全文获取类型
收费全文 | 1332篇 |
免费 | 94篇 |
专业分类
林业 | 46篇 |
农学 | 45篇 |
基础科学 | 11篇 |
163篇 | |
综合类 | 262篇 |
农作物 | 126篇 |
水产渔业 | 37篇 |
畜牧兽医 | 675篇 |
园艺 | 15篇 |
植物保护 | 46篇 |
出版年
2017年 | 13篇 |
2016年 | 25篇 |
2015年 | 13篇 |
2014年 | 22篇 |
2013年 | 53篇 |
2012年 | 31篇 |
2011年 | 45篇 |
2010年 | 26篇 |
2009年 | 40篇 |
2008年 | 42篇 |
2007年 | 46篇 |
2006年 | 45篇 |
2005年 | 42篇 |
2004年 | 41篇 |
2003年 | 40篇 |
2002年 | 40篇 |
2001年 | 29篇 |
2000年 | 38篇 |
1999年 | 23篇 |
1998年 | 22篇 |
1997年 | 13篇 |
1995年 | 14篇 |
1993年 | 15篇 |
1992年 | 18篇 |
1991年 | 18篇 |
1990年 | 29篇 |
1989年 | 19篇 |
1988年 | 19篇 |
1987年 | 15篇 |
1986年 | 13篇 |
1985年 | 11篇 |
1984年 | 16篇 |
1983年 | 22篇 |
1982年 | 21篇 |
1979年 | 20篇 |
1978年 | 24篇 |
1977年 | 21篇 |
1976年 | 13篇 |
1975年 | 16篇 |
1974年 | 24篇 |
1973年 | 19篇 |
1972年 | 14篇 |
1971年 | 18篇 |
1970年 | 26篇 |
1969年 | 18篇 |
1968年 | 17篇 |
1948年 | 10篇 |
1945年 | 12篇 |
1944年 | 14篇 |
1943年 | 14篇 |
排序方式: 共有1426条查询结果,搜索用时 218 毫秒
231.
Wm Tod Drost DVM Robert J. Bahr DVM George A. Henry DVM Gregory A. Campbell DVM PhD 《Veterinary radiology & ultrasound》1999,40(3):262-266
A 12-year-old, neutered female, Siberian Husky dog presented with a hind limb weakness of one month duration. To facilitate making a diagnosis multiple imaging modalities were performed. These modalities included radiography, ultrasonography, magnetic resonance imaging (MRI), magnetic resonance angiography (MRA) and selective angiography of the abdominal aorta. In this dog, the MRI/MRA studies provided the first documentation of the external iliac thrombi and the collateral circulation via the lumbar arteries. At necropsy, an aortoiliac thrombus caused by a mineralized arteriosclerotic plaque was noted. 相似文献
232.
233.
Feral swine (Sus scrofa) are one of the most threatening mammalian pest species in North America owing to the damage they cause to natural habitats and agroecosystems, and the risk of disease transmission they pose to wildlife, livestock, and humans. The long-term (> 1 year) effects of lethal control efforts on feral swine populations at local scales are largely unknown. Using a panel of molecular markers, we assessed the effects of lethal control efforts on selected populations of feral swine in southern Texas. We collected tissue samples from two sites during removal campaigns, extracted and amplified DNA, and assessed population structure, genetic clustering, and immigration. We removed 145 individuals (9.7 swine per km2) at one site and 204 individuals (6.6 swine per km2) at another site. Fixation indices, Bayesian clustering, and assignment tests based on allele frequencies all produced similar results, indicating little or no differentiation among removals at either site. Localized feral swine removals aimed at reducing damage had no long-term impact on population parameters. Removals occurred at sites in which the swine groups were contiguous with neighboring feral swine family units and groups. This may have resulted in immigration of adjacent, but not genetically distinct, feral swine onto sites following the initial removals. To achieve long-term reduction of damage by feral swine populations, additional information is needed to enable genetic populations and corresponding management units to be defined. 相似文献
234.
235.
236.
237.
Kate L. Baker Silke Langenheder Graeme W. Nicol Dean Ricketts Kenneth Killham Colin D. Campbell James I. Prosser 《Soil biology & biochemistry》2009,41(11):2292-2298
Soil physicochemical properties and microbial communities are highly heterogeneous and vary widely over spatial scales, necessitating careful consideration of sampling strategies to provide representative and reproducible soil samples across field sites. To achieve this, the study aimed to establish appropriate sampling methodology and to determine links between the variability of parameters, utilising two sampling strategies. The first (design 1) involved extracting 25 cores from random locations throughout the field and pooling them into five sets of five cores. The second (design 2) involved a further 25 cores within five 1 m2 sub-plots. Sub-samples from each sub-plot were pooled in order to determine between and within sub-plot variability. All samples were analysed independently and as pooled sub-samples. Results indicate that pooling spatially separated samples significantly reduced the variability in pH, compared to individual samples. Pooling samples from a small area resulted in lower within sub-plot variability than between sub-plots for pH and bacterial community composition assessed by terminal-restriction fragment length polymorphism analysis. Following multivariate statistical analysis, a large amount of variation in community composition was explained by soil pH, which is remarkable given the relatively small size of the sampling area and minor differences in pH. Moisture content was also important in determining bacterial communities in the random design (design 1). In the 1 m2 sub-plot design (design 2), the spatial location of the plots explained a large degree of the variation in bacterial community composition between plots, which was due to spatial autocorrelation of pH and possible additional environmental parameters. This study emphasises the importance of sampling design for obtaining representative samples from soil. 相似文献
238.
Rachel E. Creamer Pat Bellamy Helaina I. J. Black Clare M. Cameron Colin D. Campbell Paul Chamberlain Jim Harris Nisha Parekh Mark Pawlett Jan Poskitt Dote Stone Karl Ritz 《Biology and Fertility of Soils》2009,45(6):623-633
The use of indicators in soil monitoring schemes to detect changes in soil quality is receiving increased attention, particularly
the application of soil biological methods. However, to date, the ability to compare information from different laboratories
applying soil microbiological techniques in broad-scale monitoring has rarely been taken into account. This study aimed to
assess the consistency and repeatability of two techniques that are being evaluated for use as microbiological indicators
of soil quality: multi-enzyme activity assay and multiple substrate-induced respiration (MSIR). Data were tested for intrinsic
(within-assay plate) variation, inter-laboratory repeatability (geometric mean regression and correlation coefficient) and
land-use discrimination (principal components analysis). Intrinsic variation was large for both assays suggesting that high
replicate numbers are required. Inter-laboratory repeatability showed diverging patterns for the enzyme assay and MSIR. Discrimination
of soils was significant for both techniques with relatively consistent patterns; however, combined laboratory discrimination
analyses for each technique showed inconsistent correspondence between the laboratories. These issues could be addressed through
the adoption of reliable analytical standards for biological methods along with adequate replication. However, until the former
is addressed, dispersed analyses are not currently advisable for monitoring schemes. 相似文献
239.
240.
Murray D. Jelinski John R. Campbell Jonathan M. Naylor Karen L. Lawson Dena Derkzen 《The Canadian veterinary journal. La revue veterinaire canadienne》2009,50(6):621-629
The objective of this study was to generate demographic data on veterinarians working in western Canada. A sample of 551 veterinarians was randomly selected from a population of 2474 veterinarians employed in western Canada, 425 (77.1%) of whom responded to the survey. The respondents were evenly split between males (53.1%) and females (46.9%). More than half (58.0%) of the private practitioners practised exclusively on companion animals (small animals and horses), while 2.9% devoted 100% of their time to food animals. There were 351 respondents who had had ≥ 2 employers since graduation; 80% of those who had begun their careers in companion animal (CA) practice had remained in this type of practice, while 54.3% of those who had begun their careers in mixed animal practice had switched to CA practice. Analyses of wage and workload data from 85 full-time veterinary employees showed that CA practitioners worked the fewest hours/week (47.0), had the least number of evenings on-call/month (3.7), and earned the highest hourly wage ($35.79) as compared with non-CA practitioners. 相似文献