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   146412篇
  免费   8554篇
  国内免费   74篇
林业   6344篇
农学   5271篇
基础科学   1000篇
  18028篇
综合类   23194篇
农作物   5552篇
水产渔业   7637篇
畜牧兽医   76663篇
园艺   1863篇
植物保护   9488篇
  2019年   1348篇
  2018年   2212篇
  2017年   2525篇
  2016年   2337篇
  2015年   1997篇
  2014年   2423篇
  2013年   5837篇
  2012年   4280篇
  2011年   5114篇
  2010年   3411篇
  2009年   3444篇
  2008年   5115篇
  2007年   4770篇
  2006年   4589篇
  2005年   4103篇
  2004年   3930篇
  2003年   4045篇
  2002年   3678篇
  2001年   4908篇
  2000年   4784篇
  1999年   3788篇
  1998年   1536篇
  1997年   1623篇
  1996年   1428篇
  1995年   1684篇
  1994年   1467篇
  1993年   1457篇
  1992年   2959篇
  1991年   3158篇
  1990年   3088篇
  1989年   3094篇
  1988年   2771篇
  1987年   2754篇
  1986年   2857篇
  1985年   2654篇
  1984年   2267篇
  1983年   1934篇
  1979年   2057篇
  1978年   1570篇
  1977年   1397篇
  1976年   1376篇
  1975年   1518篇
  1974年   1785篇
  1973年   1794篇
  1972年   1689篇
  1971年   1618篇
  1970年   1591篇
  1969年   1546篇
  1968年   1367篇
  1967年   1447篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
311.
312.
Sera from 1186 apparently healthy sheep in Hokkaido were examined by enzyme-linked immunosorbent assay (ELISA) and immunodiffusion (ID) for the presence of antibodies against Corynebacterium pseudotuberculosis. ELISA-positives were 466 (39.3%) while ID-positives were 330 (27.8%). Spread of caseous lymphadenitis in sheep in Hokkaido was thus clarified. Although ID was less sensitive than ELISA in detecting the antibodies against C. pseudotuberculosis it did not give any non-specific reaction. From the results and in view of the simplicity of the test procedure, ID was found to be of practical diagnostic value. Distribution by age group of anti-C. pseudotuberculosis antibodies in 758 sheep in a herd detected by both tests showed that the ratio of positives was low in sheep aged less than 1 year, and the ratio increased significantly in those aged 1 year and continued to increase with age until it reached a plateau at the age of 4-5 years.  相似文献   
313.
Spraying in so-called space cultures (orchards, vineyards) is determined by two main aims which require contrary technical measures for their realization: (1) optimal even distribution of droplet deposition on the target. This generally requires sufficient air assistance of droplet transportation and penetration in order to bridge the distance between the nozzle outlet and the target; (2) the surplus energy which normally exists should be minimized in order to avoid any droplet flow outside the target. Several possibilities exist to approach these requirements: (1) optimal blower design and correct adjustment for each specific purpose; (2) catching and recycling of misdirected droplets by recycling units; (3) sensoring of gaps in the canopy and activation of a nozzle shut-off. The legal requirements of the BBA are given, together with the results of experiments on drift and ground deposition of chemicals from air-assisted sprayers, with special attention to recycling units and sensored sprayers.  相似文献   
314.
Jergensen, C. B., J. S. Agerholm, J. Pedersen and P. D. Thomsen: Bovine leukocyte adhesion deficiency in Danish Holstein-Friesian Cattle I. PCR screening and allele frequency estimation. Acta vet. Scand. 1993,34,231-236.–A screening program for bovine leukocyte adhesion deficiency (BLAD) in Danish Holstein-Friesian cattle has been initiated. During the first months 1611 animals were tested by a PCR based assay. Of these animals 1256, 346, and 8 were assigned normal, BLAD carriers, and BLAD affected animals, respectively One bull, born as a co-twin, showed weak reaction for the BLAD allele on DNA isolated from leukocytes, but a normal genotype on DNA isolated from semen. Chromosome analysis showed that this bull was a blood chimaera. Estimation of the BLAD allele frequency upon the PCR test results showed that around 450 Danish calves born in 1991 might have been affected with the recessive disorder.  相似文献   
315.
A field experiment was conducted during the winter seasons of 1992–93 and 1993–94 at Anand to study the effect of FYM, nitrogen and source of fertilizer on growth and yield of mustard [ Brassica juncea (L.) Czernj & Cosson]. The results showed significant variation in leaf area index (LAI), crop growth rate (CGR), dry matter production and seed yield. The direct effect of farmyard manure (FYM) was conspicuous in improving the growth of mustard. FYM application at 10 tonnes ha−1 significantly increased the LAI, CGR and dry matter accumulation per plant at almost all the stages during first year study (1992–93) and in pooled analysis. Similarly, nitrogen application registered maximum LAI, CGR at 75 kg level and RGR and NAR at 50 kg level at almost all the during both years. Sulphur carrying source (Ammonium sulphate plus single super phosphate) increased all stages growth characters. Maximum dry matter accumulation per plant and seed yield were recorded with highest levels of FYM (20 tonnes ha−1), N (75 kg ha−1) and source having S. Seed yield was strongly associated with LAI and dry matter accumulation per plant at all the stages.  相似文献   
316.
MOSEDALE  J.R.; CHARRIER  B.; JANIN  G. 《Forestry》1996,69(2):111-124
Pressler cores of young clones of Quercus petraea and Q. roburwere analysed from two sites in Germany. Variation of wood colour(defined by the CIELAB system), density and heartwood ellagitanninswas examined between clones, species and sites. Similar studieswere made of cores from another trial of 20 half-sib familiesof parent trees deriving from five German forests. The resultsfrom both the clonal and progeny trials indicate that heartwoodellagitannin content and wood density are under strong geneticcontrol, while sapwood and heartwood colour are less so. A largeproportion of between-clone variation in wood density and ellagitanninlevels occurred between the two species, while these propertiesvaried little among ramets of the same clone grown on two contrastingsites, despite a difference in growth rates. A large proportionof the total variation among progeny was attributed to forestorigins and could not be attributed clearly to either geneticor environmental causes because of limitations of the samplingdesign  相似文献   
317.
The history of hyperplastic canker of almond, caused by Pseudomonas amygdali , its evolution and the present situation in Greece are reviewed. Its main aspects, causal agent, symptomatology, epidemiology and control measures are described. The main characteristics differentiating the pathogen from some other pseudomonads pathogenic to fruit trees are also reported.  相似文献   
318.
Abstract. Usual residue-management options are to remove the residue, use it as mulch with or without undercutting or to incorporate it into the soil. While the role of surface mulch in evaporation has been widely studied, the information on the effect on evaporation of mulch with undercutting or residue incorporated into soil, particularly in relation to soil type and evaporativity (Eo) is lacking. We studied the effect of wheat straw used in various ways on the course of evaporation loss from soil columns with three soils at Ludhiana, India and one soil at Bushland, Texas, USA, under two Eo's Energy-limited evaporation rates under mulch (Eom) followed the soil-specific relation Eom/Eo= a e(bRes+cEo), where Res is residue rate t/ha and a, b and c are constants; Eo, is expressed in mm/d. In an effort to model the total evaporation (CE) during the energy-limited stage ‘U’ was obtained from appropriate CE versus time curves and (CE-U) was regressed over (t - ti)0.5 to obtain the slope ‘α’ (Ritchie 1972) for the soil-limited evaporation stage. The observed ‘U’ was independent of mulch rate and Eo but was strongly affected by soil type, Values of ‘α’ decreased with increase in mulch rate and decrease in Eo and coarseness of soil. The otherwise short lived benefit of evaporation reduction with mulch per se, which peaked after a few days was maintained when residue was mixed with soil at the stage when evaporation reduction reached a maximum; this benefit continued for several weeks. Cumulative evaporation values computed from ‘U’ and ‘α’ agreed closely with the observed values under straw mulch for loamy sand and clay loam soils and for ‘undercut’ and ‘residue mixed’ treatments on all soils regardless of Eo, and for all situations under small Eo. However, for sandy loam and silt loam soils under Eo of 10 mm/d, the modified square root of the time function of Jalota et al. (1988) gave a better fit.  相似文献   
319.
Epidural Morphine in Goats after Hindlimb Orthopedic Surgery   总被引:1,自引:0,他引:1  
Morphine (0.1 mg/kg) diluted with 0.9% saline to a volume of 0.13 mL/kg was administered into the epidural space at the lumbosacral junction in 10 halothane-anesthetized goats immediately before discontinuation of halothane. The same volume of 0.9% saline was given to control group of eight anesthetized goats. Both groups had undergone an orthopedic procedure that replaced the anterior cruciate ligament with a patellar tendon autograft. The appearance and unprovoked behavior of goats in the morphine group were significantly different (p < .05) from the saline groups. The goats in the morphine group were more sedate and struggled less during recovery. Epidural morphine did not produce respiratory depression or bloat during a 9 hour observation period. Heart rate, respiratory rate, and blood pressure (mean, systolic, and diastolic) of the morphine group did not differ from those of the control group.  相似文献   
320.
The reagent carrier Reflotron-Creatinine was tested for its suitability for whole blood and plasma samples of dogs and cats. It showed a very good correlation between the results of Reflotron-Creatinine and the reference method. Using plasma the results of both methods were extremely close. Using whole blood with a hematocrit of up to 50% good results could be achieved. If the hematocrit exceeded 50% large deviations could result.  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