全文获取类型
收费全文 | 1435篇 |
免费 | 74篇 |
专业分类
林业 | 156篇 |
农学 | 70篇 |
基础科学 | 12篇 |
357篇 | |
综合类 | 46篇 |
农作物 | 300篇 |
水产渔业 | 223篇 |
畜牧兽医 | 244篇 |
园艺 | 19篇 |
植物保护 | 82篇 |
出版年
2023年 | 14篇 |
2022年 | 53篇 |
2021年 | 65篇 |
2020年 | 69篇 |
2019年 | 77篇 |
2018年 | 101篇 |
2017年 | 112篇 |
2016年 | 102篇 |
2015年 | 50篇 |
2014年 | 85篇 |
2013年 | 168篇 |
2012年 | 101篇 |
2011年 | 103篇 |
2010年 | 85篇 |
2009年 | 63篇 |
2008年 | 89篇 |
2007年 | 54篇 |
2006年 | 30篇 |
2005年 | 15篇 |
2004年 | 12篇 |
2003年 | 12篇 |
2002年 | 7篇 |
2001年 | 3篇 |
2000年 | 10篇 |
1998年 | 1篇 |
1997年 | 2篇 |
1996年 | 6篇 |
1995年 | 2篇 |
1994年 | 3篇 |
1993年 | 3篇 |
1990年 | 1篇 |
1989年 | 3篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1972年 | 1篇 |
1939年 | 1篇 |
排序方式: 共有1509条查询结果,搜索用时 15 毫秒
991.
In the present study the antigenotoxic effect of apigenin was studied against a genotoxic dose of ethinylestradiol using the damage parameters of chromosomal aberrations (CAs), sister chromatid exchanges (SCEs) and cell cycle kinetics (CCK). Human peripheral blood lymphocytes were cultured and treated with 10 μM of ethinylestradiol along with doses of 5, 10, 15 and 20 μM of apigenin. A clear decrease in the genotoxic damage induced by ethinylestradiol was observed with increasing doses of apigenin, suggesting a protective role for apigenin during ethinylestradiol therapy. 相似文献
992.
在过去的几十年里,遥感与地理信息系统融合技术应用于土壤侵蚀评价的研究得到了显著的发展。本研究采用遥感资料、土壤和土地调查数据、数字地面模型和气象观测数据,利用地理信息系统技术来分别产生通用土壤流失方程(USLE)的各个参数,从而来评价水土流失量和空间分布状况。 相似文献
993.
994.
995.
Asgari Q Sarnevesht J Kalantari M Sadat SJ Motazedian MH Sarkari B 《Tropical animal health and production》2011,43(2):389-392
Toxoplasmosis is a widespread zoonotic disease that causes significant morbidity and mortality in human fetus and in immunocompromised
patients. Moreover, it becomes a major cause of abortion in sheep and goats. Since consumption of meat of infected lamb and
goat is considered as the main sources of human infection in Iran, this study was undertaken to determine the prevalence of
Toxoplasma infection in edible tissues of sheep and goats in Shiraz in 2008. Samples of brain, tongue, liver, and muscles of neck, intercostals,
and femoral were taken from 56 sheep and 22 goats and tested by PCR. The total prevalence of Toxoplasma infection among animals was found to be 33.3%. Five out of 22 goats (22.7%) and 21 out of 56 sheep (37.5%) were infected
by Toxoplasma. Differences between the prevalence rate of infection among females (nine out of 14 = 46%) and males animals (12 out of 45
= 29.5%) was significant (P = 0.013). Furthermore, a positive correlation was found between the age of animals and the rate of infection; animals greater
than 2 years old showed a higher rate of infection (47%) in comparison with those less than 2 years old (25%, P = 0.04). The highest infected tissue was tongue (21.8%) followed by brain (19.2%) and femoral and intercostal muscles (17.9%).
This study demonstrated a high level of Toxoplasma infection in slaughtered animals in Shiraz and these should be considered as the main sources of infection for human population
in the region. 相似文献
996.
Shirzad MR Seifi S Gheisari HR Hachesoo BA Habibi H Bujmehrani H 《Tropical animal health and production》2011,43(8):1601-1604
Coccidiosis is a disease of almost universal importance in Poultry production. The subclinical form of coccidiosis has most
significant economic impact due to impaired growth rate and feed conversion. Some factors such as: age, size of flock, season,
etc., may effect on severity of this disease. In this study, the relationships between some major risk factors and prevalence
of subclinical coccidiosis were investigated. This study was done in 120 broiler farms in five different cities of Mazandaran
province, north of Iran. Five chicks (3–8 weeks of ages) were taken randomly from every 120 farms and post-mortem and parasitological
examinations were performed. Five Eimeria spp. were recognized: Eimeria tenella, Eimeria maxima, Eimeria acervulina, Eimeria brunetti, and Eimeria necatrix. The prevalence rate of subclinical coccidiosis among them is 75% (90 farms out of 120). E. acervulina was the most prevalent species (65.5%) followed by E. maxima (17.7%), E. tenella (15.5%), E. brunetti (10%), and E. necatrix (5.5%). According to what the results approve, the occurrence of subclinical coccidiosis is significantly related to the
age and size of flock, whereas the other factors such as the season of year, industrial strains, chicken’s keeping system,
and anticoccidial drugs do not affect this phenomenon remarkably. 相似文献
997.
Freeman L Rahmani EY Burgess RC Al-Haddad M Selzer DJ Sherman S Constable P 《Veterinary surgery : VS》2011,40(2):140-150
Objective: Study the learning curve for canine Natural Orifice Transluminal Endoscopic Surgery (NOTES) ovariectomy by evaluating operative times and complications. Study Design: Preclinical research study. Animals: Adult female dogs (n=20). Methods: NOTES ovariectomy procedures were performed as follows: Feasibility Group 1 (n=5), Feasibility Group 2 (n=5), and Early Clinical Group 3 (n=10). Six steps of the procedure were identified, timed separately, and the overall time was recorded from introduction to removal of the endoscope. Complications were recorded. Repeated measures analysis of variance using ranked data compared the effect of group (3 levels) on the time for each step. Nonlinear regression using an exponential model with nonzero asymptote was used to model the operative time–procedure number relationship. Results: Overall median operative time was significantly longer for Group 1 (195 minutes; range, 160–265 minutes) than Group 2 (108 minutes; range, 81–148 minutes; P=.048) and Group 3 (77 minutes; range, 41–136 minutes; P=.0008). The estimated asymptotic operative time was 71 minutes (95% confidence interval, 41–100 minutes); this time was reached after 10 procedures. Gastric cleansing and removing the left ovary took significantly longer in Group 1 than in Group 2 or 3. Operative complications included incomplete ovarian excision, dropping an ovary during retrieval, and conversion to an open procedure. No intraoperative complications occurred in Group 3. Conclusion: NOTES procedures result in longer operative times in the early part of the learning curve and require considerable experience before reaching proficiency. 相似文献
998.
Mohammad Mofizur Rahman Jahangir Dries Roobroeck Oswald Van Cleemput Pascal Boeckx 《Biology and Fertility of Soils》2011,47(7):753-766
Nitrous oxide (N2O) emissions, soil microbial community structure, bulk density, total pore volume, total C and N, aggregate mean weight diameter
and stability index were determined in arable soils under three different types of tillage: reduced tillage (RT), no tillage
(NT) and conventional tillage (CT). Thirty intact soil cores, each in a 25 × 25-m2 grid, were collected to a depth of 10 cm at the seedling stage of winter wheat in February 2008 from Maulde (50°3′ N, 3°43′ W),
Belgium. Two additional soil samples adjacent to each soil core were taken to measure the spatial variance in biotic and physicochemical
conditions. The microbial community structure was evaluated by means of phospholipid fatty acids analysis. Soil cores were
amended with 15 kg NO3−-N ha−1, 15 kg NH4+-N ha−1 and 30 kg ha−1 urea-N ha−1 and then brought to 65% water-filled pore space and incubated for 21 days at 15°C, with regular monitoring of N2O emissions. The N2O fluxes showed a log-normal distribution with mean coefficients of variance (CV) of 122%, 78% and 90% in RT, NT and CT, respectively,
indicating a high spatial variation. However, this variability of N2O emissions did not show plot scale spatial dependence. The N2O emissions from RT were higher (p < 0.01) than from CT and NT. Multivariate analysis of soil properties showed that PC1 of principal component analysis had
highest loadings for aggregate mean weight diameter, total C and fungi/bacteria ratio. Stepwise multiple regression based
on soil properties explained 72% (p < 0.01) of the variance of N2O emissions. Spatial distributions of soil properties controlling N2O emissions were different in three different tillages with CV ranked as RT > CT > NT. 相似文献
999.
Mariano M. Amoroso Lori D. Daniels Mohammad BatainehDavid W. Andison 《Forest Ecology and Management》2011,262(12):2240-2249
This study presents new evidence of historic low-to-moderate-severity fires, intermixed with high-severity fires, in the foothills of the Rocky Mountains of west-central Alberta, Canada. High-severity fires that burned 120-300 years ago initiated even-aged cohorts of fast-growing lodgepole pine at each of the six study sites. Evidence of subsequent, low-to-moderate-severity fires included single and double fire scars on thin-barked lodgepole pine that were as small as 3.6 cm in diameter at the time of the fire, but survived. These low-to-moderate-severity fires resulted in structurally complex stands with a broad range of tree diameters and multiple cohorts of lodgepole pine, white and black spruce, and subalpine fir. At the site level, fire return intervals were variable, ranging from 29 to 167 years, but most were <80 years. Of the 9 years in which we documented low-to-moderate-severity fires, only the fires in 1889 and 1915 scarred trees at more than one site, indicating that these fires were small and had local effects. The new knowledge of historical, low-to-moderate-severity fires provided by this study has important implications for ecologically sustainable forest management. Although we recognize that further research needs to determine the extent of low-to-moderate-severity fires at the landscape scale, our results clearly indicate that a mixed-severity fires occurred at least locally. A broader range of silvicultural systems than is currently practiced would be consistent with historic forest dynamics. 相似文献
1000.
Two new bergenin derivatives, named rivebergenin A and B (1, and 2), together with two known compounds, bergenin (3) and norbergenin (4) were isolated from the stem of Rivea hypocrateriformis (Desr.) Choisy. The structures of the new compound have been assigned from 1H and 13C NMR spectra, DEPT, and by 2D COSY, HMQC, and HMBC experiments. Compounds 1-4 showed the strong antioxidant activity. 相似文献