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   446篇
  免费   23篇
林业   30篇
农学   10篇
基础科学   2篇
  109篇
综合类   28篇
农作物   17篇
水产渔业   34篇
畜牧兽医   184篇
园艺   17篇
植物保护   38篇
  2023年   2篇
  2022年   8篇
  2021年   17篇
  2020年   18篇
  2019年   22篇
  2018年   20篇
  2017年   20篇
  2016年   14篇
  2015年   13篇
  2014年   13篇
  2013年   23篇
  2012年   34篇
  2011年   37篇
  2010年   26篇
  2009年   14篇
  2008年   27篇
  2007年   35篇
  2006年   22篇
  2005年   13篇
  2004年   22篇
  2003年   20篇
  2002年   7篇
  2001年   4篇
  2000年   5篇
  1999年   5篇
  1998年   1篇
  1997年   3篇
  1996年   1篇
  1995年   2篇
  1994年   2篇
  1993年   1篇
  1992年   5篇
  1990年   1篇
  1989年   4篇
  1986年   1篇
  1982年   2篇
  1980年   1篇
  1973年   1篇
  1970年   1篇
  1964年   2篇
排序方式: 共有469条查询结果,搜索用时 15 毫秒
91.
Abstract

The present review summarizes the available data on the concentration of trace elements (TEs) in soils and their availability to plants with a view to reflect the quality and safety of food and fodder crops. Most soils in western Balkan countries are not contaminated. However, soils around industrial centers and historical mining sites do create concern for food and feed safety. Also high concentrations of TEs are related to their geochemical original. For example, ultrabasic rocks and serpentinites in western Serbia and western Bosnia are very rich in chromium, nickel, magnesium, iron (Fe), and cobalt, and cover an area of 5200 km2. High TE concentrations caused by anthropogenic activities are also reported. In vineyard soils in Croatia, concentrations of cadmium, copper (Cu), and zinc (Zn) were much higher than their background concentrations. On the other hand, TE deficiency in plants is also prevalent in the regions. For example, Zn and Fe deficiencies in eastern parts of Croatia and northern parts of Serbia, Cu deficiency in pasture and sheep's blood at Nisici Plateau of Bosnia and Herzegovina, and selenium (Se) deficiency almost in the whole region have been observed. Therefore, information on TEs' behavior and soil factors affecting their mobility and availability is highly needed in order to separate the areas of contamination and then propose agrotechnical measures to protect the entry of TEs into the food chain. Research is also required to assess the influence of agronomic management on TE supply to plants and for achieving a better utilization of essential TEs. Concentrations of Se in wheat in Serbia are so low that if people were fed exclusively with wheat, their daily requirements for Se would not be met. There is also a need for full implementation of new food safety regulations in the Balkan countries in accordance with the legislations of the European Union.  相似文献   
92.
93.

Context

Socio-ecological landscapes typically characterized by non-linear dynamics in space and time are difficult to be analyzed using standard quantitative methods, due to multiple processes interacting on different spatial and temporal scales. This poses a challenge to the identification of appropriate approaches for analyzing time series that can evaluate system properties of landscape dynamics in the face of disturbances, such as uncontrolled fires.

Objective

The purpose is the application of non-linear methods such as recurrence quantification analysis (RQA) to landscape ecology. The examples concern the time series of burnt and unburnt Mediterranean rangelands, to highlight potential and limits of RQA.

Methods

We used RQA together with joint recurrence analysis (JRA) to compare the evolutionary behavior of different land uses.

Results

Time series of forests and grasslands in rangelands present both periodic and chaotic components with a rather similar behavior after the fire and clear transitions from less to more regular/predictable dynamics/succession. Results highlight the impacts of fire, the recovery capacity of land covers to pre-burnt levels, and the decay of synchronization towards the previous regime associated with vegetation secondary succession consistent with early successional species.

Conclusions

RQA and JRA with their set of indices (recurrence rate: RR, laminarity: LAM, determinism: DET, and divergence: DIV) can represent new sensitive measures that may monitor the adaptive capacity and the resilience of landscapes. However, future applications are needed to standardize the analysis by strengthening the accuracy of this approach in describing the ongoing transformations of natural and man-managed landscapes.
  相似文献   
94.
This experiment evaluated the effects of subnutrition during early gestation on hematology in cows (Bos Taurus) and on hematological, metabolic, endocrine, and vitality parameters in their calves. Parda de Montaña and Pirenaica dams were inseminated and assigned to either a control (CONTROL, 100% requirements) or a nutrient‐restricted group (SUBNUT, 65%) during the first third of gestation. Dam blood samples were collected on days 20 and 253 of gestation, and calf samples were obtained during the first days of life. Pirenaica dams presented higher red series parameters than Parda de Montaña dams, both in the first and the last months of gestation. During early pregnancy, granulocyte numbers and mean corpuscular hemoglobin were lower in Pirenaica‐SUBNUT than in Pirenaica‐CONTROL cows. Calves from the SUBNUT cows did not show a physiological reduction in red series values in early life, suggesting later maturation of the hematopoietic system. Poor maternal nutrition affected calf endocrine parameters. Newborns from dystocic parturitions showed lower NEFA concentrations and weaker vitality responses. In conclusion, maternal nutrition had short‐term effects on cow hematology, Pirenaica cows showing a higher susceptibility to undernutrition; and a long‐term effect on their offspring endocrinology, SUBNUT newborns showing lower levels of IGF‐1 and higher levels of cortisol.  相似文献   
95.
A 7‐year‐old male Boxer with a 3.5‐year history of atopy and food hypersensitivity was presented with multiple poorly circumscribed nodules and maculae of the skin and tongue, and jaundiced mucosal membranes. Cytologic and histopathologic examination of the skin lesions revealed cutaneous epitheliotropic lymphoma. Cells were CD3+ and CD8+ in flow cytometry. The CBC showed a moderate leukocytosis with 16% atypical lymphocytes with irregularly cleaved nuclei. Flow cytometric phenotyping of peripheral blood showed an elevated proportion of the CD8+ T‐lymphocyte subpopulation, indicating a malignant population of T‐cell origin, and the electropherogram of the PCR antigen receptor rearrangement produced a monoclonal peak for TCRγ. Liver enzyme activities were markedly increased and abdominal ultrasound examination showed increased echogenicity of the liver and enlarged abdominal lymph nodes. Fine‐needle aspirates of the liver confirmed infiltration with lymphocytes exhibiting the same morphology as the cells detected in skin and peripheral blood. Treatment was induced with L‐asparaginase, lomustine, and prednisone. Partial clinical remission of the skin and tongue lesions was achieved within 10 days, and hematologic abnormalities resolved. Despite further treatment with L‐asparaginase and lomustine, the dog relapsed within one month and was euthanized. Presence of malignant lymphocytes in skin, peripheral blood, and liver indicate a rare variant of leukemic cutaneous T‐cell lymphoma, equivalent of Sézary syndrome in a dog. This case report describes the use of flow cytometry as a complementary tool for lymphocyte characterization of skin lesions for the first time.  相似文献   
96.
This report describes the clinical and histologic recovery of a 2‐year‐old mixed‐breed dog presented with hypovolemic shock, markedly increased serum alanine amino transferase activity, and hemoabdomen. Emergency exploratory surgery revealed a friable liver with multiple capsule hemorrhages necessitating removal of the left lateral lobe. Histologic evaluation showed acute massive hepatic necrosis with centrilobular and midzonal distribution. The dog survived, and all monitored laboratory values normalized within 7 weeks. A liver biopsy taken 8 weeks after presentation revealed normal hepatic architecture with a few, randomly distributed neutrophilic foci. Follow‐up included intermittent determination of liver variables including liver function tests for a period of 7 years. The dog's health status, and all test results remained normal during this time. Complete recovery and good long‐term quality of life after life‐threatening acute liver failure secondary to massive hepatic necrosis is possible in dogs.  相似文献   
97.
98.
Nowadays, the use of foetal calf serum (FCS) during in vitro embryo culture is very controversial. Whilst some authors have encouraged its use, others reject it because of its harmful effects. Although in vitro embryo production in red deer is a promising assisted reproductive technique, it is still in its infancy and a great effort is needed to update the protocols used. The aim of this study was to assess whether FCS supplementation in red deer embryo culture medium is necessary to produce blastocyst and, if so, when is the best time to add it in terms of blastocyst production and quality. In vitro blastocysts were cultured with FCS added at 24, 48 or 96 hours post‐insemination (hpi). In addition, a treatment without FCS was used as control. Six hundred and ninety‐four cumulus–oocyte complexes were collected for in vitro fertilization. Cleavage rate was examined at 48 hpi, and blastocyst yield was recorded on days 6, 7 and 8. FCS had no influence on cleavage and blastocyst rate for any of the treatments studied. However, the number of cells was higher (p = .025) in those blastocysts cultured with FCS from 48 hpi compared with FCS‐free culture media (93.88 ± 7.76 vs. 54.11 ± 8.36). In conclusion, the addition of FCS to the embryo culture medium at 48 hpi improves the quality of red deer blastocyst, although it does not affect the percentage of embryos obtained.  相似文献   
99.
The aim of the work was to infect pyrethrum (Tanacetum cinerariifolium (Trevir.) Schultz-Bip.), an important secondary metabolite pyrethrin producing plant, with MA phytoplasma (16S rRNA group III, subgroup B) to initiate tissue culture of phytoplasma-infected plants, and to examine possible effects of infection on pyrethrin content, morphology, and regenerative capacity in shoot culture.  相似文献   
100.
Noroviruses (NoVs) are a major cause of epidemic gastroenteritis in children and adults. Several pieces of evidence suggest that viruses genetically and antigenically closely related to human NoVs might infect animals, raising public health concerns about potential cross‐species transmission. The natural susceptibility of non‐human primates (NPHs) to human NoV infections has already been reported, but a limited amount of data is currently available. In order to start filling this gap, we screened a total of 86 serum samples of seven different species of NPHs housed at the Zoological Garden (Bioparco) of Rome (Italy), collected between 2001 and 2017, using an enzyme‐linked immunosorbent assay (ELISA) based on virus‐like particles (VLPs) of human GII.4 and GIV.1 NoVs. Antibodies specific for both genotypes were detected with an overall prevalence of 32.6%. In detail, IgG antibodies against GII.4 NoVs were found in 18 Japanese macaques (29.0%, 18/62), a mandrill (10.0%, 1/10), a white‐crowned mangabey (16.6%, 1/6) and in an orangutan (33.3%, 1/3). Twelve macaques (19.3%, 12/62), five mandrills (50.0%, 5/10), two chimpanzees (100%, 2/2) and a white‐crowned mangabey (16.6%, 1/6) showed antibodies for GIV.1 NoVs. The findings of this study confirm the natural susceptibility of captive NHPs to GII NoV infections. In addition, IgG antibodies against GIV.1 were detected, suggesting that NHPs are exposed to GIV NoVs or to antigenically related NoV strains.  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