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   7294篇
  免费   56篇
  国内免费   87篇
林业   359篇
农学   113篇
基础科学   58篇
  748篇
综合类   2449篇
农作物   271篇
水产渔业   98篇
畜牧兽医   3017篇
园艺   122篇
植物保护   202篇
  2018年   29篇
  2014年   72篇
  2013年   28篇
  2012年   493篇
  2011年   534篇
  2010年   127篇
  2009年   71篇
  2008年   402篇
  2007年   430篇
  2006年   422篇
  2005年   411篇
  2004年   363篇
  2003年   323篇
  2002年   339篇
  2001年   231篇
  2000年   254篇
  1999年   165篇
  1998年   40篇
  1997年   64篇
  1996年   60篇
  1995年   73篇
  1994年   85篇
  1993年   81篇
  1992年   81篇
  1991年   74篇
  1990年   95篇
  1989年   77篇
  1988年   114篇
  1987年   106篇
  1986年   62篇
  1985年   84篇
  1984年   103篇
  1983年   95篇
  1982年   71篇
  1981年   44篇
  1980年   41篇
  1979年   39篇
  1975年   25篇
  1974年   40篇
  1973年   42篇
  1972年   43篇
  1971年   23篇
  1970年   29篇
  1963年   50篇
  1960年   31篇
  1959年   28篇
  1958年   30篇
  1957年   34篇
  1955年   25篇
  1886年   25篇
排序方式: 共有7437条查询结果,搜索用时 31 毫秒
91.
92.
Paratuberculosis (Johne's disease) is one of the most important diseases in ruminants today. Its contribution is worldwide and the disease is causing severe financial losses among cattle producers in some countries [Hasanova, L., Pavlik, I., 2006. Economic impact of paratuberculosis in dairy cattle herds: a review. Vet. Med.-Czech. 51, 193–211]. Paratuberculosis is untreatable; diagnosis limited to the early stages of the infection and control of the disease is difficult. The prevalence of serologically positive Austrian cattle farms rose significantly to 19.0% during the past years [Baumgartner, W., Damoser, J., Khol, J.L., 2005. Comparison of two studies concerning the prevalence of bovine paratuberculosis (Johne's disease) in Austrian cattle in the years 1995–1997 and 2002/2003 (Article in German with extended English summary). Vet. Med. Austria/Wien. Tierärztl. Mschr. 92, 274–277]. Based on these findings clinical paratuberculosis in ruminants was declared a notifiable disease in Austria in April 2006.

A survey of the current situation in Austria, the most important parts of the new compulsory measures and their practical implementation and impacts are presented in this short communication.  相似文献   

93.
We conducted a study on 81 initially bulk-milk ELISA negative dairy herds taken from a random sample of Dutch dairy herds to evaluate variation in bulk-milk S/P ratios and to study reasons for bulk-milk conversion. These herds were repeatedly (3-month intervals) tested between April 2004 and August 2005 and serostatus of all animals had previously been established as negative (N), low-positive (LP) or high-positive (HP). Of these herds, herd- and test-related factors associated with variation in sample over positive (S/P) ratios were analysed using a multivariable linear-mixed model with ‘herd’ as random effect. In addition, changes of animal serostatus in converting herds were described. S/P ratios were calculated as the optical density of the bulk-milk sample minus the optical density of the negative serum control divided by the difference in optical density between the positive and negative serum control.

Sixteen bulk-milk conversions in 12 dairy herds were seen with few indications of serological conversion in lactating cattle except for one herd in which recrudescence of infection appeared likely in nine cows. The effect of HP serostatus on bulk-milk S/P ratio was 2–3 times stronger compared with LP serostatus. In addition, bulk-milk S/P-ratio increased when the proportion of HP animals between 1 and 60 days in milk increased and decreased when the average milk-production level of the herd increased. Besides these herd-related factors, the use of different ELISA-testkits between test rounds had a significant effect on the S/P-ratio in bulk-milk samples.  相似文献   

94.
OBJECTIVES: To compare the recovery times and complication rates between the standard ventral slot and the modified slanted ventral slot for the treatment of cervical disc disease in dogs with the same neurological grade. METHODS: A retrospective analysis of an equal number of cervical disc cases for each procedure was carried out specifically to determine the recovery times and complication rates. To allow for meaningful comparison only dogs that had the same neurological grade were selected. Outcomes were determined by time to recover and the resolution of symptoms and complications were graded in terms of severity. RESULTS: The standard ventral slot cases had a higher complication rate than the modified slanted slot cases with one case dying in the postoperative period. However, the modified slanted slot cases on average had a marginally slower recovery time than standard ventral slot cases. CLINICAL SIGNIFICANCE: Ventral decompression using the modified slanted slot instead of the standard ventral slot will allow for spinal decompression with potentially less destabilisation, and incur a similar incidence of complications and rate of recovery.  相似文献   
95.
96.
天津市农作物品种审定委员会于4月11日至13日召开成立大会。各专业组成员,以及有关代表,各区、县种子公司经理等49人参加了会议。会议通过了委员会的组成人选、工作任务及天津市农作物品种审定试行办法。  相似文献   
97.
作为干、支管上喷头间距的函数的灌水均匀度和灌水深度的计算机模拟结果,用来作为研究无风条件下固定、半固定喷灌系统性能评价的简化方法。传统的在整个有效控制面积上格形布置的全套雨量筒设备可减少为等间距地布置于同一支管上两喷头间的10个雨量筒设备。根据模拟的结果,在干管间距小于0.65倍的湿润直径(0.65D)的情况下,简化法能取代精确法。这个间距值不超过詹森1980年提出的无风条件下,干管间距为0.65D的标准。同时,由于建改支管间距不超过干管间距的一半的规定,那么支管的最大适用间距为(0.65/2)D。而且,对于支管间距大于此限值的情形,简化法与精确法的应用结果也是一致的。尽管简化法已显示出可用于测估均匀度,然而对于估测灌水深度却不怎么行。平均灌水深度可通过喷嘴流量和喷头间距的测定值计算得到。  相似文献   
98.
多功能水田管道喷药设备及技术研究项目设计郑贞水,华维斌农业是南方地区的优势,特别是改革开放以来,“一优两高”农业得到了迅猛的发展。但是,每年在农作物生产期、无论是水稻、棉花等农作物,还是林、果、茶等经济特产,都有大面积的病虫害发生和蔓延,对确保农业的...  相似文献   
99.
<正> 本文讨论近红外反射式土壤湿度仪的设计和特性,这台仪器是小型的,手持式的,它是通过测量求积圆柱体和两小窄带干涉漏光镜反射1.8μm,和1.94μm的光谱反射强度来取得土壤湿度读数。 这种不需进行繁重的求积而直接提供土壤含水率的土壤湿度仪,设计和特性如下图所示。  相似文献   
100.
从1978年3月到5月,于5尾斑节对虾(Penaeus monodon)雌虾获得的34批卵子中,孵化出了二百多万无节幼体。头胸甲长50~63毫米的雌虾,切除一眼柄,在20~30天的脱皮时期内,不断产出活力强的卵子,这表明一次受精后,足够使在一次脱皮时期内所产的几批卵子受精。每次脱皮后重新受精是必要的。每批的产卵量从19000到460000粒(平均217635粒)不等,而且卵量没有随着相断的批数而减少。平均孵化率为30.5%。平均每批孵化无节幼体数为66000个。在靠近脱皮末期产的几批卵,有孵化率较低的趋势。从15批孵化的幼体中,抽样培养到仔虾期,其密度在40~250个/升。在3升水体的培养中,平均成活率为79%,但在大规模试验中,平均成活率小于13%。  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