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   164篇
  免费   7篇
林业   15篇
农学   5篇
  33篇
综合类   13篇
农作物   9篇
水产渔业   13篇
畜牧兽医   65篇
园艺   4篇
植物保护   14篇
  2023年   1篇
  2022年   1篇
  2021年   5篇
  2020年   1篇
  2019年   2篇
  2018年   9篇
  2017年   4篇
  2016年   6篇
  2015年   3篇
  2014年   9篇
  2013年   5篇
  2012年   15篇
  2011年   13篇
  2010年   8篇
  2009年   6篇
  2008年   14篇
  2007年   9篇
  2006年   7篇
  2005年   15篇
  2004年   15篇
  2003年   8篇
  2002年   1篇
  2001年   1篇
  2000年   3篇
  1999年   1篇
  1998年   1篇
  1996年   1篇
  1993年   1篇
  1990年   1篇
  1989年   2篇
  1975年   1篇
  1972年   1篇
  1937年   1篇
排序方式: 共有171条查询结果,搜索用时 15 毫秒
1.
Richness of Ancient Woodland Indicator plant species was analysed in 308 woodland patches that were surveyed during the Countryside Survey of Great Britain carried out in 1998. The Countryside Survey recorded vegetation plots and landscape structure in 569 stratified 1 km sample squares and developed a remotely-sensed land cover map of the UK. Using these datasets, we tested the hypothesis that Ancient Woodland Indicator species richness in woodland fragments was limited by patch area, shape and spatial isolation and that woodland patches located in the lowland region of Great Britain would respond differently than those in the upland region. The variation in Ancient Woodland Indicator species richness in the British lowlands (n = 218) was mainly explained by patch area and two measures of connectivity, the length of hedgerows and lines of trees in the 1 km square and the area of woodland within 500 m of the vegetation plot. By contrast, variation in Ancient Woodland Indicator species richness in the British uplands (n = 90) was related to Ellenberg scores of the vegetation communities sampled – a surrogate for habitat quality – and no significant effect of spatial structure was detected. It therefore appears that the degree of fragmentation of woodland in the British lowlands limits the distribution of Ancient Woodland Indicator species, while in the uplands, failed colonisation is a matter of habitat quality rather than a result of landscape structure.This revised version was published online in May 2005 with corrections to the Cover Date.  相似文献   
2.
3.
We have studied the structural effects of application to the soil of a potentially detrimental herbicide, 4,6-dinitroorthocresol (DNOC) by analysing amplified ribosomal DNA restriction analysis (ARDRA) and terminal restriction fragment length polymorphism (T-RFLP) signatures of 16S rDNA fragments of culturable bacterial communities isolated from diluted soil suspensions. This approach has the potential to reveal changes induced by stressing the soil microflora with DNOC. This paper shows that, whereas only few changes of the ARDRA and T-RFLP profiles result from ageing of the soil, treatment of the soil with DNOC induces major modifications of these profiles. Therefore, for the practical purpose of pesticide registration, ARDRA and T-RFLP analysis performed on the dominant culturable fraction of the soil bacteria, implemented using conventional gel electrophoresis, offers the means of a routine, simple and meaningful test for detecting some of the changes affecting the structure of the soil microflora in response to pesticide application.  相似文献   
4.
Climate data are dramatically increasing in volume and complexity, just as the users of these data in the scientific community and the public are rapidly increasing in number. A new paradigm of more open, user-friendly data access is needed to ensure that society can reduce vulnerability to climate variability and change, while at the same time exploiting opportunities that will occur.  相似文献   
5.
Several human and animal Ebola outbreaks have occurred over the past 4 years in Gabon and the Republic of Congo. The human outbreaks consisted of multiple simultaneous epidemics caused by different viral strains, and each epidemic resulted from the handling of a distinct gorilla, chimpanzee, or duiker carcass. These animal populations declined markedly during human Ebola outbreaks, apparently as a result of Ebola infection. Recovered carcasses were infected by a variety of Ebola strains, suggesting that Ebola outbreaks in great apes result from multiple virus introductions from the natural host. Surveillance of animal mortality may help to predict and prevent human Ebola outbreaks.  相似文献   
6.
Microtubules display dynamic instability, with alternating phases of growth and shrinkage separated by catastrophe and rescue events. The guanosine triphosphate (GTP) cap at the growing end of microtubules, whose presence is essential to prevent microtubule catastrophes in vitro, has been difficult to observe in vivo. We selected a recombinant antibody that specifically recognizes GTP-bound tubulin in microtubules and found that GTP-tubulin was indeed present at the plus end of growing microtubules. Unexpectedly, GTP-tubulin remnants were also present in older parts of microtubules, which suggests that GTP hydrolysis is sometimes incomplete during polymerization. Observations in living cells suggested that these GTP remnants may be responsible for the rescue events in which microtubules recover from catastrophe.  相似文献   
7.
Automatic distinction of weed species using form parameters by means of digital image processing
In former investigations the possibility to distinguish weed from soil background by computerized quantitative image analysis has been reported. In the present studies image analysis technique are applied to identify different weed species. From six typical winter weed species including various growth stages, 300 color slides were taken at the research station in Bonn on the 4th of February, 1988. From each slide a number of form parameters were measured automatically out of the digitized shape of each weed plant. Form parameters including area, perimeter, minimum and maximum diameter of the plants are taken from the digitized shape of each weed as well as figures representing the relative proportions of these parameters. For automatic discrimination up to 9 of the parameters were selected to compute five so-called canonical variables. The values of these variables are typical for each weed species and can be interpreted as their "finger prints". The average rate of correct identification was 82.3 %. The scientific and practical implications of the results are discussed.  相似文献   
8.
9.
One hundred pigs from the NE Index Line (NEI) and 100 Hampshire-Duroc cross pigs (HD) were inoculated intranasally with porcine respiratory and reproductive syndrome virus (PRRSV 97-7895 strain) at 26 d of age to determine whether genetic variation in response to PRRSV exists. An uninfected littermate to each infected pig served as a control. Pigs were from 163 dams and 83 sires. Body weight and rectal temperature were recorded, and blood samples were drawn from each pig on d 0 before inoculation and on d 4, 7, and 14 after inoculation. Pigs were sacrificed on d 14. Lung and bronchial lymph nodes were collected, placed in optimal cutting temperature compound, and frozen at -80 degrees C. The presence of PRRSV in serum and in lung tissue and bronchial lymph nodes was determined by isolation in cell culture. The presence of antibodies in serum collected on d 14 was determined by a commercial ELISA test. Lung tissue was examined microscopically and scored for incidence and severity of lesions (score of 1 to 3; 1 = no or few lesions, and 3 = severe interstitial pneumonia). Data were analyzed with a mixed model that included random sire and dam effects. The interaction of line x treatment was significant (P < 0.001) for weight change and rectal temperature. Un-infected HD pigs gained 0.67 kg more from d 0 to 14 and averaged 0.32 degrees C higher rectal temperature than uninfected NEI pigs (P < 0.001), whereas infected NEI pigs gained 0.34 kg more and had -0.54 degrees C lower temperature than infected HD pigs (P < 0.001). Viremic titer (cell culture infectious dose 50%/mL) was greater (P < 0.05) in HD than NEI at d 4 (10(4.52) vs. 10(4.22)), 7 (10(4.47) vs. 10(3.99)), and 14 (10(3.49) vs. 10(3.23)). Viral titer loads in lung (P = 0.11) and bronchial lymph nodes tended (P = 0.07) to be greater in HD than NEI pigs. Antibody signal-to-positive (S/P) ELISA ratios in infected pigs ranged from 0.18 to 3.38, and 88% had levels > or = 0.40, which is the positive threshold for this ELISA. The S/P range in uninfected pigs was 0 to 1.11, and 99% had levels < or = 0.40. Mean S/P ratio for infected pigs was 0.23 units higher in HD than in NEI (P < 0.001). The HD pigs had a greater incidence of interstitial pneumonia and 0.65 higher mean lesion scores than NEI pigs (P < 0.001). In summary, responses of pigs of the two lines to infection with PRRSV differed, indicating that underlying genetic variation existed.  相似文献   
10.
Little is known about the prevalence of caprine yersiniosis in Germany. Only few cases are reported every year. The intention of the survey was to provide representative data on the seroprevalence of anti-Yersinia antibodies in goats in the German state of Lower Saxony. A commercially available Western blot kit was used to identify caprine and ovine anti-Yersinia antibodies against five proteins [YopM, H, D, E and V-antigen (V-Ag)]. Of the 681 investigated goat sera, 449 (66%) had anti-Yop/V-Ag antibodies. Only two of 28 animal holdings housed sero-negative goats. Boxplot analysis showed that the number of non-reactive animals is correlated to the size of a herd and the fact of milk production, respectively. A tendency was observed that various management factors may influence the anti-Yersinia antibody status. No statement was possible on the impact of keeping additional carrier animals such as pigs, cows or sheep on a farm or the type of husbandry on the seroprevalence of anti-Yersinia antibodies. This study provides trend-setting data for yersiniosis in goat-holdings. The impact on consumer health, i.e. especially for risk groups-like people allergic to cow milk and the impact on the profit of a farm will have to be elucidated in further studies.  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