全文获取类型
收费全文 | 4086篇 |
免费 | 221篇 |
国内免费 | 332篇 |
专业分类
林业 | 489篇 |
农学 | 298篇 |
基础科学 | 394篇 |
485篇 | |
综合类 | 1862篇 |
农作物 | 158篇 |
水产渔业 | 229篇 |
畜牧兽医 | 459篇 |
园艺 | 94篇 |
植物保护 | 171篇 |
出版年
2024年 | 12篇 |
2023年 | 35篇 |
2022年 | 73篇 |
2021年 | 85篇 |
2020年 | 98篇 |
2019年 | 125篇 |
2018年 | 102篇 |
2017年 | 137篇 |
2016年 | 208篇 |
2015年 | 144篇 |
2014年 | 170篇 |
2013年 | 237篇 |
2012年 | 303篇 |
2011年 | 310篇 |
2010年 | 266篇 |
2009年 | 279篇 |
2008年 | 247篇 |
2007年 | 302篇 |
2006年 | 258篇 |
2005年 | 196篇 |
2004年 | 160篇 |
2003年 | 129篇 |
2002年 | 100篇 |
2001年 | 105篇 |
2000年 | 74篇 |
1999年 | 74篇 |
1998年 | 63篇 |
1997年 | 50篇 |
1996年 | 42篇 |
1995年 | 41篇 |
1994年 | 41篇 |
1993年 | 39篇 |
1992年 | 38篇 |
1991年 | 23篇 |
1990年 | 25篇 |
1989年 | 11篇 |
1988年 | 21篇 |
1987年 | 9篇 |
1986年 | 6篇 |
1962年 | 1篇 |
排序方式: 共有4639条查询结果,搜索用时 93 毫秒
941.
为探究体况评分(Body Condition Score,BCS)和步态评分(Locomotion Score,LS)对奶牛泌乳性能的影响,本试验在2016年7月至8月通过对北京地区7个奶牛场近5 000头泌乳牛进行体况和步态评分并收集相关的DHI数据,采用固定模型分析BCS和LS的影响因素及场、胎次、泌乳阶段、LS和BCS对日产奶量、体细胞数及乳成分的影响。结果表明:BCS和LS对日产奶量均有极显著影响(P0.01),BCS对乳蛋白率有显著影响(P0.05);胎次对LS有极显著影响(P0.01),而对BCS无显著影响,场-测量人、泌乳阶段对BCS和LS均有极显著影响(P0.01);LS与BCS之间回归关系显著,LS较高的个体BCS较低。在生产上可以综合运用BCS和LS加强管理,提升产奶量,改善乳品质量,使牛场获得更大的经济利益。 相似文献
942.
Effective and efficient segmentation of vegetation from digital plant images is an actively studied topic in crop phenotyping. Many of the formerly proposed methods showed good performance in the extraction under controlled light conditions but it is still hard to properly extract only vegetation from RGB images taken under natural light condition where the images can contain shadowed and lighted parts with specularly reflected parts of plants. In this paper, we propose a robust method to extract vegetation from the plant images taken under natural light conditions using wheat images. The method is based on a machine learning process, decision tree and image noise reduction filters. We adopted the CART algorithm to create a decision tree in the training process and examined its performance using test images, comparing it with the performances of other methods such as ExG, ExG-ExR and Modified ExG which are widely used recently. The results showed that the accuracy of the vegetation extraction by the proposed method was significantly better than that of the other methods particularly for the images which include strongly shadowed and specularly reflected parts. The proposed method also has an advantage that the same model can be applied to different images without requiring a threshold adjustment for each image. 相似文献
943.
Lan Mu 《Acta Agriculturae Scandinavica, Section B - Plant Soil Science》2013,63(8):735-746
Mulching is considered a desirable management technology for improving and stabilizing agricultural crop production. The aim of the present study was to evaluate the effect of different mulching practices on hot pepper (Capsicum annuum L.) performance in terms of leaf photosynthetic capacity, photosynthetic light response (PLR) curves, and growth parameters, under greenhouse conditions. Consequently, field experiments were conducted during the 2011 and 2012 growing seasons in Northwest China using four types of mulching practices (without mulch; wheat straw mulch; plastic film mulch; and combined mulch with plastic film and wheat straw). The results showed that mulching increased the leaf net photosynthesis rate, stomatal conductance to water vapor, intercellular CO2 concentration, water-use efficiency, and transpiration rate. Mulching significantly affected the PLR curves, and important parameters (such as the apparent quantum yield, maximum net photosynthetic rate, light compensation point, and light saturation point) were all improved under combined mulching conditions when compared with other mulching practices. Therefore, it is inferred that hot pepper leaf photosynthetic capacity and light-use efficiency were significantly improved under combined mulching, and thereby the combined mulching is recommended for hot pepper cultivation in greenhouses due to working well to facilitate soil condition (moisture and temperature), plant growth, and marketable photosynthetic characteristics. 相似文献
944.
945.
立地条件与柳杉幼林生长量的关系 总被引:1,自引:0,他引:1
采用样地调查的方法,运用数量化理论Ⅰ与通径分析方法,对福建省周宁县不同立地条件下5年生柳杉人工林的生长量调查分析,探讨立地条件与柳杉生长的关系。结果表明,影响林木生长的主导因子是:土壤松紧度、质地、厚度;间接因子是:坡位、海拔。 相似文献
946.
947.
Fenglan Li Xianfeng Jiang Meili Sun Hui-lian Xu Lijuan Shi Weiyu Shan 《Acta Agriculturae Scandinavica, Section B - Plant Soil Science》2013,63(8):694-699
Dry rot is a severe fungal disease during potato storage. Fusarium spp. is the pathogen causing severe rot of potato tubers, producing Fusarium toxins and contaminating food. The disease leads to a huge loss to China's potato industry. The present research optimized and screened the culture conditions of the major pathogens causing potato dry rot including Fusarium culmorum, Fusarium avenaceum, Fusarium sambucinum, Fusarium solani var. coeruleum, Fusarium trichothecioides, and Fusarium sporotrioides. The results showed that F. sambucinum and F. culmorum were sensitive to temperature and pH. F. sambucinum, F. trichothecioides, F. Solani var. coeruleum, and F. sporotrioides are favorable to the glucose-based culture while F. culmorum and F. avenaceum were to sucrose-based culture. Illumination stimulated the growth of F. sambucinum and F. sporotrioides but inhibited the growth of F. culmorum and F. avenaceum. It is concluded that the culture conditions are different with species of Fusarium and the understanding of the culture conditions is expected useful in research and the disease control. 相似文献
948.
949.
950.
土壤矿物作为土壤重要活性组分,可驱动土壤有毒有机物化学转化,降低污染风险。以往土壤矿物与有毒有机物界面行为研究主要集中于水环境或矿物悬浊液体系,然而实际环境中土壤及其矿物常处于干燥、湿润等水分非饱和状态。近年来,水分非饱和条件下土壤矿物界面有毒有机物转化及机制已成为研究热点,相关研究获得一系列新发现。低含水量铁锰矿物、黏土矿物和金属离子饱和黏土矿物能驱动多环芳烃、抗生素等疏水性有毒有机物化学转化。水分非饱和环境会减弱矿物界面水分子与有毒有机物竞争活性位点,并使矿物发生脱水、向高活性结构转变。此外,土壤矿物水分状态也会影响有毒有机物转化产物,水分非饱和环境更有利于持久性自由基和卤代二噁英等中间产物的形成和稳定。以往研究认为,电子转移反应是土壤矿物界面有毒有机物转化机制,随着检测技术与理论计算的发展,自由基催化和水解作用机制逐渐被发现,相关机制研究精准至矿物晶型和晶面层面。虽然水分非饱和条件下土壤矿物界面有毒有机物转化及机制已逐渐清晰,但其研究广度和深度有待进一步拓宽和加深。建议未来在实际水分非饱和土壤和矿物中开展有毒有机物转化研究,深入探究还原转化过程,研发原位反应装置及检测方法,尝试从微纳米尺度和分子水平解析有毒有机物在矿物界面转化机制。 相似文献