全文获取类型
收费全文 | 8175篇 |
免费 | 268篇 |
国内免费 | 738篇 |
专业分类
林业 | 290篇 |
农学 | 1044篇 |
基础科学 | 532篇 |
463篇 | |
综合类 | 3195篇 |
农作物 | 1610篇 |
水产渔业 | 319篇 |
畜牧兽医 | 367篇 |
园艺 | 1269篇 |
植物保护 | 92篇 |
出版年
2024年 | 17篇 |
2023年 | 60篇 |
2022年 | 135篇 |
2021年 | 165篇 |
2020年 | 249篇 |
2019年 | 193篇 |
2018年 | 154篇 |
2017年 | 278篇 |
2016年 | 379篇 |
2015年 | 302篇 |
2014年 | 452篇 |
2013年 | 322篇 |
2012年 | 540篇 |
2011年 | 536篇 |
2010年 | 447篇 |
2009年 | 437篇 |
2008年 | 457篇 |
2007年 | 552篇 |
2006年 | 463篇 |
2005年 | 443篇 |
2004年 | 320篇 |
2003年 | 305篇 |
2002年 | 259篇 |
2001年 | 214篇 |
2000年 | 201篇 |
1999年 | 174篇 |
1998年 | 171篇 |
1997年 | 164篇 |
1996年 | 137篇 |
1995年 | 128篇 |
1994年 | 72篇 |
1993年 | 126篇 |
1992年 | 53篇 |
1991年 | 61篇 |
1990年 | 57篇 |
1989年 | 47篇 |
1988年 | 27篇 |
1987年 | 24篇 |
1986年 | 9篇 |
1985年 | 9篇 |
1984年 | 4篇 |
1982年 | 2篇 |
1981年 | 14篇 |
1980年 | 5篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1976年 | 9篇 |
1963年 | 1篇 |
1955年 | 1篇 |
排序方式: 共有9181条查询结果,搜索用时 0 毫秒
71.
72.
73.
依据场内巡视电动车的使用条件并按照法规对电动车的总体设计研究,完成悬架、驱动电机布置、转向系、制动系的选型和设计。 相似文献
74.
传统的防病毒技术难以抵御蠕虫的传播,为此本文提出实时混合对抗技术,该技术在被对抗蠕虫感染的主机与易感染主机数量相等时转换对抗策略,并建立实时混合对抗蠕虫的传播模型.最后,通过仿真实验从对抗有效性、资源消耗和应对突发事件能力三个方面验证实时混合对抗技术的性能.理论分析和实验结果表明,实时混合对抗技术能更好地满足对抗有效性和低资源消耗要求,并能应对突发事件. 相似文献
75.
Sorghum [Sorghum bicolor (L.) Moench] is a drought‐tolerant crop, and its productivity in rain fed environments has increased since the 1950s. This increase is due to changes in agronomic practices and hybrid improvement. The objective of this study was to determine what aspects of grain sorghum morphology, physiology and water use have changed with hybrid improvement and might have contributed to this yield increase. A 2‐year greenhouse experiment was conducted with one hybrid from each of the past five decades. The hybrids were studied in well‐watered and pre‐ and post‐flowering water deficit conditions. Total water use, transpiration, stomatal conductance and photosynthesis were measured during the growing period. Biomass and biomass components were measured at harvest. There was no consistent change in the leaf physiological parameters resulting from hybrid advancement. In contrast, total water use increased in rate of 8.5 cm3 kg soil?1 year?1 from old to new hybrids in the well‐watered treatments. Root biomass also increased in rate of 0.2 g plant?1 year?1. Leaf biomass and panicle length also was greater for the newest compared with the older hybrids. Hybrid advancement was related to increase in panicle length but decrease in peduncle length. Results indicated that hybrid development programmes created hybrids with improved drought avoidance, due to better root density of newly released hybrids, or hybrids with better resource use which ultimately increased yield under rain fed conditions. 相似文献
76.
农村电力排灌站电气设备主要是利用电力进行农业用水资源调配的水利设施。笔者根据自身工作经验,主要对农村电力排灌站电气设备最优配置进行了一番分析与研究。 相似文献
77.
Genetic divergence in two tropical maize composites after four cycles of reciprocal recurrent selection 下载免费PDF全文
Adesike O. Kolawole Abebe Menkir Melaku Gedil Essie Blay Kwadwo Ofori Jennifer G. Kling 《Plant Breeding》2017,136(1):41-49
Two tropical maize composites were subjected to four cycles of reciprocal recurrent selection to develop divergent inbred lines with good combining ability. This study was conducted to examine the extent of genetic diversity, changes in allele composition and genetic structure, of 100 randomly selected S1 lines each from the original (C0) and advanced (C4) selection cycles of TZL COMP3 and TZL COMP4, genotyped using single nucleotide polymorphism (SNP) markers. Results revealed that the proportion of alleles at both low and high frequencies decreased from C0 to C4, whereas those at intermediate frequencies increased at C4 in the two composites. More unique and other alleles were lost at C4 in TZL COMP3 relative to those in TZL COMP4. The changes in different measures of genetic diversity were either small or negligible with selection in the two composites. The proportion of markers departing from Hardy–Weinberg equilibrium (HWE) decreased with selection, whereas the total number of pairs of markers in linkage disequilibrium increased with selection in the two composites. Examination of changes in population structures using a model‐based approach as well as cluster and multivariate analyses found a high degree of concordance in stratifying the 400 S1 lines into four non‐overlapping groups corresponding to the two selection cycles each within the reciprocal composites. The observed molecular‐based divergence between cycles within the same composite and the clear differentiation between the complementary composites highlight the importance of reciprocal recurrent selection for preserving genetic diversity for long‐term selection. This increases the potential of the advanced selection cycles to sustain genetic gain in productivity of hybrids adapted to the savannas in West and Central Africa. 相似文献
78.
79.
80.
吕天阳 《农业装备与车辆工程》2020,58(3):50-54,78
以并联式双能量源(锂电池组&超级电容)纯电动汽车为研究对象,提出一种改进的功率分配策略。这种功率分配策略在模糊控制的基础上增加深度神经网络训练过程,以获得更精确的功率分配因子。通过整车模型构建、功率分配模型构建、功率分配策略制定、仿真验证结果分析后得出结论:与电量消耗功率分配策略相比,这种改进的功率分配策略能优化锂电池组和超级电容二者之间的功率分流,限制锂电池组充放电的峰值电流,延长复合储能系统的工作寿命,提升动力系统的工作效率。 相似文献