全文获取类型
收费全文 | 8694篇 |
免费 | 756篇 |
国内免费 | 820篇 |
专业分类
林业 | 922篇 |
农学 | 727篇 |
基础科学 | 199篇 |
1137篇 | |
综合类 | 2995篇 |
农作物 | 565篇 |
水产渔业 | 1180篇 |
畜牧兽医 | 1196篇 |
园艺 | 228篇 |
植物保护 | 1121篇 |
出版年
2024年 | 35篇 |
2023年 | 169篇 |
2022年 | 242篇 |
2021年 | 311篇 |
2020年 | 351篇 |
2019年 | 367篇 |
2018年 | 302篇 |
2017年 | 405篇 |
2016年 | 407篇 |
2015年 | 389篇 |
2014年 | 430篇 |
2013年 | 519篇 |
2012年 | 598篇 |
2011年 | 630篇 |
2010年 | 487篇 |
2009年 | 495篇 |
2008年 | 466篇 |
2007年 | 484篇 |
2006年 | 409篇 |
2005年 | 336篇 |
2004年 | 274篇 |
2003年 | 260篇 |
2002年 | 210篇 |
2001年 | 190篇 |
2000年 | 190篇 |
1999年 | 151篇 |
1998年 | 140篇 |
1997年 | 126篇 |
1996年 | 140篇 |
1995年 | 113篇 |
1994年 | 116篇 |
1993年 | 104篇 |
1992年 | 93篇 |
1991年 | 60篇 |
1990年 | 87篇 |
1989年 | 48篇 |
1988年 | 33篇 |
1987年 | 31篇 |
1986年 | 29篇 |
1985年 | 7篇 |
1984年 | 5篇 |
1982年 | 5篇 |
1981年 | 6篇 |
1979年 | 3篇 |
1978年 | 5篇 |
1977年 | 5篇 |
1976年 | 2篇 |
1962年 | 1篇 |
1956年 | 1篇 |
1955年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Trout and charr, members of the salmonid family, have high conservation value but are also susceptible to anthropogenic threats in part due to the specificity of their habitat requirements. Understanding historical and future threats facing these species is necessary to promote their recovery. Of freshwater trout and charr in the Canadian Rocky Mountain region, westslope cutthroat trout (Oncorhynchus clarkii lewisi), bull trout (Salvelinus confluentus; a charr species) and Athabasca rainbow trout (Oncorhynchus mykiss) are of conservation concern. And indeed, range contractions and declining populations are evident throughout much of their ranges. Range contraction was most evident in the southern Alberta designatable unit (DU) of westslope cutthroat trout. Diminished populations were also evident in the downstream watersheds of the Alberta bull trout range, and throughout the Athabasca rainbow trout range. We assessed historical and future threats to evaluate the relative importance of individual threats to each DU and compare their impact among species. Individual threats fall into the broad categories of angling, non-native species and genes, habitat loss and alteration, and climate change. Severity of each threat varies by DU and reflects the interaction between species’ biology and the location of the DU. Severity of threats facing each DU has changed over time, reflecting extirpation of native populations, changes in management and industry best practices, expansion of non-native species and progressing climate change. The overall threat impact for each DU indicates a high probability of substantial and continuing declines and calls for immediate action. 相似文献
2.
3.
Shi-Zhi Wang Guang-Xin E Yan Zeng Yan-Guo Han Yong-Fu Huang Ri-Su Na 《Reproduction in domestic animals》2021,56(6):936-941
The aim of this study was to analyse the association between single-nucleotide polymorphisms within INHA and ACVR2B and litter size in Dazu black goats. In total, twenty-two SNPs were genotyped in 190 individuals by SNaPshot and resequencing. The results showed that three SNPs (SNP_1, SNP_12 and SNP_13 in this study) were detected to have significant additive genetic effect on the recorded goat litter size (p < .05). The SNP_1 (NC_030809.1), a non-synonymous substitution of G for T at chr2-g. 28314990 in the exon 2 of INHA gene (NM_001285606.1), resulted in homozygote 2 (HOM2) contributed 0.25 and heterozygote (HET) contributed 0.12 larger litter than homozygote 1 (HOM1). Meanwhile, SNP_12 (Chr22-g. 11721225 A > T) and SNP_13 (Chr22-g. 11721227 A > C) (NC_030829.1) simultaneously mutated at the first and third position of a triplet AAA (lysine, K) in the exon 4 of ACVR2B gene (XM_018066623.1) had estimated genetic effects of HOM1 (0.00) and HOM2 (0.03) larger than HET (−0.12). In conclusion, one SNPs (chr2-g. 28314990 T > G) within the exon 2 of INHA and two SNPs (Chr22-g. 11721225 A > T and Chr22-g. 11721227 A > C) i n the exon 4 of ACVR2B gene were highly recommended as candidate markers of litter size in Dazu black goats. A large-scale association study to assess the impact of these variants on litter size is still necessary. 相似文献
4.
杨红 《农业图书情报学刊》2004,15(1):80-82
对《四川农业科技》2 0 0 0年~ 2 0 0 2年读者群现状分析结果表明 ,影响读者群的因素有期刊自身原因和地域性原因等 ,扩大和稳定读者群关键在合理定位、栏目设计有特色、多渠道宣传等。 相似文献
5.
分散松香施胶剂性能影响因素的探讨 总被引:7,自引:0,他引:7
对逆转法制备分散松香施胶剂过程中的主要因素如乳化剂的种类及用量、马来松香加合物含量、搅拌转速、加料速度和乳化温度等系统地进行了探讨,提出了合理的工艺条件,并对实际生产中可能出现的问题及解决办法进行了阐述。 相似文献
6.
波尔山羊杂交后代体尺、体重与肉用性能的相关性研究 总被引:1,自引:0,他引:1
本文利用 SPSS数据处理软件 (10 .0版 )统计分析了 2 3只 (约 8~ 9月龄 )波尔山羊与唐山奶山羊杂交后代公羊的体尺、体重和肉用性能。结果表明 :(1)波尔山羊与唐山奶山羊杂交 F1 代体尺、体重与肉用性能指标的变异程度大于 F2 代 ,其中宰前重、胴体重、净肉重和胸宽四项指标变异程度在 F1 和 F2 代中都很大 ;(2 )杂交 F1 代、F2 代各项指标 t检验结果 ,除胸宽外其余指标差异显著或极显著 ;(3 )波尔山羊杂交后代中胸宽与各项指标间相关不显著 ,宰前重、胴体重、净肉重与体尺指标相关显著或不显著。 相似文献
7.
张彬 《湖南农业大学学报(自然科学版)》1990,(1)
用经改进的聚丙烯酰胺凝胶电泳铁染法,测定了二花脸猪血浆转铁蛋白多态类型,分析了其基因型频率和基因频率,并探讨了猪血浆转铁蛋白多态性与产仔性能和增重速度的相关性。 相似文献
8.
The population of male Kirtland's warbler (Dendroica kirtlandii) in the breeding season has averaged 206 from 1971 to 1987. The Kirtland's warbler occupies dense jack pine (Pinus banksiana) barrens from 5 to 23 years old and from 1.4 to 5.0 m high, formerly of wildfire origin. In 1984, 73% of the males censused were found in habitat naturally regenerated from wildfire or prescribed burning. The rest were in plantations (11%) or in harvested, unburned jack pine stands stocked by natural regeneration (16%). Twenty-two percent (630 of 2,886) of the Kirtland's warbler males counted in the annual censuses from 1971 through 1984 were found in 26 stands that were unburned and naturally regenerated following harvest. From 1982 to 1987, suitable regenerating areas were barely sufficient to replace currently occupied maturing stands, so population growth was impeded. Ecosystems of suitable size and regeneration characteristics (wildfire and plantation) doubled in area by 1989. In response, the population of Kirtland's warblers increased from 167 to 398 males between 1987 and 1992, but they withdrew almost entirely from the unburned, unplanted barrens by 1989 when the area of more suitable regeneration types increased. Minimum (368 males) and maximum (542 males) population estimates for 1996 were calculated based on 1984 average density (1.9 males per 40 ha) and peak population in burns (2.8 males per 40 ha). 相似文献
9.
M. HEDDELL-COWIE 《Fisheries Management and Ecology》2005,12(2):137-142
Abstract The body mass distributions and variations in abundance of the spring and autumn catches of Atlantic salmon, Salmo salar L., were compared between fisheries above and below the confluence of the rivers Tweed and Teviot, Scotland. Significant differences between the distributions were found over three periods, one in spring and two in autumn, when salmon catches were high. Correlations were found between abundance trends for periods when distributions were similar or dissimilar. Abundance of salmon returning to the River Teviot was proportional to the main river. However, its population structure differed when abundances were high, suggesting different survival strategies. It appears that changes in the numbers returning to the river were resultant of changes in the marine rather than freshwater environment. 相似文献
10.
Late blight, caused by Phytophthora infestans , is an ongoing threat to potato and tomato crop production worldwide and considerable fundamental and applied research is conducted with the long-term aim of improved disease control. Understanding the mechanisms, processes and rates of P. infestans evolution is an important factor in predicting the effectiveness and durability of new management practices. A range of phenotypic and genotypic tests has been applied to achieve this goal, but each has limitations and new methods are sought. Recent progress in P. infestans genomics is providing the raw data for such methods and new high-throughput codominant biomolecular markers are currently being developed that have tremendous potential in the study of P. infestans population biology, epidemiology, ecology, genetics and evolution. This paper reviews some key applications, recommends some changes in approach and reports on the status and potential of new and existing methods for probing P. infestans genetic diversity. 相似文献