全文获取类型
收费全文 | 95720篇 |
免费 | 5757篇 |
国内免费 | 54篇 |
专业分类
林业 | 3940篇 |
农学 | 3761篇 |
基础科学 | 716篇 |
12046篇 | |
综合类 | 13858篇 |
农作物 | 3624篇 |
水产渔业 | 5141篇 |
畜牧兽医 | 50751篇 |
园艺 | 1270篇 |
植物保护 | 6424篇 |
出版年
2020年 | 810篇 |
2019年 | 1036篇 |
2018年 | 1671篇 |
2017年 | 1941篇 |
2016年 | 1770篇 |
2015年 | 1483篇 |
2014年 | 1833篇 |
2013年 | 4124篇 |
2012年 | 3202篇 |
2011年 | 3909篇 |
2010年 | 2510篇 |
2009年 | 2490篇 |
2008年 | 3768篇 |
2007年 | 3498篇 |
2006年 | 3388篇 |
2005年 | 2910篇 |
2004年 | 2835篇 |
2003年 | 2830篇 |
2002年 | 2584篇 |
2001年 | 3582篇 |
2000年 | 3478篇 |
1999年 | 2713篇 |
1998年 | 1025篇 |
1997年 | 1096篇 |
1996年 | 927篇 |
1995年 | 1073篇 |
1994年 | 926篇 |
1993年 | 935篇 |
1992年 | 1928篇 |
1991年 | 2019篇 |
1990年 | 1965篇 |
1989年 | 1959篇 |
1988年 | 1694篇 |
1987年 | 1667篇 |
1986年 | 1679篇 |
1985年 | 1591篇 |
1984年 | 1286篇 |
1983年 | 1070篇 |
1982年 | 743篇 |
1979年 | 1102篇 |
1978年 | 839篇 |
1975年 | 837篇 |
1974年 | 924篇 |
1973年 | 891篇 |
1972年 | 839篇 |
1971年 | 823篇 |
1970年 | 821篇 |
1969年 | 801篇 |
1968年 | 726篇 |
1967年 | 756篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
102.
103.
104.
The effects of canopy cover, seeding depth, and soil moisture on emergence of Centaurea maculosa Lam. and C. diffusa Lam. were studied. Canopy cover had no effect on ermergence rate (seedlings/day/100 seeds) of either species. Seeds of both species emerged faster when placed on the soil surface. Emergence rate decreased as seeding depth increased. Both species required more than 55% initial soil moisture to initiate emergence, with 65–70% being optimum. Percentage emergence followed exactly the same trend as emergence rate. C. maculosa had better emergence characteristics over a wider range of conditions than C. diffusa, possibly being some of the reasons for the former having a wider geographical distribution in the United States and Canada. 相似文献
105.
106.
Lipinsky ES 《Science (New York, N.Y.)》1978,199(4329):644-651
The development of fuels from biomass can lead naturally to dispersed facilities that incorporate food or materials production (or both) with fuel production, forming adaptive systems that can be modified to meet evolving needs and constraints. The technology that is appropriate to each system needs to be worked out, taking into account associated food and materials opportunities in order to decrease the ultimate cost of energy delivered to the consumer. I analyze possible systems based on sugarcane, corn, and guayule. 相似文献
107.
1. The α‐tocopherol (AT) content of abdominal fat and fat stability increased as a function of the duration of α‐tocopheryl acetate (ATA) feeding in broilers fed on diets containing either 40 g acidulated soyabean‐oil soapstock (ASS) or tallow/kg. A linear relationship was observed between AT content of the adipose tissue and its stability with both types of fat supplements.
2. The stabilities of carcass fat and meat of 9‐week‐old broilers fed on diets containing ASS with 300 mg ethoxyquin (EQ)/kg or diets containing tallow with 20 mg ATA/kg from weeks 5 to 9 were comparable with those obtained by feeding these antioxidants and fats throughout the growth period.
3. ATA added to a diet containing ASS, and EQ, added to a diet containing tallow did not improve meat stability despite such an effect on adipose tissue when fed for 9 weeks. The results emphasise the importance of using the dietary antioxidant appropriate to the fat supplement to improve carcass stability. 相似文献
108.
T S Baskett 《Pesticides monitoring journal》1975,9(2):67-78
Samples of breast muscle from 327 ducks collected from October 1970 to March 1971 in the conterminous United States were analyzed for total mercury by flameless atomic absorption spectrometry. Mercury levels for the entire collection ranged from less than 0.01 to 3.91 ppm wet weight with a median of 0.10 ppm. Twenty-five ducks had levels equalling or exceeding the 0.5 ppm guideline for fish and shellfish established by the Food and Drug Administration, U.S. Department of Health, Education, and Welfare. Dabbling ducks, which are shallow-water feeders and mostly vegetarian in fall, winter, and spring, usually had lower levels than diving and sea ducks. Levels were generally higher in ducks collected in areas where environmental mercury levels were known to be greater than in ducks from nonsuspect areas. Despite the mobility of the ducks, levels seemed more closely linked to local environmental contamination than to various factors associated with large geographic areas. 相似文献
109.
Sunlight is known to reduce the incidence of blister blight disease of tea, caused by Exobasidium vexans . The effects of the ultraviolet-B (UV-B: 290–320 nm) component of solar radiation on the disease and the development of E. vexans were studied in the field in Sri Lanka using UV-screening filter materials held over a commercial crop. Exclusion of UV-B radiation by Polyester, which reduces fluxes in the UV-B region by approximately 75–85%, increased both the number of translucent spots (immature sites of infection) and number of sporulating blisters (mature sites of infection), but it had little or no effect on the sporulation of the pathogen. When basidiospores were artificially inoculated on leaves, and were exposed to full or filtered solar radiation, their survival and germination increased when UV-B wavelengths were removed; significant differences were found 62 h after inoculation. However, UV-B did not affect the extension of germ tubes. This study indicates that the UV-B component of solar radiation plays an important role in the natural regulation of blister blight disease in the field. 相似文献
110.
Muneera S. M. Al-Saleem Wafaa H. B. Hassan Zeinab I. El Sayed Mahmoud M. Abdel-Aal Wael M. Abdel-Mageed Eman Abdelsalam Sahar Abdelaziz 《Marine drugs》2022,20(5)
Marine sponge-derived endozoic fungi have been gaining increasing importance as promising sources of numerous and unique bioactive compounds. This study investigates the phytochemical profile and biological activities of the ethyl acetate extract of Penicillium chrysogenum derived from Cliona sp. sponge. Thirty-six compounds were tentatively identified from P. chrysogenum ethyl acetate extract along with the kojic acid (KA) isolation. The UPLC-ESI-MS/MS positive ionization mode was used to analyze and identify the extract constituents while 1D and 2D NMR spectroscopy were used for kojic acid (KA) structure confirmation. The antimicrobial, antioxidant, and cytotoxic activities were assessed in vitro. Both the extract and kojic acid showed potent antibacterial activity against Staphylococcus aureus and Pseudomonas aeruginosa with MIC 250 ± 0.82 µg/mL. Interestingly, the extract showed strong antifungal activity against Candida albicans and Cryptococcus neoformans with MIC 93.75 ± 0.55 and 19.53 ± 0.48 µg/mL, respectively. Furthermore, KA showed the same potency against Fusarium oxysporum and Cryptococcus neoformans with MIC 39.06 ± 0.85 and 39.06 ± 0.98 µg/mL, respectively. Ultimately, KA showed strong antioxidant activity with IC50 33.7 ± 0.8 µg/mL. Moreover, the extract and KA showed strong cytotoxic activity against colon carcinoma (with IC50 22.6 ± 0.8 and 23.4 ± 1.4 µg/mL, respectively) and human larynx carcinoma (with equal IC50 30.8 ± 1.3 and ± 2.1 µg/mL, respectively), respectively. The current study represents the first insights into the phytochemical profile and biological properties of P. chrysoenum ethyl acetate extract, which could be a promising source of valuable secondary metabolites with potent biological potentials. 相似文献