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   942篇
  免费   56篇
林业   85篇
农学   68篇
基础科学   1篇
  148篇
综合类   164篇
农作物   32篇
水产渔业   83篇
畜牧兽医   357篇
园艺   27篇
植物保护   33篇
  2023年   11篇
  2022年   9篇
  2021年   11篇
  2020年   31篇
  2019年   19篇
  2018年   30篇
  2017年   30篇
  2016年   21篇
  2015年   17篇
  2014年   30篇
  2013年   28篇
  2012年   52篇
  2011年   47篇
  2010年   26篇
  2009年   18篇
  2008年   43篇
  2007年   48篇
  2006年   45篇
  2005年   34篇
  2004年   27篇
  2003年   40篇
  2002年   28篇
  2001年   28篇
  2000年   26篇
  1999年   19篇
  1998年   12篇
  1996年   8篇
  1995年   7篇
  1994年   7篇
  1993年   7篇
  1992年   14篇
  1991年   8篇
  1990年   10篇
  1989年   8篇
  1988年   6篇
  1987年   16篇
  1986年   6篇
  1985年   7篇
  1984年   6篇
  1982年   7篇
  1979年   5篇
  1974年   12篇
  1973年   8篇
  1972年   12篇
  1971年   8篇
  1970年   8篇
  1969年   8篇
  1968年   7篇
  1966年   6篇
  1949年   4篇
排序方式: 共有998条查询结果,搜索用时 15 毫秒
131.
A 2‐year‐old, male castrated German shepherd dog was presented to the University of Tennessee Veterinary Medical Center (UTVMC) with periorbital swelling and conjunctival mucopurulent discharge 2 days following removal of a twig from the medial canthus by the owner. Diagnostic imaging was pursued due to the suspicion of a retrobulbar foreign body (FB). A cylindrical FB approximately 3.0 cm in length and 1.0 cm in diameter with concentric rings, suspected to be wooden material, was identified on computed tomography (CT) imaging. An attempt to remove the FB via a stab incision using ultrasound guidance was unsuccessful, and postmanipulation ultrasound confirmed the FB position was unchanged. An exploratory orbitotomy was performed, using the acquired CT images for guidance in locating the FB; however, the FB was not present at the predicted site. The CT imaging was repeated and showed that the FB had migrated rostrally approximately 3.0 cm, compared to the originally acquired study and its same location during attempted ultrasound‐guided removal. A combination of CT‐guided needle placement and contrast injection was then used with repeat imaging in an attempt to better localize the FB and its soft tissue tract. The dog was taken back into the operating room, and the wooden FB was successfully removed.  相似文献   
132.
A 7‐week‐old male pig was presented with signs of a central nervous system disorder. An MRI of the head and cervical spine was performed immediately after euthanasia. The MRI revealed multifocal bilaterally symmetric T2‐weighted hyperintense lesions in the brain and spinal cord, likely due to a toxic metabolic process. Histopathological examination supported the MRI findings and confirmed the diagnosis of edema disease due to Shiga‐like toxin produced by Escherichia coli. This is the first case published of the MRI findings in an edema disease affected pig.  相似文献   
133.
134.
The wood pastures or hardwood rangelands of the southwestern Iberian Peninsula are complex social-ecological systems created from the long-term interaction of society and the landscape. Dehesa, oak woodlands managed for grazing, cropping, and other forms of production, is the most common rangeland system and one of the most distinctive landscapes. Traditionally characterized by multifunctional low-intensity management that enhances a wide range of ecosystem services, current management has shifted from the traditional toward more intensified models. This paper assesses the coproduction of ecosystem services on dehesa properties by exploring the relationship between biophysical and sociocultural factors and dehesa management practices. Based on 42 surveys we analyze 16 quantitative indicators using multivariate techniques. The results indicate that there are four main dehesa types as defined by their characteristics and management: large heterogeneous operations with diverse products; small and homogeneous operations focused on a reduced number of products; medium-large properties focused on crop production; and midsized properties with easy public access. Management is the result of the dynamics of interacting biophysical and sociocultural factors that influence manager priorities and investments. Management decisions group around the degree of multifunctionality of the operation, the relative importance of crop production, the degree of grazing pressure in the system, and how restrictive public access policy is. We find that in the study area, interactions between all the previously mentioned elements covary consistently, generating bundles of ecosystem services associated with each of three management models based on the intensity of management.  相似文献   
135.
Iridophores are iridescent cutaneous pigment cells found in reptiles, fish, and amphibians. Neoplasms of iridophores are rarely reported, and little is known about their behaviour, metastatic potential, and prognostic indicators. This paper reports details of the clinical course and pathological findings of metastatic iridophoroma in a veiled chameleon (Chamaeleo calyptratus) and red-barred dragon (Ctenophorus vadnappa). The veiled chameleon presented with a subcutaneous mass on the right lateral elbow and was diagnosed as an iridophoroma on fine needle aspiration. It was otherwise clinically normal. Within 35 days of excision, multiple secondary skin masses developed which were often non-pigmented and required microscopic examination to identify them as iridophoroma metastases. There were in total 54 days between the first detection of the primary mass and end stage, extensive cutaneous and visceral metastatic disease, and there had been no detectable primary mass during routine clinical examination 128 days prior to death. The red-barred dragon presented with a white sub-mandibular mass. While undergoing an excisional biopsy the animal expired, post-mortem examination revealed a cutaneous malignant iridophoroma with tumour cell clusters seen in pulmonary arterial vessels, suggesting that surgical handling of iridophoroma could precipitate the release of circulating tumour cell clusters. This is the first iridophoroma described in a red-barred dragon and the second report of a malignant iridophoroma in a veiled chameleon. Timely detection and careful excision of lizard iridophoroma may be an important factor in clinical outcome.  相似文献   
136.
Objective— To evaluate trans-splenic portal scintigraphy (TSPS) and per-rectal portal scintigraphy (PRPS) for diagnosis of congenital portosystemic shunts (CPSS) in dogs, and compare these results with surgical findings.
Study Design— Prospective, randomized cross over clinical trial.
Animals— Dogs (n=42) with suspected CPSS.
Methods— Dogs had TSPS and PRPS 48 hours apart; quantity of radionuclide administered was recorded. Three independent, blinded reviewers evaluated each scintigraphic study for study quality, shunt presence, number, and location of shunt termination (caudal vena cava, azygos vein). All dogs had exploratory celiotomy. Negative scintigraphic findings were confirmed with intraoperative mesenteric portography. Ameroid constrictors were placed on all extrahepatic CPSS, and hepatic biopsies were obtained.
Results— TSPS was 100% sensitive and specific for diagnosis of CPSS and significantly ( P <.05) more likely than PRPS to detect shunt number and termination. Interpretation was consistent between observers, and TSPS required significantly less radionuclide than PRPS.
Conclusion— TSPS was as sensitive as PRPS for detection of shunting vessels, and consistently yielded studies of higher quality, allowing detection of shunt number and location with consistent interpretation among radiologists.
Clinical Relevance— TSPS provides information about shunt number and location, which allows improved surgical planning. Because it requires significantly less radionuclide, TSPS improves safety, allows for more comprehensive patient care, and earlier surgical intervention.  相似文献   
137.
Molecular wheat breeding by direct gene transfer   总被引:7,自引:0,他引:7  
H. Lörz  D. Becker  S. Lütticke 《Euphytica》1998,100(1-3):219-223
A method for efficient genetic transformation of wheat has been developed using immature embryos as targets for microprojectile-mediated gene transfer and a helium driven particle delivery system. Screening and selection of transgenic cells, somatic embryos and regenerated plants are performed with the gus-gene and the phosphinothricin acetyl transferase (bar) gene coding for Basta-resistance as the selectable marker. On average, one fertile transgenic plant can be obtained from about 100 microprojectile treated, immature embryos. The number of integrated copies of the transferred gene ranges from 1 up to about 10. Stable integrated genes are inherited in most of the transgenic lines in a normal mendelian fashion segregating 3:1 in the F2. Homozygous, as well as heterozygous, lines have been followed and analysed genetically at the molecular level and up to F5. Apart from normal stable gene expression, examples have also been found which showed a loss of gene activity or unexpected segregation pattern. For applied aspects, different genes are transferred aiming for improved disease resistance, modification of quality, or other characteristics. First results from these transgenic lines are reported, and problems still existing with the production of stable transgenic wheat lines are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   
138.
For the first time concentrations of trace nitrogenous (N) air pollutants, gaseous nitric acid (HNO3), nitrous acid (HNO2), ammonia (NH3), and fine particulate nitrate (NO3) and ammonium (NH4), were measured in the montane forests of southern Poland. Determinations were performed in two forest locations of the Silesian Beskid Mountains in the western range of the Carpathian Mountains, and in an industrial/urban location in Karowice, Poland. The measurements performed in summer 1997 with honeycomb denuder/filter pack systems showed elevated concentrations of the studied pollutants. These findings agree with the low carbon/nitrogen (C/N) ratios and the results of 15N analyses of soil and moss samples. High concentrations of N air pollutants help to explain previously determined high levels of NO3 and NH4 deposition to Norway spruce (Picea abies Karst.) canopies in these mountains. Ambient concentrations of sulfur dioxide (SO2) and ozone (O3) were elevated and potentially phytotoxic. Deficiencies of phosphorus (P) and magnesium (Mg) in Norway spruce foliage were found while concentrations of other nutrients were normal.  相似文献   
139.
140.
Introduction   Cowpea (Vigna unguiculata) is a valuable protein source that can contribute towards overcoming a predicted protein deficit equivalent to more than 1 000 000 t oilcake in Southern Africa by the year 2000 (N ell et al. 1992). However, like most other legumes, cowpeas contain antinutritional factors (ANFs) (M akinde et al. 1996). It has also been shown that feeding raw cowpea causes reduced growth and histomorphometric changes in the various segments of the intestine (M akinde et al. 1997) attributable to ANFs. The major ANFs in the cowpea are trypsin inhibitors, tannins and phytates (A letor and A ladetimi 1989). Although the protease inhibitors are found in most legumes, only soybean inhibitors have been thoroughly investigated (L iener and K akade 1980), and trypsin inhibitors in other legumes have received little attention.  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