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   4544篇
  免费   324篇
  国内免费   1篇
林业   303篇
农学   128篇
基础科学   36篇
  812篇
综合类   850篇
农作物   193篇
水产渔业   233篇
畜牧兽医   1870篇
园艺   109篇
植物保护   335篇
  2023年   31篇
  2022年   28篇
  2021年   65篇
  2020年   79篇
  2019年   82篇
  2018年   102篇
  2017年   90篇
  2016年   112篇
  2015年   80篇
  2014年   121篇
  2013年   201篇
  2012年   229篇
  2011年   284篇
  2010年   179篇
  2009年   153篇
  2008年   254篇
  2007年   244篇
  2006年   258篇
  2005年   239篇
  2004年   189篇
  2003年   222篇
  2002年   181篇
  2001年   72篇
  2000年   81篇
  1999年   84篇
  1998年   45篇
  1997年   39篇
  1996年   40篇
  1995年   36篇
  1994年   29篇
  1993年   34篇
  1992年   40篇
  1991年   58篇
  1990年   49篇
  1989年   47篇
  1988年   33篇
  1987年   60篇
  1986年   40篇
  1985年   58篇
  1984年   38篇
  1983年   46篇
  1982年   25篇
  1981年   22篇
  1980年   24篇
  1979年   34篇
  1978年   25篇
  1976年   26篇
  1973年   27篇
  1970年   24篇
  1969年   31篇
排序方式: 共有4869条查询结果,搜索用时 78 毫秒
991.
Ineffective erythropoiesis was diagnosed in an 8‐year‐old male castrated Labrador Retriever. Despite treatment with immunosuppressive therapy for suspected immune‐mediated erythrocyte maturation arrest, resolution of the nonregenerative anemia was not achieved. Following documentation of Bartonella henselae bacteremia by Bartonella alpha proteobacteria growth medium (BAPGM) enrichment blood culture, immunosuppressive therapy was discontinued, and the anemia resolved following prolonged antibiotic therapy. Bartonella immunofluorescent antibody testing was negative, whereas B henselae western blot was consistently positive. The contribution of B henselae bacteremia to ineffective erythropoiesis remains unknown; however, the potential role of B henselae in the pathophysiology of bone marrow dyscrasias warrants additional investigation.  相似文献   
992.
We examined utilitarian and hedonic values as motivations for rhino horn use in Vietnam. We also evaluated consumers’ response to consequences of the illegal trade in behavior modification campaigns and the likely outcome of a legalized trade. The most prevalent use was for treatment of hangovers indicating utilitarian values, although difficult to separate from the hedonic value in projecting success in business. A ritualized way of honoring terminally ill relatives represented a hedonic value replacing belief in effective treatment. Demand reduction campaigns need to appropriately reflect all relevant values determining specific uses. The plight of rhino populations, Vietnam’s penal code, and the possible contribution to international crime mattered little to consumers. Horn from wild rhino was preferred over farmed, and respondents would demand more if available in a legalized trade. This suggested that a legalized trade could maintain or even increase demand for poached rhino horn.  相似文献   
993.
Der Statische Düngungsversuch Bad Lauchstädt wurde 1902 auf Löß‐Schwarzerde (21% Ton, 8,7°C Jahresdurchschnittstemperatur und 484 mm Jahresniederschlag) angelegt. Nach 96 Versuchsjahren wurde im Jahre 1998 auf Schlag I von allen Varianten der Gehalt an Gesamt‐P, doppellaktatlöslichem P und K, Mg, Cu, Zn, Mn und pH‐Wert bestimmt. Die P‐Entzüge liegen zwischen 9 kg/ha.a (ungedüngt) und 30 kg/ha.a. Die Gehalte an Gesamt‐P liegen zwischen 47 und 100 mg/100g Boden. Mit 10t/ha.a Stalldung wird bereits eine optimale Zufuhr an den Nährstoffen P, K und Mg erreicht bzw. überschritten. Mg und Zn reagieren deutlich auf die Stalldunggabe, weniger Cu. Die pH‐Werte liegen mit Kalkdüngung im Mittel aller Prüfglieder bei 7,3 (Spannweite 7,2 bis 7,5), ohne Kalk bei 6,6 (6,3–7,3). Stalldung wirkt nicht auf die pH‐Werte. Es besteht eine deutliche Wechselwirkung zwischen pH‐Wert und Mn. Mit Kalk wird Mn festgelegt und liegt zwischen 27 und 40 ppm, ohne Kalk zwischen 32 und 100 ppm.  相似文献   
994.
Catadromy among freshwater eels is increasingly recognised as being facultative, with some individuals carrying out growth exclusively in brackish or coastal marine waters, or switching between brackish or marine waters and freshwater habitats. In an attempt to reconstruct habitat use of yellow‐stage American eels in a large river‐lake ecosystem, trace element line scans were obtained, using LA‐ICP‐MS, from the otoliths of 110 eels sampled at various locations throughout the St. Lawrence River–Lake Ontario (SLRLO) system. Elemental profiles for strontium (Sr), barium (Ba), manganese (Mn) and magnesium (Mg) enabled us to distinguish three chemical signatures that appear to represent three distinct habitats within the SLRLO. Of these, one was shown to likely correspond to the brackish estuary (high strontium values). The other two signatures, characterised by low strontium but variable concentrations of barium and manganese, may correspond to habitats within the main‐stem St. Lawrence River and one or more of its tributaries. Most (78%) of the switches among habitats occurred within the first four years after recruitment suggesting an increasing likelihood for eels to maintain residence in one habitat as they grow older. This suggests that tributaries may provide important habitats for American eels during the first several years after recruiting to the SLRLO. In addition, our results suggest that a small proportion American eels in the SLRLO can undertake movements on the order of at least 200 km during the early growth stage. This information has important implications for the management and conservation of this species in the system.  相似文献   
995.
This study describes a safe, reliable, and accessible means to measure heart rate (HR) and HR variability (HRV) and evaluates the use of HRV as a physiological correlate of stress in the Asian elephant. A probabilistic model indicates that HRV measurements may adequately distinguish between stressed and non-stressed elephants.  相似文献   
996.
997.
Annual fishes, which inhabit temporary pools with extremely limited habitat complexity and niche availability, display remarkable sexual dimorphism, rapid growth, and enormous investment into reproduction, all traits associated with high energy requirement. This study tests three hypotheses for two syntopic annual fishes (Austrolebias minuano and Cynopoecilus fulgens) found in six wetlands of southern Brazil: (i) considerable morphological differences result in low dietary overlap, (ii) sexual dimorphism in both species leads to intraspecific diet segregation, and (iii) dietary richness increases during ontogenetic development, and is narrower in C. fulgens than A. minuano due to morphological limitations imposed by reduced size. The diet of 82 A. minuano and 211 C. fulgens individuals was analyzed over two annual cycles. The morphology was characterized by 26 measurements covering the entire body of both species. There was no evidence of morphological specialization related to food competition and the diet of A. minuano and C. fulgens showed high overlap. High food availability, high predator abundance, and high connectivity of adjacent wetlands are likely the main mechanisms allowing coexistence of both species. Within species, sexual dimorphism did not result in a decrease in dietary overlap, which reinforces the idea that morphological differences between the sexes did not evolve as a mechanism to decrease food competition. Large A. minuano did not have a more diverse diet than the smaller C. fulgens; however, increase in body size allowed both species to ingest larger prey. Morphological variability in both species was mainly related to ontogenetic development and reproduction.  相似文献   
998.
Abstract –  Fishery biologists have documented small home ranges, relative to available habitat, for many littoral freshwater fishes. Home ranges for pelagic species, such as white bass Morone chrysops , are generally not well described, yet are thought to be large. We studied white bass movement using acoustic telemetry in two irrigation reservoirs of the Republican River basin in south-western Nebraska. Acoustic transmitters were implanted in fall of the previous year and tracking occurred a minimum of once per week throughout spring (mid-March to May) 2007 and 2008. Linear home ranges were calculated from observed locations of individual fish. Twelve of the twenty-seven tagged fish with at least five locations exhibited localised home ranges throughout the spring whereas the remaining fish exhibited home ranges extending across large portions of each reservoir. Home range size was not correlated with fish size or condition.  相似文献   
999.
Sequence-related amplified polymorphism (SRAP) and morphological markers to estimate the genetic relations among forty pea varieties (Pisum sativum L.) were studied. Data on 15 morphological traits were collected and analyzed. A total of 162 polymorphic SRAP's bands were scored using seven combinations of primers. Cluster analysis and both principal component and principal coordinate analysis were carried out. The varieties were grouped in four clusters through procrustes generalized analysis. Relationships among varieties revealed by molecular markers were significantly correlated with those based on the agronomic traits, suggesting that the two systems give similar estimates of genetic relations among the varieties. Parents selection depend on the specific objectives in further breeding programs.  相似文献   
1000.
Abstract –  Ultrasonic telemetry and hydrodynamic modelling were used to study the migratory behaviour of 54 wild Atlantic salmon ( Salmo salar ) smolt captured in freshwater during their downstream migration and tracked in 2 years through a shallow estuary system. A high-density, fixed array of receivers provided detailed spatial and temporal resolution of behaviour in the second year of study. Smolt migration in the river occurred mostly at night and downstream migration was slower during the day. In the estuary, smolt moved seaward on ebbing tides and landward on flooding tides. The effect of current velocity was greater during the night than during the day. We documented for the first time that current velocity and diurnal period only accounted for approximately one-third of the variation in smolt ground speeds in the estuary, indicating that smolt movements were far less passive than previously reported. Smolt energetic status had no effect on smolt swimming behaviour or migratory performance. With an increase in salinity, smolt seaward movements during flooding tides were more frequent, and overall seaward ground velocity increased. The increase in salinity experienced by the smolt during their migration through the leading edge of saltwater intrusion thus induced a behavioural transition from a more passive, fluvial migration to a more active- and seaward-oriented migration.  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