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   655篇
  免费   17篇
林业   67篇
农学   17篇
基础科学   4篇
  213篇
综合类   122篇
农作物   28篇
水产渔业   22篇
畜牧兽医   145篇
园艺   16篇
植物保护   38篇
  2023年   5篇
  2022年   8篇
  2021年   15篇
  2020年   9篇
  2019年   13篇
  2018年   26篇
  2017年   16篇
  2016年   13篇
  2015年   20篇
  2014年   18篇
  2013年   36篇
  2012年   49篇
  2011年   40篇
  2010年   29篇
  2009年   21篇
  2008年   45篇
  2007年   25篇
  2006年   33篇
  2005年   32篇
  2004年   25篇
  2003年   27篇
  2002年   31篇
  2001年   13篇
  2000年   6篇
  1999年   7篇
  1998年   5篇
  1997年   3篇
  1992年   4篇
  1991年   6篇
  1990年   3篇
  1989年   5篇
  1988年   4篇
  1987年   5篇
  1986年   6篇
  1985年   2篇
  1984年   6篇
  1979年   2篇
  1978年   4篇
  1975年   2篇
  1974年   3篇
  1973年   2篇
  1972年   3篇
  1971年   3篇
  1970年   2篇
  1969年   4篇
  1960年   2篇
  1958年   2篇
  1940年   2篇
  1931年   2篇
  1922年   2篇
排序方式: 共有672条查询结果,搜索用时 15 毫秒
71.
One approach to ongrow juvenile European lobster, Homarus gammarus, is to utilize land based rearing systems, incorporating automated feeding, individual culture and provision of stable pelleted feeds, preferably using sustainable ingredients. We initiated three feeding experiments to investigate the general suitability of ingredients produced from seafood by‐products as novel feeds for H. gammarus, in terms of promoting survival, development and growth of post‐larval lobsters from post‐larvae (PL) stage IV to the first juvenile stage (stage V). The first experiment was designed to screen an array of candidate, locally produced, novel protein sources on growth performance parameters. This initial experiment revealed that PL reared on a raw (i.e. wet, unprocessed shrimp) feed used as a reference showed superior performance to those reared on experimental feeds containing fishmeal, herring protein isolate or mussel meal; however, a novel type of shrimp meal, produced by flocculation from waste water, promoted the best PL performance of any experimental feed. A second experiment was designed to test the effect of drying method and to optimize the form of a wet shrimp reference feed used by lobster hatcheries. This showed that the performance of PL reared on experimental freeze‐dried shrimp feed was not significantly different to those reared on the wet, unprocessed shrimp used as a reference feed. However, lobsters offered experimental oven‐dried shrimp feed (with or without an immune supplement) resulted in significantly lower survival or growth performance. A third and final experiment was designed in an attempt to improve a candidate herring‐based protein source, by supplementing with nutrients found in shrimp. However, the results showed that PL reared on the wet reference shrimp feed still showed superior growth and survival than those reared on a herring feed alone, or supplemented with additives found in shrimp meal (either glucosamine, astaxanthin or both supplements combined). The high survival and growth, low incidence of moulting problems and high availability of waste shrimp material, suggest that non‐heat‐treated shrimp products are a promising feed ingredient for post‐larval European lobsters.  相似文献   
72.
The effect of forest structure and health on the relative surface temperature captured by airborne thermal imagery was investigated in Norway Spruce-dominated stands in Southern Finland. Airborne thermal imagery, airborne scanning light detection and ranging (LiDAR) data and 92 field-measured sample plots were acquired at the area of interest. The surface temperature correlated most negatively with the logarithm of stem volume, Lorey’s height and the logarithm of basal area at a resolution of 254?m2 (9?m radius). LiDAR-derived metrics: the standard deviations of the canopy heights, canopy height (upper percentiles and maximum height) and canopy cover percentage were most strongly negatively correlated with the surface temperature. Although forest structure has an effect on the detected surface temperature, higher temperatures were detected in severely defoliated canopies and the difference was statistically significant. We also found that the surface temperature differences between the segmented canopy and the entire plot were greater in the defoliated plots, indicating that thermal images may also provide some additional information for classifying forests health status. Based on our results, the effects of forest structure on the surface temperature captured by airborne thermal imagery should be taken into account when developing forest health mapping applications using thermal imagery.  相似文献   
73.
The dependency of the retention of dissolved organic carbon (DOC) on mineral phase properties in soils remains uncertain especially at neutral pH. To specifically elucidate the role of mineral surfaces and pedogenic oxides for DOC retention at pH 7, we sorbed DOC to bulk soil (illitic surface soils of a toposequence) and corresponding clay fraction (< 2 μm) samples after the removal of organic matter and after removal of organic matter and pedogenic oxides. The DOC retention was related to the content of dithionite‐extractable iron, specific surface area (SSA, BET‐N2 method) and cation exchange capacity (pH 7). The reversibility of DOC sorption was determined by a desorption experiment. All samples sorbed 20–40 % of the DOC added. The DOC sorption of the clay fractions explained the total sorption of the bulk soils. None of the mineral phase properties investigated was able to solely explain the DOC retention. A sorption of 9 to 24 μg DOC m–2 indicated that DOC interacted only with a fraction of the mineral surface, since loadings above 500 μg m–2 would be expected for a carbon monolayer. Under the experimental conditions used, the surface of the silicate clay minerals seemed to be more important for the DOC sorption than the surface of the iron oxides. The desorption experiment removed 11 to 31 % of the DOC sorbed. Most of the DOC was strongly sorbed.  相似文献   
74.
75.
The ground reaction force patterns from 20 clinically sound Dutch Warmblood horses (Group A) were recorded at the normal walk. The data from four to 10 stance phases of each limb were computer averaged after normalisation to the animal's body mass and to the stance time. This analysis method allowed comparison of data from left and right fore- and hindlimbs within and between horses. The left-to-right symmetry in the reaction force peaks of contralateral limbs of one horse exceeded 90 per cent. The time in the stance phase at which the peaks occur were even more symmetrically distributed. A characteristic force-force diagram was constructed by plotting the longitudinal horizontal and the vertical ground reaction forces against each other; in this way the symmetry of loading of contralateral fore- and hindlimbs could be interpreted easily. Force plate tracings were obtained from eight horses (Group B) in three successive years. The similarity of the tracings from a sound, well-trained horse over that period was better than the differences between horses of the same breed.  相似文献   
76.
The amplitudes, impulses and times of occurrence of a number of selected peaks in the ground reaction force tracings of 17 horses with various clinical histories were compared with those of 20 horses used to establish values for the 'standard' Dutch Warmblood horse. The resulting factors were combined, using different weighting factors, into indices characterising each limb. The symmetry between the loading of the forelimbs and the hindlimbs was used to calculate amplitude and peak-time symmetry indices. Limb and symmetry indices were combined in one H(orse) INDEX. This method of quantifying the ground reaction force pattern, together with appropriate graphic display of the tracings, was useful in clinical evaluation of force plate measurements.  相似文献   
77.
78.
This study determined the uptake of three model compounds, applied in the presence and absence of surfactants, into the leaves of three plant species (Chenopodium album L, Hedera helix L and Stephanotis floribunda Brongn). The results with 2-deoxy-D-glucose, 2,4-dichlorophenoxyacetic acid and epoxiconazole in the presence ofsurfactants (the polyethylene glycol monododecyl ethers C12EO3, C12EO6, C12EO10 and a trisiloxane ethoxylate with mean EO of 7.5, all used at one equimolar concentration and therefore different percentage concentrations) illustrate that the initial dose (nmol mm(-2)) of xenobiotic applied to plant foliage is a strong positive determinant of uptake. This held true for all the xenobiotics studied over a wide concentration range in the presence of these surfactants. Uptake on a unit area basis (nmol mm(-2)) could be related to the initial dose of xenobiotic applied per unit area (ID) by an equation of the form: Uptake = a [ID]b at time t = 24h. ID is given by the mass of xenobiotic applied, M divided by the droplet spread area, A. Total mass uptake is then calculated from an equation of the form: Total Uptake = a [ID]b x A.  相似文献   
79.
Current tools used to assess the safety of food and feed derived from modern biotechnology emphasize the investigation of possible unintended effects caused directly by the expression of transgenes or indirectly by pleiotropy. These tools include extensive multisite and multiyear agronomic evaluations, compositional analyses, animal nutrition, and classical toxicology evaluations. Because analytical technologies are rapidly developing, proteome analysis based on two-dimensional gel electrophoresis (2DE) was investigated as a complementary tool to the existing technologies. A 2DE method was established for the qualitative and quantitative analysis of the seed proteome of Arabidopsis thaliana with the following validation parameters examined: (1) source and scope of variation; (2) repeatability; (3) sensitivity; and (4) linearity of the method. The 2DE method resolves proteins with isoelectric points between 4 and 9 and molecular masses (MM) of 6-120 kDa and is sensitive enough to detect protein levels in the low nanogram range. The separation of the proteins was demonstrated to be very reliable with relative position variations of 1.7 and 1.1% for the pI and MM directions, respectively. The mean coefficient of variation of 254 matched spot qualities was found to be 24.8% for the gel-to-gel and 26% for the overall variability. A linear relationship (R2 > 0.9) between protein amount and spot volume was demonstrated over a 100-fold range for the majority of selected proteins. Therefore, this method could be used to interrogate proteome alterations such as a novel protein, fusion protein, or any other change that affects molecular mass, isoelectric point, and/or quantity of a protein.  相似文献   
80.
This article describes educational approaches for training veterinary students, veterinary graduates, and practicing veterinarians in the area of aquatic animal health and lists a range of general research, training, internship/residency, and continuing-education resources.  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