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   568篇
  免费   72篇
林业   27篇
农学   10篇
  83篇
综合类   65篇
农作物   28篇
水产渔业   58篇
畜牧兽医   324篇
园艺   20篇
植物保护   25篇
  2024年   4篇
  2023年   18篇
  2022年   16篇
  2021年   34篇
  2020年   36篇
  2019年   55篇
  2018年   32篇
  2017年   37篇
  2016年   35篇
  2015年   21篇
  2014年   27篇
  2013年   19篇
  2012年   48篇
  2011年   54篇
  2010年   24篇
  2009年   24篇
  2008年   30篇
  2007年   37篇
  2006年   31篇
  2005年   25篇
  2004年   8篇
  2003年   10篇
  2002年   10篇
  2000年   1篇
  1991年   2篇
  1983年   1篇
  1977年   1篇
排序方式: 共有640条查询结果,搜索用时 15 毫秒
21.
In military planning, it is important to be able to estimate not only the number of fatalities but how often attacks that result in fatalities will take place. We uncovered a simple dynamical pattern that may be used to estimate the escalation rate and timing of fatal attacks. The time difference between fatal attacks by insurgent groups within individual provinces in both Afghanistan and Iraq, and by terrorist groups operating worldwide, gives a potent indicator of the later pace of lethal activity.  相似文献   
22.
Interest in the consumption of fresh fruits and vegetables is, to a large extent, due to its content of bioactive nutrients and their importance as dietary antioxidants. Among all of the selected fruits and vegetables, strawberries and black grapes have relatively high antioxidant capacities associated with high contents of total phenolic compounds, ascorbic acid, and flavonols. More interesting, the results of this study indicated that in most fruits and vegetables storage did not affect negatively the antioxidant capacity. Better, in some cases, an increase of the antioxidant capacity was observed in the days following their purchase, accompanied by an increase in phenolic compounds. In general, fruits and vegetables visually spoil before any significant antioxidant capacity loss occurs except in banana and broccoli. When ascorbic acid or flavonoids (aglycons of flavonols and anthocyanins) were concerned, the conclusions were similar. Their content was generally stable during storage.  相似文献   
23.
Apple and pear fruits are important sources of secondary plant metabolites and one of the major sources of dietary phenolics consumed all year round. The aim of this work was to identify the main variables influencing phenolic content and antioxidant capacity in apples. Higher phenolic and antioxidant contents were observed in some varieties (such as the Delbar Estival apple and Durondeau pear). Storage conditions were important. Our results also showed that fruits should be consumed rapidly after purchase and with their peel. After one week of domestic storage, the ascorbic acid content was found to decrease by 75%. Peeling led to a more than 25% decrease in total phenolics and ascorbic acid. The harvest time (at normal ripeness) had only a limited impact, but significant year-to-year variations were observed. In conclusion, well-chosen and well-stored apples and pears may contribute to an antioxidant-rich diet if consumed rapidly and with their peel.  相似文献   
24.
OBJECTIVE: To determine demographic, clinical, and radiographic features of bronchiectasis in dogs. DESIGN: Retrospective study. ANIMALS: 289 dogs identified through the Veterinary Medical Database (VMDB) and 27 dogs examined at the North Carolina State University Veterinary Teaching Hospital. PROCEDURE: Demographic characteristics of dogs identified through the VMDB were compared with characteristics of the entire population of dogs entered in the VMDB. Medical records of dogs examined at the teaching hospital were reviewed; the diagnosis was confirmed through review of thoracic radiographs. RESULTS: Analysis of data from the VMDB indicated that American Cocker Spaniels, West Highland White Terriers, Miniature Poodles, Siberian Huskies, English Springer Spaniels, and dogs > 10 years old had an increased risk of bronchiectasis. Among dogs examined at the teaching hospital, coughing was the most common clinical sign. There was evidence for excessive airway mucus but not hemorrhage. A variety of bacterial organisms were isolated from tracheal wash and bronchoalveolar lavage samples. On thoracic radiographs, cylindrical bronchiectasis, generalized disease, and right cranial lung lobe involvement were most common. Seven of 14 dogs for which follow-up radiographs were available did not have any progression of radiographic lesions. Median duration of clinical signs prior to diagnosis of bronchiectasis was 9 months (range, 1 day to 10 years). Median survival time was 16 months (range, 2 days to 72 months). CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that despite substantial clinical abnormalities, dogs with bronchiectasis may survive for years. Certain purebred dogs and older dogs may have an increased risk of developing bronchiectasis.  相似文献   
25.
The objective of this study was to develop a reliable Taqman 5' Nuclease Assay for genotyping sheep for scrapie susceptibility. The sheep prion gene contains 2 single nucleotide polymorphisms (SNPs) that may mediate resistance to classical scrapie, one at codon 136, alanine (A) or valine (V), and another at codon 171, arginine (R) or glutamine (Q). The R allele appears to confer resistance to classical scrapie, with the AA(136) RR(171) genotype the most resistant to scrapie and QR(171) only rarely infected in the US sheep population. The Assays by Design protocol was used for development of probes and primers for codon 136 and Primer Express for codon 171. Commercially available kits were used to isolate genomic DNA from blood or muscle. For validation, 70 SNP determinations for each codon were compared to commercial testing with an error rate of less than 1%. Then, 935 samples from blood (n = 818) and muscle (n = 117) were tested for both codons with 928 successful determinations and only 7 samples (<1% of total samples) that needed repeating. Genotypes were AA QQ (n = 102; 11.0%), AV QQ (n = 28; 3.0%), AA QR (n = 396; 42.7%), AV QR (n = 54; 5.8%), and AA RR (n = 348; 37.5%). Thus, 86% of the sheep tested (n = 798) contained R at codon 171 and were expected to be scrapie-resistant. This new Taqman 5' Nuclease SNP genotyping assay is accurate, easy to perform, and useful in the study of classical scrapie in sheep and its prevention through selective breeding programs to eliminate highly susceptible animals.  相似文献   
26.
Microchips are commonly used in companion, research, and zoo animal medicine for easy, reliable, and cost-effective identification with relatively low risk of side effects. This report describes development of a metastatic leiomyosarcoma associated with a microchip in an Egyptian fruit bat (Rousettus aegyptiacus).  相似文献   
27.

Context

North American grassland songbird populations have declined significantly due to habitat loss and fragmentation. Understanding the influence of the surrounding landscape on prairie fragment occupancy is vital for predicting the fate of grassland birds in these heavily altered landscapes.

Objectives

We examined the relative importance of local and landscape variables on grassland bird occupancy of prairie fragments using a focal-patch study. We also investigated the spatial scale at which landscape variables were most influential.

Methods

We surveyed birds on 29 unplowed prairie fragments in western Minnesota and eastern North and South Dakota. We quantified local habitat on the fragment using vegetation surveys and aerial photographs and the landscape surrounding the fragment out to 4 km using aerial photographs. We analyzed occupancy using multi-model approaches applied to multiple logistic regression.

Results

Of 38 species encountered, nine were neither too rare nor too abundant to be analyzed. Predictors of patch occupancy were unique for each bird species, yet general patterns emerged. For eight species, landscape variables were more important than local variables. Mostly, those landscape variables measured configuration (e.g., edge density) and not composition (e.g., percent cover of a particular matrix element). Landscape effects were mostly from variables measured at the greatest extents from the prairie fragment.

Conclusions

Using a focal-patch study design we demonstrated the importance of the surrounding landscape, often out to 4 km from the fragment edge, on prairie occupancy by grassland birds. Effective management of grassland songbirds will require attention to the landscape context of prairie fragments.
  相似文献   
28.
Increasing urbanization of rural landscapes has created new challenges for wildlife management. In addition to changes in the physical landscape, urbanization has also produced changes in the socio-cultural landscape. The greater distancing from direct interaction with wildlife in urbanized societies has led to the emergence of a culture whose meanings for wildlife are less grounded in the utilitarian/instrumental orientation of rural agrarian systems. Urban perspectives on wildlife are comprised of more highly individualized emotional/symbolic values. This shift creates two problems with respect to managing wildlife in an urbanizing landscape. First the increased diversity in values and meanings increases the likelihood for social conflicts regarding wildlife management while at the same time making socially acceptable resolutions more intractable. This in turn requires fundamental changes in decision-making paradigms and the research approaches used to inform decision making. Second, as remaining rural communities feel the pressures of urbanization, wildlife conflicts become conflicts not just over wildlife but conflict over larger socio-political concepts such as equity, tradition, private property rights, government control, power, and acceptable forms of knowledge. This paper examines the wildlife management implications of changes associated with increasing urbanization and employs two case studies to illustrate these issues. First a study of a controversy over urban deer management provides insights into how to map conflicting values and search for common ground in an urban culture with increasingly individualistic values for wildlife. Specifically, the analysis illustrates that common ground may, at times, be found even among people with conflicting value systems. The second case study examined a ranching community faced with predator reintroduction. This case study illustrates tensions that occur when the community of interest (i.e. a national public) is broader than the community of place in which the problem occurs. In this latter situation, the debate centers around more than just different views about the rights of animals. It also entailed the rights of individuals and communities to decide their future. The conclusion discusses the need for wildlife institutions to adapt their underlying decision making philosophy including the way science is integrated into decision making processes in light of the changes in social context caused by urbanization.  相似文献   
29.
Between 1998 and 2008, feline fecal specimens were submitted to provincial veterinary diagnostic laboratories in Regina and Saskatoon, Saskatchewan, for sucrose centrifugation-flotation (n = 635), parasite identification (n = 17), and/or Giardia (n = 283) or Cryptosporidium (n = 266) commercial direct immunofluorescence assay (IFA). The most commonly detected parasites on flotation were Toxocara cati (4.7%), Isospora (3.8%), and taeniid eggs (Echinococcus or Taenia) (1.3%). Cats less than 2 years of age were twice as likely to have a positive parasite test as cats older than 2 years. Using IFA, Giardia was detected in 9.9% of samples, and Cryptosporidium in 2.3% of samples. Relative to IFA, flotation had sensitivity values of 39% and 50% for detection of Giardia and Cryptosporidium, respectively. Giardia and Isospora were detected in a higher proportion of samples in our study population than reported in the general cat population in western Canada. This study highlights the importance of sensitivity when interpreting diagnostic tests and provides information to guide region-specific recommendations for helminth parasite prevention and treatment.  相似文献   
30.
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