首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   3017篇
  免费   174篇
  国内免费   493篇
林业   211篇
农学   278篇
基础科学   848篇
  576篇
综合类   897篇
农作物   86篇
水产渔业   104篇
畜牧兽医   534篇
园艺   52篇
植物保护   98篇
  2024年   16篇
  2023年   51篇
  2022年   111篇
  2021年   116篇
  2020年   139篇
  2019年   159篇
  2018年   134篇
  2017年   169篇
  2016年   172篇
  2015年   159篇
  2014年   166篇
  2013年   234篇
  2012年   203篇
  2011年   231篇
  2010年   189篇
  2009年   152篇
  2008年   153篇
  2007年   174篇
  2006年   137篇
  2005年   127篇
  2004年   94篇
  2003年   88篇
  2002年   80篇
  2001年   53篇
  2000年   47篇
  1999年   42篇
  1998年   41篇
  1997年   49篇
  1996年   29篇
  1995年   27篇
  1994年   33篇
  1993年   25篇
  1992年   17篇
  1991年   19篇
  1990年   13篇
  1989年   17篇
  1988年   9篇
  1987年   4篇
  1986年   2篇
  1985年   1篇
  1984年   2篇
排序方式: 共有3684条查询结果,搜索用时 296 毫秒
81.
Comparison of test characteristics allows a clinician to choose the optimal diagnostic test method for an individual patient. This study assessed the comparative test characteristics of noninvasive (NI) blood pressure measurement methods (oscillometric and Doppler) and used this information to develop optimal cutoff values for diagnosis of systolic hypertension in dogs by these NI methods. Simultaneous NI (oscillometric or Doppler methods) and invasive (arterial puncture [AP]) systolic blood pressure (SBP) measurements were obtained prospectively from normal dogs and dogs suspected of having systemic hypertension based on clinical signs. Oscillometric SBP readings were obtained from the distal hind limb (Osc-L, n = 54) or the proximal tail (T. n = 27). Doppler BP measurements were obtained using a forelimb cuff (n = 57). AP-SBP was categorized as hypertensive if > or = 160 mmHg, and sensitivity (Se). specificity (Sp), and likelihood ratios (LR) were calculated for diagnostic cutoff values ranging from 130 to 220 mmHg. Receiver operator characteristic (ROC) curves were analyzed to determine optimal cutoff values for diagnosis of AP-SBP > or = 160 mmHg. Optimal NI SBP cutoff values considered to reflect AP values > or = 160 mmHg were: Osc-L = 160 mmHg (Se: 65%, Sp: 85%. LR = 4.33: 1), Osc-T = 150 mmHg (Se: 84%, Sp: 75%, LR = 3.36: 1), and Doppler = 160 mmHg (Se: 71%,  相似文献   
82.
The combined occurrence of ocular pigment deposition and glaucoma has been described in Cairn Terriers. Recently, this condition was also observed in two other breeds: the Boxer (two cases) and the Labrador Retriever (one case). Six dogs were referred to the Ophthalmology section of the Department of Clinical Sciences of Companion Animals and to a private referral clinic because of glaucoma or blindness in one or both eyes. In five cases ophthalmic examination showed pigment depositions in the sclera around the entire circumference of the perilimbal zone. Eight enucleated eyes (four eyes of two Cairn Terriers, three eyes of two Boxers and one eye of a Labrador Retriever) were examined microscopically. All eyes showed the same findings: an extensive infiltration of large melanin-containing cells with an eccentric nucleus, located in the iris, ciliary body, retina, choroids and sclera. Transmission electron microscopy of two of the examined eyes revealed that the morphology of most of these cells was consistent with melanophages. While reports in the veterinary literature concerning this condition are limited the cells concerned have been described to be melanocytes. Further research is needed to conclusively identify the cell type. As described in the present report, the histologic and transmission electron microscopic findings suggest a different etiology of the ocular pigment deposition and glaucoma compared with the pigment dispersal syndrome in humans.  相似文献   
83.
Objective To study the course of intraocular pressure (IOP) after cataract surgery in 50 dogs. Design Prospective study. Animals Fifty dogs without preoperative ocular hypertension were selected for cataract surgery. Methods All dogs underwent cataract surgery: 25 by manual extracapsular extraction and 25 by phacoemulsification. For each dog, intraocular pressure was measured before surgery, and 1, 3, 5, 18 h, 1 week and 1 month post surgery. Results No significant difference of mean intraocular pressure between the two surgical methods was observed for each time measurement. Nine dogs had postoperative hypertension (IOP > 25 mmHg) during the first 5 hours post surgery. Incidence of postoperative hypertension was not significantly different with manual extracapsular extraction (16%) vs. phacoemulsification (20%). A decrease of mean IOP was observed 1 h after surgery (8.49 mmHg vs. 10.91 mmHg), then an increase 3 and 5 h post surgery (12.3 and 13.32 mmHg, respectively). At 18 h, 1 week and 1 month post surgery, mean IOP decreased. Mean IOP was 10.38, 10.38 and 8.84 mmHg, respectively. Conclusion In this study incidence of POH is not high. However, a follow‐up of IOP in the first hours after cataract surgery is required to avoid complications of the retina and optic nerve and to administer hypotensive treatment if necessary.  相似文献   
84.
Oba  Gufu  Post  Eric  Syvertsen  P.O.  Stenseth  N.C. 《Landscape Ecology》2000,15(6):535-546
Progressive growth of bush cover in dry savannahs is responsible for declines in range conditions. In southern Ethiopia, the Booran pastoralists assisted our understanding of spatial patterns of bush cover and range conditions in 54 landscape patch types grouped into six landscape units within an area of 30000 km2. The size of landscape patches sampled was 625 m2. We assessed the relationships between bush cover, grass cover and bare soil and grazing pressure and soil erosion and changes in range condition. Externally, political conflicts and internally, break down of land use, and official bans on the use of fire promoted bush cover and the decline in range conditions. Bush cover was negatively correlated with grass cover, and positively correlated with bare soil. Grass cover was negatively correlated with bare soil and grazing pressure in most landscape patch types. Grazing pressure was not significantly correlated with bush cover or bare soil, while soil erosion was directly related to bare soil. Soil erosion was absent in 64% of the landscape patch types, and seemingly not a threat to the rangelands. The relationship between bush cover, grass cover, bare soil and soil erosion is complex and related to climate, landscape geology, and patterns of land use. Main threats to range conditions are bush climax, loss of grass cover and unpalatable forbs. Currently, >70% of the landscape patch types are in poor to fair range conditions. Decline in range conditions, unless reversed, will jeopardise the pastoral production system in southern Ethiopia.  相似文献   
85.
本试验在121℃条件下,采用不同灭菌时间对不同包装规格的SPF鸡饲料进行高压蒸汽灭菌处理,通过细菌培养和灭菌指示剂对不同处理后饲料的灭菌效果进行检查,以确定饲料最短有效灭菌时间。结果表明,饲料包装尺寸影响高压蒸汽灭菌效果,在包装尺寸合理的前提下,经121℃、20min高压蒸汽处理过的饲料,在30d的保存期内未检测到细菌,可以满足SPF鸡饲料的灭菌要求,生物灭菌指示剂试验得出同样结果。  相似文献   
86.
利用2年沙漠边缘至绿洲4个观测点的气象资料,计算得到感热通量、湍流热量、净辐射、同一高度的气压等,讨论这些要素的时间和空间变化规律,解释绿洲至沙漠气温逐渐增加的原因,以对沙漠增温效应做出进一步的分析,为发挥特定地区的农业气候资源优势,促进农业结构调整和区域经济发展及构建新型农业技术体系提供理论基础.  相似文献   
87.
Distribution of intraocular pressure in dogs   总被引:2,自引:0,他引:2  
Intraocular pressure (IOP) was measured by four different applanation tonometers in normal dogs. By MacKay-Marg tonometry in 391 dogs (772 eyes) the mean ± SD IOP was 18.8 ± 5.5 mmHg (range 8–52 mmHg). Using Tono-Pen XL tonometry in 421 dogs (823 eyes) the mean IOP was 19.2 ± 5.9 mmHg, and the range was 4.42 mmHg. With MMAC-II tonometry in 80 dogs (158 eyes), the mean IOP was 15.7 ± 2.8 mmHg with a range of 10–30 mmHg. By pneumatonograph tonometry in 135 dogs (255 eyes), the mean IOP was 22.9 ± 6.1 mmHg and the range was 10–47 mmHg. In this study 53 breeds were represented. Of those breeds with six animals or more, no significant differences were detected in IOP between breeds ( P > 0.353) or sex ( P > 0.270). There was a significant decline of 2–4 mmHg ( P > 0.0001) in IOP as age increased from less than 2 years to greater than 6 years of age. This trend was present with all of the four tonometers. There were no significant differences between the MacKay-Marg and TonoPen-XL tonometers ( P > 0.198), but significant differences with the MMAC-II ( P > 0.001) and pneumatonograph ( P > 0.001) tonometers existed compared to the first two instruments. Based on this study and the literature, the mean IOP for the normal dog is 19.0 mmHg with a range of 11 (5%) and 29 (95%) mmHg.  相似文献   
88.
Intraocular pressure (IOP) was measured in normal dairy cows by applanation tonometry. In the first study of 15 Holstein and 17 Jersey cows the mean IOP by Mackay-Marg tonometry was 27.5 ± 4.8 mmHg (range 16–39 mmHg); no significant differences ( P < 0.92) were observed between the Holstein and Jersey breeds. In the second study of 15 Holstein and 12 Jersey cows, the mean IOPs by Mackay-Marg and TonoPen-XL tonometry were 28.2 ± 4.6 mmHg (range 19–39 mmHg) and 26.9 ± 6.7 mmHg (range 16–42 mmHg), respectively. Comparisons of the Mackay-Marg and TonoPen tonometers indicated no significant differences ( P < 0.16). The mean and range of IOP in normal dairy cows within 2 SD (95% of the population) is 27 mmHg with a range of 16–36 mmHg.  相似文献   
89.
Tonometry was performed to estimate intraocular pressure (IOP) in 12 Nubian ibexes ( Capra ibex nubiana ), 10 Grant zebras ( Equus burchelli  ) and five Arabian oryxes ( Oryx leucoryx ), using both applanation (Tono-Pen) and/or indentation (Schiotz) tonometers. Animals were anesthetized with a mixture of etorphine hydrochloride and acepromazine maleate. Mean (± SD) IOP in the ibex was 17.95 ± 4.78 mmHg (24 eyes, indentation tonometry). In the zebra, indentation tonometry (20 eyes) yielded a mean IOP of 25.30 ± 3.06 mmHg, and applanation tonometry (six eyes) yielded a mean IOP of 29.47 ± 3.43 mmHg. In the oryx, indentation tonometry (five eyes) yielded a mean IOP of 22.68 ± 8.15 mmHg, and applanation tonometry (10 eyes) yielded a mean IOP of 11.76 ± 3.43 mmHg. There were no significant effects of gender, age, weight, side or reading number on the IOP measured in any of the three species. No significant differences were found between the IOP of the three species, nor between the readings of the two instruments, although some of the P -values were close to the significance level.  相似文献   
90.
Predictions of future climate change include shifts in patterns of precipitation, evapotranspiration and water run‐off, resulting in increased periods of drought as well as variability and intensity of rainfall events. In the United Kingdom, the non‐native North American sunfish, pumpkinseed Lepomis gibbosus (L.), is expected to benefit from these changes. We examine how hydrological variability induced by predicted changes in climate will affect the dispersal and spread of pumpkinseed in England by: (i) determining the relationship between discharge regime and pumpkinseed propagule pressure; (ii) examining a newly‐established pumpkinseed population following a flood event in 2007; and (iii) comparing the growth and life‐history traits of this new population with fish collected from the source population to demonstrate how the pumpkinseed's life‐history plasticity contributes to its success as a coloniser. Using Bayesian modelling, we determined that the number of pumpkinseed escapees is likely to increase with increasing discharge. The newly‐established pumpkinseed population showed fast juvenile growth, early age at maturity and small size at maturity. These traits differed significantly from the source population, specifically total length (TL) means at ages 1 and 2 were significantly greater in the new population, whereas TL at age 4 was significantly greater in the source population, and a significantly higher proportion of mature females were found at smaller size classes in the newly established pumpkinseed population. This study demonstrates the potential link between hydrological variability (current and future) and the dispersal of non‐native pumpkinseed, leading to the establishment of new populations.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号