全文获取类型
收费全文 | 133960篇 |
免费 | 7356篇 |
国内免费 | 94篇 |
专业分类
林业 | 5474篇 |
农学 | 4329篇 |
基础科学 | 837篇 |
17318篇 | |
综合类 | 21649篇 |
农作物 | 5505篇 |
水产渔业 | 6804篇 |
畜牧兽医 | 68520篇 |
园艺 | 1843篇 |
植物保护 | 9131篇 |
出版年
2020年 | 1209篇 |
2019年 | 1405篇 |
2018年 | 2059篇 |
2017年 | 2359篇 |
2016年 | 2126篇 |
2015年 | 1769篇 |
2014年 | 2264篇 |
2013年 | 5357篇 |
2012年 | 4180篇 |
2011年 | 5182篇 |
2010年 | 3289篇 |
2009年 | 3444篇 |
2008年 | 5048篇 |
2007年 | 4750篇 |
2006年 | 4438篇 |
2005年 | 4153篇 |
2004年 | 3928篇 |
2003年 | 4028篇 |
2002年 | 3602篇 |
2001年 | 4175篇 |
2000年 | 4278篇 |
1999年 | 3392篇 |
1998年 | 1492篇 |
1997年 | 1403篇 |
1996年 | 1301篇 |
1995年 | 1539篇 |
1994年 | 1369篇 |
1993年 | 1243篇 |
1992年 | 2610篇 |
1991年 | 2710篇 |
1990年 | 2619篇 |
1989年 | 2632篇 |
1988年 | 2382篇 |
1987年 | 2369篇 |
1986年 | 2439篇 |
1985年 | 2358篇 |
1984年 | 1900篇 |
1983年 | 1680篇 |
1982年 | 1155篇 |
1979年 | 1757篇 |
1978年 | 1404篇 |
1977年 | 1158篇 |
1976年 | 1228篇 |
1975年 | 1298篇 |
1974年 | 1511篇 |
1973年 | 1525篇 |
1972年 | 1408篇 |
1971年 | 1309篇 |
1970年 | 1253篇 |
1969年 | 1289篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
D J Larson L G Morehouse R F Solorzano D A Kinden 《American journal of veterinary research》1979,40(4):477-486
Fourteen neonatal dogs (4 through 11 days of age) were exposed orally to the Purdue strain of transmissible gastroenteritis (TGE) virus, and six dogs of similar age were noninoculated controls. Clinical signs of enteric disease did not develop. Both exposed and control dogs had normal fecal passages and appetite throughout the experiment. Jejunal epithelium from dogs euthanatized at 12, 24, 48, and 96 hours and at 10 days after exposure did not exhibit morphologic alterations detectable by light microscopy. Electron microscopic examination indicated that jejunal epithelial cells contained TGE viral particles as early as 12 hours after dogs were exposed. There were no apparent morphologic alterations or signs of desquamation of virus-infected cells, however. Results of pig transmission studies indicated that viable TGE virus was in jejunal tissue of the dogs as early as 12 hours and as late as 10 days after exposure to the virus. 相似文献
962.
M A Albassam B J Houston P Greaves N Barsoum 《Journal of the American Veterinary Medical Association》1989,194(11):1595-1597
A 13-month-old Beagle became anorectic and had fever, stiff gait, and tenderness in the inguinal region. Clinical signs of disease were associated with neutrophilia and a decrease in the albumin-to-globulin ratio. The dog became clinically normal for 5 days after 3 days of treatment with penicillin G and dihydrostreptomycin. Clinical signs of disease recurred, and the dog was euthanatized after failing to respond to administration of a trimethoprim-sulfamethoxazole combination for 9 days. Disseminated arteritis was seen in the testes, epididymides, mesentery, coronary arteries, aorta, and thyroid gland. Lesions were seen in large and medium-sized arteries and varied from acute necrotizing arteries to a chronic lesion with organization and recanalization of thrombi. The clinical signs of disease resembled those of Beagle pain syndrome, described in laboratory Beagles. 相似文献
963.
Jose A Morales Ligbel Sanchez Harvi Velasquez Beatriz De Borrego Marlene De Nava Denis Portillo Yulixis Cano Angel Morillo Ana Albornoz Elsa Socorro 《Water, air, and soil pollution》2001,130(1-4):511-516
Nutrient content and pH of rain samples collected at nine sites during 1988-1996 were studied to determine the amounts of N, P and S added by precipitation over Maracaibo Lake basin. The basin is a wide depression occupied by Lake Maracaibo and extensive alluvial plains and delimited by a mountainous zone of variable physiolgraphic characteristics with different cattle ranching and industrial activities. Rain pHs averaged ≈ 4 to 6, individual samples occasionally reached as low as pH 3 in the south basin. NH4 + levels were significantly higher (up to 2.6 mg/l NH4-N) than NO3 ? plus NO2 ?. Phosphorus levels were low, ranging from undetectable to 1.5 mg/l. The amounts of inorganic N/ha/yr added ranged from ≈ 5.4 Kg in the north to ≈ 8 Kg in the south basin; around the lakeshore, it was estimated that ≈ 9.8–16.8 Kg TN/ha (≈ 55 % as organic-N) are annually deposited. The annual amounts of SO4-S/ha added range from ≈ 10 to 13 Kg. The contribution of precipitation to P in the ecosystem is very small; ≈ 0.02 – 0.8 Kg of PO4-P/ha/yr was added in basin. The results of this study show that precipitation represents a significant factor for the total nitrogen and sulfur loading to Maracaibo Lake Basin. 相似文献
964.
Field studies were conducted at Alupe in western Kenya in 1995 and 1996 to evaluate the efficacy of crop and species mixtures for the management of sorghum anthracnose (caused by Colletotrichum sublineolum) and leaf blight (caused by Exserohilum turcicum). The progress of these diseases developing simultaneously on a susceptible sorghum cultivar planted in inter- or intra-row mixtures of varying proportions with either maize or resistant sorghum was monitored. The effects of host type and mixture patterns on disease progress were compared by parameter estimates derived from fitted Lotka-Volterra competition equations and nonlinear logistic models. Competition coefficients were not significant and their confidence intervals included zero in most cases, suggesting that interactions between C. sublineolum and E. turcicum did not occur. Mixtures of the susceptible sorghum with either the nonhost maize or the resistant sorghum delayed the time when disease is first observed and reduced the rate of disease progress and carrying capacity for both anthracnose and leaf blight, with a more pronounced effect on the latter disease. The lower efficacy of mixtures in reducing anthracnose was attributed to an aggregated spatial pattern, coupled with higher rates of progress for this disease. Intra-row mixtures were more efficient than inter-row mixtures in reducing disease development in all years. The implications of these observations for the management of sorghum diseases under small-scale farming systems are discussed. 相似文献
965.
966.
Wing SL Harrington GJ Smith FA Bloch JI Boyer DM Freeman KH 《Science (New York, N.Y.)》2005,310(5750):993-996
Rapid global warming of 5 degrees to 10 degrees C during the Paleocene-Eocene Thermal Maximum (PETM) coincided with major turnover in vertebrate faunas, but previous studies have found little floral change. Plant fossils discovered in Wyoming, United States, show that PETM floras were a mixture of native and migrant lineages and that plant range shifts were large and rapid (occurring within 10,000 years). Floral composition and leaf shape and size suggest that climate warmed by approximately 5 degrees C during the PETM and that precipitation was low early in the event and increased later. Floral response to warming and/or increased atmospheric CO2 during the PETM was comparable in rate and magnitude to that seen in postglacial floras and to the predicted effects of anthropogenic carbon release and climate change on future vegetation. 相似文献
967.
Atlas R Campbell P Cozzarelli NR Curfman G Enquist L Fink G Flanagin A Fletcher J George E Hammes G Heyman D Inglesby T Kaplan S Kennedy D Krug J Levinson R Marcus E Metzger H Morse SS O'Brien A Onderdonk A Poste G Renault B Rich R Rosengard A Salzburg S Scanlan M Shenk T Tabor H Varmus H Wimmer E Yamamoto K;Journal Editors Authors Group 《Science (New York, N.Y.)》2003,299(5610):1149
968.
969.
The atomic force microscope (AFM) was used to image an electrode surface at atomic resolution while the electrode was under potential control in a fluid electrolyte. A new level of subtlety was observed for each step of a complete electrochemical cycle that started with an Au(111) surface onto which bulk Cu was electrodeposited. The Cu was stripped down to an underpotential-deposited monolayer and finally returned to a bare Au(111) surface. The images revealed that the underpotential-deposited monolayer has different structures in different electrolytes. Specifically, for a perchloric acid electrolyte the Cu atoms are in a close-packed lattice with a spacing of 0.29 +/- 0.02 nanometer (nm). For a sulfate electrolyte they are in a more open lattice with a spacing of 0.49 +/- 0.02 nm. As the deposited Cu layer grew thicker, the Cu atoms converged to a (111)-oriented layer with a lattice spacing of 0.26 +/- 0.02 nm for both electrolytes. A terrace pattern was observed during dissolution of bulk Cu. Images were obtained of an atomically resolved Cu monolayer in one region and an atomically resolved Au substrate in another in which a 30 degrees rotation of the Cu monolayer lattice from the Au lattice is clearly visible. 相似文献
970.