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   96353篇
  免费   5911篇
  国内免费   41篇
林业   4014篇
农学   3097篇
基础科学   567篇
  12072篇
综合类   16612篇
农作物   4001篇
水产渔业   4957篇
畜牧兽医   49152篇
园艺   1341篇
植物保护   6492篇
  2020年   854篇
  2019年   961篇
  2018年   1669篇
  2017年   1795篇
  2016年   1698篇
  2015年   1469篇
  2014年   1743篇
  2013年   4034篇
  2012年   3322篇
  2011年   3724篇
  2010年   2366篇
  2009年   2243篇
  2008年   3522篇
  2007年   3380篇
  2006年   3117篇
  2005年   2995篇
  2004年   2816篇
  2003年   2799篇
  2002年   2583篇
  2001年   3104篇
  2000年   2942篇
  1999年   2411篇
  1998年   1027篇
  1997年   909篇
  1996年   808篇
  1995年   1000篇
  1994年   891篇
  1993年   875篇
  1992年   1862篇
  1991年   1859篇
  1990年   1786篇
  1989年   1851篇
  1988年   1605篇
  1987年   1717篇
  1986年   1778篇
  1985年   1708篇
  1984年   1342篇
  1983年   1192篇
  1982年   856篇
  1979年   1136篇
  1978年   916篇
  1977年   791篇
  1976年   749篇
  1975年   821篇
  1974年   1020篇
  1973年   1059篇
  1972年   1014篇
  1971年   967篇
  1970年   913篇
  1969年   834篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
101.
Abstract. A two year field experiment was carried out in a semiarid Mediterranean area in order to evaluate, the effect on soil erosion of adding different urban organic wastes: a stabilized municipal waste (compost), an unstabilized municipal waste, and an aerobic sewage sludge. All the treatments significantly reduced soil erosion, compared to the control soil. The soil amended with compost was the most effective treatment, reducing soil loss by 94% and runoff by 54%.  相似文献   
102.
Flavonoids are important dietary constituents owing to their health-promoting properties. As a result, simplified analytic techniques are required for the population of databases with food values so that associations between dietary intake and disease risk/incidence can be established. The current research provides a simplified sample preparation procedure for the accurate estimation of food anthocyanidins, flavones, and flavonols as aglycons. Traditionally, flavonoid aglycons have been formed by acidic hydrolysis. However, some flavonoid aglycons are slowly degraded by acid. A procedure has been developed whereby anthocyanidins and flavonols are deglycosylated with HCl in 50% aqueous methanol and the resulting aglycons subsequently quantified by application of pseudo-first-order kinetics to their degradation. Flavones are also deglycosolated under similar conditions but, at appropriate temperatures, their aglycons are stable in acid, so kinetics were not required for the quantitation of this subclass of flavonoids. Catechins and flavanones were rapidly degraded under the hydrolytic conditions used in these studies.  相似文献   
103.
104.
Ceftiofur is a new broad spectrum cephalosporin marketed for the treatment of acute bovine respiratory disease. In this investigation ceftiofur was administered by intramuscular injection, at 24 h intervals, to healthy beef-bred calves for four days at dosages of 2.2 and 4.4 mg/kg of body weight, with 4 wk intervals between dosing regimens. Serum, tissue chamber fluid (TCF), and bronchial secretion (BS) concentrations of ceftiofur were measured by microbiological assay after the first and fourth dose of each dosing regimen. Peak serum concentrations (Cmax) of 8.8 micrograms/mL and 17.3 micrograms/mL were obtained approximately 2 h (Tmax), the time of mean peak concentration) after single injections of 2.2 mg/kg and 4.4 mg/kg, respectively. The Cmax was increased approximately twofold following multiple doses of 2.2 mg/kg (Cmax = 13.1 micrograms/mL) and 4.4 mg/kg (Cmax = 24.1 micrograms/mL). Ceftiofur accumulated slowly into TCF and peak concentrations were found to be approximately 14% of those observed in serum after the first dose and approximately 24% after multiple dosing. Concentrations of ceftiofur in BS were obtained rapidly with peak concentrations reaching 45% of the serum Cmax after the first dose. After multiple dosing the Cmax for BS was approximately 25% of the serum Cmax. This study found that both the 2.2 mg/kg and 4.4 mg/kg dosing regimens resulted in continuous serum, TCF and BS concentrations of ceftiofur that exceeded the minimal concentration required to inhibit the bacteria most frequently isolated from calves with acute bovine respiratory disease.  相似文献   
105.
A study of the basis of virulence variation of bovine rotaviruses.   总被引:3,自引:0,他引:3  
Rotaviruses are enteric pathogens of cattle but sub-clinical infections are common. Virulence variation has been identified with bovine rotaviruses and some rotaviruses replicated without clinical signs in non-immune calves. The rotavirus genome is composed of eleven segments of double-stranded RNA and the fourth largest segment codes for a non-glycosylated surface protein, VP4, which has been linked with virulence. In the present study the biological basis of rotavirus virulence variation was studied in vivo and compared with the known properties of the fourth gene. Calves were inoculated orally with a virulent rotavirus or a rotavirus of low virulence which multiplied but failed to cause diarrhoea. They were taken for necropsy at intervals of 2 days after inoculation. Clinical signs, virus in faeces and the percentage of infected small intestinal epithelium were determined. Damage to the small intestine was assessed by measurement of villus heights and crypt-cell production rates. Virulence was associated with a greater level of colonization of the small intestinal epithelium, greater enterocyte damage and preferential infection of the upper small intestine. The fourth gene determines the ability of rotaviruses to spread in vitro and the finding that virulence was associated with greater colonization in vivo raises the possibility that this gene may have an important role in rotavirus virulence.  相似文献   
106.
We investigated changes in hemostatic function after infusion of 6% dextran 70 (high molecular weight dextran) at 2 rates. Six healthy dogs underwent 3 regimens: 20 ml of dextran/kg of body weight administered in 1 hour (trial A), 20 ml of dextran/kg administered in 30 minutes (trial B), and 0.9% sodium chloride solution as a control administered over 1 hour to achieve hemodilution equivalent to that for 20 ml of dextran/kg (trial C). Before and at 2, 4, 8, and 24 hours after the start of trials A and B, we measured PCV, total solids (TS) concentration, amount of von Willebrand factor antigen (vWf:Ag), factor VIII coagulant activity (VIII:C), prothrombin time, activated partial thromboplastin time (APTT), platelet retention in a glass bead column, and buccal mucosa bleeding time (BMBT). Values were not obtained at 8 and 24 hours for trial C. Saline-induced changes in hemostasis were significant (P less than 0.05) from baseline throughout the sample collection period. Significant differences (P less than 0.05) between trial A and control were observed for vWf:Ag, VIII:C, BMBT, APTT, TS, and PCV values at 2 hours, and for VIII:C at 4 hours. Significant differences (P less than 0.05) between trial B and control were observed for APTT, TS, and PCV values at 2 hours, and for vWf:Ag, VIII:C, BMBT, APTT, TS, and PCV values at 4 hours. During trials A and B, mean values of analytes infrequently deviated from reference intervals, and clinical signs of bleeding were not observed in any dog.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
107.
Recombinant strains of herpesvirus of turkeys (HVT) were constructed that contain either the fusion protein gene or the hemagglutinin-neuraminidase gene of Newcastle disease virus (NDV) inserted into a nonessential gene of HVT. Expression of the NDV antigens was regulated from a strong promoter element derived from the Rous sarcoma virus long terminal repeat. Recombinant HVT strains were stable and fully infectious in cell culture and in chickens. Chickens receiving a single intra-abdominal inoculation at 1 day of age with recombinant HVT expressing the NDV fusion protein had an immunological response and were protected (> 90%) against lethal intramuscular challenge at 28 days of age with the neurotropic velogenic NDV strain Texas GB. Recombinant HVT expressing the NDV hemagglutinin-neuraminidase provided partial protection (47%) against the same challenge. Chickens vaccinated with recombinant HVT vaccines had low levels of protection against NDV replication in the trachea when challenged ocularly. Recombinant HVT vaccines and the parent HVT strain provided similar levels of protection to chickens challenged with the very virulent RB1B strain of Marek's disease virus, indicating that insertion of foreign sequences into the HVT genome did not compromise the ability of HVT to protect against Marek's disease.  相似文献   
108.
The practice of compensating wetland losses through wetland construction, restoration, or enhancement has become more commonplace; however, an appropriate method for assessing replacement of wetland function has been lacking. The Evaluation for Planned Wetlands (EPW) was developed to meet this need. It is a rapid assessment procedure which documents and highlights differences between a wetland assessment area and planned wetland based on their capacity to provide six functions: shoreline bank erosion control, sediment stabilization, water quality, wildlife, fish (tidal, non-tidal stream/river, and non-tidal pond/lake), and uniqueness/heritage. The differences between wetlands are expressed in terms of individual elements. Functional Capacity Indices, and Functional Capacity Units. The results provide information on individual design elements and measures of functional capacity which are a necessity under current regulatory programs that require tangible goals and a method for calculating planned wetland size. EPW includes functional assessment models, a procedure for using these models during the planning/mitigation process, and guidelines for functional design.  相似文献   
109.
Prepubertal gilts of obese (n = 24) or lean (n = 24) genetic lines were injected (s.c.) daily with 0, 2, or 4 mg of porcine somatotropin (pST) for 6 wk starting at 160 d of age to determine whether pST affects follicular function. Blood and ovaries were collected at slaughter 24 h after the last injection. Surface follicles greater than or equal to 1.0 mm in diameter were counted, and pools of follicular fluid (FFL) and granulosa cells were collected from 1.0- to 3.9-mm (small) and 4.0- to 6.9-mm (medium) follicles. Oocytes were collected from small and medium follicles and evaluated for maturational stage and viability. Porcine somatotropin increased (P less than .08) the numbers of small but not the numbers of medium follicles per gilt (P greater than .10). Oocyte maturation and viability were not affected by pST or genetic line. Porcine somatotropin increased (P less than .05) concentrations of insulin-like growth factor I (IGF-I) in serum and FFL of both obese and lean gilts; IGF-I was lower (P less than .01) in lean gilts. Treatment with pST decreased (P less than .05) IGF-II in FFL of lean but not in that of obese gilts. Dose of pST and line had no effect on concentrations of progesterone in FFL of small or medium follicles or on concentrations of estradiol in FFL of small follicles.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
110.
A strain of Yersinia enterocolitica of O serogroup 6,30 isolated from the liver of an aborted ovine fetus was inoculated intravenously into a group of pregnant ewes at about 90 days gestation and produced placentitis with abortion or delivery of infected lambs about 50 days later. Y. enterocolitica of the same serogroup was recovered from the necrotic placental cotyledons and most other fetal tissues and could be isolated from vaginal discharges of the ewes for a least 2 weeks after abortion. Histological changes were consistent with an acute bacterial necrotizing placentitis and systemic infection of the fetus. Subsequent pregnancies in the ewes proceeded to term without evidence of infection.  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