全文获取类型
收费全文 | 6234篇 |
免费 | 272篇 |
国内免费 | 1篇 |
专业分类
林业 | 454篇 |
农学 | 139篇 |
基础科学 | 49篇 |
1052篇 | |
综合类 | 1149篇 |
农作物 | 362篇 |
水产渔业 | 398篇 |
畜牧兽医 | 2197篇 |
园艺 | 231篇 |
植物保护 | 476篇 |
出版年
2023年 | 37篇 |
2022年 | 42篇 |
2021年 | 118篇 |
2020年 | 120篇 |
2019年 | 131篇 |
2018年 | 113篇 |
2017年 | 95篇 |
2016年 | 122篇 |
2015年 | 109篇 |
2014年 | 145篇 |
2013年 | 272篇 |
2012年 | 329篇 |
2011年 | 395篇 |
2010年 | 214篇 |
2009年 | 179篇 |
2008年 | 408篇 |
2007年 | 406篇 |
2006年 | 404篇 |
2005年 | 408篇 |
2004年 | 420篇 |
2003年 | 406篇 |
2002年 | 369篇 |
2001年 | 78篇 |
2000年 | 61篇 |
1999年 | 67篇 |
1998年 | 50篇 |
1997年 | 40篇 |
1996年 | 68篇 |
1995年 | 53篇 |
1994年 | 51篇 |
1993年 | 52篇 |
1992年 | 52篇 |
1991年 | 33篇 |
1990年 | 50篇 |
1989年 | 43篇 |
1988年 | 36篇 |
1987年 | 35篇 |
1986年 | 29篇 |
1985年 | 42篇 |
1984年 | 44篇 |
1983年 | 39篇 |
1982年 | 28篇 |
1981年 | 22篇 |
1980年 | 20篇 |
1979年 | 17篇 |
1978年 | 22篇 |
1977年 | 29篇 |
1976年 | 20篇 |
1975年 | 22篇 |
1974年 | 26篇 |
排序方式: 共有6507条查询结果,搜索用时 15 毫秒
201.
Robert N Streeter D L Step 《Veterinary Clinics of North America: Food Animal Practice》2007,23(3):541-74, vii
The use of diagnostic ultrasound equipment is becoming widespread within various sectors of veterinary practice. Ultrasonographic examination has several advantages over other imaging modalities and it can be applied in hospital and ambulatory settings. It has the potential for widespread use in the diagnosis of disorders of several body systems in food animal species. This article describes its application to gastrointestinal, hepatobiliary, cardiothoracic, urogenital, and umbilical disorders. Normal and abnormal findings are portrayed and discussed. Accurate and timely use of this diagnostic modality requires a modest amount of training and practice, but it allows for relatively rapid, inexpensive, and noninvasive acquisition of clinically relevant data. 相似文献
202.
203.
Robert G. Latta 《Landscape Ecology》2006,21(6):809-820
Landscape geneticists can take considerable advantage of differences in the action of evolutionary forces (mutation, migration,
selection, and drift) on different loci within the genome. Appropriate comparisons among loci allow researchers to isolate
and study the effects of these processes. For example, the organelles are typically inherited maternally (but occasionally
paternally), and so will experience migration only when females or seeds disperse (males or pollen in the paternally inherited
organelles). Thus, the comparison with biparentally inherited loci allows inferences about the differential migration rates
of male vs. female animals or of seeds vs. pollen in plants. Conversely, all biparentally inherited nuclear loci should experience
the same levels of migration and drift. Thus, loci that show unusually large levels of variation across the landscape (as
compared with the bulk of loci) may be reflecting the action of spatially varying natural selection (local adaptation). Such
comparisons are conceptually straightforward, but are complicated by the high intrinsic variability of stochastic neutral
processes. Careful statistical analysis is needed to avoid over-interpreting differences among loci. Inferences will be most
robust when both genetic and non-genetic data can be integrated, and the collaboration of landscape ecologists with geneticists
promises to be particularly fruitful. 相似文献
204.
Comparison of the Sensitivity of Landscape-fire-succession Models to Variation in Terrain, Fuel Pattern, Climate and Weather 总被引:1,自引:0,他引:1
Geoffrey J. Cary Robert E. Keane Robert H. Gardner Sandra Lavorel Mike D. Flannigan Ian D. Davies Chao Li James M. Lenihan T. Scott Rupp Florent Mouillot 《Landscape Ecology》2006,21(1):121-137
The purpose of this study was to compare the sensitivity of modelled area burned to environmental factors across a range of
independently-developed landscape-fire-succession models. The sensitivity of area burned to variation in four factors, namely
terrain (flat, undulating and mountainous), fuel pattern (finely and coarsely clumped), climate (observed, warmer & wetter,
and warmer & drier) and weather (year-to-year variability) was determined for four existing landscape-fire-succession models
(EMBYR, FIRESCAPE, LANDSUM and SEM-LAND) and a new model implemented in the LAMOS modelling shell (LAMOS(DS)). Sensitivity
was measured as the variance in area burned explained by each of the four factors, and all of the interactions amongst them,
in a standard generalised linear modelling analysis. Modelled area burned was most sensitive to climate and variation in weather,
with four models sensitive to each of these factors and three models sensitive to their interaction. Models generally exhibited
a trend of increasing area burned from observed, through warmer and wetter, to warmer and drier climates with a 23-fold increase
in area burned, on average, from the observed to the warmer, drier climate. Area burned was sensitive to terrain for FIRESCAPE
and fuel pattern for EMBYR. These results demonstrate that the models are generally more sensitive to variation in climate
and weather as compared with terrain complexity and fuel pattern, although the sensitivity to these latter factors in a small
number of models demonstrates the importance of representing key processes. The models that represented fire ignition and
spread in a relatively complex fashion were more sensitive to changes in all four factors because they explicitly simulate
the processes that link these factors to area burned.
The US Government's and the Canadian Government's right to retain a non-exclusive, royalty-free license is acknowledged 相似文献
205.
Mesher CI Rej R Stokol T 《Veterinary clinical pathology / American Society for Veterinary Clinical Pathology》1998,27(1):26-30
Unusually low serum activity of alanine aminotransferase (ALT) was detected in a Rottweiler dog with gastric dilatation-volvulus. Activity of ALT in the same sample was found to be much higher (estimated increase of 14,225%) when measured by methods adding the cofactor pyridoxal-5'-phosphate (P5P), indicating that nearly all serum ALT was in the apoenzyme form. An investigation was undertaken to determine the frequency of high serum aminotransferase apoenzyme levels in dogs. Eighty canine serum samples submitted to the Clinical Pathology Laboratory at Cornell University were assayed for ALT and aspartate aminotransferase (AST) in the presence or absence of exogenous P5P. In 79 dogs, inclusion of P5P in the LAT assay resulted in a median decrease in AST activity of -6.3% (range -33.3% to 25.0%) in all 80 dogs. One dog had an increase of 336% in ALT activity after inclusion of P5P in the assay, but lacked a similar increase in AST activity. The reason for the high levels of ALT apoenzyme in the 2 dogs was not determined. 相似文献
206.
Fintan J. McEvoy MVB PhD Myra A. Forster-van Hijfte DVM Robert N. White BSc BVetMed 《Veterinary radiology & ultrasound》1998,39(3):234-237
This study reports data obtained from per-rectal 99m Tc-pertechnetate portal scintigraphy in normal cats. It examines the effects of chemical restraint and the methods employed in defining regions of interest (ROIs) on the shunt index derived from this data. Six normal cats were used for the study; all six were chemically restrained for imaging using propofol and later four of them were manually restrained for comparison. Portal blood flow was studied and the mean shunt index was found to be 5.9%± 3.9 when ROIs were operator defined and 9.2%± 4.4 when ROIs were defined using an isocontour program. In cats that were restrained using propofol and operator defined ROIs, the mean value for the time between detection of radioactivity in the liver and in the heart was 14 ± 1 seconds. 相似文献
207.
Robert M. Kirberger MMedVet Remo G. Lobetti MMedVet 《Veterinary radiology & ultrasound》1998,39(4):313-317
The thoracic radiographic changes of Pneumocystis carinii in 7 miniature Dachshunds were reviewed. The dogs were 7–12 months old and presented with polypnea, exercise intolerance and clinical signs suggestive of immune-incompetence. P. carinii pneumonia was diagnosed in all the dogs using transtracheal aspirate cytology and confirmed at postmortem in 3 dogs that died. Radiographically, diffuse pulmonary changes were present and varied from a mild interstitial and bronchial pattern to an alveolar pattern. Radiographic evidence of cor pulmonale was present in 1 dog. The most severe radiographic changes were seen in 2 of the dogs that died. 相似文献
208.
The toxic action of a series of O-alkyl, O-substituted-phenyl alkyl- and aryl-phosphonates and phosphonothionates have been evaluated by correlating the linear free energy parameters for steric (Es), electronic (σ), and polar () effects with topical LD50 to the house fly and oral LD50 to the white mouse. In molecules free from major steric interactions with the reactive P atom, variations in these linear free energy parameters account for >90% of the variations in the LD50 values, and the degree of correlation with LD50 is at least as precise as that with the biomolecular rate constants for inhibition of the target-site enzyme acetylcholinesterase. The value of correlations of linear free energy parameters with LD50 in understanding quantitative structure-activity relationships is illustrated. 相似文献
209.
Genchi C Cody R Pengo G Büscher G Cavalleri D Bucci V Junquera P 《Veterinary parasitology》2004,122(4):287-292
The efficacy of a combination of milbemycin oxime and praziquantel in preventing the establishment of experimentally induced heartworm (Dirofilaria immitis) infection was investigated in a study involving 24 young domestic short-hair cats. The animals were inoculated with 50 infective larvae on day 0. Subsequently they were divided into two groups of 12 animals each. The animals in group 1 were treated once with medicated tablets containing 4 mg milbemycin (minimum dose 2 mg/kg body weight) and 10 mg praziquantel (MILBEMAX) on day 30 after infection. Cats in group 2 received placebo tablets on the same day. On day 183 post-infection a blood sample was taken from each animal before euthanasia and necropsy. The blood samples were tested for the presence of microfilariae and the necropsied animals were examined for the presence of adult worms. Microfilariae were not found in any of the investigated cats. No heartworms were found in the animals in group 1 (treated with medicated tablets). Out of the 12 placebo-treated cats 1 was heartworm-free, whereas all the others were found to be infected with 1-3 adult heartworms. 相似文献
210.