby removing the need to measure FCE directly.  相似文献   
993.
Interrelationships between N-acetyl-beta-D-glucosaminidase (indicating cellular damage) and antitrypsin (indicating increased permeability between the blood and milk compartments) were evaluated in 1,411 quarter-milk samples collected during routine herd surveys. N-Acetyl-beta-D-glucosaminidase was antitrypsin, whereas, in more severe mastitis, antitrypsin had a more constant deflection. The sensitivity of both determinants was associated with the virulence of bacteria. Production of bacterial hemolytic toxins was associated with a significant increase in both determinants. Penicillinase production by staphylococci was associated with selective increases of antitrypsin.  相似文献   
994.
Samples of bulk tank milk and cow-composite milk from 23,138 dairy cows from 50 California dairies were examined by use of microbiologic procedures. The number of colonies of mastitis pathogens isolated per milliliter of bulk tank milk (used as a predictor of the percentage of infected cows in the herd) was evaluated, using simple regression analysis and Spearman's rank correlation. Correlations between the pathogens and the percentage of cows in each herd shedding the pathogens were found for Streptococcus agalactiae (r = 0.71) and mycoplasma (r = 0.59), but were considerably lower for other pathogens. When greater than or equal to 4,000 colonies of Streptococcus agalactiae were found per milliliter of bulk tank milk, at least 7% of the cows in the herd was shedding this organism. However, a pattern was not found between the number of mycoplasma colonies per milliliter of bulk tank milk and the percentage of infected cows in the herd.  相似文献   
995.
Evaluation of renal function in cats, using quantitative urinalysis   总被引:2,自引:0,他引:2  
Two consecutive 24-hour quantitative urinalyses were performed on each of 12 healthy adult cats to evaluate the technique and obtain reference values for measurements of urinary excretion of several substances. Endogenous creatinine clearance (2.31 +/- 0.47 ml/min/kg) and urinary protein excretion (17.43 +/- 9.05 mg/kg/day) were determined. Additionally, clearances and ratios to creatinine clearances were calculated for phosphate, sodium, potassium, and chloride. The endogenous creatinine clearance value was compared with another estimate of glomerular filtration rate that was based on 99mTc(Sn) diethylene-triaminepentaacetic acid clearance (2.52 +/- 0.58 ml/min/kg). Evaluation of feline renal function, using 24-hour quantitative urinalysis techniques, has potential for clinical application, but has several important limitations as well.  相似文献   
996.
A Y-maze avoid-avoid choice test was used to elucidate pregnant ewes' relative preference for electro-immobilization as opposed to restraint by a squeeze-tilt table. Choices in successive trials evaluating three commercial electro-immobilizers were: electro-immobilizer-13, 13 and 8% for respective models; squeeze-tilt table-79, 57 and 71%; and no choice-8, 30 and 21%. In all trials combined, 56% of the ewes never chose the electro-immobilizer after once experiencing it, while 94% did choose the squeeze-tilt table one or more times after being restrained by it. Most ewes became more willing to enter the table as experience with it increased, but those that had been both electro-immobilized and table-restrained became more hesitant to pass the test facility's entrance gate as these experiences increased. Ewes accepted a feed reward only reluctantly if at all after being electro-immobilized, but readily after table restraint. Electro-immobilization was clearly more aversive to the ewes than was restraint by a squeeze/tilt table. When restraint by either electro-immobilization or squeeze/tilt table is necessary, use of the table would be indicated in terms of its being less aversive.  相似文献   
997.
The therapeutic efficacy and pharmacokinetics of oxytetracycline (10 mg kg-1), ampicillin (20 mg kg-1) and a combination (TSS) of trimethoprim (20 mg kg-1), sulphadimidine (50 mg kg-1) and sulphamethylphenazole (50 mg kg-1) were investigated in normal dwarf goats and in those infected with Ehrlichia phagocytophila. Goats given oxytetracycline or TSS intravenously showed improvement, whereas ampicillin was ineffective. The infected goats had significantly prolonged elimination half-life values for sulphadimidine and oxytetracycline. The disposition kinetics of ampicillin and sulphamethylphenazole showed no marked differences between the healthy and infected animals. The tick-borne fever model used in the present study can be of value in testing the therapeutic efficacy and pharmacokinetics of chemotherapeutic agents in rickettsial infections.  相似文献   
998.
Detomidine is a sedative-analgesic which has a pharmacological profile similar to xylazine. There is evidence that the sedative effects are mediated through alpha-2 adrenoceptors.Cardiopulmonary responses were determined using detomidine as the principal agent and as a preanesthetic prior to the induction of general anesthesia. Compatibility with guaifenesin, sodium thia-mylal and halothane were determined.As in the case of xylazine, detomidine produces a slowing of heart rates. This was found to be either sinus bradycardia or heart block. There may be a corresponding increase in systolic blood pressures. The respiratory pattern is altered through the arterial blood gases and pH data supported evidence of adequate ventilation. The heart rate response to detomidine without anticholinergic treatment was transient and related to he duration of drug action.Atropine sulfate, 0.02 mg/kg i.v. was effective in preventing or treating bradycardia or heart block from detomidine. Heart rates also increased during the administration of guaifenesin and sodium thia-mylal when given 50 min poisit-detomidine.  相似文献   
999.
Fourteen limbal biopsy specimens from 11 dogs with chronic superficial keratitis (CSK) were examined histologically and immunohistochemically. Ten of the 14 specimens had corneal epithelial hyperplasia and/or atrophy. Eleven of the 14 specimens had thickened epithelial basement membranes. Each specimen had cellular infiltration and lamellar disruption of the stroma. An avidin-biotin immunoperoxidase complex stain was used to detect immunoglobulin (Ig) deposition. Twelve of the 14 specimens stained positive for Ig. The staining pattern was consistent and characterized by diffuse deposition of stain in the superficial conjunctival stroma near the limbus. Four of the 12 Ig-positive specimens also stained positive in the superficial corneal stroma with 1 of these 4 also staining positive along the epithelial cell basement membrane. The diffuse pattern of stain deposition and the absence of staining of specific epithelial structures indicated that CSK is not a classical autoimmune disease similar to any disease in the pemphigus group or similar to systemic lupus erythematosus. Although the results may implicate CSK as an immune-mediated disease, nonspecific factors could not be ruled out.  相似文献   
1000.
Microorganisms associated with the vagina, cervix and uterus of rabbits were isolated and identified. The predominant microorganisms isolated from the vaginas and cervices were coagulase-negative staphylococci, micrococci, and nonfermentative bacilli. Coagulase-negative staphylococci were isolated frequently, but in small numbers, from the uteri. The pH of the rabbit vagina was found to be near neutrality. Our data indicate that the genital flora of female rabbits is relatively simple, regarding the number and type of microorganisms.  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