全文获取类型
收费全文 | 299476篇 |
免费 | 16595篇 |
国内免费 | 703篇 |
专业分类
林业 | 24419篇 |
农学 | 14180篇 |
基础科学 | 2993篇 |
46276篇 | |
综合类 | 37487篇 |
农作物 | 17641篇 |
水产渔业 | 17940篇 |
畜牧兽医 | 124091篇 |
园艺 | 6450篇 |
植物保护 | 25297篇 |
出版年
2020年 | 2940篇 |
2019年 | 3765篇 |
2018年 | 4425篇 |
2017年 | 4879篇 |
2016年 | 5035篇 |
2015年 | 4449篇 |
2014年 | 5804篇 |
2013年 | 16403篇 |
2012年 | 7797篇 |
2011年 | 10107篇 |
2010年 | 8062篇 |
2009年 | 8546篇 |
2008年 | 9450篇 |
2007年 | 8641篇 |
2006年 | 8873篇 |
2005年 | 8241篇 |
2004年 | 7983篇 |
2003年 | 7776篇 |
2002年 | 7005篇 |
2001年 | 7711篇 |
2000年 | 7451篇 |
1999年 | 6619篇 |
1998年 | 4307篇 |
1997年 | 4353篇 |
1996年 | 4066篇 |
1995年 | 4755篇 |
1994年 | 4123篇 |
1993年 | 3822篇 |
1992年 | 5202篇 |
1991年 | 5414篇 |
1990年 | 5151篇 |
1989年 | 5116篇 |
1988年 | 4559篇 |
1987年 | 4652篇 |
1986年 | 4654篇 |
1985年 | 4821篇 |
1984年 | 4345篇 |
1983年 | 3983篇 |
1982年 | 3160篇 |
1981年 | 3020篇 |
1980年 | 2988篇 |
1979年 | 3862篇 |
1978年 | 3368篇 |
1977年 | 3051篇 |
1976年 | 2932篇 |
1975年 | 2898篇 |
1974年 | 3203篇 |
1973年 | 3141篇 |
1972年 | 2861篇 |
1971年 | 2825篇 |
排序方式: 共有10000条查询结果,搜索用时 312 毫秒
991.
992.
Annual sex hormonal profiles, gonad development and age determination of 18 (13–14 kg body weight) and three (145–226 kg body weight) Mekong giant catfish (MGC) (Pangasianodon gigas, Chevey) reared in earthen ponds in Chiang Mai and from the Mekong River in Chiang Rai, Thailand, respectively, were investigated. The hormonal profiles were determined from blood samples of the fish by electrochemiluminescence immunoassay during May to August in 2000. The highest testosterone levels of 0.06 ng mL?1 in both sexes and the highest oestradiol of 47.8 and 14.23 pg mL?1 in females and males, respectively, were observed in May. The gonadosomatic index was found to be 0.07 for males and 0.38 for females from ponds in comparison with 2.27 for males and 8.29 for females from the Mekong River. Higher development stages of spermatocytes and oocytes of the cultured fish in May than in February and November were demonstrated. No mature germ cells were obtained from either the males or females, indicating no sexual maturity. The average age of fish was determined from the annual rings of the cross‐section of the pectoral fins by a stereomicroscope. The average age of fish in earthen pond determined from the annual ring was 8 years, which agreed with the actual culture records, while those from the Mekong River were 21 years. This information will be beneficial for the breeding programmes and conservation of the MGC. 相似文献
993.
994.
Field experiments were conducted to characterize the demography of Abutilon theophrasti and Setaria faberi in a conventionally managed 2‐year (maize/soya bean) rotation, and in 3‐year (maize/soya bean/triticale + red clover) and 4‐year (maize/soya bean/triticale + lucerne/lucerne) rotations managed with 72% and 79% lower herbicide inputs respectively. Rates of weed seedling recruitment, seedling survival and adult plant fecundity were determined for populations in each phase of each rotation and used to calculate annual rates of weed population change, Δ. In both years of the study, Δ for A. theophrasti populations declined or remained stable in all three rotation systems. Despite greater rates of seedling survival and fecundity in maize and soya bean in the 3‐ and 4‐year rotations, increases in Δ for A. theophrasti populations were prevented in these systems because of low fecundity in triticale and low seedling survival and fecundity in lucerne. For Setaria faberi populations, Δ remained stable in the 2‐year rotation, increased in the 3‐year rotation in both years, and increased in the 4‐year rotation in 1 year. The results of this study indicate that when herbicide use is reduced, rotations that include triticale and lucerne can facilitate the suppression of A. theophrasti. Rotations that include lucerne can contribute to restraining S. faberi population growth, given adequate levels of seedling mortality in this crop. 相似文献
995.
A. Wjtowicz 《EPPO Bulletin》2003,33(3):515-516
A computer system Agrogis has been developed to record information on pest infestation of plants in different regions of Poland. Data can be entered manually or from disks, and can be displayed in the form of charts and maps. Agrogis will be made available on the Internet. 相似文献
996.
糖类寄生螨快速分离检验方法 总被引:1,自引:0,他引:1
本文对糖类寄生螨的快速分离检验方法进行了改进。将糖类溶解后,经过500目/英寸标准检验筛进行过筛分离。此方法对各种可溶性糖类中的寄生螨类能取得100%的快速分离检验结果。 相似文献
997.
Enzyme Immunoassays for the Determination of Ovine LH and FSH 总被引:2,自引:0,他引:2
GM Peclaris A Pappa K Deligiannis K Koutsotolis 《Reproduction in domestic animals》2003,38(5):367-372
The development of competitive enzyme immunoassays for ovine plasma LH (oLH) and FSH (oFSH) is described. Standards and plasma samples were preincubated with diluted antiserum to oLH or oFSH and the reacted solution (100 μl per well) was transferred to plates previously coated with oLH or oFSH, respectively. The second antibody used was anti‐rabbit IgG horseradish peroxidase. The measuring range was 0.39–50 ng/ml for each hormone and the 50% relative binding sensitivity was 9 ng/ml for oLH. The respective value for oFSH was 3.5 or 34 ng/ml with different hormone and antibody preparations used for the assay. The enzyme immunoassays were used to determine oLH and oFSH levels in plasma from ewes of two breeds during the oestrous cycle. The assays detected the first FSH surge coincident with the LH surge, the second FSH surge about 24 h later and the periodic fluctuations of FSH concentrations during the luteal phase of the oestrous cycle. These enzyme immunoassays are an efficient and economic alternative to the established radioimmunoassays (RIA) for oLH and oFSH. 相似文献
998.
999.
浙江淳安新安江开发公司封山育林效益研究 总被引:2,自引:0,他引:2
浙江省淳安县新安江开发公司所属16个国营林场22475.53hm~3荒山迹地,封山育林30年,现有天然次生林总蓄积量135万m~3,年总生长率8.07%,活立木直接经济价值2.25亿元。到1987年止公司已获中间利用净收入2135万元。近20年累计天然次生林比无林地多涵蓄水10亿m~3,每年进入水库的泥沙量减少80%,对调节水库水源、延长发电年限、纯洁水质都起到良好作用。公司长期实施的5项管护技术,把封山育林提高到科学经营水平,在千岛湖区成功地建立了森林生态屏障。 相似文献
1000.