全文获取类型
收费全文 | 3247篇 |
免费 | 234篇 |
国内免费 | 248篇 |
专业分类
林业 | 112篇 |
农学 | 143篇 |
基础科学 | 198篇 |
497篇 | |
综合类 | 1164篇 |
农作物 | 113篇 |
水产渔业 | 1067篇 |
畜牧兽医 | 247篇 |
园艺 | 20篇 |
植物保护 | 168篇 |
出版年
2024年 | 26篇 |
2023年 | 102篇 |
2022年 | 92篇 |
2021年 | 106篇 |
2020年 | 119篇 |
2019年 | 130篇 |
2018年 | 69篇 |
2017年 | 125篇 |
2016年 | 135篇 |
2015年 | 115篇 |
2014年 | 162篇 |
2013年 | 211篇 |
2012年 | 215篇 |
2011年 | 226篇 |
2010年 | 176篇 |
2009年 | 209篇 |
2008年 | 183篇 |
2007年 | 184篇 |
2006年 | 160篇 |
2005年 | 178篇 |
2004年 | 139篇 |
2003年 | 101篇 |
2002年 | 92篇 |
2001年 | 77篇 |
2000年 | 73篇 |
1999年 | 41篇 |
1998年 | 36篇 |
1997年 | 37篇 |
1996年 | 37篇 |
1995年 | 36篇 |
1994年 | 30篇 |
1993年 | 23篇 |
1992年 | 20篇 |
1991年 | 19篇 |
1990年 | 8篇 |
1989年 | 15篇 |
1988年 | 13篇 |
1987年 | 4篇 |
1986年 | 3篇 |
1985年 | 2篇 |
排序方式: 共有3729条查询结果,搜索用时 31 毫秒
1.
Large‐scale patterns of fish diversity and assemblage structure in the longest tropical river in Asia
下载免费PDF全文
![点击此处可从《Ecology of Freshwater Fish》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Although the Mekong River is one of the world's 35 biodiversity hot spots, the large‐scale patterns of fish diversity and assemblage structure remain poorly addressed. This study aimed to investigate the fish distribution patterns in the Lower Mekong River (LMR) and to identify their environmental determinants. Daily fish catch data (i.e. from December 2000 to November 2001) at 38 sites distributed along the LMR were related to 15 physicochemical and 19 climatic variables. As a result, four different clusters were defined according to the similarity in assemblage composition and 80 indicator species were identified. While fish species richness was highest in the Mekong delta and lowest in the upper part of the LMR, the diversity index was highest in the middle part of the LMR and lowest in the delta. We found that fish assemblages changed along the environmental gradients and that the main drivers affecting the fish assemblage structure were the seasonal variation of temperature, precipitation, dissolved oxygen, pH and total phosphorus. Specifically, upstream assemblages were characterised by cyprinids and Pangasius catfish, well suited to low temperature, high dissolved oxygen and high pH. Fish assemblages in the delta were dominated by perch‐like fish and clupeids, more tolerant to high temperatures, and high levels of nutrients (nitrates and total phosphorus) and salinity. Overall, the patterns were consistent between seasons. Our study contributes to establishing the first holistic fish community study in the LMR. 相似文献
2.
郑书高 《畜牧兽医科学(电子版)》2021,(4):182-183
近年,畜牧养殖业蓬勃发展,过去的区域化散养已经基本不存在,养殖结构逐渐向集约化、一体化、规模化过渡,养殖规模也逐渐增大。养殖副产品肉、蛋、奶等极大满足了人们的生产生活,养殖中产生的尿液、粪污对生态环境造成极大影响。为促进规模化养殖实现绿色健康发展,该文分析规模化畜牧养殖对生态环境的破坏及防治措施。 相似文献
3.
湟源县草地面积有 85 1 85 .1 3hm2 ,占全县土地总面积的 5 6.45 % ,草地资源丰富 ,牧草种类较多 ,农业生产秸秆来源广泛 ,但由于利用不当 ,造成了草地严重破坏和秸秆的浪费 ,如何合理利用草地和秸秆资源 ,扼制草地退化 ,是促进该县畜牧业发展的重要因素 相似文献
4.
政府渔业补助政策与博弈 总被引:1,自引:0,他引:1
渔业补助政策是政府为弥补市场经济缺陷而采取的重要措施,本文应用博弈论对政府与渔民的互动博弈展开研究,并确立了混合策略模型下的政府和渔民各自最佳选择概率,达到纳什均衡,这对于解释当前政府渔业补助政策内涵与指导修订补助政策有参考意义。 相似文献
5.
6.
7.
Modern biotechnology promises a number of new applications in animal breeding and production. Although conventional pig breeding has achieved a high level of efficiency and productivity numerous problems have been encountered with animal health and the loss of meat quality. Selection based on phenotypic performance data of individual animals does not take into account the importance of specific genes and their relevance within a complex regulatory system. In most cases it is therefore difficult to trace back the genetic origins of clinically important disorders. The application of genetic engineering techniques in pig production will facilitate diagnosis, improvement of productivity, and animal health by allowing direct genetic manipulation. Attention must be focussed on the physical and genetic analysis of the procine genome. The isolation and characterisation of genes, DNA-markers, polymorphic DNA-fragments, and their chromosomal assignment will be important prerequisites and tools for the elucidation of genetic disorders. Especially the detection of heterozygous carriers of recessive disorders and their elimination from the breeding stock will increase selection accuracy and decrease the generation intervals. But also the rapid and simple detection of infectious diseases, which is sometimes difficult if not impossible at present, will improve animal health and welfare. Although the production of transgenic animals either by DNA-microinjection into zygotes or the use of embryonal stem cells manipulated in vitro is less straightforward than DNA-based diagnosis it will play an important role in the direct manipulation of the porcine genome and genes. Breeding programmes including the use of transgenic livestock have already been developed. There is no doubt that genetic engineering has reached a degree of practical feasibility, allowing it to play an important role in pig breeding in particular and animal production in general. 相似文献
8.
对以QY系列为主体的农用潜水电泵修复可靠性低、寿命短的问题,提出了改进措施,从而有效提高潜水电泵修复可靠性,并延长使用寿命。 相似文献
9.
依据海涂独特的自然属性确定开发利用方案是保证土地资源持续生产的关键。采用降低土壤盐分、实行保护性耕作、用植物残余物覆盖地面、种植牧草防止土壤侵蚀和促使生草层土的发育,保证了以潜在形式集中的富含必需营养元素的腐殖质在土壤中聚积,改善土壤结构,提高土壤水分和养分的利用效率,并以牧草饲养家畜的土地经营活动形成了成功的草地农业生产。 相似文献
10.