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   1175篇
  免费   83篇
  国内免费   2篇
林业   88篇
农学   18篇
基础科学   5篇
  184篇
综合类   221篇
农作物   28篇
水产渔业   128篇
畜牧兽医   448篇
园艺   49篇
植物保护   91篇
  2024年   5篇
  2023年   14篇
  2022年   11篇
  2021年   32篇
  2020年   36篇
  2019年   48篇
  2018年   39篇
  2017年   27篇
  2016年   37篇
  2015年   35篇
  2014年   27篇
  2013年   61篇
  2012年   73篇
  2011年   102篇
  2010年   52篇
  2009年   44篇
  2008年   60篇
  2007年   79篇
  2006年   77篇
  2005年   71篇
  2004年   59篇
  2003年   49篇
  2002年   54篇
  2001年   12篇
  2000年   12篇
  1999年   6篇
  1997年   4篇
  1995年   6篇
  1994年   3篇
  1993年   8篇
  1992年   4篇
  1991年   5篇
  1990年   9篇
  1989年   8篇
  1987年   4篇
  1986年   5篇
  1985年   7篇
  1984年   8篇
  1983年   6篇
  1982年   4篇
  1981年   5篇
  1980年   4篇
  1978年   6篇
  1977年   6篇
  1976年   7篇
  1975年   4篇
  1973年   3篇
  1969年   2篇
  1963年   2篇
  1942年   2篇
排序方式: 共有1260条查询结果,搜索用时 15 毫秒
41.
There is an increasing risk of Lyme disease in Canada due to range expansion of the tick vector, Ixodes scapularis. The objectives of this article are to i) raise public awareness with the help of veterinarians on the emerging and expanding risk of Lyme disease across Canada, ii) review the key clinical features of Lyme disease in dogs, and iii) provide recommendations for veterinarians on the management of Lyme disease in dogs.  相似文献   
42.
43.
Foot and mouth disease (FMD) is a highly transmissible viral infection of cloven hooved animals associated with severe economic losses when introduced into FMD-free countries. Information on the impact of the disease in FMDV-endemic countries is poorly characterised yet essential for the prioritisation of scarce resources for disease control programmes. A FMD (virus serotype SAT2) outbreak on a large-scale dairy farm in Nakuru County, Kenya provided an opportunity to evaluate the impact of FMD on clinical mastitis and culling rate. A cohort approach followed animals over a 12-month period after the commencement of the outbreak. For culling, all animals were included; for mastitis, those over 18 months of age. FMD was recorded in 400/644 cattle over a 29-day period. During the follow-up period 76 animals were culled or died whilst in the over 18 month old cohort 63 developed clinical mastitis. Hazard ratios (HR) were generated using Cox regression accounting for non-proportional hazards by inclusion of time-varying effects. Univariable analysis showed FMD cases were culled sooner but there was no effect on clinical mastitis. After adjusting for possible confounders and inclusion of time-varying effects there was weak evidence to support an effect of FMD on culling (HR = 1.7, 95% confidence intervals [CI] 0.88-3.1, P = 0.12). For mastitis, there was stronger evidence of an increased rate in the first month after the onset of the outbreak (HR = 2.9, 95%CI 0.97-8.9, P = 0.057).

Electronic supplementary material

The online version of this article (doi:10.1186/s13567-015-0173-4) contains supplementary material, which is available to authorized users.  相似文献   
44.
The antioxidant activity of three major polyamine conjugates, N,N'-dicoumaroyl-putrescine (DCP), N-p-coumaroyl-N'-feruloylputrescine (CFP), and N,N'-diferuloyl-putrescine (DFP) isolated from corn bran, and their related hydroxycinnamic acids, p-coumaric acid and ferulic acid, were evaluated by three antioxidant in vitro assay systems, including 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical and superoxide and hydroxyl radicals generated by enzymatic and nonenzymatic reactions. Additionally, five phenolic compounds were evaluated for melanogenesis inhibitory activity using mushroom tyrosinase and B16 melanoma cells. Most of the phenolic compounds significantly scavenged DPPH, superoxide, and hydroxyl radicals in a dose-dependent manner. Particularly, DFP showed potent DPPH (IC50 = 38.46 microM) and superoxide (IC50 = 291.62 microM) radical scavenging activities, while DCP exhibited the strongest hydroxyl radical scavenging activity (IC50 = 120.55 microM). CFP also exerted moderate DPPH, superoxide, and hydroxyl radical scavenging activities. Meanwhile, DCP (IC50 = 181.73 microM) showed potent tyrosinase inhibitory activity toward l-tyrosine as the substrate, whereas DFP (IC50 = 733.64 microM) significantly inhibited melanin synthesis in B16 melanoma cells. These current results indicate that these three polyamine conjugates from corn bran may be useful potential sources of natural antioxidants and skin-whitening agents.  相似文献   
45.
46.
A simulation tool for site-specific vulnerability assessments of pesticide leaching to groundwater was developed, based on the pesticide fate and transport model MACRO, parameterized using pedotransfer functions and reasonable worst-case parameter values. The effects of uncertainty in the pedotransfer functions on simulation results were examined for 48 combinations of soils, pesticides and application timings, by sampling pedotransfer function regression errors and propagating them through the simulation model in a Monte Carlo analysis. An uncertainty factor, f(u), was derived, defined as the ratio between the concentration simulated with no errors, c(sim), and the 80th percentile concentration for the scenario. The pedotransfer function errors caused a large variation in simulation results, with f(u) ranging from 1.14 to 1440, with a median of 2.8. A non-linear relationship was found between f(u) and c(sim), which can be used to account for parameter uncertainty by correcting the simulated concentration, c(sim), to an estimated 80th percentile value. For fine-textured soils, the predictions were most sensitive to errors in the pedotransfer functions for two parameters regulating macropore flow (the saturated matrix hydraulic conductivity, K(b), and the effective diffusion pathlength, d) and two water retention function parameters (van Genuchten's N and alpha parameters). For coarse-textured soils, the model was also sensitive to errors in the exponent in the degradation water response function and the dispersivity, in addition to K(b), but showed little sensitivity to d. To reduce uncertainty in model predictions, improved pedotransfer functions for K(b), d, N and alpha would therefore be most useful.  相似文献   
47.
Testing MACRO (version 5.1) for pesticide leaching in a Dutch clay soil   总被引:2,自引:0,他引:2  
Testing of pesticide leaching models against comprehensive field-scale measurements is necessary to increase confidence in their predictive ability when used as regulatory tools. Version 5.1 of the MACRO model was tested against measurements of water flow and the behaviour of bromide, bentazone [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one-2,2-dioxide] and imidacloprid [1-(6-chloro-3-pyridylmethyl)-N-nitroimidazolidin-2-ylideneamine] in a cracked clay soil. In keeping with EU (FOCUS) procedures, the model was first calibrated against the measured moisture profiles and bromide concentrations in soil and in drain water. Uncalibrated pesticide simulations based on laboratory measurements of sorption and degradation were then compared with field data on the leaching of bentazone and imidacloprid. Calibrated parameter values indicated that a high degree of physical non-equilibrium (i.e. strong macropore flow) was necessary to describe solute transport in this soil. Comparison of measured and simulated bentazone concentration profiles revealed that the bulk of the bentazone movement in this soil was underestimated by MACRO. Nevertheless, the model simulated the dynamics of the bentazone breakthrough in drain water rather well and, in particular, accurately simulated the timing and the concentration level of the early bentazone breakthrough in drain water. The imidacloprid concentration profiles and its persistence in soil were simulated well. Moreover, the timing of the early imidacloprid breakthrough in the drain water was simulated well, although the simulated concentrations were about 2-3 times larger than measured. Deep groundwater concentrations for all substances were underestimated by MACRO, although it simulated concentrations in the shallow groundwater reasonably well. It is concluded that, in the context of ecotoxicological risk assessments for surface water, MACRO can give reasonably good simulations of pesticide concentrations in water draining from cracking clay soils, but that prior calibration against hydrologic and tracer data is desirable to reduce uncertainty and improve accuracy.  相似文献   
48.
Dry mixtures of lactose and caseinate were heated at 60 degrees C for up to 96 h at different relative humidities (RHs) ranging from 29 to 95%. The resulting nonenzymatic browning was studied by determining lactulosyl lysine formation in the caseinate (as measured by the conversion to furosine), amount of reacted lactose, loss of lysine, color formation, and fluorescent intensity. For each measurement, the maximum reaction occurred at intermediate RHs. While there is general agreement between the results obtained by different methods, discrepancies are understandable given the complex nature of nonenzymatic browning. It was shown that the degradation of the Amadori product, lactulosyl lysine, increased with RH. Moreover, the Maillard reaction, as opposed to caramelization of lactose, was the major pathway at all RHs. Visible browning occurred when the destruction of Amadori product became dominant, and interactions between sugar fragments and caseinate were not the rate-limiting steps in the nonenzymatic browning.  相似文献   
49.
50.
● Virtual joint centers on N agronomy were established between UK and China. ● Key themes were improving NUE for fertilizers, utilizing livestock manures, and soil health. ● Improved management practices and technologies were identified and assessed. ● Fertilizer emissions and improved manure management are key targets for mitigation. Two virtual joint centers for nitrogen agronomy were established between the UK and China to facilitate collaborative research aimed at improving nitrogen use efficiency (NUE) in agricultural production systems and reducing losses of reactive N to the environment. Major focus areas were improving fertilizer NUE, use of livestock manures, soil health, and policy development and knowledge exchange. Improvements to fertilizer NUE included attention to application rate in the context of yield potential and economic considerations and the potential of improved practices including enhanced efficiency fertilizers, plastic film mulching and cropping design. Improved utilization of livestock manures requires knowledge of the available nutrient content, appropriate manure processing technologies and integrated nutrient management practices. Soil carbon, acidification and biodiversity were considered as important aspects of soil health. Both centers identified a range of potential actions that could be taken to improve N management, and the research conducted has highlighted the importance of developing a systems-level approach to assessing improvement in the overall efficiency of N management and avoiding unintended secondary effects from individual interventions. Within this context, the management of fertilizer emissions and livestock manure at the farm and regional scales appear to be particularly important targets for mitigation.  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