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   173篇
  免费   10篇
林业   12篇
农学   3篇
  9篇
综合类   27篇
农作物   5篇
水产渔业   5篇
畜牧兽医   108篇
园艺   8篇
植物保护   6篇
  2021年   2篇
  2020年   2篇
  2019年   4篇
  2018年   11篇
  2017年   5篇
  2014年   2篇
  2013年   12篇
  2012年   2篇
  2011年   10篇
  2010年   10篇
  2009年   5篇
  2008年   8篇
  2007年   4篇
  2006年   6篇
  2005年   6篇
  2004年   3篇
  2003年   5篇
  2001年   5篇
  2000年   4篇
  1999年   3篇
  1994年   2篇
  1993年   2篇
  1992年   2篇
  1990年   5篇
  1989年   3篇
  1988年   4篇
  1987年   3篇
  1986年   2篇
  1985年   2篇
  1984年   2篇
  1981年   3篇
  1980年   2篇
  1979年   2篇
  1978年   2篇
  1976年   2篇
  1975年   3篇
  1974年   2篇
  1973年   2篇
  1970年   2篇
  1969年   3篇
  1968年   2篇
  1967年   2篇
  1963年   1篇
  1940年   1篇
  1932年   1篇
  1926年   1篇
  1921年   2篇
  1918年   1篇
  1915年   1篇
  1913年   1篇
排序方式: 共有183条查询结果,搜索用时 31 毫秒
91.
A common approach used to estimate landscape resistance involves comparing correlations of ecological and genetic distances calculated among individuals of a species. However, the location of sampled individuals may contain some degree of spatial uncertainty due to the natural variation of animals moving through their home range or measurement error in plant or animal locations. In this study, we evaluate the ways that spatial uncertainty, landscape characteristics, and genetic stochasticity interact to influence the strength and variability of conclusions about landscape-genetics relationships. We used a neutral landscape model to generate 45 landscapes composed of habitat and non-habitat, varying in percent habitat, aggregation, and structural connectivity (patch cohesion). We created true and alternate locations for 500 individuals, calculated ecological distances (least-cost paths), and simulated genetic distances among individuals. We compared correlations between ecological distances for true and alternate locations. We then simulated genotypes at 15 neutral loci and investigated whether the same influences could be detected in simple Mantel tests and while controlling for the effects of isolation-by-distance using the partial Mantel test. Spatial uncertainty interacted with the percentage of habitat in the landscape, but led to only small reductions in correlations. Furthermore, the strongest correlations occurred with low percent habitat, high aggregation, and low to intermediate levels of cohesion. Overall genetic stochasticity was relatively low and was influenced by landscape characteristics.  相似文献   
92.
Whilst the benefits of agroforestry are widely recognised in tropical latitudes few studies have assessed how agroforestry is perceived in temperate latitudes. This study evaluates how stakeholders and key actors including farmers, landowners, agricultural advisors, researchers and environmentalists perceive the implementation and expansion of agroforestry in Europe. Meetings were held with 30 stakeholder groups covering different agroforestry systems in 2014 in eleven EU countries (Denmark, France, Germany, Greece, Hungary, Italy, Netherlands, Portugal, Spain, Sweden and the United Kingdom). In total 344 valid responses were received to a questionnaire where stakeholders were asked to rank the positive and negative aspects of implementing agroforestry in their region. Improved biodiversity and wildlife habitats, animal health and welfare, and landscape aesthetics were seen as the main positive aspects of agroforestry. By contrast, increased labour, complexity of work, management costs and administrative burden were seen as the most important negative aspects. Overall, improving the environmental value of agriculture was seen as the main benefit of agroforestry, whilst management and socio-economic issues were seen as the greatest barriers. The great variability in the opportunities and barriers of the systems suggests enhanced adoption of agroforestry across Europe will be most likely to occur with specific initiatives for each type of system.  相似文献   
93.
Potential benefits and costs of agroforestry practices have been analysed by experts, but few studies have captured farmers’ perspectives on why agroforestry might be adopted on a European scale. This study provides answers to this question, through an analysis of 183 farmer interviews in 14 case study systems in eight European countries. The study systems included high natural and cultural value agroforestry systems, silvoarable systems, high value tree systems, and silvopasture systems, as well as systems where no agroforestry practices were occurring. A mixed method approach combining quantitative and qualitative approaches was taken throughout the interviews. Narrative thematic data analysis was performed. Data collection proceeded until no new themes emerged. Within a given case study, i.e. the different systems in different European regions, this sampling was performed both for farmers who practice agroforestry and farmers who did not. Results point to a great diversity of agroforestry practices, although many of the farmers are not aware of the term or concept of agroforestry, despite implementing the practice in their own farms. While only a few farmers mentioned eligibility for direct payments in the CAP as the main reason to remove trees from their land, to avoid the reduction of the funded area, the tradition in the family or the region, learning from others, and increasing the diversification of products play the most important role in adopting or not agroforestry systems.  相似文献   
94.
The degree of similarity between rice milled in a McGill #2 laboratory mill and commercial milling processes was evaluated using eight physical, physicochemical, and end‐use properties. There was no statistical difference between the two milling systems with respect to color parameters L* and a*, final viscosity, texture, and end‐use cooking properties (α = 0.05). Overall, the kernel dimensions of length, width, and thickness were less in the McGill #2 laboratory‐milled rice than the same rice milled commercially. The incidence of bran streaks and peak viscosity values were each higher when the rice sample was milled commercially in 27, and 28, respectively, of the 29 samples by means comparison. The decrease in kernel dimensions and incidence of bran streaks were attributed to the more aggressive nature of the single‐pass, batch milling system of the McGill #2 laboratory mill as compared with multipass, continuous milling systems that are used commercially. Finally, as surface lipid content (SLC) decreased, L* increased and a*, b*, and the incidence of bran streaks decreased for both milling systems.  相似文献   
95.
The oral and intravenous disposition of the anti-thyroid drug propylthiouracil (PTU) was determined in six clinically healthy cats and four cats with naturally occurring hyperthyroidism. Compared with the normal cats, the mean plasma elimination half-life of PTU was significantly (P less than 0.001) shorter in the hyperthyroid cats (77.5 +/- 5.8 minutes compared with 125.5 +/- 3.7 minutes) and the total body clearance of PTU was significantly (P less than 0.05) more rapid in the cats with hyperthyroidism (5.1 +/- 0.8 ml kg-1 min-1 compared with 2.7 +/- 0.2 ml kg-1 min-1). Following oral administration, both the bioavailability (59.7 +/- 4.9 per cent compared with 73.3 +/- 3.7 per cent) and peak plasma concentrations (14.5 +/- 1.6 micrograms ml-1 compared with 18.9 +/- 0.9 micrograms ml-1) of PTU were significantly (P less than 0.05) lower in the hyperthyroid cats than in the control cats. No difference was noted, however, between the apparent volume of distribution for PTU in the two groups of cats. Overall, results of this study indicate that the oral bioavailability of PTU is decreased and PTU disposition is accelerated in cats with hyperthyroidism.  相似文献   
96.
97.
A Holstein fetus was delivered by Caesarean section at a gestational age of 441 days. The pituitary pars distalis was aplastic and the adrenal and thyroid glands were severely hypoplastic. Arrested or retarded cartilage cell maturation resulted in absence or minimal development of epiphyseal ossification centers, delayed ossification of carpal bones, and arrest of longitudinal growth of bones. The pathophysiology of prolonged gestation and of skeletal pathology is discussed.  相似文献   
98.
In laboratory studies, acute toxicity tests (96-hour) showed strong main effect on crawfish when Propanil, Ordram, and Furadan were combined; this combination was the most toxic treatment to crawfish. Furthermore, a strong interaction among the three suggests a non-additive response. Ordram alone was the least toxic pesticide. Additive effect was observed when Furadan was combined with Ordram or Propanil, but antagonism was detected in the combination of Propanil with Ordram. Concentrations higher than the recommended rate for use at normal conditions had a significant (P<0.05) lethal effect on crawfish. Soil moderated the toxicity of some pesticides and their combinations.  相似文献   
99.
Nonspecific cytotoxic cells (NCC) from fish (Ictalurus punctatus) were treated with different concentrations of retinolacetate and poly I:C. Both in vitro and in vivo experiments were conducted. Retinolacetate significantly increased NCC activity against chromium-51 labeled human B-cell lymphoma target cells (NC-37). Preincubation (treatment prior to adding the labeled target cells) of NCC for 4 to 8 h in 10?3 to 10?12 M concentration of retinolacetate produced significant increases in NCC activity compared to treatment during the killing assay only. Similar experiments with different concentrations of poly I:C had no NCC augmenting effects when tested by adding poly I:C either during preincubation periods or during the cytotoxicity assay. Retinolacetate probably produces positive modulation of cytotoxicity by increasing the killing effectiveness of individual NCC, rather than recruiting larger numbers of cytolytic cells. In vivo studies were also conducted by injecting catfish (i.p.) with 1 ×, 3 × and 5 × the daily recommended vitamin A dosages and determining NCC activity after 24, 48 and 72 h treatment. The 1 × dose significantly increased NCC activity at 72 h. This increase was not transient because NCC activity after 33–37 days' treatment was significantly higher than controls in the 1 ×, 2 × and 3 × groups. Intraperitoneal injections of fish with poly I:C produced no significant increases in NCC activity at 24 or 72 h post-inoculation.  相似文献   
100.
Episodes of hemolysis and leukocytosis which were associated with Babesia bigemina followed each of 3 challenge exposures to influenza A viruses. It is possible that viral infection altered the immunologic host-parasite equilibrium. Acute thrombocytopenia and rouleaux formation were also observed. Death, attributed to liver flukes, occurred 168 days after the yak was transferred from high to low altitude. A 2nd yak died of foot-and-mouth disease, thus supporting the Nepali belief that yaks will not survive at the lower altitudes of Kathmandu.  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