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   455篇
  免费   12篇
  国内免费   2篇
林业   44篇
农学   2篇
  97篇
综合类   38篇
农作物   21篇
水产渔业   50篇
畜牧兽医   180篇
园艺   13篇
植物保护   24篇
  2023年   5篇
  2021年   12篇
  2020年   15篇
  2019年   13篇
  2018年   10篇
  2017年   7篇
  2016年   9篇
  2015年   17篇
  2014年   21篇
  2013年   23篇
  2012年   31篇
  2011年   42篇
  2010年   25篇
  2009年   19篇
  2008年   46篇
  2007年   29篇
  2006年   32篇
  2005年   24篇
  2004年   27篇
  2003年   14篇
  2002年   12篇
  2001年   8篇
  2000年   2篇
  1996年   1篇
  1995年   4篇
  1994年   1篇
  1993年   1篇
  1992年   1篇
  1990年   2篇
  1987年   1篇
  1986年   3篇
  1984年   1篇
  1983年   3篇
  1982年   1篇
  1980年   1篇
  1979年   1篇
  1977年   1篇
  1974年   1篇
  1972年   1篇
  1967年   1篇
  1965年   1篇
排序方式: 共有469条查询结果,搜索用时 31 毫秒
31.
Pulmonary dysfunction was evaluated in pigs infected with porcine reproductive and respiratory syndrome virus (PRRSV, isolate VR-2332) and compared to clinical and pathological findings. Infected pigs developed fever, reduced appetite, respiratory distress and dullness at 9 days post-inoculation (dpi). Non-invasive pulmonary function tests using impulse oscillometry and rebreathing of test gases (He, CO) revealed peripheral airway obstruction, reduced lung compliance and reduced lung CO-transfer factor. PRRSV-induced pulmonary dysfunction was most marked at 9–18 dpi and was accompanied by a significantly increased respiratory rate and decreased tidal volume. Expiration was affected more than inspiration. On histopathological examination, multifocal areas of interstitial pneumonia (more severe and extensive at 10 dpi than 21 dpi) were identified as a possible structural basis for reduced lung compliance and gas exchange disturbances.  相似文献   
32.
The prevalence of methicillin resistant Staphylococcus aureus (MRSA) in pigs at abattoirs is higher than in pigs sampled on farms. This study investigated whether MRSA negative pigs can become MRSA positive during transportation from the farm to the abattoir after exposure to other pigs and environmental sources of MRSA. Nasal swabs were collected from four batches of pigs during loading at the farm, on arrival at the abattoir and after stunning. Environmental wipes were taken from lorries after transporting pigs and from lairages after holding pigs. All pigs (n = 117) tested MRSA negative before transportation. On arrival at the abattoir, 12/117 (10.3%) pigs in two batches tested MRSA positive. In lorries that tested positive after transportation, the prevalence of MRSA positive pigs was 21.1%, whereas no MRSA was detected in pigs that had been transported in lorries that tested negative after transportation. At stunning, all batches and 70/117 (59.8%) pigs tested MRSA positive. Pigs can become MRSA positive in the short period of time during transportation from the farm to stunning at the abattoir.  相似文献   
33.
Discrimination between nestmates and non‐nestmates in social insects is thought to rely on the pattern of cuticular hydrocarbons. We investigated the cuticular hydrocarbon profiles of 2 parapatric sibling ant species, Temnothorax nylanderi (Förster, 1850) and Temnothorax crassispinus (Karavaiev, 1926), and their hybrid. We found that although the profiles show considerable similarities, a discriminant analysis based on the relative peak areas of cuticular hydrocarbons separates the 3 taxa. The profiles of hybrids were not consistently intermediate between those of the paternal species, suggesting either non‐additive interactions among the parental biosynthetic pathways or systematic differences in environment‐derived odor cues.  相似文献   
34.
Botrytis cinerea is a major crop pathogen infesting >220 hosts worldwide. A cryptic species has been identified in some French populations but the new species, B. pseudocinerea, has not been fully delimited and established. The aim of this study was to distinguish between the two species, using phylogenetic, biological, morphological, and ecological criteria. Multiple gene genealogies confirmed that the two species belonged to different, well-supported phylogenetic clades. None of the morphological criteria tested (spore size, germination rate, or mycelial growth) was able to discriminate between these two species. Sexual crosses between individuals from the same species and different species were carried out. Only crosses between individuals from the same species were successful. Moreover, population genetics analysis revealed a high level of diversity within each species and a lack of gene flow between them. Finally, a population survey over time showed that B. cinerea was the predominant species but that B. pseudocinerea was more abundant in spring, on floral debris. This observation could not be explained by temperature adaptation in tests carried out in vitro or by aggressiveness on tomato or bean leaves. This study clearly establishes that B. cinerea and B. pseudocinerea constitute a complex of two cryptic species living in sympatry on several hosts, including grapevine and blackberry. We propose several biological or molecular tools for unambiguous differentiation between the two species. B. pseudocinerea probably makes a negligible contribution to gray mold epidemics on grapevine. This new species has been deposited in the MycoBank international database.  相似文献   
35.
Creaming in black tea   总被引:1,自引:0,他引:1  
Tea cream is the precipitate formed as tea cools. Its formation has been studied by X-ray scattering, and it is shown that a higher tea concentration leads to earlier onset of creaming and larger particles and that addition of theaflavin and calcium promotes creaming. Association constants between the major components of black tea have been obtained using NMR and show that calcium and glucose enhance the self-association of caffeine, polyphenols, and theaflavin but have little effect on hetero-association. Glycosylation of a polyphenol reduced self-association and reduced binding to caffeine. We conclude that theaflavin is important for the initiation of creaming, forming nanoclusters of typically 3 nm diameter, whereas caffeine acts more to fill in the gaps within the clusters and thus adds to the bulk of tea cream without being necessary for its initiation. Tea creaming may be reduced by increasing the solubility of the polyphenols (i.e., by glycosylation) or by removing calcium. Tea cream; theaflavin; caffeine; small-angle X-ray scattering; NMR; colloid.  相似文献   
36.
The myrmecophilous Platyarthrus schoblii Budde-Lund, 1885 is widely distributed and native in the Mediterranean region. In Hungary it was first found at Budapest, in 2001, in a colony of Lasius neglectus van Loon, Boomsma and Andrásfalvy, 1990 (Hymenoptera: Formicidae). This invasive and polygynous ant species is dispersing in an antropochorous way throughout Europe. There are 16 known colonies of L. neglectus in Hungary. Fourteen of them have been surveyed for the isopod, which was detected in eight cases (57%). In addition to L. neglectus, the isopod has been recently found with other native ant species [Lasius niger (Linnaeus, 1758), Lasius emarginatus (Olivier, 1791) and Tetramorium caespitum (Linnaeus, 1758)] in the country. We have also found the joint occurrence of P. hoffmannseggii Brandt, 1833 and P. schoblii. The co-occurrence and joint expansion of the ranges of L. neglectus and P. schoblii indicates their co-habitation and antropochorous dispersal while the appearance with L. emarginatus, L. niger and T. caespitum supports our hypothesis about possible adoption by different ant species.  相似文献   
37.
Significance of microbial biomass and non-exchangeable ammonium with respect to the nitrogen transformations in loess soils of Niedersachsen during the growing season of winter wheat. I. Change of pool sizes Nitrogen transformations in loess soils have been examined by laboratory and field experiments. After straw application (· 8 t · ha?1), N in microbial biomass (Nmic) increased by about 20 mg · kg?1 soil (· 90 kg N · ha?1 · 30 cm?1) after 9 days of incubation (20 °C). Another laboratory experiment yielded an increase of about 400 mg of NH4+-N · kg?1 fixed by minerals within 1 h after addition of 1 M NH4+-acetate. Defixation of the recently fixed NH4+ after addition of 1 M KCl amounted to only 60 mg · kg?1 within 50 days. In a field experiment with winter wheat 1991, an increase in Nmic of about 80 kg N · ha?1 · 30 cm?1 was observed from March to June. After July, growth of the microbes was limited by decreased soluble carbon concentrations in the rhizosphere. Different levels of mineral N-fertilizer (0, 177 and 213 kg N · ha?1) did not affect significantly the microbial biomass. The same field experiment yielded a decrease of non-exchangeable ammonium on the “zero”-fertilized plot in spring by 200 kg N · ha?1 · 30 cm?1. The pool of fixed ammonium increased significantly after harvest. After conventional mineral N-fertilizer application (213 kg N · ha?1). NH4+-defixation was only about 120 kg N · ha?1 · 30 cm?1 until July.  相似文献   
38.
39.
Die Dauer der biologischen Rastzeit ist eine variable Grβe und wird von ver-schiedenen endogenen und exogenen Faktoren beeinfluβt. Eine Rastzeit unter 60 Ta-gen ist nur bei einem kleinen Prozentsatz der Tiere als biologisch anzusehen. Eine indi-viduelle durchschnittliche Zwischenkalbezeit von 341 bis 360 Tagen ist in der Landes-zucht trotz frühzeitiger Besamung post partum nur bei einem relativ kleinen Prozentsatz zu erreichen (16,5 %). Eine individuelle durchschnittliche Zwischenkalbezeit von 361 Bis 380 Tagen dürfte als Ausdruck guter Fruchtbarkeit bei den heutigen Fütte-rungs- und Haltungsbedingungen zu werten sein. Bei einer individuellen durchschnitt-lichen Zwischenkalbezeit von 381 bis 400 Tagen liegt der Verdacht vor, daβ Fütte-rungs- und Haltungsfehler sowie Geburts- und Puerperalstörungen die Ursache der ver-langerten Zwischenkalbezeit sind. Bei Kühen mit einer sehr hohen Milchleistung kann diese Zwischenkalbezeit noch als vertretbar angesehen werden. Tiere mit einer individuellen durchschnittlichen Zwischenkalbezeit von über 400 Tagen sind im allgemeinen ats Minusvarianten hinsichtlich Fruchtbarkeit einzustufen. Dies gilt allerdings nicht für Tiere mit einer weit über dem Herdendurchschnitt liegenden Milchleistung. Eine Ver-kurzung der individuellen durchschnittlichen Zwischenkalbezeit erscheint möglich durch: Verbesserung von Fütterung und Haltung; gutes Management; Fruchtbarkeits-uberwachung; Verbesserung der Geburtshygiene; tierärztliche Überwachung von Puer-perium und Gustreit; Prophylaxe und rechtzeitige Behandlung diagnostizierter Erkran-kungen .  相似文献   
40.
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