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   136997篇
  免费   7878篇
  国内免费   70篇
林业   5673篇
农学   4705篇
基础科学   863篇
  17834篇
综合类   19004篇
农作物   5208篇
水产渔业   7306篇
畜牧兽医   73393篇
园艺   1816篇
植物保护   9143篇
  2020年   1238篇
  2019年   1487篇
  2018年   2309篇
  2017年   2547篇
  2016年   2360篇
  2015年   2054篇
  2014年   2520篇
  2013年   5803篇
  2012年   4476篇
  2011年   5284篇
  2010年   3559篇
  2009年   3612篇
  2008年   5237篇
  2007年   5053篇
  2006年   4723篇
  2005年   4303篇
  2004年   4146篇
  2003年   4210篇
  2002年   3832篇
  2001年   4779篇
  2000年   4641篇
  1999年   3748篇
  1998年   1473篇
  1997年   1453篇
  1996年   1342篇
  1995年   1601篇
  1994年   1330篇
  1993年   1364篇
  1992年   2763篇
  1991年   2902篇
  1990年   2866篇
  1989年   2898篇
  1988年   2610篇
  1987年   2592篇
  1986年   2539篇
  1985年   2402篇
  1984年   1957篇
  1983年   1715篇
  1982年   1154篇
  1979年   1649篇
  1978年   1284篇
  1977年   1135篇
  1976年   1100篇
  1975年   1164篇
  1974年   1357篇
  1973年   1449篇
  1972年   1378篇
  1971年   1195篇
  1970年   1247篇
  1969年   1146篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
A five-year-old male, West Highland terrier was treated for a gastrointestinal foreign body via a gastrotomy incision and enterotomies. The dog was asymptomatic for several months but was eventually re-presented because of vomiting. Endoscopic and surgical evaluation confirmed a large mass within the pyloric canal that probably resulted from a reaction to the polypropylene suture material used to close the original gastrotomy incision. The mass was excised via a Y-U pyloroplasty incision and the dog has been free of vomiting for over a year.  相似文献   
972.
Genetic factors affecting female reproductive performance in lines of mice with a known history of selection were estimated from a 5 X 5 diallel cross. Lines were selected as follows: large litter size at birth (L+); large 6-wk body weight (W+); an index for large litter size and small 6-wk body weight (L+W-); the complementary index (L-W+) and randomly (K). Partitioning of direct and correlated responses for litter size, 6-wk body weight and related traits into average direct genetic (li) and average maternal genetic (mi) effects indicated that the magnitude of differences in li exceeded those in mi. Lines having positive responses in li were W+ greater than L+ greater than L-W+ for dam body weight, L+ greater than L+W- greater than W+ for litter size and L+ greater than (W+, L+W-) for litter birth weight, whereas L-W+ responded negatively for litter size. A positive association was found between mi for litter size and dam body weight, W+ and L-W+ being high and L+ and L+W- low for both traits. Female infertility and time from male exposure to parturition had relatively small correlated responses. Line rankings in general combining ability (gi) and net line effects were similar for the respective traits. Depending upon the line and trait involved, the relative contribution of average direct genetic and line direct heterotic (hi) effects to general combining ability [gi = (1/2) li + hi] varied. Line heterosis refers to average heterosis in crosses involving that line. Direct heterosis ( hij ) for each trait differed considerably among crosses. The three crosses showing the highest hij for litter size at birth, W+ X L-W+ (1.78), L+ X W+ (1.28) and L-W+ X L+W- (1.22), possibly had loci contributing directional dominance to litter size with frequencies of parental lines deviating in opposite directions relative to mean gene frequency. The correlation between absolute difference in parental line means and hij for litter size was not significant, suggesting that the magnitudes of absolute differences in parental means were not reliable predictors of divergence in gene frequency. Crossbred performance increased linearly with midparent values for litter size at birth (b = .88 +/- .09, R2 = .92) and dam parturition body weight (b = 1.13 +/- .04, R2 = .99), the latter trait showing an increase (P less than .01) in heterosis as midparent values increased.  相似文献   
973.
Fifty-one treponemas were isolated from pigs. Twenty-three isolates with typical morphology and growth characteristic were beta hemolytic, enteropathogenic, produced indole and with exception of three strains did not ferment fructose. These strains were classified as typical T. hyodysenteriae and were usually isolated from pigs with symptoms of mucohemorrhagic diarrhoea. The seventeen other isolates were weakly beta hemolytic after 48 h incubation, enteropathogenic, 12 out of 17 produced indole, 10 out 17 fermented fructose. These strains were usually isolated from pigs with symptoms of gray-green diarrhoea and classified as T. hyodysenteriae 2 biotype or intermediate type. They may be compared with Treponema sp. isolated by Taylor et al. Eleven non enteropathogenic strains showed typical characteristic for T. innocens. Gas chromatography analysis of the fatty acids production from glucose, showed that all isolated treponemas produced acetate and butyrate. Typical T. hyodysenteriae produced additionally propionate. Strains of T. hyodysenteriae biotype 2 produced propionate or isobutyrate as well.  相似文献   
974.
975.
The antigenic relationships, antigenic spectrum, and immunogenicity of seven IBV-Massachusetts-41 isolates were investigated using the hemagglutination-inhibition (HI) test. HI titers equal to 32 are considered suspicious, titers lower than 32 are considered negative, and titers higher than 32 are considered positive immune responses to infectious bronchitis virus (IBV). Some isolates of Massachusetts-41 ( M41 ) were capable of inducing large quantities of antibodies in chickens following inoculation and demonstrated a wider antigenic spectrum than others. Variations in antigenic spectrum observed within M41 isolates in this study are in agreement with previous reports. This variation is of importance in selecting a proper vaccine strain for a successful immunization program for IBV.  相似文献   
976.
Indirect immunofluorescence testing for pemphigus-like antibodies was performed on 79 horses: 28 horses with various nonpemphigus dermatologic diseases, 21 horses with various nondermatologic diseases, and 30 normal horses. Pemphigus-like antibodies were detected in 6 horses: 3 normal horses with titers of 1:40, 2 horses with dermatophilosis at titers of 1:10 and 1:80, and 1 horse with lymphosarcoma at a titer of 1:320. It was concluded that equine pemphigus-like antibodies are a potential source of misinterpretation and misdiagnosis in indirect immunofluorescence testing. Direct immunofluorescence testing for whole immunoglobulin, IgG, IgM, and IgA was performed on skin lesions from 2 horses with dermatophilosis. Diffuse intercellular deposition of whole immunoglobulin and IgG was found in both horses. It was concluded that equine dermatophilosis is a potential source of misinterpretation and misdiagnosis in direct immunofluorescence testing.  相似文献   
977.
978.
Skeletal muscle degeneration and necrosis in a wild population of bluegills (Lepomis macrochirus) were investigated. Hemorrhage and large areas of muscle necrosis were evident at necropsy. Histologically, muscle fibers were granular, vacuolated, and fragmented. Ultrastructural alterations included mitochondrial swelling, clumping and loss of actin and myosin fibrils, swelling of T tubules, and loss of continuity of the sarcolemma. An etiologic agent was not identified.  相似文献   
979.
Antagonism of xylazine sedation in steers by doxapram and 4-aminopyridine   总被引:1,自引:0,他引:1  
Five groups of 6 fasted crossbred steers were injected IM with standard dosages of xylazine hydrochloride (0.3 to 0.5 mg/kg). At maximal sedation, the steers were injected IV with the antagonists' doxapram (1.0 mg/kg), doxapram + yohimbine (0.125 mg of yohimbine/kg), doxapram + 4-aminopyridine (4-AP; 0.3 mg of 4-AP/kg), or 4-AP + yohimbine. One group was given 1.0 ml of saline solution IV instead of antagonists. Doxapram, doxapram + yohimbine, doxapram + 4-AP, and 4-AP + yohimbine decreased mean standing time (time from antagonist injection until animal could stand unaided) to 17.0, 4.3, 3.3, and 4.5 minutes, respectively--significantly (P less than 0.05) down from a control value of 49.8 minutes. Mean total recovery time (time from xylazine injection until animal resumed eating) was decreased to 78 minutes by doxapram and 81.6 minutes by doxapram + 4-AP--significantly (P less than 0.05) down from the control value of 142.9 minutes. Respiratory character was improved (depth of respiration was increased) only by doxapram + 4-AP. Relapses to recumbency and marked sedation were not seen in steers given doxapram + 4-AP or the saline solution. One steer given doxapram, 2 given doxapram + yohimbine, and 1 given 4-AP + yohimbine relapsed to recumbency and sedation. Recovery was relatively smooth in steers given doxapram + 4-AP or 4-AP + yohimbine. Animals given doxapram or doxapram + yohimbine had difficult recoveries.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
980.
Measurement of plasma antithrombin III activity in healthy horses   总被引:1,自引:0,他引:1  
A fluorometric assay was used to determine plasma antithrombin III (AT III) activities in 15 healthy adult horses. Nearly all plasma samples had an initial value of greater than 100% thrombin inhibited, so a 1:1 dilution of the prepared samples was performed. Following dilution, the mean value of the animals was 59.17 +/- 7.4% thrombin inhibited. Mares had significantly greater AT III activity than did geldings (P less than 0.01). The results of this study indicate the horse has more AT III activity than did other domestic species in which AT III activity has been reported.  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