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   918篇
  免费   91篇
林业   119篇
农学   35篇
基础科学   1篇
  272篇
综合类   3篇
农作物   57篇
水产渔业   94篇
畜牧兽医   340篇
园艺   19篇
植物保护   69篇
  2023年   10篇
  2022年   8篇
  2021年   30篇
  2020年   30篇
  2019年   32篇
  2018年   78篇
  2017年   68篇
  2016年   78篇
  2015年   49篇
  2014年   54篇
  2013年   65篇
  2012年   64篇
  2011年   85篇
  2010年   42篇
  2009年   18篇
  2008年   33篇
  2007年   35篇
  2006年   27篇
  2005年   37篇
  2004年   29篇
  2003年   27篇
  2002年   27篇
  2001年   11篇
  2000年   8篇
  1999年   8篇
  1998年   5篇
  1997年   6篇
  1996年   5篇
  1995年   4篇
  1994年   2篇
  1993年   5篇
  1992年   1篇
  1991年   2篇
  1990年   3篇
  1989年   3篇
  1986年   1篇
  1984年   2篇
  1982年   2篇
  1981年   4篇
  1980年   1篇
  1978年   1篇
  1977年   1篇
  1976年   4篇
  1971年   1篇
  1968年   1篇
  1966年   1篇
  1938年   1篇
排序方式: 共有1009条查询结果,搜索用时 421 毫秒
41.
In the technological processes of ore flotation in the non-ferrous metal industry flotation, tailings constitute up to 98% of the total of the processed material. They are stored in sedimentation ponds as fine-grained drift containing an excess of heavy metals, mainly zinc, cadmium, lead and copper. After the dehydration and drying up of the surface of sedimentation ponds, these metals are carried with dust by the wind and they are accumulated in the vicinity of the ponds in humus horizons of soils causing considerable degradation and even devastation. In this publication, we have presented the results of research on the negative influence of such a structure on the soils of an adjacent fragment of forest and its dependence on the distance from the sedimentation pond. The level of degradation of the soils under research has been assessed on the basis of changes and transformations of the morphological soil profiles, the level of alkalization and accumulation of heavy metals (Zn, Pb, Cd) in the topsoil layers and their translocation in the soil horizons. The following soils have been distinguished in the investigated area: Haplic Podzols distorted and degraded as well as Urbic Anthrosols. The acquired results have served to depict the spatial accumulation of these heavy metals in the topsoil layers. The highest accumulation of heavy metals and the degradation of the forest sites connected with it has occurred in the vicinity of a sedimentation pond drain channel. Soil remediation has been recommended in this area.  相似文献   
42.
ABSTRACT

The content of macronutrients in potato tubers arouses interest because of their substantial consumption in the world and significant role in elements' budget in human diet. The research objective has been to evaluate the content of macronutrients in tubers of potatoes grown in different systems of crop production in Poland. In 2012–2014, an experiment was conducted to test potato cultivation in three-crop production systems: conventional, integrated and organic. In each of the six-field crop rotation systems, there were five potato cultivars classified into different earliness groups. The following macronutrients in dry mass of potato tubers were determined: N, P, K, Ca, Mg, Na, S and Cl, and the various ratios between them were calculated. The research carried out proved that potatoes production systems affect the chemical composition of tubers, which is additionally modified by whether condition and genetic features of cultivars. Potato tubers from organic farming contained by about 20% less N than tubers from conventional or integrated systems. Potato tubers from organic production system displayed lower content of Ca and Na in comparison with the conventional and integrated systems. The least of P and S were detected in tubers of the potatoes grown conventionally.  相似文献   
43.
Using Tn5 mutagenesis of lysogenic Sinorhizobium meliloti strain L5–30 (L54), three neomycin-resistant transconjugants differing in phage-resistance profiles were isolated. Two of them increased the dry weight of plants and were capable of establishing root nodules, whereas the third one was ineffective. The bacterium-phage interaction did not have observable consequences in the Medicago sativa - S. meliloti symbiosis because it did not affect the number of nodules on M. sativa or plant dry weight.  相似文献   
44.
In this paper, the contamination degree of the Rybnik Reservoir with cadmium, copper and nickel was analyzed. Quality of the water from the reservoir was determined by drawing comparisons between the metal content in the water and both the officially permitted levels (contamination factor) and levels of metals occurring in the water of non-contaminated areas (enrichment factor). Contamination of bottom sediment with chosen metals was analyzed with reference to the metal content in mudstone (geoaccumulation index, enrichment factor, contamination factor). Trends towards changing the metal content in the bottom sediment was analyzed by determining the enrichment factor of the surface layer of the bottom sediments in relation to a deeper layer. Enrichment of the bottom sediments with metals coming from the water was also determined.  相似文献   
45.
The purpose of this study was to evaluate transepidermal water loss (TEWL), skin hydration and skin pH in normal cats. Twenty shorthaired European cats of both sexes were examined in the study. Measurements were taken from five different sites: the lumbar region, the axillary fossa, the inguinal region, the ventral abdominal region and the left thoracic region. In each of the regions, TEWL, skin hydration and skin pH were measured. The highest TEWL value was observed in the axillary fossa (18.22g/h/m(2)) and the lowest in the lumbar region (10.53g/h/m(2)). The highest skin hydration was found in the inguinal region (18.29CU) and the lowest in the lumbar region (4.62CU). The highest skin pH was observed in the inguinal region (6.64) and the lowest in the lumbar region (6.39). Statistically significant differences in TEWL were observed between the lumbar region and the left side of the thorax region (P=0.016), the axillary fossa (P=0.0004), the ventral region (P=0.005), and the inguinal region (P=0.009). There were significant differences in skin hydration between the lumbar region and the left thorax (P=0.000003), the axillary fossa (P=0.002), the ventral abdomen (P=0.03), and the inguinal region (P=0.0003) as well as between the thorax and the ventral abdomen (P=0.005). TEWL was higher in females (15g/h/m(2)) than in males (4.57g/h/m(2)). Skin hydration was higher in females (13.89CU) than in males (12.28CU). Significant differences were not found between males and females for TEWL and skin hydration. Skin pH was higher in males (6.94) than in females (6.54), which was significant (P=0.004).  相似文献   
46.
Local immunity of the respiratory mucosal system in chickens and turkeys   总被引:1,自引:0,他引:1  
This review article presents fundamental mechanisms of the local mucosal immunity in selected regions of the respiratory tract in healthy birds and in some pathological conditions. The respiratory system, whose mucosa come into direct contact with microorganisms contaminating inhaled air, has some associated structures, such as Harderian gland (HG), conjunctive-associated lymphoid tissue (CALT) and paranasal glands (PG), whose participation in local mechanisms of the mucosal immunity has been corroborated by numerous scientific studies. The nasal mucosa, with structured clusters of lymphoid tissue (NALT - nasal-associated lymphoid tissue) is the first to come into contact with microorganisms which contaminate inhaled air. Lymphoid nodules, made up of B cells with frequently developed germinal centres (GC), surrounded by a coat of CD4+ cells, are the major NALT structures in chickens, whereas CD8+ cells are situated in the epithelium and in the lamina propria of the nasal cavity mucosa. Studies into respiratory system infections (e.g. Mycoplasma gallisepticum) have shown the reactivity of the tracheal mucosa to infection, despite a lack of essential lymphoid tissue. Bronchus-associated lymphoid tissue (BALT) takes part in bronchial immune processes and its structure, topography and ability to perform defensive function in birds is largely age-dependent. Mature BALT is covered by a delicate layer of epithelial cells, called follicle-associated epithelium (FAE). Germinal centres (GC), surrounded by CD4+ cells are developed in most mature BALT nodules, while CD8+ lymphocytes are dispersed among lymphoid nodules and in the epithelium, and they are rarely present in GC. Macrophages make up the first line of defence mechanisms through which the host rapidly responds to microorganisms and their products in the respiratory mucosal system. Another very important element are polymorphonuclear cells, with heterophils being the most important of them. Phagocytic cells obtained from lung lavages in birds are referred to as FARM (free avian respiratory macrophages). Their number in chickens and turkeys is estimated to be 20 times lower than that in mice and rats, which indicates a deficit in the first-line of defence in the birds' respiratory system. There are numerous B cells and antibody secreting cells (ASC) present throughout the respiratory system in birds. Their role comes down to perform antigen-specific protection by producing antibodies (IgM, IgY or IgA class) as a result of contact with pathogenic factors.  相似文献   
47.
Real-time PCR directed to intergenic spacer (IGS) noncoding region between 16S and 23S rRNA genes was used for species specific detection of Mycoplasma felis in conjunctival scrapings. Samples were collected from 57 cats suffering from chronic conjunctivitis in 2008-2010 (Wroc?aw, Poland). Samples from 36 cats (63.2%) were shown to be positive for Mycoplasma felis. Our research gives a first insight in the occurrence of Mycoplasma felis among domestic cats in Poland suggesting that this pathogen may constitute an underestimated cause of chronic conjunctivitis.  相似文献   
48.
Interactions between extracellular matrix (ECM) and epithelial cells are necessary for proper organisation and function of the epithelium. In the present study we show that bovine mammary epithelial cell line BME-UV1 cultured on ECM components, commercially available as Matrigel, constitutes a good model for studying mechanisms controlling functional differentiation of the bovine mammary gland. In contact with Matrigel BME-UV1 cells induce apicobasal polarity, and within 16 days form three dimensional (3D) acinar structures with a centrally localized hollow lumen, which structurally resemble mammary alveoli present in the functionally active mammary gland. We have shown that the 3D culture system enables a high expression and proper localisation of integrin receptors and tight junction proteins in BME-UV1 cells to be induced. This effect was not obtained in cells grown in the classical 2D culture system on plastic. Moreover, ECM highly stimulated the synthesis of one of the major milk proteins, beta-casein, even in the absence of prolactin. Our results show that contact with ECM plays an important role in the lactogenic activity of bovine MECs, however, prolactin is necessary for the efficient secretion of milk proteins.  相似文献   
49.
As immunosuppression in pigeons is common and results in reduced post-vaccination immunity and lower health status of the birds, studies have been taken up aimed at evaluation of the effect of three doses of methisoprinol on the percentage of CD4+ and CD8+ T lymphocyte subpopulation in peripheral blood and in the spleen and the titre of anti-NDV antibodies in the serum of pigeons in four groups (A, B, C, D), with 20 birds each. Pigeons in each group were immunised against paramyxovirosis at week 6 and 9 of life. Water for injection (group A - control) or methisoprinol at 100 mg/kg of body weight (group B), 200 mg/kg of body weight (group C) and 600 mg/kg of body weight (group D) was administered intramuscularly for 3 days before each vaccination. The immunological analyses were carried out by flow cytometry and the ELISA test. The findings indicate that methisoprinol administered intramuscularly at 100 and 200 mg/kg of body weight for 3 successive days before vaccination against paramyxovirosis mainly stimulates the mechanisms of non-specific humoral and cellular immunity, which is indicated by a higher percentage of the subpopulation of CD4+ T lymphocytes in peripheral blood and in the spleen and a higher titre of anti-NDV antibodies.  相似文献   
50.
The aim of this study was to determine the effect of fish-based feed materials, as a source of readily available protein contained in fish broth and essential polyunsaturated fatty acids (PUFAs) found in fish oil, on the health of piglets and rearing results. The experiment was conducted on a commercial pig fattening farm. The study involved a total of 80 weaners with an approximate body weight of 15 kg. The experiment was carried out over a period of 40 days. Feed samples were subjected to laboratory analyses. Blood samples were collected from experimental group animals to determine serum biochemical and immunological parameters. The body weight gains of weaners, mortality rates and average feed intake per animal were calculated for the entire experimental period. The addition of fish broth and fish oil significantly improved the n3:n6 fatty acid ratio in diets. The presence of EPA and DHA in the experimental diet could have had a positive health effect on piglets, comparable with that exerted by therapeutic doses of zinc often administered to pigs of this age group. During the experiment, feed conversion ratio (FCR) gain was considerably reduced in the experimental group, with similar daily gains in the control and experimental group.  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