全文获取类型
收费全文 | 90872篇 |
免费 | 5225篇 |
国内免费 | 391篇 |
专业分类
林业 | 4460篇 |
农学 | 3438篇 |
基础科学 | 671篇 |
12282篇 | |
综合类 | 11967篇 |
农作物 | 3413篇 |
水产渔业 | 4667篇 |
畜牧兽医 | 48850篇 |
园艺 | 1141篇 |
植物保护 | 5599篇 |
出版年
2020年 | 819篇 |
2019年 | 923篇 |
2018年 | 1593篇 |
2017年 | 1709篇 |
2016年 | 1596篇 |
2015年 | 1386篇 |
2014年 | 1663篇 |
2013年 | 3885篇 |
2012年 | 2893篇 |
2011年 | 3462篇 |
2010年 | 2337篇 |
2009年 | 2312篇 |
2008年 | 3245篇 |
2007年 | 3128篇 |
2006年 | 2979篇 |
2005年 | 2667篇 |
2004年 | 2507篇 |
2003年 | 2508篇 |
2002年 | 2320篇 |
2001年 | 3259篇 |
2000年 | 3152篇 |
1999年 | 2450篇 |
1998年 | 1002篇 |
1997年 | 982篇 |
1996年 | 901篇 |
1995年 | 1003篇 |
1994年 | 869篇 |
1993年 | 814篇 |
1992年 | 1791篇 |
1991年 | 2019篇 |
1990年 | 1999篇 |
1989年 | 1924篇 |
1988年 | 1779篇 |
1987年 | 1747篇 |
1986年 | 1712篇 |
1985年 | 1580篇 |
1984年 | 1315篇 |
1983年 | 1073篇 |
1979年 | 1270篇 |
1978年 | 997篇 |
1976年 | 853篇 |
1975年 | 863篇 |
1974年 | 1031篇 |
1973年 | 1026篇 |
1972年 | 1036篇 |
1971年 | 912篇 |
1970年 | 929篇 |
1969年 | 904篇 |
1968年 | 799篇 |
1967年 | 829篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Abstract. The Pan‐European Soil Erosion Risk Assessment (pesera ) model was evaluated using existing soil erosion data collected under various types of climate, vegetation, landscape and soil conditions. The data used represent a variety of typical Mediterranean land uses such as winter wheat, vines, olives and bare, stony land prevailing in hilly areas. Using this data, the model was calibrated for sediment transport by overland flow and results compared to measured soil erosion values from runoff plots and a watershed on a monthly basis. The performance of the model was assessed statistically, showing that it can be satisfactorily used for predicting soil erosion rates under the conditions included in the study. The overall model estimate including all the available experimental data was 0.69 t ha?1 yr?1 with a maximum error of 1.49 t ha?1 yr?1. After validation, the model was applied to a small watershed (60 ha) of great ecological importance for the sea turtle Caretta caretta. For this purpose, soil and vegetation maps were compiled from all the necessary data for applying the model. The model was run for three years using daily data from an existing nearby meteorological station. The predicted and measured soil erosion rates for a 7‐month period were 0.31 t and 0.18 t, respectively. Application of the model to each mapping unit showed the over‐riding importance of land use for sediment generation under the given climatic conditions. Bare land, occupying 5.5% of the watershed area, generated up to 69% of the total sediments estimated for the watershed. It is concluded that the pesera model can be used as a regional diagnostic tool under a range of soil, topographic and climatic conditions for identifying the best land use type and vegetation cover to protect hilly areas from soil erosion. The calculated overall root mean square error for the model is 0.06 t ha?1 yr?1, compared to a soil erosion rate of 0.04 t ha?1 yr?1, which can be tolerated for protecting the area for the sea turtle. 相似文献
72.
73.
将土、基础及上部结构物视为一个整体,在计入土的弹性抗力后对多跨无铰拱和连续刚构的上下部结构进行联合计算,并给出了算例,可供设计参考。 相似文献
74.
75.
76.
Lockyer JN 《Science (New York, N.Y.)》1886,7(169):386-387
77.
Intrajugular injection of a purified E. coli lipopolysaccharide induced a biphasic fever in sheep after a latent period of 12 to 20 min. The changes in the blood flow from the liver and from the viscera drained by the portal vein were: (a) in the latent period, decreases in total hepatic blood flow (THF) due to decreased portal venous blood flow (PVF); (b) during the first febrile phase, increases in THF due to increased hepatic arterial blood flow and, (c) in the second febrile phase, increases in THF due to decreased PVF. Although there were large variations in the oxygen supply to the viscera drained by the portal vein and to the liver, there were relatively small or no changes in their oxygen consumption. 相似文献
78.
A.J.A. Vinten D.R. Lewis D.R. Fenlon K.A. Leach R. Howard I. Svoboda I. Ogden 《Soil Use and Management》2002,18(3):223-231
Abstract. Slurry from farm animals may contaminate water supplies, rivers and bathing waters with faecal coliforms, such as Escherichia coli . Where animals harbour the O157 strain the hazard to human health is particularly high, but both the hazard level, and the low incidence and sporadic nature of the excretion of E. coli O157 make it difficult to study this strain under field conditions. The survival of total E. coli and of E. coli O157 were compared in the laboratory for two soils under controlled temperature and moisture. E. coli O157 die-off rate was the same as or quicker than for total E. coli . This result meant that field experiments studying the fate of total E. coli should give a satisfactory evaluation of the risk of water contamination by the O157 strain. In four field experiments at three sites, slurry containing total E. coli numbers of 2.2 × 104 to 5.7 × 105 colony forming units per mL (c.f.u. mL–1 ) was applied to drained field plots. Field die-off was faster than expected from laboratory experiments, especially in one experiment where two weeks dry weather followed application. In all but this experiment, the first drain flow events after slurry application led to very high E. coli concentrations in the drains (103 to 104 c.f.u. mL–1 ). E. coli O157 was present in the slurry used for two of the experiments (33 c.f.u. per 100 mL in each case). However the proportion of E.coli O157 was very low (about 1 in 105 ) and it was not detected in the drainage water. After the first week E. coli drainage water numbers decreased rapidly but they were 1–10 c.f.u. mL–1 for much of the sampling period after slurry application (1–3 months). 相似文献
79.
S. A. Selim K. M. S. Aziz A. J. Sarker H. Rahman 《Veterinary research communications》1991,15(4):327-333
Faecal samples from 434 calves under 1 year of age (307 diarrhoeal and 127 normal) were collected from three dairy farms and one village in selected areas of Bangladesh. The samples were tested by an enzyme-linked immunosorbent assay (ELISA) to detect the presence of rotavirus antigen. Of 402 dairy calves tested, 28 (7.0%) were positive, of which 21 (7.2%) were from diarrhoeic calves and 7 (6.3%) from non-diarrhoeic calves. Rotavirus infection varied from farm to farm (2.7–9.2%) and there was no positive response from any of the 32 village calves. Rotavirus was most commonly found in calves of 1 week of age or less (up to 22.2% in one group) but was not found in any calves later than 6 months of age. More than 80% of rotavirus-positive samples from diarrhoeic calves exhibited a titre of 128 or more (geometric mean 345±4.5), whereas non-diarrhoeal calves had titres less than or equal to 128 (geometric mean=29±1.9), suggesting that rotavirus infection in calves in Bangladesh was mostly associated with diarrhoea. 相似文献
80.
Bayesian analysis via Gibbs sampling, restricted maximum likelihood (REML), and Method R were used to estimate variance components for several models of simulated data. Four simulated data sets that included direct genetic effects and different combinations of maternal, permanent environmental, and dominance effects were used. Parents were selected randomly, on phenotype across or within contemporary groups, or on BLUP of genetic value. Estimates by Bayesian analysis and REML were always empirically unbiased in large data sets. Estimates by Method R were biased only with phenotypic selection across contemporary groups; estimates of the additive variance were biased upward, and all the other estimates were biased downward. No empirical bias was observed for Method R under selection within contemporary groups or in data without contemporary group effects. The bias of Method R estimates in small data sets was evaluated using a simple direct additive model. Method R gave biased estimates in small data sets in all types of selection except BLUP. In populations where the selection is based on BLUP of genetic value or where phenotypic selection is practiced mostly within contemporary groups, estimates by Method R are likely to be unbiased. In this case, Method R is an alternative to single-trait REML and Bayesian analysis for analyses of large data sets when the other methods are too expensive to apply. 相似文献