全文获取类型
收费全文 | 119篇 |
免费 | 5篇 |
专业分类
林业 | 7篇 |
农学 | 1篇 |
64篇 | |
综合类 | 4篇 |
农作物 | 5篇 |
水产渔业 | 3篇 |
畜牧兽医 | 33篇 |
园艺 | 2篇 |
植物保护 | 5篇 |
出版年
2022年 | 1篇 |
2021年 | 2篇 |
2020年 | 2篇 |
2019年 | 1篇 |
2018年 | 1篇 |
2016年 | 2篇 |
2015年 | 4篇 |
2014年 | 4篇 |
2013年 | 11篇 |
2012年 | 5篇 |
2011年 | 5篇 |
2010年 | 2篇 |
2009年 | 1篇 |
2008年 | 11篇 |
2007年 | 3篇 |
2006年 | 9篇 |
2005年 | 9篇 |
2004年 | 7篇 |
2003年 | 6篇 |
2002年 | 4篇 |
2000年 | 4篇 |
1999年 | 5篇 |
1996年 | 2篇 |
1995年 | 2篇 |
1994年 | 1篇 |
1993年 | 2篇 |
1992年 | 3篇 |
1991年 | 4篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1982年 | 2篇 |
1975年 | 1篇 |
1940年 | 1篇 |
1939年 | 1篇 |
1936年 | 1篇 |
排序方式: 共有124条查询结果,搜索用时 15 毫秒
61.
Heini Kujala Miguel B. Araújo Wilfried Thuiller Mar Cabeza 《Biological conservation》2011,(10):2450-2458
Gap analysis is a widely used method for assessing the representation of species in protected area (PA) networks. However, representation does not imply persistence. Here, we investigated whether gap analysis may result in misleading conservation guidelines by comparing the representation to two indicators of persistence. We ran a gap analysis with Finnish breeding birds and identified conservation priorities based on current distribution patterns. We tested the sensitivity of these results by using two target setting schemes and several thresholds defining the amount of protected area, and found the levels of representation identified by gap analysis to be robust. We then compared the gap analysis results with recent population trends and projected changes in potential suitable climate under different climate change scenarios for the year 2050. We show that although high latitude species are well represented in PAs, they are currently declining and are projected to lose climatic suitability in the near future. In contrast, low latitude species with poor representation in PAs have increasing population trends and are generally expected to expand their ranges into protected areas in the near future. This study demonstrates with empirical data a mismatch between representation in PAs and population trends, resulting in misleading understanding of current PA effectiveness. The mismatch is linked to the latitude of species distributions and corresponds to expected future changes, indicating that the patterns are potentially driven by climate change. We therefore urge practitioners and researchers to include better indicators of persistence in gap-analysis frameworks even for short term assessments. 相似文献
62.
63.
Using high-resolution transmission electron microscopy, we obtained structure images of strontium titanate (SrTiO3) with a clearly resolved oxygen sublattice along different crystallographic directions in the bulklattice and for a Sigma3 tilt grain boundary. Comparison with image simulations showed that the grain boundary contains oxygen vacancies. Measurements of atom displacements near the grain boundary revealed close correspondence with theoretical calculations. 相似文献
64.
Kuczynski LA Schwartz P Peddle G Huang S Mai W Giger U 《Journal of the American Animal Hospital Association》2012,48(4):278-283
At approximately 4-5 mo of age, three Polish Lowland sheepdog puppies from a single litter of eight puppies presented to their respective primary veterinarians with bilateral subcutaneous masses in their ventral cervical regions. Evaluation, including thyroid function testing, surgical exploration with resection, computed tomography, and angiography, identified the masses as enlarged thyroid glands with severely dilated and abnormal vasculature in the regions of the glands. The dogs were also found to have serum concentrations of thyroid hormones that were below the reference ranges. None of the three dogs showed clinical signs of hypothyroidism, except for the presence of goiter. One dog also had a patent ductus arteriosus that was surgically repaired. All dogs were clinically normal at 2 yr of age. This is the first report of major vascular anomalies associated with goiter in any species. The mechanism is unknown. 相似文献
65.
Céline Jolivet Aki M. H?ltken Heike Liesebach Wilfried Steiner Bernd Degen 《European Journal of Forest Research》2012,131(4):1055-1069
Although pollen dispersal has been extensively studied in trees, parameters influencing between-population variation are still poorly understood. In this study, we conducted paternity analyses on open-pollinated seeds in four natural populations of wild cherry (Prunus avium) with contrasting density and clonal propagation, using eight microsatellite loci and one self-incompatibility system locus. We also measured four quantitative traits and spatial positions as potential correlates of reproductive success. Levels of polyandry differed among populations and 30% of the seed families exhibited unequal paternal contributions, suggesting variation in reproductive success rather than variation in mate availability. Mating occurred preferentially among neighbours in all populations, suggesting that it is a common pattern in wild cherry and probably results from pollinator behaviour. Paternal success was positively correlated with diameter at breast height, as indicated in previous studies and tree dominance only resulted in higher paternal success in low density plots. Mating patterns were thus also affected by both density and tree size. Large-scale studies are needed to disentangle relative influences of these factors on the mating system and pollination success. 相似文献
66.
Wilfried Mai DVM PhD Johann Le Floc''h PhD Didier Vray PhD Jacques Samarut PhD Paul Barthez DVM PhD Marc Janier MD PhD 《Veterinary radiology & ultrasound》2004,45(6):568-573
The purpose of this study was to evaluate color- and spectral Doppler ultrasound in the establishment of normal functional cardiovascular development features in the mouse fetus. Mouse fetuses (129Sv strain) were studied in utero between embryonic day (EDs) 9.5 and 19.5. Time-velocity curves were derived from Doppler interrogation of the aorta and umbilical artery. The sample volume was accurately placed on the vessels of interest based on color-Doppler images. From these curves, the following parameters were obtained: heart rate (HR), acceleration time (AT), and deceleration time (DT). HR increased between EDs 9.5 and 19.5 from 102.9 to 303.2 b.p.m. For the other parameters, the most significant change observed was the increase of DT in the umbilical artery at the end of pregnancy, corresponding to the appearance of a diastolic flow. We report the use of a commercially available, clinical, ultrasound unit to obtain quantitative data on the cardiovascular development in the mouse fetus. These results may be useful for the recognition of in utero cardiovascular dysfunction in transgenic or knock-out fetus. 相似文献
67.
The potential of five different commercial DNA isolation methods to remove real-time PCR inhibitors from the synovial fluid of horses, cattle and pigs was investigated. All kits with the exception of one included a silica column-based purification of the DNA. With the fifth kit, DNA purification is achieved by removing contaminating macromolecules by a desalting process. We used a recently developed method based on comparison of the real-time PCR signal of an artificial target incorporated into each PCR reaction in the presence of the isolated DNA from the sample, and in control samples containing water instead of isolated DNA. This was followed by statistical analysis of the data. Inhibition and subsequent reduction of the endpoint fluorescence in the real-time PCR reaction was encountered in many cases. Less frequently, the target copy number in the samples was underestimated. However, we found no experimental evidence of a negative influence of the reduced endpoint fluorescence signal on the detection limit of the real-time PCR assay. All kits tested were useful for analyzing pelleted synovial fluid from horses, cattle and pigs. When analyzing non-pelleted synovial fluid, three kits - two based on silica columns and one employing a desalting process - yielded inhibitor-free DNA for real-time PCR analysis. 相似文献
68.
Claus A Schreiter P Weber A Graeff S Herrmann W Claupein W Schieber A Carle R 《Journal of agricultural and food chemistry》2006,54(23):8968-8976
Because the impact of agronomical factors on bakery products quality is still an insufficiently studied field, acrylamide contents of breads produced from flours of nine wheat, two rye, and two spelt varieties harvested in 2003 and 2004 were investigated. It could be demonstrated that acrylamide content in bread strongly depends on the cultivar, with extremes differing by a factor of 5.4 due to marked differences in free asparagine and crude protein contents. Nitrogen fertilization also resulted in elevated amino acid and protein contents, thus increasing acrylamide levels from 10.6 to 55.6 mug/kg. Independent of fertilization, harvest year turned out to be another factor influencing acrylamide formation. Breads produced from 2003 flours showed significantly higher acrylamide contents than those of 2004, which was ascribed to favorable light and temperature conditions during the cultivation period, thus enhancing amino acid and protein contents. Sprouting of the grain also resulted in significantly higher acrylamide levels, which was attributed to elevated enzyme activities and the formation of precursors from protein and starch. Furthermore, bakery products made from flours with higher extraction rates were shown to contain higher acrylamide levels resulting from extracted free asparagine and protein from the aleuron layers of the cereal grain. 相似文献
69.
Effect of mechanical stress on structure and productivity of a loess‐derived Luvisol with conventional and conservation tillage In Germany farmers are committed to caring for the land by a soil protection law. Yet vehicles with ever increasing axle load endanger productivity and environmental quality of arable soils. In spring of 1995 a field experiment was startet on a wet silty Luvisol to test the effect of single mechanical loading on soil and crop characteristics, when managed by mouldboard ploughing (PL) or conservation tillage (CT). CT soils are considered to be more resistant against compactive stresses and to recover from degeneration more rapidly than PL soils. Beside an unwheeled control the loading treatments were light (2 × 2.5 t; number of wheel passes times wheel load); medium (2 × 5 t) and high (6 × 5 t). In 1995 even light loading of the PL soil caused a significant yield decline by 50% in spring barley, but this happened on CT soil only with high loading. In subsequent years with winter wheat and winter barley yield decline was less distinct. Loading of PL soil reduced total root length (from 4 to 1 km m−2) and rooting depth (from 70—90 to 40—70 cm), but on CT soil only root length was diminished by high loading. A tillage‐traffic pan (30—35 cm) hindered subsoil rooting in PL, which was favored in CT by earthworm channels. High loading caused compaction to at least 50 cm depth. Within the pan of the PL soil, penetration resistance attained 5 MPa and bulk density 1.65 g cm−3. In the CT soil the zone of maximum compaction was closer to the surface (15—25 cm). In PL soil the saturated hydraulic conductivity and the O2‐diffusion coefficient gradually decreased with loading, but in CT soil only with heavy loading. The compacted top soil was broken in subsequent years by ploughing (PL: 25 cm) or rotary implements (CT: 5—8 cm). With PL, structure in the pan layer and subsoil did not recover, and rooting depth was still limited. Some restoration, however, was indicated with CT. Here transmitting properties increased in time, which was attributed to the reconstruction of root and earthworm channels, as demonstrated by computer tomography. We conclude that in silty soils compacted layers below ploughing depth will hardly be regenerated by internal processes. CT soils are less susceptible to loading, but high stresses are harmful too. Therefore recommending CT as a measure for protecting soil from compaction would not be enough, considering the present development towards heavy field machinery. 相似文献
70.
Wilfried Drilling 《林产工业》2008,35(2):43-44
钢模板和钢模带的正确使用、处理和维护有助于生产出更好品质的产品,从而使使用者投资获得最大的回报. 相似文献