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   4304篇
  免费   330篇
  国内免费   310篇
林业   388篇
农学   414篇
基础科学   92篇
  751篇
综合类   1292篇
农作物   125篇
水产渔业   365篇
畜牧兽医   615篇
园艺   39篇
植物保护   863篇
  2024年   25篇
  2023年   173篇
  2022年   199篇
  2021年   246篇
  2020年   235篇
  2019年   244篇
  2018年   164篇
  2017年   235篇
  2016年   288篇
  2015年   254篇
  2014年   273篇
  2013年   306篇
  2012年   407篇
  2011年   339篇
  2010年   192篇
  2009年   210篇
  2008年   144篇
  2007年   172篇
  2006年   113篇
  2005年   96篇
  2004年   83篇
  2003年   64篇
  2002年   60篇
  2001年   57篇
  2000年   47篇
  1999年   51篇
  1998年   43篇
  1997年   27篇
  1996年   31篇
  1995年   44篇
  1994年   20篇
  1993年   22篇
  1992年   22篇
  1991年   15篇
  1990年   10篇
  1989年   11篇
  1988年   9篇
  1987年   3篇
  1986年   3篇
  1985年   1篇
  1984年   3篇
  1979年   1篇
  1962年   2篇
排序方式: 共有4944条查询结果,搜索用时 19 毫秒
101.
呼伦贝尔岭东南旱作丘陵区气候资源潜力分析   总被引:4,自引:1,他引:4  
对呼伦贝尔市阿荣旗三道沟镇商品粮生产基地主要粮食作物的光合、光温、气候、气候-土壤资源潜力进行分析,结果表明热量条件是限制该地区粮食增产的主要非可控因素,如能改善热量条件,还有20%~30%的增产潜力;该地区的降水条件对玉米、大豆、马铃薯、水稻四种主要粮食作物的满足程度在80%左右,表明降水量及其季节分布是该镇粮食产量低的制约因素之一,存在的问题是水分利用率较低;土壤养分状况对马铃薯和水稻增产未表现明显的限制作用,而对玉米和大豆增产却是最主要限制因素。  相似文献   
102.
气候变化对陇东塬区冬小麦生长发育及产量的影响   总被引:20,自引:3,他引:20  
分析西峰农业气象试验站冬小麦生育期的观测资料表明,气候变暖使陇东塬区冬小麦全生育期天数缩短,主要是越冬期天数缩短,而冬小麦实际生长的天数并未缩短;气候变暖使冬小麦播种期推迟,成熟期提前,从而使麦田的休闲期延长,延长了复种作物的生长期,为提高气候资源的利用率和单位面积的产量提供了保证。气候变化对冬小麦气候产量的影响具有明显的周期性。  相似文献   
103.
新疆三工河流域气候暖湿化特征分析   总被引:1,自引:9,他引:1  
对三工河流域近40a中山区、近山区、近沙漠区的气温、降水、蒸发资料进行月线性趋势分析,得出流域增温主要在2月和9月。2月增温趋势大于9月。流域春夏季气温多年变化较为稳定。降水增湿在1,2月和夏秋季节,夏秋季节降水增湿趋势大于冬季。整个流域夏季蒸发呈减小趋势,平原区蒸发减小趋势大于中山区。夏季降水增大与蒸发减小有较好的反相关性。流域冬季气候增温增湿显著,夏季增湿作用显著。各地暖湿化在空间和时间上有所差异,中山区相对较弱,平原区较强,近山平原区是气候暖湿化最为显著的地域。  相似文献   
104.
内蒙古北部荒漠草原带的严重荒漠化及其治理   总被引:11,自引:3,他引:11  
位居干旱气候区的内蒙古高原中北部的荒漠草原地带 ,面临着严重荒漠化的威胁。目前已有 90 %以上的天然草场退化与沙化 ,变成寸草不生的不毛之地。大大扩展了我国北部沙尘暴频繁发生的地带 ,成为北方生态安全最危险的地区。这是多年来盲目增畜 ,超载过牧 ,滥垦乱挖的掠夺式生产经营中又加上干旱气候因素的耦合作用所造成的必然恶果。面对这一严峻形势 ,急需采取果断措施抢救荒漠草原地带的整体生态环境。应立即实行禁牧封育 ,人畜转移 ,调整结构 ,移民扩镇。使荒漠草原地带与农牧交错地带以及城市之间建立起产业化可持续发展的新途径  相似文献   
105.
甘肃第四纪气候期划分   总被引:10,自引:2,他引:10  
第四纪时期气候变化是由冷和暖及干和湿组成的。冷和暖及干和湿是相对的,是呈循环变化的,是有强、弱之分和持续时间长短不同的。本文根据第四纪地层中植物孢粉和CaCO3的含量及岩石风化系数、黄土中古土壤、第四纪泥炭的造炭期等资料,通过分析研究,用比拟方法确定第四纪气候期的性质,用冷和暖及干和湿作为第四纪气候期划分的尺度。将第四纪气候期划分为四次温暖期、五次寒冷期、一次冷热频繁交替期。  相似文献   
106.
Using field plots where rhizomania had not previously been detected, different inoculum levels of beet necrotic yellow vein virus (BNYVV) were created by application of infested soil. A susceptible sugar beet cultivar (cv. Regina) was grown for two consecutive years (1988 and 1989), in the presence or absence of drip irrigation. In soil samples taken in spring 1989, the different initial inoculum levels of 1988 could be distinguished using a quantitative bioassay estimating most probable numbers (MPNs) of infective units per 100 g dry soil. The first sugar beet crop resulted in a tenthousandfold multiplication of inoculum of BNYVV (viruliferousPolymyxa betae). Mean MPNs of BNYVV ranged from 0.6 and 7 per 100 g soil for the lowest inoculum level to 630 and 1100 per 100 g for the highest level, in plots without and with irrigation, respectively. In spring 1990, MPNs had again increased. In both years, the initial inoculum level of 1988 had a significant linear effect on log-transformed MPNs of BNYVV determined. Log-transformed MPNs for 1990 and 1989 showed a positive linear correlation, despite a decreasing multiplication ratio at higher inoculum levels. Drip irrigation during one or two years enhanced the increase in MPN of BNYVV, which was reflected by the enhancement of multiplication ratios at all inoculum levels. The totalP. betae population was also higher after growing two irrigated crops than after growing two non-irrigated ones.  相似文献   
107.
人工接种及病区调查研究结果表明,在种植感病品种情况下,气温决定了水稻细菌性条斑病的发生与否和潜育期的和短衣病斑长度;病害发生的最低温为12℃,发病最适温度为25-30℃以上,而湿度决定此病发病率及严重度。结合主成分分析,该病在田间的流行取决于温度/湿度及降雨量。台风暴雨造成的伤口有利于病害的流行。  相似文献   
108.
Twenty each of healthy lactating Friesian and Holstein cows were studied. They were of similar age and body weight, nonpregnant in their third lactation and 80–100 days post partum. The investigation was carried out for 8 weeks on two farms at the same time during the hot summer season in Egypt. The first farm was in Demietta (North east of the Nile Delta, 31° 40 N) on 20 Friesian cows and the second was in Fakous (East of the Nile Delta, 30° 40 N) on 20 Holstein cows. On each of the two farms, 10 cows were newly imported and 10 were born in Egypt.The average daily milk yield and total milk production in the third lactation of the newly imported cows were significantly higher than those of the locally born cows for both Friesians and Holsteins. At the same time, the T4, T3, urea-N, haematological values and AST enzyme activity in the newly imported cows were significantly lower than those in the locally born ones in both breeds, while the locally born cows showed significantly lower values for rectal temperature and respiration rate, as well as for AST and Alk-P enzyme activities, than the newly imported cows of either breed.Holstein cows surpassed the Friesians in milk production, as well as thyroid hormone secretion and cholesterol, haemoglobin, packed cell volume and erythrocyte count values. The opposite was found for serum total protein, urea-N and creatinine concentrations, leukocyte count and AST, ALT and Alk-P enzyme activities.  相似文献   
109.
The central and regional organisation of the campaign to eradicate the CSF epidemic in the Netherlands in 1997/1998 is described. The main instruments used in the campaign were based on stamping-out and movement restrictions specified by the European Union. Additional instruments were used for the first time, namely, pre-emptive culling of contact and neighbouring farms, compartmentalisation of transport, monthly serological screening in established surveillance areas and supervised repopulation of all farms in the former surveillance zone. Two other measures, the killing of very young piglets and a breeding ban were introduced to reduce production in established surveillance zones. Several factors complicated the eradication campaign, for instance, the late detection of the first infection; artificial insemination as a source of infection; the organisation of pig farming in the Netherlands, with its highly concentrated production and dependence on the transport of stock from one unit to another; insufficient rendering capacity; decreasing sensitivity of clinical inspection; and extremely high costs.  相似文献   
110.
应用GCMs和历史气候资料生成我国在CO2倍增时的气候情景   总被引:21,自引:2,他引:19  
采用3种大气环流模型(GCMs)的输出结果,并结合我国各地有代表性气象台站20-30年的逐日气象资料,在微机上生成了我国在CO_2倍增时的3种气候情景.进而分析了我国未来温度、降水和太阳辐射3要素可能的时空分布特征和季节变化趋势,并推测我国易受全球气候变化不利影响的脆弱地区.  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