全文获取类型
收费全文 | 3986篇 |
免费 | 275篇 |
国内免费 | 564篇 |
专业分类
林业 | 293篇 |
农学 | 883篇 |
基础科学 | 222篇 |
652篇 | |
综合类 | 1795篇 |
农作物 | 375篇 |
水产渔业 | 91篇 |
畜牧兽医 | 315篇 |
园艺 | 94篇 |
植物保护 | 105篇 |
出版年
2024年 | 63篇 |
2023年 | 101篇 |
2022年 | 150篇 |
2021年 | 151篇 |
2020年 | 181篇 |
2019年 | 184篇 |
2018年 | 135篇 |
2017年 | 177篇 |
2016年 | 179篇 |
2015年 | 196篇 |
2014年 | 215篇 |
2013年 | 212篇 |
2012年 | 303篇 |
2011年 | 317篇 |
2010年 | 275篇 |
2009年 | 252篇 |
2008年 | 254篇 |
2007年 | 233篇 |
2006年 | 250篇 |
2005年 | 216篇 |
2004年 | 173篇 |
2003年 | 115篇 |
2002年 | 81篇 |
2001年 | 58篇 |
2000年 | 56篇 |
1999年 | 43篇 |
1998年 | 43篇 |
1997年 | 28篇 |
1996年 | 32篇 |
1995年 | 25篇 |
1994年 | 24篇 |
1993年 | 17篇 |
1992年 | 19篇 |
1991年 | 11篇 |
1990年 | 12篇 |
1989年 | 12篇 |
1988年 | 10篇 |
1987年 | 6篇 |
1986年 | 3篇 |
1984年 | 1篇 |
1977年 | 1篇 |
1963年 | 1篇 |
1962年 | 7篇 |
1956年 | 1篇 |
1955年 | 2篇 |
排序方式: 共有4825条查询结果,搜索用时 15 毫秒
1.
Abstract. Recent developments in in situγ ray spectrometry offer a new approach to measuring the activity of radionuclides such as 137Cs and 40K in soils, and thus estimating erosion or deposition rates and field moist bulk density (ρm). Such estimates would be rapid and involve minimal site disturbance, especially important where archaeological remains are present. This paper presents the results of a pilot investigation of an eroded field in Scotland in which a portable hyper pure germanium (HPGe) detector was used to measure γ ray spectra in situ. The gamma (γ) photon flux observed at the soil surface is a function of the 137Cs inventory, its depth distribution characteristics and ρm. A coefficient, QCs, derived from the forward scattering of 137Cs γ ray photons within the soil profile relative to the 137Cs full energy peak (662 keV), was used to correct the in situ calibration for changes in the 137Cs vertical distribution in the ploughed field, a function of tillage, soil accumulation and ρm. Based on only 8 measurements, the agreement between in situγ ray spectrometry and soil sample measurements of 137Cs inventories improved from a non significant r2=0.05 to a significant r2=0.62 (P<0.05). Erosion and deposition rates calculated from the corrected in situ137Cs measurements had a similarly good agreement with those calculated from soil cores. Mean soil bulk density was also calculated using a separate coefficient, QK, derived from the forward scattering γ photons from 40K within the soil relative to the 40K full energy peak (1460 keV). Again there was good agreement with soil core measurements (r2=0.64; P<0.05). The precision of the in situ137Cs measurement was limited by the precision with which QCs can be estimated, a function of the low 137Cs deposition levels associated with the weapons testing fallout and relatively low detector efficiency (35%). In contrast, the precision of the in situ ρm determination was only limited by the spatial variability associated with soil sampling. 相似文献
2.
Modern biotechnology promises a number of new applications in animal breeding and production. Although conventional pig breeding has achieved a high level of efficiency and productivity numerous problems have been encountered with animal health and the loss of meat quality. Selection based on phenotypic performance data of individual animals does not take into account the importance of specific genes and their relevance within a complex regulatory system. In most cases it is therefore difficult to trace back the genetic origins of clinically important disorders. The application of genetic engineering techniques in pig production will facilitate diagnosis, improvement of productivity, and animal health by allowing direct genetic manipulation. Attention must be focussed on the physical and genetic analysis of the procine genome. The isolation and characterisation of genes, DNA-markers, polymorphic DNA-fragments, and their chromosomal assignment will be important prerequisites and tools for the elucidation of genetic disorders. Especially the detection of heterozygous carriers of recessive disorders and their elimination from the breeding stock will increase selection accuracy and decrease the generation intervals. But also the rapid and simple detection of infectious diseases, which is sometimes difficult if not impossible at present, will improve animal health and welfare. Although the production of transgenic animals either by DNA-microinjection into zygotes or the use of embryonal stem cells manipulated in vitro is less straightforward than DNA-based diagnosis it will play an important role in the direct manipulation of the porcine genome and genes. Breeding programmes including the use of transgenic livestock have already been developed. There is no doubt that genetic engineering has reached a degree of practical feasibility, allowing it to play an important role in pig breeding in particular and animal production in general. 相似文献
3.
4.
数字乡村发展的实践与探索——基于北京的调研 总被引:1,自引:0,他引:1
研究旨在通过分析北京市数字乡村建设实践,发现存在问题,找出推进路径,为数字乡村发展提供可借鉴经验.利用文献分析、典型案例和专家咨询法,探讨数字乡村发展理论与实践.数字化是现代农业4.0阶段的关键特征,数字经济是农业农村高质量发展的新引擎、新动能.北京是数字乡村发展的先行者,为北京乡村振兴和农业现代化建设打下了坚实基础.... 相似文献
5.
DENNIS BALLANCE 《Veterinary radiology & ultrasound》2008,49(S1):S29-S32
The elements of a digital imaging system are bound together by the network, so careful attention must be paid to this essential component. Networking hardware and cable choice will affect the speed of image transmission between devices within a network. Wireless networking offers convenience at the expense of speed and potentially, security. If a facility allows its network to connect to the Internet, security precautions are essential. Firewalls prevent unauthorized and destructive access to the network; virtual private networks allow encrypted communication with the network; and email and web browser encryption allow data transmitted from the network to other users on the Internet safely. This article presents an overview of this broad array of technologies. Readers are encouraged to seek additional depth as needed to address individual networking needs. 相似文献
6.
J. D. C. Anderson S. M. Puchalski R. F. Larson M. L. Delco J. R. Snyder 《Equine Veterinary Education》2008,20(7):383-388
Insertional tendinopathies of the DDFT have been reported both as the sole lesion and as part of a multifocal lesion (Dyson et al. 2003). Computed tomography (CT) and magnetic resonance imaging allow specific diagnosis of deep digital flexor tendon lesions within the hoof capsule; however, direct intralesional treatment of such lesions is difficult because of the hoof's rigid structure. A technique designed to mimic intralesional injection of insertional tendinopathies of the DDFT in the standing horse using radiographic guidance was assessed. Radiographic and contrast CT imaging and sectioning of the limbs confirmed accurate injection in all cases although inadvertant administration of injectate into adjacent structures was also evident. 相似文献
7.
多年生黑麦草叶片长度数量性状位点(QTLs)研究 总被引:2,自引:1,他引:2
选择固定有益等位基因,研究多年生黑麦草叶片长度数量性状位点,结果表明:与叶长最为相关的3个标记所表达的变异占表型总变异的50.9%(r2);在选择中如能将这3个标记最优化地组合在一起,其综合共同作用有望使叶片长度延长和缩短26 mm;鉴于供试材料在最初3个世代的群体选择中所估测到叶片长度的遗传力为0.32,这样在今后的选择工作中,即使只使用现有的两个标记,其效果亦优于传统的表型选择法. 相似文献
8.
Olivier M. Lepage DMV MSc Renée Léveillé DMV Luc Breton DMV MSc Marcel Marcoux DMV MSc 《Veterinary radiology & ultrasound》1995,36(5):384-386
An 11-month-old Thoroughbred colt was presented with a hard swelling at the proximal third of the right 4th metatarsal bone. A medial dislocation of the deep digital flexor tendon ( flexor digitorum profundus ) was also observed on the same leg. On the plantaroproximal-plantarodistal projection of the calcaneus, there was flattening and shortening of the sustentaculum tali. The smooth bony proliferation at the proximal third of the right 4th metatarus was compatible with a chronic splint bone fracture. This report describes a medial deep digital flexor dislocation associated with hypoplasia of the sustentaculum tali . 相似文献
9.
AFLP是一种新的分子遗传标记技术,它结合了RFLP和PCR技术的优点,具有RFLP的稳定性和PCR的高效性,被称为“最有威力的分子遗传标记”或“下一代分子标记”。相对于国外而言,我国在这方面的研究还比较少。本文综述了扩增片段长度多态性(AFLP)这种遗传标记的原理、技术特点、技术发展及其在动物遗传多样性、种质鉴定、遗传图谱构建、基因定位和标记辅助育种等研究中的应用。 相似文献
10.
WILLIAM R. WIDMER 《Veterinary radiology & ultrasound》2008,49(S1):S2-S8
Use of digital radiography is growing rapidly in veterinary medicine. Two basic digital imaging systems are available, computed radiography (CR) and direct digital radiography (DDR). Computed radiographic detectors use a two-step process for image capture and processing. Image capture is by X-ray sensitive phosphors in the image plate. The image plate reader transforms the latent phosphor image to light photons that are converted to an analog electrical signal. An analog to digital converter is used to digitize the electrical signal before computer analysis. Direct digital detectors provide digital data by direct readout after image capture—a reader unnecessary. Types of DDR detectors are flat panel detectors and charge coupled device (CCD) detectors. Flat panel detectors are composed of layers of semiconductors for image capture with transistor and microscopic circuitry embedded in a pixel array. Direct converting flat panel detectors convert incident X-rays directly into electrical charges. Indirect detectors convert X-rays to visible light, then to electrical charges. All flat panel detectors send a digitized electrical signal to a computer using a direct link. Charge coupled device detectors have a small chip similar to those used in digital cameras. A scintillator first converts X-rays to a light signal that is minified by an optical system before reaching the chip. The chip sends a digital signal directly to a computer. Both CR and DDR provide quality diagnostic images. CR is a mature technology while DDR is an emerging technology. 相似文献