全文获取类型
收费全文 | 1319篇 |
免费 | 54篇 |
国内免费 | 106篇 |
专业分类
林业 | 39篇 |
农学 | 77篇 |
基础科学 | 135篇 |
346篇 | |
综合类 | 573篇 |
农作物 | 37篇 |
水产渔业 | 3篇 |
畜牧兽医 | 38篇 |
园艺 | 220篇 |
植物保护 | 11篇 |
出版年
2024年 | 4篇 |
2023年 | 17篇 |
2022年 | 23篇 |
2021年 | 30篇 |
2020年 | 30篇 |
2019年 | 29篇 |
2018年 | 31篇 |
2017年 | 39篇 |
2016年 | 66篇 |
2015年 | 68篇 |
2014年 | 82篇 |
2013年 | 82篇 |
2012年 | 122篇 |
2011年 | 129篇 |
2010年 | 82篇 |
2009年 | 95篇 |
2008年 | 79篇 |
2007年 | 110篇 |
2006年 | 81篇 |
2005年 | 77篇 |
2004年 | 31篇 |
2003年 | 30篇 |
2002年 | 19篇 |
2001年 | 14篇 |
2000年 | 19篇 |
1999年 | 11篇 |
1998年 | 14篇 |
1997年 | 18篇 |
1996年 | 11篇 |
1995年 | 5篇 |
1994年 | 5篇 |
1993年 | 10篇 |
1992年 | 6篇 |
1991年 | 2篇 |
1990年 | 2篇 |
1989年 | 2篇 |
1988年 | 1篇 |
1984年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
排序方式: 共有1479条查询结果,搜索用时 235 毫秒
31.
32.
33.
Comparison of ammonia and greenhouse gas emissions during the fattening of pigs, kept either on fully slatted floor or on deep litter 总被引:7,自引:0,他引:7
Five successive batches of fattening pigs were raised, each during a four month period, on a totally concrete slatted floor in one experimental room and on straw based deep litter in another. The rooms were automatically ventilated to maintain a constant ambient temperature. Available floor space was of 0.75 m2 per pig kept on the slatted floor and 1.20 m2 per pig kept on the deep litter. With this last system, about 46 kg of straw were supplied per pig throughout a fattening period. The slurry pit was emptied and the litter removed after each batch. Once a month, the emissions of ammonia (NH3), nitrous oxide (N2O), methane (CH4), carbon dioxide (CO2), and water vapour (H2O) were measured continuously for 6 consecutive days by infra-red photoacoustic detection.The performance of the animals was not significantly different according to the floor type.Gaseous emissions from pigs raised on the slatted floor and on the deep litter were, respectively, 6.2 and 13.1 g per pig per day for NH3, 0.54 and 1.11 g per pig per day for N2O, 16.3 and 16.0 g per pig per day for CH4, 1.74 and 1.97 kg per pig per day for CO2 and 2.48 and 3.70 kg per pig per day for H2O. Except for the CH4 emissions, all the differences were significant (P < 0.001). Thus, pig fattening on deep litter releases nearly 20% more greenhouse gases than on slatted floor, with 2.64 and 2.24 kg of CO2-equivalents, respectively (P < 0.001). Whatever the floor type, emissions increased from the beginning to the end of the fattening periods by about 5 times for NH3, 4 times for N2O, 3 times for CH4 and 2 times for CO2 and H2O. Correlation coefficients between CO2-emissions and H2O, NH3 and CH4 emissions were, on average for both floor types, 0.82, 0.77 and 0.74, respectively.Although rearing pigs on straw generally has a good brand image for the consumer, this rearing system produces more pollutant gases than keeping pigs on slatted floors. 相似文献
34.
蜜蜂是设施农业的重要授粉昆虫,授粉蜂群的状况可能影响授粉效率的高低。通过意大利蜂(Apismellifera)对温室草莓、油桃的6次授粉重复试验,采集授粉蜂群的蜂子比值与出勤率的数据,分析蜂子比值与出勤率的相关性。结果表明,蜂子比值与工蜂出勤率之间呈显著的负相关(P<0.05),其相关系数分别为-0.8470、-0.6543、-0.7616、-0.6969、-0.6740和-0.8477,说明在正常范围内,温室授粉蜂群的出勤率随蜂子比值的减小而提高,也就是说,授粉蜂群内的子脾越好,蜜蜂出勤采集的积极性越高。 相似文献
35.
Scaevola aemula is a popular ornamental crop cultivated as a bedding plant or for hanging baskets. We characterized gas exchange properties of S. aemula ‘New Wonder’ in response to photosynthetically active radiation (PAR), carbon dioxide concentration, and leaf temperature. Net CO2 assimilation rate (A) was responsive to CO2, exhibiting a saturation when intercellular CO2 concentration (Ci) was greater than 600 μmol mol−1. Net CO2 assimilation rate and dark respiration rate (Rd) were 23.1 and 2.3 μmol m−2 s−1, respectively, at 25 °C and PAR = 1500 μmol m−2 s−1. Net CO2 assimilation rates were similar at leaf temperatures between 20 and 30 °C but significantly reduced at 15 °C. These gas exchange results were used to test the extendibility of a coupled gas exchange model previously developed for cut-roses. Utilizing the gas exchange data measured at 25 °C leaf temperature, several model parameters were independently determined for S. aemula. Model predictions were then compared with observations at different leaf temperatures. The model predicted the rates of net CO2 assimilation and transpiration of S. aemula reasonably well. Without additional calibration, the model was capable of predicting the temperature dependence of net CO2 assimilation and transpiration rates. Applying the model to predict the effects of supplemental lighting and CO2 enrichment on canopy photosynthesis and transpiration rates, we show that this model could be a useful tool for examining environmental control options for S. aemula production in the greenhouse. 相似文献
36.
以Navier-Stokes方程为模型基础,运用ANSYS/FLOTRAN软件对华北型机械通风式连栋塑料温室在不同山墙门打开度情况下室内的气流场进行三维稳态模拟.同时通过试验测得温室内各测点的气流流速,并分析试验数据得到温室内的气流流速的平均值.结果表明:在试验条件下山墙门打开度为2.3×2 m2,造成室内局部风速过大,流场分布紊乱;山墙门打开度为2.3×3m2,使室内风速平均值太小,达不到温室通风的要求;山墙门打开度为2.3×2.4m2时在同等条件下温室内的气流场分布均匀,平均风速最大,通风效果较理想. 相似文献
37.
Andreas Kofoet Mathias Fink 《European journal of plant pathology / European Foundation for Plant Pathology》2007,117(4):369-381
Epidemics of Peronospora parasitica are strongly affected by temperature and air moisture, and the interaction of these factors. Because a significant percentage
of radish plants are grown in greenhouses, it may be possible to influence epidemics by altering the greenhouse climate. The
objective of this study was to test the hypothesis that epidemics of P. parasitica can be modelled by the effects of air temperature and moisture in the greenhouse. Such a model could then be used to analyse
greenhouse climate control strategies with regard to managing downy mildew. Five radish crops were grown under greenhouse
conditions with set-points for heating and ventilation intended to obtain favourable conditions for disease development during
the first part of the growing cycle. Subsequent to this first phase, unfavourable conditions were set until harvest. Disease
incidence was measured once a week until the radishes reached marketable size. In addition, experiments were carried out in
growth chambers in which inoculated plants were subjected to air temperatures between 8 and 27°C, and disease incidence and
sporulation intensity were measured. Data from these two experiments were then used to estimate model parameters. In this
model, the interactions of air temperature (T) and water vapour saturation deficit (SD) were adequately described by a multiplicative
relationship. The simulated epidemics by the fitted model were highly correlated with the observed epidemics (r = 0.91, R
2 = 0.83, n = 29). Parameter estimates indicated that T of ca. 20°C and SD < 0.03 hPa resulted in the highest rates of disease development
and that the rate was zero when SD > 2.0 hPa. Both experimental data and simulations showed that epidemics of P. parasitica can be effectively controlled by managing the greenhouse climate. 相似文献
38.
39.
Energy and resource consumption of land-based Atlantic salmon smolt hatcheries in the Pacific Northwest (USA) 总被引:1,自引:0,他引:1
John Colt Steve Summerfelt Tim Pfeiffer Sveinung Fivelstad Michael Rust 《Aquaculture (Amsterdam, Netherlands)》2008,280(1-4):94-108
This paper evaluates the resource and energy requirements of six different types of land-based, hatchery production systems located in the U.S. Pacific Northwest: flow-through with a gravity water supply, flow-through with a pumped water supply, flow-through with pure oxygen, partial reuse system, partial reuse with heating, and a reuse system for the production of Atlantic salmon (Salmo salar) smolts. Key parameters used in the evaluation include direct energy, indirect energy, transportation energy, greenhouse gas emissions, and pollutant discharges.Power (electricity and natural gas) and feed energy accounted for the majority of the required energy for all the rearing option evaluated. The sum of the fixed capital and chemicals components accounted for less than 2–12% of the total energy budget for any rearing option. The energy efficiency (energy output/energy input) of the six options ranges from 0.97% for flow-through with pumped supply to 3.49% for the flow-through with gravity supply. The rearing options with the three highest energy efficiencies were flow-through with gravity supply (3.49%), partial reuse (2.75%), and reuse (2.64%).On a kg of smolt produced basis, the six rearing options showed a wide range in performance. The reuse system had the lowest water (2 m3 kg− 1) and land (0.13 m2 kg− 1) requirements and the third lowest total energy requirement (288 MJ kg− 1). The partial reuse system had the second lowest total power requirement (276 MJ kg− 1), a low land requirement (0.21 m2 kg− 1), and moderate water requirements (33 m3 kg− 1). The partial reuse with temperature control had the second highest total power requirement (657 MJ kg− 1) and land and water requirements similar to the partial reuse system without temperature control. The flow-through system with pumped water supply had the highest water (289 m3 kg− 1), land (2.19 m2 kg− 1), and energy requirements (786 MJ kg− 1) of any of the rearing options. By comparison, the flow-through system with gravity water supply had the lowest energy requirement (218 MJ kg− 1), a moderate land requirement (0.78 m2 kg− 1), and a high water requirement (214 m3 kg− 1). The ranking of the six rearing options based capital and operating costs are likely to be quite different from those based on energy, water, and greenhouse gas emissions. 相似文献
40.
基于温室内植物冠层能量平衡关系,建立了与温室内、外气象条件和温室结构相关的冠层温度模拟机理模型,并在华北地区文洛型温室内对该模型进行了试验验证.结果表明:模型能较好地模拟冬季温室内植物冠层温度,模拟值和实测值之间的相关系数为0.797 5,均方根误差为1.3℃.建立了冠层温度的BP神经网络模型,模型相关系数为0.783 5,均方根误差为0.6℃.在所建神经网络模型基础上,运用敏感性分析法对影响冠层温度的各因素进行重要性分析和排序,得出影响冠层温度的最重要因子是室内温度,其次为蒸腾速率、室外太阳辐射和室内相对湿度. 相似文献