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   279篇
  免费   9篇
林业   74篇
农学   4篇
  57篇
综合类   32篇
农作物   9篇
水产渔业   17篇
畜牧兽医   87篇
园艺   6篇
植物保护   2篇
  2021年   2篇
  2020年   3篇
  2019年   3篇
  2018年   8篇
  2017年   6篇
  2016年   9篇
  2014年   7篇
  2013年   8篇
  2012年   9篇
  2011年   7篇
  2010年   11篇
  2009年   20篇
  2008年   16篇
  2007年   15篇
  2006年   5篇
  2005年   16篇
  2004年   17篇
  2003年   13篇
  2002年   13篇
  2001年   6篇
  2000年   2篇
  1999年   10篇
  1997年   4篇
  1996年   4篇
  1995年   9篇
  1993年   2篇
  1992年   5篇
  1990年   3篇
  1989年   5篇
  1988年   3篇
  1986年   3篇
  1985年   4篇
  1984年   3篇
  1983年   3篇
  1982年   1篇
  1981年   3篇
  1980年   1篇
  1979年   3篇
  1978年   1篇
  1977年   1篇
  1974年   4篇
  1973年   3篇
  1972年   1篇
  1971年   2篇
  1970年   3篇
  1969年   1篇
  1968年   1篇
  1966年   4篇
  1965年   2篇
  1960年   1篇
排序方式: 共有288条查询结果,搜索用时 15 毫秒
71.
72.
Transport by road can induce significant stress in cattle. Thermal stress is among the main stress producing factors during transport. The provision of ventilation in livestock transport vehicles is usually through openings along the sides of the vehicle. The incoming air will affect air quality inside by regulating temperature, relative humidity, gas levels and levels of other contaminants. The aim of the present investigation was to map out the air quality in a commercial cattle transport vehicle under various climatic conditions and with varying stocking densities and transport times. Distributions of air temperature, relative humidity and concentrations of ammonia, carbon dioxide, oxygen and methane have been determined during 35 experimental journeys. In average the mean temperature inside the compartment was about 3 degrees C and 6 degrees C higher than outside temperature in summer (+7.8(-)+24.0 degrees C) and winter (-24.3(-)+12.7 degrees C) conditions respectively. The temperature increment inside, as could be expected from theory, increased with reduced ventilation and increased animal density. Many stops to load new animals lowered the temperature increment and relative humidity in winter time. In summer more stops made the compartment temperature and relative humidity increase. The inside temperature distribution was less than about 3 degrees C during both summer and winter season. Average ammonia level varied between 3 and 6 ppm depending on stocking density and number of stops with a maximum value of 18 ppm. No detectable methane levels could be found inside the compartment at any time.  相似文献   
73.
In the absence of commercially viable methods for cryopreserving turkey spermatozoa, new processing methods are required to extend the functional life of stored turkey spermatozoa for artificial insemination. The present study evaluates the efficacy of a new extender (Turkey Semen Extend) and investigates the use of density gradient centrifugation in processing turkey spermatozoa for artificial insemination. The new extender is compared with two commercially available turkey semen extenders, Beltsville Poultry Semen Extender and Ovodyl. Turkey spermatozoa in Turkey Semen Extend were still motile 20 h after collection, representing a considerable improvement over the other semen extenders (40%, 0% and 8% for Turkey Semen Extend, Beltsville Poultry Semen Extender and Ovodyl, respectively). A field trial on a commercial turkey farm showed improved fertilization rates following insemination of turkey hens with semen extended in Turkey Semen Extend (89.7%) compared with Beltsville Poultry Semen Extender (86.9%). This difference is statistically significant (p < 0.05). Processing on a density gradient, optimized for turkey spermatozoa, also increased sperm survival (50% gradient-prepared spermatozoa still motile after 18 h compared with <10% non-processed spermatozoa). Preliminary studies indicate that gradient preparation of spermatozoa may aid survival during cryopreservation.  相似文献   
74.
The occurrence of coagulase-negative, novobiocin-resistant staphylococci, i.e. Staphylococcus cohnii, Staphylococcus saprophyticus, Staphylococcus sciuri and Staphylococcus xylosus, on the skin of animals and man has been studied. On cultures from cats, cows, dogs, guinea pigs, mice, rabbits and sheep studied, such organisms were predominant among the coagulase-negative staphylococci. From the skin of the hands of 21 of 38 persons whose professions brought them into contact with animals, e.g. inséminât ors, slaughterhouse workers and veterinarians, coagulase-negative, novobiocin-resistant staphylococci were isolated. This finding contrasted with that regarding 50 persons lacking such contacts, of whom only 1 harboured such bacteria. S. saprophyticus was isolated only from those slaughterers presenting with wounds on their hands. Coagulase-negative, novobiocin-resistant staphylococci were also isolated from every second specimen collected from the surface of meat at a slaughterhouse. No difference in the culture results could be demonstrated from specimens collected before and after cutting-up of the carcasses. Of 26 strains of coagulase-negative, DNase-negative staphylococci isolated from milk with pathological CMT, all but 5 were novobiocin-resistant. Fifteen were classified as S. xylosus, 4 as S. sciuri and 1 as S. cohnii. Of another 15 DNase-positive strains, 3 were resistant to novobiocin. Finally, clinical infections with coagulase-negative, novobiocin-resistant staphylococci in man, e.g. urinary tract infections caused by S. saprophyticus, are considered in relation to possible contagious reservoirs and modes of spread.  相似文献   
75.

Background

Digital dermatitis in cattle is an emerging infectious disease. Ulcerative lesions are typically located on the plantar skin between the heel bulbs and adjacent to the coronet. Spirochetes of the genus Treponema are found in high numbers in the lesions and are likely to be involved in the pathogenesis. The aim of this study was to obtain pure cultures of spirochetes from cattle with digital dermatitis and to describe them further.

Methods

Tissue samples and swabs from active digital dermatitis lesions were used for culturing. Pure isolates were subjected to, molecular typing through 16S rRNA gene sequencing, pulsed-field gel electrophoresis (PFGE), random amplified polymorphic DNA (RAPD) and an intergenic spacer PCR developed for Treponema spp. as well as API-ZYM and antimicrobial susceptibility tests. The antimicrobial agents used were tiamulin, valnemulin, tylosin, aivlosin, lincomycin and doxycycline.

Results

Seven spirochete isolates from five herds were obtained. Both 16S rRNA gene sequences, which were identical except for three polymorphic nucleotide positions, and the intergenic spacer PCR indicated that all isolates were of one yet unnamed species, most closely related to Treponema phagedenis. The enzymatic profile and antimicrobial susceptibility pattern were also similar for all isolates. However it was possible to separate the isolates through their PFGE and RAPD banding pattern.

Conclusion

This is the first report on isolation of a Treponema sp. from cattle with digital dermatitis in Scandinavia. The phylotype isolated has previously been cultured from samples from cattle in the USA and the UK and is closely related to T. phagedenis. While very similar, the isolates in this study were possible to differentiate through PFGE and RAPD indicating that these methods are suitable for subtyping of this phylotype. No antimicrobial resistance could be detected among the tested isolates.  相似文献   
76.
测定了香红椿(Cedrela odorata)、神圣愈疮木(Guaiacum sanctum)和柠檬树(Calycophyllum candidissimum)种子萌发所需要的条件、并对光强对在尼加拉瓜开阔地、半开阔地和干旱森林林冠下所营造的香红椿和神圣愈疮木的苗存活率和生长的影响进行了研究。结果表明,在20-35℃常温条件下,柠檬树在有光照条件下种子萌发率显著高于黑暗条件的种子萌发率;香红椿和神圣愈疮木的种子在光照和黑暗条件下都有一定程度的萌发,最适萌发温度均为20-25℃。研究还表明,变温条件培养并没有提高3个树种的种子萌发率。在林冠层下和开阔地上神圣愈疮木苗木的存活率分别为70%和80%,两种条件下的周径相对生长速率分别为1.42mm/月和1.88mm/月。总的来说,在3个树种中,香红椿幼苗存活率较低,特别是在冠层内生长的幼苗。以上结果说明:柠檬树种子萌发需要光照;而香红椿和神圣愈疮木种子萌发对温度条件较为敏感。神圣愈疮木幼苗在裸地条件下存活率较高,可以作为退化土地造林备选树种。图2表2参43。  相似文献   
77.
The concentrations of Cd, Co, Cu, Pb, W and Zn were measured in road runoff and total deposition at two Swedish field sites during one year. It was found that the concentrations of most elements increased significantly during the winter, up to one order of magnitude. For cobalt and tungsten, it was found that around 90% of the total mass transport occurred during the winter, whereas for Cu, Pb, Cd, Zn and Na, the corresponding figures were 70–90, 40–80, 60–90, 50–70 and ≥99% depending on site specific conditions. The deicing salts (rock salts) did not significantly contribute to the increase in trace element concentrations. Instead, the increased concentrations were due to more intense wearing of the pavement during the winter because of the use of studded tires in combination with the chemical effects caused by the use of deicing salts. New potential elemental markers for roads and traffic are suggested.  相似文献   
78.
Abstract –  This study used passive telemetry (passive integrated transponders) to evaluate winter migration in three species of cyprinids (roach ( Rutilus rutilus (L.)), white bream ( Blicca bjoerkna (L.)) and rudd ( Scardinius erythrophthalmus (L.))) and their potential predators (pike ( Esox lucius (L.)) and perch ( Perca fluviatilis (L.))) between a shallow lake and its streams. Migration patterns were investigated from October to June, and a substantial part of the roach (40%) and white bream (55%) populations tagged in the lake during autumn migrated during winter into the streams, whereas only very few piscivores (<2%) migrated. In contrast to roach and white bream, only few rudd (<6%) migrated, which is likely a consequence of different overwintering strategies, e.g., rudd overwintering in shallow highly structured habitats. Small rudd migrated more than larger rudd, whereas there were no size-differentiated migration patterns for roach or white bream. Migration of the cyprinid fishes was generally initiated in late October and ended in May, and specific synchronised bursts of migration were observed in December, January and April, suggesting that migration is triggered by one or more proximate environmental cues. The cyprinid fishes generally entered the streams in late afternoon or in the morning, depending on season, but overall migration patterns varied between the three streams. We suggest and discuss that our results have great implications for lake management as well as for the interpretation of seasonal trophic dynamics in shallow lakes.  相似文献   
79.
Recent years have seen the introduction of the concept of urban greening, defined as embracing the planning and management of all urban vegetation to create or add values to the local community. Green-space development has become recognised by international agencies and donors as important tool in improving the quality of urban livelihoods and urban environment. This paper evaluates an example of an urban greening aid project, carried out by Danish and Russian partners in the city of St. Petersburg, Russia. The project aimed to contribute to conservation and development of the cultural–historical, social and ecological values of St. Petersburg's urban green areas by implementing a structured, socially inclusive, well-informed planning and management approach. The project had three main components: (1) the development of a GIS-based information system to assist green-space planning and management; (2) on-site improvements in selected green areas and (3) awareness raising and public involvement activities. Ex post evaluation of the project showed that in spite of the limits of time and resources, important results were achieved. A more strategic approach to urban green-space planning and management, as promoted by urban greening, was adapted by some of the Russian project partners. Achievements also included notable physical improvements to one park. But the main project impacts were improved communication and collaboration between the local park department and local academia, as well as expertise developed in running a complex urban greening project. The project failed, however, in its public involvement ambitions.Moreover, the project should have facilitated discussion on some of the current premises of urban green-space planning and management in St. Petersburg, which insufficiently consider changing values and public preferences related to green spaces.  相似文献   
80.
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