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   415篇
  免费   33篇
  国内免费   1篇
林业   39篇
农学   14篇
基础科学   1篇
  107篇
综合类   51篇
农作物   12篇
水产渔业   36篇
畜牧兽医   135篇
园艺   8篇
植物保护   46篇
  2023年   4篇
  2022年   5篇
  2021年   8篇
  2020年   7篇
  2019年   8篇
  2018年   15篇
  2017年   14篇
  2016年   7篇
  2015年   18篇
  2014年   10篇
  2013年   21篇
  2012年   20篇
  2011年   30篇
  2010年   15篇
  2009年   12篇
  2008年   23篇
  2007年   23篇
  2006年   28篇
  2005年   21篇
  2004年   20篇
  2003年   21篇
  2002年   13篇
  2001年   13篇
  2000年   7篇
  1999年   9篇
  1998年   7篇
  1997年   4篇
  1994年   4篇
  1993年   3篇
  1992年   6篇
  1991年   3篇
  1990年   3篇
  1989年   3篇
  1988年   3篇
  1986年   3篇
  1985年   4篇
  1982年   6篇
  1981年   2篇
  1978年   1篇
  1972年   3篇
  1971年   2篇
  1965年   4篇
  1963年   3篇
  1960年   1篇
  1958年   1篇
  1930年   1篇
  1928年   1篇
  1926年   1篇
  1914年   1篇
  1906年   1篇
排序方式: 共有449条查询结果,搜索用时 31 毫秒
411.
Lime-trees (Tilia begonifolia Stev.) have countless ecological and economical values in the northern forests of Iran. Bole wounds on residual trees are commonly caused by logging. The healing ability of bole wounds and the effect of bole wounds on diameter growth in lime-trees were assessed in a natural mixed uneven-age stand. The susceptibility to decay in relation to the wound severity was also evaluated. Condition and location of lime-trees before and after selective logging were determined through systematic plot sampling. The position of each lime-tree was identified on a topographical map using the global positioning system. After 10 years from wound occurrence, tree and wound characteristics were also re-measured. Overall, 68 stems (9.7 stem ha?1) of lime-trees were found in the sampled area, 25 stems (36.8%) were damaged in the form of bole wound, and 43 stems (63.2%) were undamaged. Statistical differences between wounded or non-wounded lime-trees in terms of diameter and height were not found. Most of the wounds occurred at the height of < 1 m; the average wound size was 370.5 cm2. Wound severity was related to wound size and wound position. Wound width healing rate was 6.2 mm year?1. The wound shape was found to be a healing factor as the healing rate by width was higher than that by length. Healed wounds were only 12, 60% of bole wounds resulted in decay. Diameter growth of wounded trees was 43.5% lower than undamaged trees. The average annual diameter growth decreased by increasing wound severity. The results indicated the lime-tree is a very sensitive species to damage caused by logging (bole wound). Single-tree selection cutting needs more careful logging operations to reduce damage on residual lime-trees in order to safeguard this species, valuable in ecologic and economic terms.  相似文献   
412.
Fats, oils, and grease (FOG) and source separated organics (SSO) were treated with the microwave-enhanced advanced oxidation process (MW-AOP) at 90 and 110 °C, with varying amounts of hydrogen peroxide dosages. The treatment efficiency, in terms of soluble substrates and volatile fatty acids (VFA), increased with an increase in both temperature hydrogen peroxide dosages. Fatty acids and compounds with carbonyl group and/or hydroxyl group in both initial and treated FOG samples were identified by gas chromatography-mass spectrometry. MW-AOP treatment temperatures and hydrogen peroxide dosages dictated the formation of degradation products. The degradation followed peroxidation mechanism to produce lower molecular weight substrates such as short chain fatty acids which would be less inhibitory to microbes. After the MW-AOP treatment, both SSO and FOG comprised of more soluble and low molecular weight compounds. These compounds included VFA and nutrients that would be readily available for bacterial or plant uptake.  相似文献   
413.
This paper deals with the problem of mineral N leaching from arable lands due to the fertilization method. The influence of different doses of compost (50 and 100?Mg ha?1) and N-mineral fertilizer (35-70-140 kg N ha?1) on leaching of Nmin in a lysimetric experiment with winter wheat. The area of our interest represents the main source of drinking water for the city of Brno and its neighborhoods. To demonstrate the effect of compost and mineral nitrogen addition, the lysimetric experiment was established there. Seven variants of the experiment with different fertilization schemes were studied during two vegetation seasons (2013 and 2014), each with three repetitions. The experiment was carried out in plastic experimental containers of 0.3 m diameter and 0.5 m height. Therefore, each lysimeter was the same size and was filled with 25 kg of subsoil and 25 kg of topsoil. The highest leaching of Nmin was detected in the variant C2 where 140 kg N ha?1 was applied, in both vegetation periods (5.97 kg Nmin ha?1 after the first vegetation period and 17.02 kg Nmin ha?1 after the second vegetation period). The positive effect of compost application (individually or in combination with the mineral N) on decrease in mineral N leaching was found during both vegetation period in comparison with variant C2. The highest doses of compost (100?Mg per ha) significantly decreased the concentration of mineral nitrogen in the soil eluate in both periods (3.03 kg Nmin ha?1 and 5.79 kg Nmin ha?1, respectively), by 197% and 293% in comparison with variant C2. There is evidence that the application of compost has a positive effect on the reduction of Nmin leaching.  相似文献   
414.
In order to promote the use of lupin in pig nutrition, in this research the nutritional characteristics (i.e. dietary fibre, alkaloid and fatty acid profile) and the in vitro gas production of 12 lupin varieties grown in the Mediterranean basin and belonging to three lupin species (Lupinus albus, Lupinus angustifolius and Lupinus luteus) were assessed. Four varieties of Lalbus (Asfer, Lublanc, Lutteur and Multitalia) were grown in South Campania. Three varieties of Lluteus (Dukat, Mister and Taper), three of L. angustifolius (Jindalee, Sonet and Wonga) and two of Lalbus (Rosetta and Luxor) were grown in Eastern Sicily. Lupinus albus varieties showed interesting nutritional and dietetic characteristics (i.e. high protein and low fibre content); the lipid fraction, rather elevated, is well represented by monounsaturated fatty acids (544 g/kg), whereas saturated fatty acids (SFAs) are less represented (167 g/kg) and the n‐3/n‐6 ratio (0.510) is the most favourable. Lupinus luteus varieties presented the most remarkable dietetic aspects, in terms of polyunsaturated fatty acid (PUFA) content (569 g/kg), n‐6 PUFA series (490 g/kg), UFA/SFA (5.24) and PUFA/SFA (3.56) ratios and atherogenic (0.059) and thrombogenic (0.100) indices and very low alkaloid content (1.07 mg per 100 g). Lupinus angustifolius varieties showed the least interesting nutritional and dietetic characteristics: low protein and fat content, high fibre level, high SFA amount (248 g/kg) and the lowest favourable nutritional indices (IA: 0.164 and IT: 0.334). Regarding the fermentation process, in L. albus, the tendency to increase the rate of gas production during the early stages of fermentation suggests that the high presence of alkaloids did not affect the in vitro degradability, production of short‐chain fatty acids and fermentation process, probably due to their concentration and/or water solubility. Lupinus angustifolius and L. luteus showed intermediate and slightly worse in vitro fermentation patterns respectively. From a nutritional and dietetic point of view, lupin may represent an interesting alternative to soya bean in pig feeding.  相似文献   
415.
Noncardiogenic pulmonary edema is an important cause of respiratory disease in dogs and cats but few reports describe its radiographic appearance. The purpose of this retrospective case series study was to describe radiographic findings in a large cohort of dogs and cats with presumed noncardiogenic pulmonary edema and to test associations among radiographic findings versus cause of edema. Medical records were retrieved for dogs and cats with presumed noncardiogenic edema based on history, radiographic findings, and outcome. Radiographs were reviewed to assess lung pattern and distribution of the edema. Correlation with the cause of noncardiogenic pulmonary edema was evaluated with a Fisher's exact test. A total of 49 dogs and 11 cats were included. Causes for the noncardiogenic edema were airway obstruction (n = 23), direct pulmonary injury (n = 13), severe neurologic stimulation (n = 12), systemic disease (n = 6), near‐drowning (n = 3), anaphylaxis (n = 2) and blood transfusion (n = 1). Mixed, symmetric, peripheral, multifocal, bilateral, and dorsal lung patterns were observed in 44 (73.3%), 46 (76.7%), 55 (91.7%), 46 (76.7%), 46 (76.7%), and 34 (57.6%) of 60 animals, respectively. When the distribution was unilateral, pulmonary infiltration involved mainly the right lung lobes (12 of 14, 85.7%). Increased pulmonary opacity was more often asymmetric, unilateral, and dorsal for postobstructive pulmonary edema compared to other types of noncardiogenic pulmonary edema, but no other significant correlations could be identified. In conclusion, noncardiogenic pulmonary edema may present with a quite variable radiographic appearance in dogs and cats.  相似文献   
416.
In October 2014 samples were collected from olive trees growing in South‐Eastern Sicily (Italy) that were showing wilting symptoms. In the field, clear signs of flagging appeared on young twigs and lateral shoots, and many leaves had fallen to the ground. Initial laboratory examinations of attacked twigs revealed the presence of a large number of adults of Rhyzopertha dominica (F., 1792). Sampling was repeated during the following 2 years, and while an outbreak of the pest did not occur, continuous damage was observed. In 2016, exclusively using pheromone‐baited traps in olive trees, a large number of adults were captured. The main local climatic parameters were monitored from 2014 to 2016 and compared with the population levels. The incidence of damage and survival of R. dominica in an environment different from its usual one (stored grain) deserves attention, especially considering that climate change could greatly affect pest demographics and put olive trees in this area and further afield at risk of increased damage.  相似文献   
417.
The long pentraxin 3 (PTX3) plays an important role in host defence and its over-expression may contribute to airway injury. The aim of the present study was therefore to characterize in more detail PTX3 and its expression in the horses’ airway. Six healthy horses and six horses affected by recurrent airway obstruction (R.A.O.) were submitted to a dusty environment challenge. PTX3 DNA and cDNA were cloned and sequenced. PTX3 expression was evaluated by RT-qPCR, Western blotting and immuno-histochemistry in bronchoalveolar lavage fluid (BALF) cells, BALF supernatant and bronchial epithelial cells. An alternative splicing of the second exon of PTX3 occurred, resulting in two forms of the protein: “spliced” (32 kDa) and “full length” (42 kDa). PTX3 was detected in BALF macrophages, neutrophils and bronchial epithelial cells. It was over-expressed in the BALF supernatant from R.A.O.-affected horses in crisis. However, dust was unable to induce PTX3 in BALF cells ex vivo, indicating that dust is an indirect inducer of PTX3. Dust exposure in-vivo induced PTX3 in BALF macrophages but there was no significant difference between healthy and R.A.O.-affected horses. Conversely, PTX3 was over-expressed in the bronchial epithelial cells from R.A.O.-affected horses in crisis. These data indicate a differential regulatory mechanism in inflammatory and bronchial epithelial cells and offer therapeutically interesting perspectives.  相似文献   
418.
Cryptosporidium parvum, Tyzzer, 1912 is identified as a common cause of diarrhoea in immunocompetent individuals. In immunocompromised, especially HIV-infected subjects, cryptosporidiosis causes severe chronic diarrhoea. In this study, nitazoxanide (NTZ) was compared for curative activity with sinefungin (SNF) and paromomycin (PRM) in immunosuppressed rats, a screening model for anticryptosporidial agents. NTZ at either 50 mg/kg/day, 100 mg/kg/day or 200 mg/kg/day resulted in seven days in a dose-dependent inhibition of oocyst shedding similar to that obtained with SNF (10 mg/kg/day) and PRM (100 mg/kg/day). Further discontinuation of SNF or PRM 100 mg/kg/day therapy resulted in early relapse of oocyst shedding which reached the pre-treatment levels in 2-4 days. In contrast, seven days after discontinuation of therapy, shedding inhibition was unchanged in NTZ-treated rats. Data prompt further assessment of the activity of NTZ on sequestered C. parvum.  相似文献   
419.
The discovery and investigation of a novel family of herbicides containing a diaryl acetal are described. The stability of the acetal limited herbicidal efficacy and recognizing that fact led to the design of analogs with commercial levels of post-emergence activity on broadleaf weeds. These compounds inhibited acetolactate synthase and in vitro activity data were used to guide target design. However, no members of this family provided a commercially valuable combination of herbicidal efficacy and crop selectivity.  相似文献   
420.
OBJECTIVE: To compare structural properties of a plate-rod combination-bone construct (PRCbc) and interlocking nail-bone construct (ILNbc) by use of an experimentally induced gap fracture in canine tibiae. SAMPLE POPULATION: 12 paired canine tibiae. PROCEDURE: Specimens were implanted with a plate-rod combination consisting of a 3.5-mm, limited-contact, dynamic-compression plate combined with an intramedullary rod or 6-mm interlocking nail. Ostectomy (removal of 10-mm segment) was performed. Paired constructs were loaded for bending, compression, or torsion measurements (4 constructs/group). Compliance was determined by fitting regression lines to the load-position curves at low (initial compliance) and high (terminal compliance) loads. RESULTS: Bending compliances did not differ significantly between constructs. For the ILNbc, initial compliance was greater than terminal compliance in compression and torsion. Initial compliance and terminal compliance for the PRCbc were similar in compression and torsion. Initial compliance in compression and torsion was greater for the ILNbc, compared with initial compliance for the PRCbc. Maximum deformations in bending and compression were similar between constructs; however, maximum torsional angle was significantly greater for the ILNbc, compared with values for the PRCbc. CONCLUSIONS AND CLINICAL RELEVANCE: The study documented that for an experimentally induced gap fracture in canine tibiae, a plate-rod combination is a significantly less compliant fixation method in torsion and compression, compared with an interlocking nail. Considering the deleterious effects of torsional deformation on bone healing, a plate-rod combination may represent a biomechanically superior fixation method, compared with an interlocking nail, for the treatment of dogs with comminuted tibial diaphyseal fractures.  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