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   91356篇
  免费   4391篇
  国内免费   6524篇
林业   10543篇
农学   6959篇
基础科学   5931篇
  12025篇
综合类   45344篇
农作物   3658篇
水产渔业   2872篇
畜牧兽医   8134篇
园艺   2373篇
植物保护   4432篇
  2024年   508篇
  2023年   1251篇
  2022年   1948篇
  2021年   2144篇
  2020年   2136篇
  2019年   2785篇
  2018年   1437篇
  2017年   2657篇
  2016年   3450篇
  2015年   3024篇
  2014年   4350篇
  2013年   4300篇
  2012年   6698篇
  2011年   6955篇
  2010年   5634篇
  2009年   5680篇
  2008年   5263篇
  2007年   6375篇
  2006年   5621篇
  2005年   4687篇
  2004年   3556篇
  2003年   3083篇
  2002年   2418篇
  2001年   2107篇
  2000年   1919篇
  1999年   1509篇
  1998年   1295篇
  1997年   1166篇
  1996年   1167篇
  1995年   1145篇
  1994年   1100篇
  1993年   937篇
  1992年   886篇
  1991年   713篇
  1990年   637篇
  1989年   581篇
  1988年   447篇
  1987年   301篇
  1986年   148篇
  1985年   54篇
  1984年   25篇
  1983年   26篇
  1982年   21篇
  1981年   21篇
  1980年   23篇
  1979年   20篇
  1978年   15篇
  1977年   12篇
  1976年   10篇
  1962年   8篇
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
11.
有人说战争是媒体的狂欢节,而美国媒体向来是擅长同战争“共舞”的。在伊拉克战争中美国媒体就发挥了重要作用,成为战争的一个组成部分。如今,媒体对现代化战争所起的作用越来越重要,如何在战争中充分发挥媒体的作用,值得我们认真思考。美国媒体在伊拉克战争中,扮演着一个全新的角色,这为如何在现代战争巾运用媒体留下了宝贵的经验,带给我们重要启示。  相似文献   
12.
Although the molluscicide Frescon is a strong neurotoxin to the Lymnaea stagnalis central nervous system in vitro, it is probable that the exposure of the whole animal to this molluscicide fails to result in central nervous system abnormalities: Frescon does not appear to reach the brain in sufficient quantity to disrupt its normal activity. However, only those Frescon analogs found to be neurotoxic were molluscicidal, suggesting some related mode, if not site, of action. Frescon and its analogs may act by affecting excitable tissues other than the nervous system (e.g., the snail musculature) by altering certain functional and/or structural membrane properties.  相似文献   
13.
1999- 2 0 0 1年进行了草生栽培对园温湿度和果实品质影响的试验。结果表明 :草生栽培可以有效地改善果园生态环境 ,降低久旱暴雨后的裂果率 ,有利于提高果的商品质量和经济效益。  相似文献   
14.
Cowpea, Vigna unguiculata (L.) Walpers, is a major dietary staple in tropical Africa where it is most often cultivated. The production is, however, greatly hampered by severe infestation and damage by insect pests including the cowpea aphid, Aphis craccivora Koch. The damage and bionomics of the aphid are briefly introduced. Chemical, cultural, physical and biological methods, and use of plant resistance for A. craccivora control in cowpea are reviewed. Finally, prospects for integrated pest management are discussed  相似文献   
15.
根据中国药典对注射剂的要求,对花粉多糖的蛋白质、鞣质、树脂、草酸盐、炽灼残渣、钾离子、砷盐、重金属含量进行测定,均符合规定,达到质量标准。  相似文献   
16.
日粮中添加柑桔皮粉对产蛋后期蛋鸡钙磷代谢的影响   总被引:1,自引:1,他引:1  
将 48周龄黄金褐蛋鸡 10只随机分成两组 ,对照组喂基础日粮 ,试验组用 2 5 %的柑桔皮粉等量替代基础日粮中的玉米 ,结果表明 :试验组钙磷表观代谢率为 65 79%、46 3 9% ,钙比对照组 (62 3 0 % )提高了 3 49个百分点 (P <0 0 5 ) ,磷比对照组 (4 7 5 2 % )降低了 1 13个百分点 (P >0 0 5 )。  相似文献   
17.
纤维素酶渣替代黄豆饲喂杂交奶牛试验   总被引:1,自引:0,他引:1  
采用纤维素酶渣替代黄豆对西黄杂奶牛(西门塔尔×本地黄牛)作饲喂对比试验,并对其产奶量及奶品质进行观察。结果表明:试验组比对照组牛日产奶量平均提高1 88kg,其乳蛋白和乳糖含量也有不同程度的提高。  相似文献   
18.
ABSTRACT

1. Enzymes have been used commercially for nearly 40 years and save significant costs through sparing of expensive nutrients but the mechanism by which this is achieved is still debated.

2. The research focused on non-starch polysaccharidase (NSPase) enzymes is used as an example of where greater progress could have been made if the details of the work had been described more fully and the analysis of the data generated had been broader in scope and more critical.

3. Lack of standardisation of the details presented in the materials and methods has been identified as a significant barrier to meaningful retrospective analysis and thus limits advances in the understanding of the mode of action of these enzymes.

4. The identity of the enzyme employed and its activity is often lacking, and more importantly the purity is rarely disclosed. Contaminant activities which are neither listed nor assayed could play a significant role in the responses observed.

5. The dose optimum of most enzymes is often considerably higher than that employed in most studies. Thus studies claiming synergy between two ‘activities’ should ensure that the response is not related to each enzyme simply augmenting the dose of just one activity in the finished feed. This is a common problem, and coupled with the lack of factorial experiments to justify the presence of each enzyme in a multi-enzyme product, it is not surprising that there is still debate as to whether single or multi-enzymes are best suited poultry rations.

6. The three proposed mechanisms for NSPases (viscosity, cell wall and prebiotic) are discussed, and along with their strengths and weaknesses it is suggested that a re-evaluation of each is needed. Viscosity may have to be re-evaluated as being a function not only of the cereal being fed, but of the age of the animal as well. The cell wall theory as described is poorly modelled in vitro and hence the validity of these data is questioned. The prebiotic theory may need significant modification as it appears that the quantities of oligomers produced are insufficient to generate the additional volatile fatty acids (VFA)’s reported. It is likely that all three mechanisms play a role in the responses observed, but the prebiotic mechanism probably plays by far the most important part in low viscosity diets.

7. Future research would be improved if it considered all potential mechanisms when designing a trial. Significant failings are apparent as a result of adherence to tenets in explanation of the results. Most importantly, it should be emphasised that a hypothesis is there to be tested, not defended.  相似文献   
19.
湖南小溪国家级自然保护区生态旅游资源的SWOT分析   总被引:1,自引:0,他引:1  
对湖南小溪国家级自然保护区进行了生态旅游资源的SWOT分析,其优势是:良好的原始次森林和高品位的自然资源、森林浴的理想场所、优秀独特的人文资源、丰富的植物基因库存.但是可进入性差、知名度低、接待设施及能力较差.周边存在张家界国家森林公园和栖凤湖风景区的竞争威胁.分析了发展机会.  相似文献   
20.
运筹学是关于做决定的科学,运筹学集数学和其他科学方法为一体,对现实问题进行模型化并提出解决方案,把它科学地应用到各企业管理中,可有效地帮助企业做出合理的决策.  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