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   278篇
  免费   22篇
林业   18篇
农学   9篇
基础科学   8篇
  69篇
综合类   10篇
农作物   29篇
水产渔业   28篇
畜牧兽医   88篇
园艺   11篇
植物保护   30篇
  2024年   1篇
  2023年   2篇
  2022年   7篇
  2021年   7篇
  2020年   14篇
  2019年   15篇
  2018年   18篇
  2017年   12篇
  2016年   8篇
  2015年   9篇
  2014年   10篇
  2013年   22篇
  2012年   17篇
  2011年   20篇
  2010年   12篇
  2009年   10篇
  2008年   18篇
  2007年   11篇
  2006年   16篇
  2005年   9篇
  2004年   7篇
  2003年   10篇
  2002年   11篇
  2001年   3篇
  2000年   1篇
  1999年   3篇
  1998年   2篇
  1997年   2篇
  1996年   1篇
  1995年   1篇
  1993年   1篇
  1991年   1篇
  1990年   1篇
  1989年   3篇
  1988年   1篇
  1985年   1篇
  1983年   1篇
  1981年   1篇
  1980年   1篇
  1977年   3篇
  1975年   1篇
  1974年   1篇
  1973年   2篇
  1972年   1篇
  1968年   1篇
  1935年   1篇
排序方式: 共有300条查询结果,搜索用时 562 毫秒
191.
The presence of multidrug resistant (MDR) Salmonella serotypes in slaughtered swine, carcasses, meat and meat handlers is scarcely evaluated. Recently we demonstrated that diverse Salmonella serotypes are frequently present in swine, pork meat and carcasses, and meat handlers at Portuguese abattoirs. Here we have characterized their antibiotic resistance phenotypes and genotypes, helping elucidate the flow of MDR Salmonella in the food chain. Testing 60 Salmonella isolates from different serotypes, the highest frequencies of resistance were observed for tetracycline (T) [70% (n = 42/60), tet(A)/tet(B)/tet(G)], streptomycin (S) [63% (n = 38/60), aadA2/strA/strB], sulfamethoxazole (Sul) [62% (n = 37/60), sul1/sul2/sul3] and ampicillin (A) [57% (n = 34/60), blaPSE-1/blaTEM]. Thirty-seven percent (n = 22/60) carried class 1 integrons and multidrug resistance was frequently observed (63% n = 38/60), including those serotypes common to human infections [S. Typhimurium 78% n = 25/32; S. 4,[5],12:i:- 67% n = 2/3; S. Rissen 75% (n = 3/4); S. London 67% n = 2/3; S. Derby 55%; n = 6/11)]. The emergent S. 4,[5],12:i:- isolates were mostly characterized by ASSuT phenotype [blaTEM/strA-strB/sul2/tet(B)], typical of the European clone, while for the first time the ST phenotype [strA-strB-tet(A)-tet(B)] was also observed. Moreover, we report a first finding of a MDR phenotype in S. London [ANSSuT; blaTEM-strA-strB-sul2-tet(A)]. Our findings suggest that the abattoir environment and the slaughter operations seem not only to harbor MDR serotypes that originated in the pig reservoir, but also propagate them through cross-contamination processes, involving meat handlers. The present study suggests a probable relationship between swine and human salmonellosis throughout the food chain, which is of interest for epidemiological, animal health and public health purposes.  相似文献   
192.
  • 1. Over the past two decades the number of fisheries targeting shark resources has increased dramatically. A combination of factors, including relatively slow growth rate, low fecundity and late age of maturity, result in low recovery rates from exploitation for most shark species. This, in turn, is reflected in the poor record of sustainability of shark fisheries.
  • 2. One of the greatest challenges is to find a way to deal with the substantial levels of shark bycatch taken in many non‐target fisheries. Poor general recording of shark landings and paucity of shark landing data at the species level also undermine the development of effective shark management strategies. 3. This paper reviews the problems that must be faced worldwide if shark resources are to be managed sustainably and lays out a comprehensive set of prioritized management strategies to facilitate the sustainable management of global shark fisheries. It is acknowledged that the majority of sharks are harvested in developing countries and that the management of shark resources in developing and developed countries will need to incorporate different management strategies relevant to local socio‐economic agendas. The management recommendations deal with methods to improve the global regulation of fisheries, ways to improve global conservation ethics and encourage active participation in management, as well as means by which specific management strategies may be implemented.
Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   
193.
Sesbania virgata(Cav.) Pers.is a pioneer species native to South America able to release allelochemicals that affect germination and development of other plant species.The aim of this work was to evaluate the allelopathic effect of S.virgata on the germination and development of co-occurring species from gallery forest and on agronomic species.Two forest native species,Enterolobium contortisiliquum(Vell.) Morong and Sapindus saponaria L.,and two agronomic species(these as control),Oryza sativa L.and Solanum lycopersicum L.were used in the lab and greenhouse assays with seeds and leaf extracts of S.virgata.Agronomic species were more affected than native species when co-germinated with seeds of S.virgata.The germination percentage and speed germination index of the agronomic species were reduced in both in vitro and greenhouse assays.In the same assays,the seeds of native species showed no significant differences in the parameters mentioned.However,the initial growth of the four species assayed was affected,with reduction in plant length and shoot diameter followed by significant reduction in plantlet shoot and root weights.In the assays with irrigation of S.virgata leaf extracts,no significant inhibitory effects on germination were observed for all species.Height and shoot diameter of the native species were not affected by the leaf extracts,nor were fresh and dry weights.However,these parameters increased in the agronomic species as they were irrigated with leaf extracts.Based on our data we conclude that seed leachates of S.virgata affect germination and seedling development of other species,suggesting that its invasive behavior is due mainly to seed allelochemicals.Although with less pronounced effects on native species,allelopathy of S.virgata might also influence Interspecific competition in its natural environment.  相似文献   
194.
Summary Based on the analysis of invasions of alien plants in Ukraine, the impact of non-native plant species upon the native flora and adverse consequences of their spread are assessed. A case study gives examples of the role of alien plants in fragmentation of populations of native species; contamination of genetic resources of rare and endangered native species, formation of new ecotypes and hybridization with native taxa, disruption of the structure of natural plant communities as a result of introduction of alien species and formation of specific plant communities with domination of aliens. Arguments are provided against uncontrolled casual introductions and subsequent escape from cultivation as a result of ill-judged deliberate introduction of plants for ornamental, agricultural, technical, forestry, and other uses without any preliminary assessment of their invasion potential in the region concerned. Invasions of alien plants promote dramatic changes in the taxonomic, geographical, and ecological patterns of local floras, disruptions in the phytosociological spectrum, spectra of biomorphs, deterioration of zonal peculiarities of the flora, and finally lead to the decline of the vegetation productivity. A list of highly invasive plant species threatening forest, steppe, and submediterranean zones of East Europe is provided.  相似文献   
195.
With the aim of determining the impact on soil caused by deforestation and replacement by pasture in the rainforests of the Venezuelan Andes, three representative plots were selected: one under natural forest (F), another deforested (D), and the third one under pasture (P). The chemical analyses of the soils were complemented with a micromorphological study. The analysis of thin sections revealed major differences in the microstructure and porosity, as well as in the characteristics of the fine fraction and organic constituents, together with other pedological features, corroborating the different dynamics and types of soil in the natural or altered forest landscape. The change in soil use leads to the reorganization of the solid fraction, evidenced in the intensity of the processes of transference and accumulation of materials in the different soil horizons. The results confirm that the change in land use is determinant in the current fertility and evolution of the soils. The main consequences of the land-use change are: a change in the soil organic-matter cycle with a greater degree of maturation in the disturbed soils (C/N in Ah horizon: 28.5 (F); 19.8 (D); 18.4 (P)), in which moder humus from undisturbed forest soils transforms into an acidic mull in the anthropogenic disturbed ones; illuviation of organic matter and minerals appears to have occurred after deforestation (colour (dry–wet) in Ah–Bt1–Bt2 horizons: 3/2–2/2, 4/4–3/4, 4.5/4–3.5/4 (F); 5/4–4.5/4, 5/6–4/6, 5/6–4.5/6 (D); 4.5/4–4/3.5, 5/6–4/4, 5/6–4/4 (P)); the amount of kaolinite and the degree of browning increased gradually with deforestation; other changes observed (bioturbation, microstructure, porosity and distribution of organic constituents) may also be related to the different activity of the soil fauna in disturbed (oribatids) and undisturbed soils (worms). Thus the porosity changes in the following form: the percentage of packing voids in the surface samples decreases from a 80% in the forest soil to a 70% and 65% in deforested and pasture soils, respectively. Moreover in depth a dominance of channels in forest-soil porosity changes into an abundance of planar voids in the other two types of soils studied.  相似文献   
196.
Rice oxalate oxidase genes (OXO) may play a role in resistance to Magnaporthe oryzae. Genome analyses showed four tandemly duplicated OXO genes, OsOXO1OsOXO4, which mapped to a blast resistance QTL in chromosome 3. These genes have >90% nucleotide and amino acid identity, but they have unique gene structures, conserved motifs, and phylogeny compared to the 70 other members of the cupin superfamily in the Nipponbare genome, which were divided into several classes. In resistant and susceptible Vandana/Moroberekan advanced backcross lines, only OsOXO4 was expressed during rice–M. oryzae interactions, and its expression increased earlier in resistant than susceptible lines. The earlier expression of OsOXO4 in resistant lines correlated with a 26-bp promoter insertion containing an additional copy of the bacterial responsive nodulation cis-element. Our results showed that OsOXO14 are in a separate class of rice cupin genes and supports a role for the promoter variant of OsOXO4 in resistance to M. oryzae.  相似文献   
197.
BACKGROUND: Chronic renal failure (CRF) is a common disease in dogs, and many metabolic disorders can be observed, including metabolic acidosis and calcium and phosphorus disturbances. Acidosis may change the ionized calcium (i-Ca) fraction, usually increasing its concentration. OBJECTIVE: In this study we evaluated the influence of acidosis on the serum concentration of i-Ca in dogs with CRF and metabolic acidosis. METHODS: Dogs were studied in 2 groups: group I (control group = 40 clinically normal dogs) and group II (25 dogs with CRF and metabolic acidosis). Serum i-Ca was measured by an ion-selective electrode method; other biochemical analytes were measured using routine methods. RESULTS: The i-Ca concentration was significantly lower in dogs in group II than in group I; 56% of the dogs in group II were hypocalcemic. Hypocalcemia was observed in only 8% of dogs in group II when based on total calcium (t-Ca) concentration. No correlation between pH and i-Ca concentration was observed. A slight but significant correlation was detected between i-Ca and serum phosphorus concentration (r = -.284; P = .022), as well as between serum t-Ca and i-Ca concentration (r = .497; P < .0001). CONCLUSION: The i-Ca concentration in dogs with CRF and metabolic acidosis varied widely from that of t-Ca, showing the importance of determining the biologically active form of calcium. Metabolic acidosis did not influence the increase in i-Ca concentration, so other factors besides acidosis in CRF might alter the i-Ca fraction, such as hyperphosphatemia and other compounds that may form complexes with calcium.  相似文献   
198.
While the modernization of irrigation techniques in recent years has improved water use efficiency, the substitution of open channels by pressurized pipes led to a substantial increase in the energy used in irrigated agriculture. Given the concern for energy savings and sustainability, official administrations have promoted several initiatives to improve energy efficiency in irrigated agriculture, and researchers have developed different tools for the same. Although energy audits have been conducted in irrigation networks managed by Water Users’ Associations, the implementation of energy conservation measures proposed in these audits has not been always successful. This paper, which reviews the energy aspects of irrigation systems modernized in recent decades in Spain and in other countries, considers the characteristics and special features of Water Users’ Associations and proposes an energy management system (EMS) for the same. The stages of the EMS are described, beginning with the definition of energy policy and the establishment, implementation and verification of energy plans, the essential conditions to guarantee the success of the EMS are described, in which energy audits are just a part of a process of continual improvement of energy efficiency. A Strategy for Efficient Energy Management has been implemented in a Water Users’ Association in southeast Spain. The improvement measures proposed in the first energy plan were implemented, and an increase from 60 to 65 % in average energy efficiency was achieved.  相似文献   
199.
Anthocyanins and carotenoids are natural pigments responsible for the color of vegetables and fruits, and they are also bioactive compounds, both demonstrating important biological, therapeutic, and preventative properties. Considering the biodiversity of edible fruits, high performance liquid chromatography coupled to photodiode array and mass spectrometry detectors (HPLC-PDA-MS) was used to establish the composition of carotenoids and anthocyanins from dovyalis and tamarillo fruits. Ten anthocyanins and 26 carotenoids were found in dovyalis, whereas tamarillo showed 3 anthocyanins and 17 carotenoids. Higher contents of anthocyanins and carotenoids were found in dovyalis, 42.0 and 6.6 mg/100 g, respectively, as compared to tamarillo fruits with 8.5 and 4.4 mg/100 g, respectively. Although these fruits belong to different families, delphinidin 3-rutinoside and beta-cryptoxanthin were found to be, respectively, the major anthocyanin and carotenoid in both fruits.  相似文献   
200.
The use of complementary and alternative medicine (CAM) by US consumers has grown in recent years. CAM therapies often utilize medicinal herbs as part of the treatment process; however, research on US practitioner preferences for medicinal herbs is limited, despite growing concern surrounding the sustainability of wild-harvested medicinal herbs. In order better to understand consumer preferences for this emerging market, a mail survey of US practitioners (licensed acupuncturists) was conducted to examine the importance of five herb attributes in practitioners?? herb selection decisions: (1) country of origin, (2) freshness, (3) production method (organic versus conventional), (4) price, and (5) traceability. The significance of these five traits is investigated using discrete choice analysis, and the implications for US medicinal herb growers are discussed.  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