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   5656篇
  免费   283篇
  国内免费   3篇
林业   2406篇
农学   90篇
基础科学   11篇
  614篇
综合类   143篇
农作物   237篇
水产渔业   433篇
畜牧兽医   1646篇
园艺   89篇
植物保护   273篇
  2020年   47篇
  2018年   94篇
  2017年   102篇
  2016年   100篇
  2015年   79篇
  2014年   106篇
  2013年   310篇
  2012年   141篇
  2011年   148篇
  2010年   123篇
  2009年   107篇
  2008年   129篇
  2007年   127篇
  2006年   130篇
  2005年   175篇
  2004年   174篇
  2003年   151篇
  2002年   140篇
  2001年   101篇
  2000年   92篇
  1999年   60篇
  1997年   64篇
  1996年   94篇
  1995年   88篇
  1994年   73篇
  1993年   81篇
  1992年   86篇
  1991年   59篇
  1990年   55篇
  1989年   46篇
  1988年   53篇
  1986年   53篇
  1984年   59篇
  1983年   61篇
  1982年   63篇
  1981年   107篇
  1980年   60篇
  1979年   86篇
  1978年   48篇
  1953年   61篇
  1938年   58篇
  1937年   47篇
  1936年   49篇
  1935年   49篇
  1934年   48篇
  1933年   53篇
  1932年   48篇
  1927年   44篇
  1926年   74篇
  1925年   55篇
排序方式: 共有5942条查询结果,搜索用时 15 毫秒
21.
22.
Neue Insektizide     
Ohne Zusammenfassung  相似文献   
23.
24.
25.
Zusammenfassung Mit theoretischen Modellen kann die Effektivität verschiedener Ansätze (z. B. Fallenzahl und-abstand, Ködermenge und-abgaberate) für den Einsatz von synthetischen Pheromonen in Pest-Management-Systemen verglichen werden. Das GAUSS-sche Modell, das von einer Normalverteilung der Konzentrationsabnahme um eine Lockstoffquelle ausgeht, wurde benutzt, um fürT. lineatum die lockwirksame Reichweite und die Konkurrenzsituation von Naturköder und Falle abzuschätzen. Bei 100 000 /ha und 2 Wochen Flugdauer ergeben sich für (+)-Lineatin (Reaktionsschwelle 0,1 ng/m3, Produktion 6,43 ng//d, Produktionsdauer 3 d/) mittlere Tagesraten von 128,6 g/ha bzw. eine Gesamtproduktion von 1,929 mg/ha. Die lockwirksame Reichweite xmax beträgt je nach Wetterlag für 1T. lineatum- 20–40 cm, für Fallen mit 1 CONREL-Faser (Abgaberate 10 g/d [+]-Lineatin) 13–16m. Für eine 10fach höhere lockwirksame Reichweite sind nach dem Modell etwa 100fach, in der Praxis nach verschiedenen Autoren sogar 200fach stärkere Köder erforderlich.In Freilandversuchen über die Relation von Fangleistung und Beifängen unterschiedlicher Fangsysteme waren die Coleoptera (bes. Rhizophagidae und Staphylinidae) am häufigsten. In Trichterfallen lagen die Beifänge mit 9,2% deutlich höher als in Schlitzfallen (1,2%). Das Giftpolter hatte demgegenüber das größte Beifangspektrum; hier traten auch Ordnungen auf, die von anderen Fangsystemen nicht betroffen waren.Massenfang kan prinzipiell in Holzhöfen und Waldbeständen erfolgreich durchgeführt werden. Allerdings ist das Problem der Beifänge noch ungelöst und die Relations von potentieller Vermehrung der Borkenkäfer und maximal möglicher Populationsreduzierung durch Fallenfang unbekannt. Zudem kann eine nach Massenfang geringere Populationsdichte im Folgciahr durch zunehmende Reproduktionsraten oder durch ersatzweise Einnischung (competive displacement) anderer Borkenkäferarten ausgeglichen und damit der Forstschutzeffekt zunichte gemacht werden.
Bark beetle control: contribution of pheromone meteorology and population dynamics
The use of semiochemicals in pest management systems of bark beetles is viewed in relation to optimization of resources investment, comparing beetle numbers in univoltine, requisitegoverned woodboringTrypodendron lineatum (Oliv.) with multigeneration bark inhabitingIps typographus (L.). The efficiency of different numbers of and distance between traps and pheromone load of dispensers was analysed using steady state models and comparing results with published and own field data. Competition between traps and beetle-infested logs, attractive distance and overlap of pheromone plume was compared forT. lineatum using models which assume a GAUSS distribution of concentration decrease around a pheromone source. For (+)-lineatin (reaction threshold 0.1 ng/m3, production 6.43 ng//d, duration of production 3 d/), 100,000 /ha and a flight period of 2 weeks, daily mean production is 128.6 g/ha. Attractive distance xmax is 20–40 cm for 1T. lineatum- and 13–16 m for a trap with 1 CONREL-dispenser (mean release rate 10 g/d (+)-lineatin). A 10-fold increase of attractive distance xmax needs a 100-fold increase of pheromone release rate according to the model, and 200-fold according to field experiments.In field experiments, Coleoptera (esp. Staphylinidae and Rhizophagidae) were the most abundant non-target trap catches. Insecticide-treated trap logs had the widest range of non-target insects including orders that were not affected by the non-insecticide trapping systems. Mass trapping can be successful in logging areas and/or forests. However, the exclusion of non-target insects still is an unsolved problem and we still have only limited knowledge on potential population increase of beetles and maximum reduction that can be achieved by trapping. Also, population levels reduced by mass trapping can be compensated by increasing reproductive effort of the succeeding generation and/or competitive displacement by secondary bark beetle species.


Mit 5 Abbildungen und 2 Tabellen

Mit Unterstützung der Deutschen Forschungsgemeinschaft.

Nach einem Vortag bei der Fortbildungstagung des Hessischen Forstvereins in Solms/Lahn am 23. Mai 1991.

Herrn Prof. Dr. Dr.G. Wellenstein zum 85. Geburtstag.  相似文献   
26.
Randomized placebo-controlled crossover studies were carried out in dogs to evaluate how two non-steroidal anti-inflammatory drugs (NSAID) might modulate an acute post-traumatic inflammatory reaction. Two "identical" surgical interventions were performed on the forelimbs of each animal with an interval of 28 days, to enable a paired comparison of the inflammatory signs and the wound/bone healing processes. At one operation 8 dogs received 300 mg phenylbutazone twice daily for 8 days starting on the day before surgery, and at the other operation matching placebo tablets were given. In a similar placebo-controlled trial another group of 8 dogs received 5 mg indomethacin twice daily. With phenylbutazone the post-operative swelling was not significantly reduced compared to placebo, but there was less pain and limping. With indomethacin the swelling was somewhat reduced, but there was no consistent difference to placebo in the pain and limping assessments. None of the drugs appeared to distinctly effect the wound or fracture healing, as evaluated by clinical inspection, comparison of radiographs and comparison of bone sections from the sites of surgery. It proved difficult to select an appropriate dosage of indomethacin due to its high potential to induce GI ulceration and bleeding in dogs. In this experimental surgical model with an acute inflammation, neither phenylbutazone nor indomethacin showed impressive anti-inflammatory or analgesic properties. In the same model paracetamol has proved to significantly and more efficiently, reduce both swelling and pain without any noticeable adverse effects, and appears to be a better alternative than the two presently tested NSAID.  相似文献   
27.
In a retrospective study, 1538 strains of -haemolysin-producing Staphylococcus species isolated from dermatitis in dogs at three veterinary clinical microbiology laboratories in Norway during 1986–87 and 1993–94 were investigated for their antimicrobial susceptibility. None of the strains was resistant to cloxacillin, cephalexin or the quinolones enrofloxacin and ciprofloxacin. More than 96% of the strains were susceptible to trimethoprim-sulphonamide, bacitracin and fucidic acid. Between 67% and 89% of the strains were susceptible to erythromycin, lincosamides, tetracycline, neomycin and chloramphenicol. Only 37.9% of the strains were susceptible to penicillin. The frequency of penicillin resistance increased significantly between the first and second periods, from 46.0% to 58.6%. The frequency of resistance to lincomycin, clindamycin and erythromycin also increased significantly between the first and second periods, from 3.0%, 2.1% and 3.3% to 25.5%, 19.5% and 24.8%, respectively. A moderate increase in resistance to tetracycline was also noted, from 20.4% in the first to 27.6% in the second period. On the other hand, the frequency of resistance to trimethoprim-sulphonamide decreased significantly from 4.1% in the first to 0.9% in the second period. Many different resistance patterns were observed in each period. However, the proportion of multiresistant strains increased from 2.1% in the first to 10.2% in the second period. There was a decrease in resistance to the combination of trimethoprim-sulphonamide and penicillin from the first to the second period. Resistance to the combination of lincosamides and pencillin increased. For the combinations penicillin-tetracycline-lincosamides, pencillin-lincosamides-erythromycin, and pencillin-tetracycline-lincosamides-erythromycin, there was a striking increase in resistance between the first and the second periods.Abbreviations CVL Central Veterinary Laboratory - NCVM Norwegian College of Veterinary Medicine - NVL Norwegian Veterinary Laboratory  相似文献   
28.
Most of 43 viruses could easily be detected directly in 53 out of 66 leaf samples dried and stored over CaCl2 for varying periods of time up to 20 1/2 years. Detection usually was with PTA pH 6.5, but alfalfa mosaic, cucumber mosaic and tomato aspermy viruses required PTA pH 3.0 to 4.0. Bean common mosaic, cowpea aphid-borne mosaic and cowpea mosaic viruses were also easily observed in newly dehydrated samples obtained for diagnosis from Morocco and Tanzania.Broad bean wilt virus, cowpea mosaic virus and cucumber mosaic virus were detected with agar gel-diffusion tests in dry leaf material ground in buffer. This serological assay demonstrated a high concentration of cucumber mosaic virus in leaf material dried over CaCl2 20 years ago. This paper further coroborates the value of the CaCl2 method of dehydration and storage of plant viruses in leaf material.Samenvatting Zeer uiteenlopende plantevirussen kunnen goed worden bewaard in bladmateriaal van geïnfecteerde planten dat is gedroogd en bewaard boven CaCl2. Bij rechtstreekse elektronenmicroscopische toetsing van 66 monsters met verschillende isolaten van 43 virussen konden in 53 monsters gemakkelijk virusdeeltjes worden waargenomen (Tabel 1 en Fig. 1 en 2). Meestal lukte dit met fosforwolfraamzuur pH 6,5 dat doorgaans voor in ruw plantesap voorkomende virussen wordt gebruikt. Bij luzernemozaïekvirus en komkommermozaïek virus gelukte dit alleen maar bij lagere pH (3,0 en 4,0). Ook het tomate-aspermievirus was dan veel gemakkelijker aantoombaar. Methylaminewolfraamzuur gaf geen beter resultaat.Draden van bonerolmozaïekvirus en van cowpea aphid-borne mosaic virus en bolletjes van cowpea mosaic virus waren snel waarneembaar in elektronenmicroscopische preparaten gemaakt van bladmateriaal van recent uit Marokko en Tanzania ontvangen monsters.Cowpea mosaic virus, tuinboneverwelkingsvirus en komkommermozaïekvirus konden eveneens gemakkelijk en snel serologisch worden aangetoond in met bufferoplossing vermalen droog blad van respectievelijk cowpea, erwt en tabak. Het laatstgenoemde, reeds 20 1/2 jaar geleden gedroogde bladmateriaal, bleek in vergelijking met pas geïnoculeerd vers blad van komkommer enChenopodium quinoa zelfs zeer veel serologisch actief virusmateriaal te bevatten (Fig. 3).De beschreven waarnemingen bevestigen nogmaals de waarde van de toegepaste methode van virusbewaring en tonen aan dat het mogelijk is vele virussen te herkennen in van elders ontvangen gedroogde bladmonsters, zonder het risico te lopen van virusontsnapping, zoals altijd aanwezig bij werk met toetsplanten in de kas.Guestworker from Istituto Sperimentale per la Patologia Vegetale, Rome, Italy, participating in the investigations from September 21 to December 21, 1977.  相似文献   
29.
In recent years in the Netherlands a second mushroom species,Agaricus bitorquis, which prefers higher temperatures thanA. bisporus and is less susceptible to certain diseases, is often commercially grown.Verticillium fungicola var.fungicola, the causal agent of dry bubble, is responsible for considerable damage in crops ofA. bisporus. InA. bitorquis, however, dry bubble has hardly been noticed, but brown spots due toV. fungicola var.aleophilum resulted in inferior mushroom quality. The latter variety also caused brown spots ina. bisporus, but to a minor degree. In variety Les Miz 60 ofA. bisporus, however, it also induced fruit-body deformation in a way different from dry bubble. Verticillium psalliotae, isolated fromA. bitorquis in England, induced more confluent brown spots inA. bitorquis. In the netherlands, where moreA. bitorquis is grown than in other countries,V. psalliotae has not yet been encountered in crops ofA. bitorquis. V. psalliotae, which has a high temperature optimum for mycelial growth, likeV. fungicola var.aleophilum andA. bitorquis, did not infectA. bisporus in our trials.Artificial infection ofA. bisporus orA. bitorquis could not be accomplished with the following related and/or fungicolous fungi:Verticillium lamellicola, V. fungicola var.flavidum, V. biguttatum, Nectriopsis tubariicola, Acremonium crotocinigenum andAphanocladium album.Samenvatting Vooral in Nederland wordt sinds een aantal jaren naastAgaricus bisporus ook de warmteminnende champignonsoortAgaricus bitorquis geteeld, die minder vatbaar is voor bepaalde ziekten. TerwijlVerticillium fungicola varfungicola in de teelt vanA. bisporus droge mollen en daardoor veel schade veroorzaakt, komen in de teelt vanA. bitorquis geen droge mollen voor maar wel bruine vlekken, die tot kwaliteitsverlies en dus schade leiden. De vlekken bleken veroorzaakt te worden doorV. fungicola var.aleophilum. Deze schimmel veroorzaakte ook inA. bisporus bruine vlekken, hoewel in minder ernstige mate, maar in het ras Les Miz 60 vanA. bisporus bovendien misvormde champignons, die wel op droge mollen leken, maar daaraan niet gelijk waren.OokV. psalliotae, in Engeland geïsoleerd vanA. bitorquis met vlekken, veroorzaakte wat meer samenvloeiende, bruine vlekken inA. bitorquis. In Nederland, waar meerA. bitorquis geteeld wordt dan in andere landen, isV. psalliotae nog niet aangetroffen in teelten vanA. bitorquis. InA. bisporus kon geen kunstmatige infectie worden verkregen metV. psalliotae, die net alsV. fungicola var.aleophilum enA. bitorquis warmteminnend genoemd zou kunnen worden.Met de volgendeVerticillium-achtige of van paddestoelen geïsoleerde schimmels kon evenmin op kunstmatige wijze een infectie worden opgeroepen inA. bisporus ofA. bitorquis: Verticillium lamellicola, V. fungicola var.flavidum, V. biguttatum, Nectriopsis tubariicola, Acremonium crotocinigenum enAphanocladium album.  相似文献   
30.
Predicted changes in average values of global climate variables (increased temperatures, altered precipitation patterns, increased concentrations of atmospheric CO2) and changes in the frequency, duration, and degree of extremes (frost, heat, drought, hail, storms, floods, etc.) will affect agricultural crops, agroecosystems, and agricultural productivity. Although forecasts of regional climate changes are still imprecise, mean temperature increases in Europe are expected to be greater in the north (2.5–4.5°C) than in the south (1.5–4.5°C). Regional forecasts for precipitation changes are also very far from precise; however, problems with drought are expected to increase, especially in Mediterranean countries. Overall, shortage of water will be the predominant factor affecting plant growth. As higher temperatures are known to enhance plant development and especially the grain-filling duration of cereals, grain yield losses are possible in a warmer climate. On the other hand, elevated atmospheric CO2 concentrations are known to stimulate photosynthesis and enhance growth and yield (“CO2 fertilization”); concomitantly, leaf transpiration is reduced, resulting in improved water use efficiency. Total biomass and yield were enhanced by 20–30% in experiments with elevated CO2 exposure (550–700 ppm) under more or less ideal growth conditions. Elucidating the interactions between positive and negative effects of climate change is of crucial importance for any prediction of future crop yields. The present paper is a brief summary mainly of the potential effects of elevated temperatures and atmospheric CO2 on crop growth, quality, and yield. Also, adaptation measures, possible interactive effects of different climate variables, and interactions of climate change components with other growth variables (pathogens, air pollutants) are briefly described.  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