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   3706篇
  免费   215篇
  国内免费   268篇
林业   300篇
农学   177篇
基础科学   125篇
  119篇
综合类   1120篇
农作物   146篇
水产渔业   333篇
畜牧兽医   1642篇
园艺   135篇
植物保护   92篇
  2024年   6篇
  2023年   27篇
  2022年   47篇
  2021年   111篇
  2020年   109篇
  2019年   148篇
  2018年   89篇
  2017年   154篇
  2016年   164篇
  2015年   156篇
  2014年   193篇
  2013年   228篇
  2012年   290篇
  2011年   280篇
  2010年   258篇
  2009年   247篇
  2008年   222篇
  2007年   218篇
  2006年   196篇
  2005年   165篇
  2004年   139篇
  2003年   124篇
  2002年   87篇
  2001年   72篇
  2000年   74篇
  1999年   59篇
  1998年   43篇
  1997年   31篇
  1996年   49篇
  1995年   26篇
  1994年   26篇
  1993年   11篇
  1992年   25篇
  1991年   19篇
  1990年   24篇
  1989年   13篇
  1988年   7篇
  1987年   4篇
  1986年   4篇
  1985年   6篇
  1984年   4篇
  1983年   7篇
  1982年   4篇
  1981年   2篇
  1980年   5篇
  1979年   2篇
  1978年   5篇
  1977年   4篇
  1973年   3篇
  1955年   1篇
排序方式: 共有4189条查询结果,搜索用时 15 毫秒
1.
"鄂椒香帅"是以BL8为母本,CA1324-4为父本育成的鲜食长线椒辣椒杂交一代品种。该品种植株生长势较强,株高55 cm,开展度52 cm,叶片绿色,卵圆形,少茸毛,单叶互生,分枝性较强且分枝处有紫色斑块,始花节位8~9节,花冠白色,花萼平展;坐果力较强,果实长28~32 cm,果实横径1.3 cm,果肉厚0.2 cm,果实长羊角形,椒条顺直,果面较光滑,三心室,单果质量约26 g,果色绿色,肉质脆,味辣,适合鲜食。抗疮痂病、中抗黄瓜花叶病毒病(CMV),挂果性好,产量高;适合长江流域地区春夏季露地和高山栽培。  相似文献   
2.
Despite the interest of meagre (Argyrosomus regius) as a fast‐growing candidate for Mediterranean aquaculture diversification, there is a lack of information on nutrition along larval development. Importance of highly unsaturated fatty acids (HUFA) and the antioxidant vitamins E and vitamin C has not been investigated yet in this species. Six diets with two levels of HUFA (0.4% and 3% dw), two of vitamin E (1500 and 3000 mg kg?1) and two of vitamin C (1800 and 3600 mg kg?1) were fed to 15 dah meagre larvae. Larval growth in total length and dry body weight was significantly lowest in larvae fed diet 0.4/150/180 and showed few lipid droplets in enterocytes and hepatocytes and lower HUFA contents than the initial larvae. Increase in dietary HUFA up to 3%, significantly improved larval growth and lipid absorption and deposition. Besides, among fish fed 3% HUFA, increase in vitamin E and vitamin C significantly improved body weight, as well as total lipid, 22:6n‐3 and n‐3 fatty acids contents in the larvae. Thus, the results showed that 0.4% dietary HUFA is not enough to cover the essential fatty acid requirements of larval meagre and a high HUFA requirement in weaning diets is foreseen for this species. Besides, the results also pointed out the importance of dietary vitamin E and C to protect these essential fatty acids from oxidation, increase their contents in the larvae and promote growth, suggesting high vitamin E and C requirements in meagre larvae (higher than 1500 and 1800 mg kg?1 for vitamin E and vitamin C respectively).  相似文献   
3.
多酚类化合物是一种大量存在于植物体内的物质,因其具有优良的抗氧化、抗癌、抗衰老等 性能而受到研究者们的广泛关注。多项研究表明具有抗氧化性能的物质之间大多具有协同作用,两 种或两种以上的物质化合,其抗氧化能力往往强于单一物质的抗氧化能力。近年来,许多研究者将目 光聚焦到多酚类物质与小分子物质化合所具有的协同抗氧化能力的研究上并做出多项试验。本文论 述了多酚类化合物的抗氧化机理及协同抗氧化机理,并以茶多酚与维生素C、维生素E 化合为例对协 同抗氧化进行论述。  相似文献   
4.
The determination of tree age is an important issue for urban green planning, forestry and dendrology; finding non-destructive and quasi-non-destructive methods for this purpose is of great theoretical and practical importance. The resistance drilling method is quasi-non-destructive because the average diameter of an opening that remains after drilling does not exceed 3 mm. Do electrically recording resistance drills allow precise assessment of tree age? The aim of this study is to assess the accuracy of determining the number of tree rings based on an examination of this special kind of drilling resistance profiles for three tree species, the pine Pinus sylvestris L., the oak Quercus robur L., and the birch Betula pendula ROTH. In 2015 and 2016, 15 pine trees, 15 oak trees, and 15 birch trees were randomly selected. For each studied tree, a measurement was conducted using the electrically recording resistance drill IML-Resi E400 with a flat-tipped 1.5/3 mm steel needle (research sample), and an increment core was taken (reference sample). The drill used was not a real Resistograph®. The analysis of the E400-profiles underestimated the number of tree rings; the mean bias error (MBE) values were –6.5, –2.5, and –6.0 years for pine, oak, and birch, respectively. The proportion of investigated trees with less than five years difference between the research and reference samples varied from 38.4 (birch) to 66.7 (oak) percent. The accuracy of tree age determination was lowest for birch and highest for oak. The binomial generalised linear model (GLM) revealed that the most accurate tree age assessments were obtained from tree rings wider than 2 mm. The measurements clearly showed that the electrically recording resistance drill IML-Resi E400 enables a quick, although approximate, tree age assessment. Future research should concentrate on electronically regulating and recording drills, providing a higher spatial and signal resolution, and a stronger correlation to wood density.  相似文献   
5.
许浒  潘研  宋铭忻 《中国畜牧兽医》2020,47(8):2561-2570
为了解中国猪瘟病毒(classical swine fever virus,CSFV)的分子流行病学及遗传变异情况,本研究应用RT-PCR方法对2018年采集自河南、河北、山东、黑龙江和辽宁5个省份的350份疑似CSFV感染的病料进行E2和NS5B基因扩增,并对PCR产物进行测序和序列分析。结果显示,350份样品中21份为CSFV阳性,共获得14株CSFV的E2基因序列和7株CSFV的部分NS5B基因序列。E2全基因、NS5B部分基因序列分析表明,21份阳性样品均属于近年来在中国流行的CSFV 2.1d亚型,且新发现的2.1d亚型CSFV与中国较早2.1d亚型CSFV毒株间同源性差异不大,新发现的2.1d亚型CSFV分离株在E2基因的6个氨基酸(R31、S34、W182、K205、K303、A331)上具有相同的分子特征,E2蛋白中15个位点上的半胱氨酸均未发生变异。韩国2.1d亚型CSFV在E2蛋白上具有3个独特的氨基酸(N97、K159、R205)特征,并且发现了韩国毒株YC11WB可能作为2.1b和2.1d亚型CSFV过渡毒株的证据,流行于中国和韩国的2.1d亚型CSFV可能分别来自于本国早期2.1b亚型CSFV的衍化。本研究证实,2018年中国及周边国家CSFV较为活跃,且流行毒株依然以2.1d亚型为主,为中国科学防控CSFV提供了依据。  相似文献   
6.
为探究高效氯氰菊酯 (beta-cypermethrin, β-CP) 对雌性小鼠卵巢生殖功能的影响及维生素E (vitamin E, VE) 的干预作用,将雌性昆白小鼠随机分为6 组:空白对照组 (花生油处理)、β-CP不同剂量 (10、20、40 mg/kg) 处理组、VE保护组 (20 mg/kg β-CP+20 mg/kg VE) 和VE组 (20 mg/kg VE),连续灌胃10 d。灌胃结束后取小鼠卵巢组织,观察组织结构的病理变化,采用免疫组化法、蛋白免疫印迹试验及RT-PCR方法检测卵巢中StAR蛋白含量及casp-3、casp-8、INHα和INHβB 基因mRNA表达的变化。结果显示:与对照相比,10、20和40 mg/kg的β-CP处理均使小鼠卵巢组织结构发生了损伤,使组织中StAR蛋白的浓度分别降低了18.8%、36.3%和40.3%,casp-3基因的表达分别升高了16.0%、26.7%和52.9%,INHα基因的表达分别升高了34.5%、83.6%和228.7%,INHβB基因的表达分别升高了7.5%、39.2%和52.7%;20和40 mg/kg的β-CP处理使得casp-8基因的表达分别升高了27.1%和36.7%。上述处理组与对照组的差异均达显著水平 (P<0.05)。与 20 mg/kg β-CP处理组相比,VE 保护组的StAR蛋白含量也显著增多 (P<0.05)。研究表明,β-CP对小鼠卵巢具有毒性作用,这与β-CP抑制StAR蛋白的合成,上调casp-3、casp-8、INHα及INHβB基因的表达有关;添加VE对小鼠卵巢有一定的保护作用,这与VE可减弱β-CP对StAR蛋白合成的抑制作用有关。  相似文献   
7.
为明确施氮量和采收时期对油蔬两用型油菜薹维生素含量的影响,以富硒1号为材料,采用裂区试验设计,设置3个施氮量(120、180和240 kg·hm-2)和4个采收时期(以株高20、30、40和50 cm表示),分析了油菜薹维生素C、维生素E、维生素B1和维生素B6含量的变化;比较了半乳糖内酯脱氢酶(GalLDH)、抗坏血酸氧化酶(AAO)、抗坏血酸过氧化物酶(APX)、脱氢抗坏血酸还原酶(DHAR)和单脱氢抗坏血酸还原酶(MDHAR)活性,并分析了维生素家族基因在菜薹中的表达量。结果表明,随着施氮量的增加,油菜薹还原型维生素C(AsA)含量降低;氧化型维生素C(DHA)含量在株高20 cm采收时增加,但在30 cm及以上株高采收时则降低;维生素E和维生素B6含量先增加后降低;维生素B1含量增加;维生素C合成关键酶GalLDH活性先略增加后降低;AAO和DHAR活性增加,APX和MDHAR活性则整体表现为先增加后降低。且施氮量增加后,维生素家族各基因在株高30和40 cm采收的菜薹中主要表现为正向调控,而在株高20和50 cm采收的菜薹中主要表现为负向调控。就采收时期而言,菜薹中的维生素含量在株高30~40 cm时高于其他时期;在同一施氮量下,随着菜薹采收株高的增加,维生素家族基因的表达量主要呈上调模式。综合分析可知,GalLDH活性的降低以及AAO/DHAR和APX/MDHAR代谢平衡是增施氮肥导致菜薹维生素C含量降低的重要原因。本研究结果为油蔬两用型油菜薹优质栽培技术提供了理论依据。  相似文献   
8.
9.
Wildlife protection and management are important priorities for landscape identity and biodiversity preservation. Feeding practices of fauna confined in facilities during temporary captivity are fundamental to support animal health and natural behavior. Appropriate provision of feedstuffs appears to be necessary to support the best practices in respect of animal species-specific natural diet. This investigation explored the variation of the metabolic profile by means of selected metabolite and respective circulating levels in a group feral Giara horses undergoing the change of the diet, moving from natural free grazing in the wild to temporary captivity. Six Giara horses (4 mares and 2 stallions; estimated age: 2.5–3 years; body weight: 163–170 kg) were captured to monitor the serological reaction to equine infectious anemia (EIA; screening at Coggins test). Animals were sheltered in a wildlife rescue center for a duration of 4 weeks, and all received the same hay-based diet (ad libitum). On 0 and 28 days of captivity, blood serum alpha-tocopherol (α-TOH) concentration was determined alongside selected metabolites (liver enzymes, total protein and fractions, cholesterol, triglycerides, and macrominerals and trace elements). Comparative feces quality and composition were also assessed. Both serum samples (0 vs. 28 days) displayed α-TOH levels below (<2 μg/mL) adequacy established for the domestic horse. Initial levels markedly (P = .020) decreased after the 4 weeks of captivity (Δ = −32.5%). Vitamin E status and ALT levels varied significantly, but serum protein fractions did not point to significant variations before and after captivity. All horses tested negative to EIA. Monitoring of vitamin E status of wild and feral herbivores may be recommendable in the context of adequate feeding practices during captivity to prevent potential deficiency or excessive depletion.  相似文献   
10.
There has always been an interest in devising breeding programs for designer foods that would benefit both the producer and consumer. The challenge today is transformation of agriculture from “subsistence farming” to “market and income generation oriented” production system for which sorghum with its diverse end uses can assume significant role. Breeding for end-use identity-specific genotypes is needed for increased profitability to the farmers. In the present study, 60 sorghum genotypes were evaluated over two years to identify genotypes suitable for semolina recovery and popping properties, i.e. popping efficiency and pop volume expansion. Semolina recovery ranged from 20.7% to 48.3%, while popping efficiency ranged from 0 to 77.5%. Semolina recovery had positive and significant association with endosperm texture (r = 0.62), grain density (r = 0.49) and grain hardness (r = 0.55) indicating that genotypes with corneous endosperm yield high semolina. Also, semolina recovery had significant positive correlation with popping efficiency (r = 0.49) indicating that genotypes suitable for semolina can also be used for popping. Genetic divergence studies indicated that out of three clusters formed, cluster II having guinea race germplasm lines are suitable for semolina and popping. The information generated and the genotypes identified will help in enhancing the demand for sorghum as an industrial crop.  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