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   208篇
  免费   2篇
  国内免费   12篇
林业   23篇
农学   9篇
基础科学   1篇
  38篇
综合类   84篇
农作物   2篇
畜牧兽医   33篇
园艺   28篇
植物保护   4篇
  2023年   3篇
  2022年   2篇
  2021年   5篇
  2020年   4篇
  2019年   6篇
  2018年   4篇
  2017年   5篇
  2016年   2篇
  2015年   6篇
  2014年   8篇
  2013年   8篇
  2012年   10篇
  2011年   19篇
  2010年   14篇
  2009年   8篇
  2008年   6篇
  2007年   16篇
  2006年   14篇
  2005年   10篇
  2004年   10篇
  2003年   7篇
  2002年   4篇
  2001年   6篇
  2000年   3篇
  1999年   2篇
  1998年   2篇
  1997年   4篇
  1996年   4篇
  1995年   4篇
  1994年   2篇
  1993年   5篇
  1992年   4篇
  1991年   3篇
  1990年   2篇
  1989年   3篇
  1988年   3篇
  1987年   4篇
排序方式: 共有222条查询结果,搜索用时 0 毫秒
81.
Broadcast seeding is one of the most widely used post-wildfire emergency response treatments intended to reduce soil erosion, increase vegetative ground cover, and minimize establishment and spread of non-native plant species. We conducted an evidence-based review to examine the effectiveness and effects of post-wildfire seeding treatments on soil stabilization, non-native species invasion, and plant community recovery in the western U.S. We reviewed 94 scientific papers and agency monitoring reports identified using a systematic search protocol. As sampling designs have become more rigorous in recent years, evidence that seeding is effective in reducing erosion has decreased. Of highest and high quality studies evaluating soil erosion, 92% (11 of 12) were published since 2000, none of which showed an effective result. Before 2000, the majority of the studies (71%) fell into the lowest quality categories, of which 72% showed seeding to be effective. The majority of studies (20 of 27, 74%) evaluating soil erosion in seeded versus unseeded controls showed that seeding did not reduce erosion relative to unseeded controls. Even when seeding significantly increased vegetative cover, seeded sites rarely supported plant cover levels considered sufficient to stabilize soils within the first and second year post-wildfire. Of the 11 studies evaluating seeding effectiveness for curtailing invasions of non-native plant species, an almost equal percentage found seeding treatments to be either effective (54%, 6 studies) or ineffective (45%, 5 studies). However, the majority of effective and ineffective treatments (83% and 80%, respectively) used non-native species. Sixteen of 26 studies (62%) evaluating seeding effects on plant communities reported that seeding suppressed recovery of native plants, although data on long-term impacts of this reduction are limited. The literature suggests that post-wildfire seeding does little to protect soil in the short-term, has equivocal effect on invasion of non-native species, and can have negative effects on native vegetation recovery, although long-term studies are needed to assess lasting impacts of seeded species.  相似文献   
82.
83.
84.
This review summarizes evidence for associations between Salmonella prevalence in market-weight swine and changes in feeding management practices or feed characteristics. A systematic review of the topic was conducted with the goal of minimizing the impact of bias on the review conclusions. Potential interventions included feed withdrawal from swine prior to slaughter, feed acidification, heat treatment of feed, pelletized feed versus mash, course versus fine grind, and wet versus dry feeds. In the reviewed literature, Salmonella prevalence was measured either by culture or by the presence of antibodies to Salmonella. The evidentiary value of studies was assessed, and studies that failed to meet predetermined standards were excluded. 7694 potentially relevant references were identified by an extensive literature search; however, 2623 references that were not published in English were excluded, because funds for translation were not available. Of the remaining references, only 277 were considered relevant to the review topic by two independent reviewers, and assessed for methodological quality. During quality assessment, 233 references were excluded because they failed to report design features that limit the introduction of bias or were conducted in a non-target population such as gnotobiotic, neonatal, nursery, or recently weaned pigs and sows. Forty-four publications passed the quality assessment conducted by 2 independent reviewers, but only 15 of the 44 publications reported studies that tested hypotheses associated with feeding management practices and feed characteristics and Salmonella prevalence in market-weight swine. The most common study design was cross-sectional (7/15). The included studies failed to provide strong evidence of an association between any of the potential interventions and Salmonella prevalence, due to the potential for confounding, and the failure to document temporal association between the intervention and Salmonella prevalence. The strongest evidence of an association was found for feed form, i.e. the use of non-pelleted may be potential interventions associated with reduced Salmonella prevalence. The uncertainty is primarily based on studies containing moderate to low evidentiary value or insufficient numbers of tested individuals, resulting in a low degree of confidence that results could be extrapolated to the target population. Therefore, the conclusion of the review is that there should be a low level of comfort among qualified scientists that the claimed association between non-pelleted feed and reduced Salmonella prevalence is scientifically valid. There is no strong evidence regarding associations between presence of Salmonella and the other feed characteristics examined.  相似文献   
85.
大麦性状特异性聚类分析   总被引:3,自引:0,他引:3  
为了明确聚类分析大麦性状特异性的效果,在对大麦性状一致性与稳定性分析的基础上,选取31个间断型性状和小区性状,对68个大麦品种的特异性进行了聚类分析研究。结果表明:一是运用系统聚类法判别品种间的性状差异,结果比较合理。分析结果与已知品种类别划分相近,所选性状可有效地区分品种差异;二是证明了穗的棱型、籽粒的皮裸性可作为大麦品种性状分组指标。同时也证明了2棱大麦与4棱大麦近似性要比与6棱大麦间的近似性要多;三是表明了东北三省育成的4棱大麦品种间距离短,性状相似性较大、亲缘关系较近。四是根据品种间差异性,能有效进行亲本和杂交组合的选配,从而为提高大麦育种效率提供理论依据。  相似文献   
86.
The objective of this study was to systematically review the efficacy of topically applied insecticide treatments of dogs (impregnated collars, spot-ons), and prophylactic medications to prevent natural Leishmania infantum (L. infantum) infection in dogs.  相似文献   
87.
Acupuncture is a popular complementary treatment option in human medicine. Increasingly, owners also seek acupuncture for their animals. The aim of the systematic review reported here was to summarize and assess the clinical evidence for or against the effectiveness of acupuncture in veterinary medicine. Systematic searches were conducted on Medline, Embase, Amed, Cinahl, Japana Centra Revuo Medicina and Chikusan Bunken Kensaku. Hand-searches included conference proceedings, bibliographies, and contact with experts and veterinary acupuncture associations. There were no restrictions regarding the language of publication. All controlled clinical trials testing acupuncture in any condition of domestic animals were included. Studies using laboratory animals were excluded. Titles and abstracts of identified articles were read, and hard copies were obtained. Inclusion and exclusion of studies, data extraction, and validation were performed independently by two reviewers. Methodologic quality was evaluated by means of the Jadad score. Fourteen randomized controlled trials and 17 nonrandomized controlled trials met our criteria and were, therefore, included. The methodologic quality of these trials was variable but, on average, was low. For cutaneous pain and diarrhea, encouraging evidence exists that warrants further investigation in rigorous trials. Single studies reported some positive intergroup differences for spinal cord injury, Cushing's syndrome, lung function, hepatitis, and rumen acidosis. These trials require independent replication. On the basis of the findings of this systematic review, there is no compelling evidence to recommend or reject acupuncture for any condition in domestic animals. Some encouraging data do exist that warrant further investigation in independent rigorous trials.  相似文献   
88.
Conservation plans are usually developed for regions that encompass only one environmental realm (terrestrial, freshwater or marine) because of logistical, institutional and political constraints. This is inadequate because these realms often interact through processes that form, utilize and maintain interfaces or connections, which are essential for the persistence of some species and ecosystem functions. We present a conceptual framework for systematic conservation prioritization that explicitly accounts for the connectivity between the terrestrial, marine, and freshwater realms. We propose a classification of this connectivity that encompasses: (1) narrow interfaces, such as riparian strips; (2) broad interfaces, such as estuaries; (3) constrained connections, such as corridors of native vegetation used by amphibians to move between natal ponds and adult habitat; and (4) diffuse connections, such as the movements of animals between breeding and feeding habitats. We use this taxonomy of inter-realm connectivity to describe existing and new spatial conservation prioritization techniques that aim to promote the persistence of processes that operate between realms.  相似文献   
89.
宁南山区玉米生长发育进程研究   总被引:1,自引:0,他引:1  
通过玉米生长发育系统观测,探讨宁南山区半干旱地区在常规生产条件下种植地膜玉米,掌握其个体静态和群体动态进程。据灌浆进程测定,在8月20日左右即开花后36d,籽粒灌浆千粒日增重最高峰达16.08g、百粒重为30.54g。  相似文献   
90.
桂豇2号是以广西南宁市农家品种甜肉豆角的优选单株M133-6为母本,以广东阳江农家品种黑籽豆角的优选单株F12-5为父本进行杂交,经系统选育而成的中熟豇豆新品种。主侧蔓均可结荚,始花节位为第4~6节,每花序可结荚2~3条,商品荚浅绿色,平均荚长55 cm,单荚质量约30 g,豆荚肉厚,味甜。春季播种至始收约55 d(天),夏秋季播种至始收约50 d(天),全生育期90~110 d(天),产量1 900~2 000 kg·(667 m~2)~(-1)。田间对锈病、白粉病、煤霉病的抗性均优于广西当地豇豆主栽品种华珍豆角。适宜华南地区春秋季种植。  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