全文获取类型
收费全文 | 4909篇 |
免费 | 359篇 |
国内免费 | 454篇 |
专业分类
林业 | 373篇 |
农学 | 270篇 |
基础科学 | 206篇 |
630篇 | |
综合类 | 2152篇 |
农作物 | 342篇 |
水产渔业 | 184篇 |
畜牧兽医 | 984篇 |
园艺 | 356篇 |
植物保护 | 225篇 |
出版年
2024年 | 40篇 |
2023年 | 109篇 |
2022年 | 232篇 |
2021年 | 229篇 |
2020年 | 203篇 |
2019年 | 199篇 |
2018年 | 186篇 |
2017年 | 240篇 |
2016年 | 154篇 |
2015年 | 262篇 |
2014年 | 248篇 |
2013年 | 306篇 |
2012年 | 373篇 |
2011年 | 415篇 |
2010年 | 390篇 |
2009年 | 352篇 |
2008年 | 370篇 |
2007年 | 314篇 |
2006年 | 242篇 |
2005年 | 188篇 |
2004年 | 154篇 |
2003年 | 93篇 |
2002年 | 97篇 |
2001年 | 83篇 |
2000年 | 84篇 |
1999年 | 36篇 |
1998年 | 13篇 |
1997年 | 6篇 |
1996年 | 6篇 |
1995年 | 7篇 |
1994年 | 10篇 |
1993年 | 15篇 |
1992年 | 16篇 |
1991年 | 13篇 |
1990年 | 5篇 |
1989年 | 14篇 |
1988年 | 8篇 |
1987年 | 4篇 |
1986年 | 1篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1956年 | 2篇 |
排序方式: 共有5722条查询结果,搜索用时 15 毫秒
1.
Jian-zhai WU Jing ZHANG Zhang-ming GE Li-wei XING Shu-qing HAN Chen SHEN Fan-tao KONG 《农业科学学报》2021,20(1):289-299
Climate change severely impacts agricultural production, which jeopardizes food security. China is the second largest maize producer in the world and also the largest consumer of maize. Analyzing the impact of climate change on maize yields can provide effective guidance to national and international economics and politics. Panel models are unable to determine the group-wise heteroscedasticity, cross-sectional correlation and autocorrelation of datasets, therefore we adopted the feasible generalized least square(FGLS) model to evaluate the impact of climate change on maize yields in China from 1979–2016 and got the following results:(1) During the 1979–2016 period, increases in temperature negatively impacted the maize yield of China. For every 1°C increase in temperature, the maize yield was reduced by 5.19 kg 667 m–2(1.7%). Precipitation increased only marginally during this time, and therefore its impact on the maize yield was negligible. For every 1 mm increase in precipitation, the maize yield increased by an insignificant amount of 0.043 kg 667 m–2(0.014%).(2) The impacts of climate change on maize yield differ spatially, with more significant impacts experienced in southern China. In this region, a 1°C increase in temperature resulted in a 7.49 kg 667 m–2 decrease in the maize yield, while the impact of temperature on the maize yield in northern China was insignificant. For every 1 mm increase in precipitation, the maize yield increased by 0.013 kg 667 m–2 in southern China and 0.066 kg 667 m–2 in northern China.(3) The resilience of the maize crop to climate change is strong. The marginal effect of temperature in both southern and northern China during the 1990–2016 period was smaller than that for the 1979–2016 period. 相似文献
2.
【目的】探究Landsat8 OLI数据和KNN算法在森林蓄积量估测中的潜力。【方法】以湖南省湘潭县为研究区,采用Landsat8 OLI数据和同时期的二类调查数据,通过距离相关系数筛选特征,分别采用线性回归模型(MLR)、K-近邻模型(KNN)、距离加权KNN模型(DW-KNN)和优化欧式KNN模型(FW-KNN)对森林蓄积量进行估测。使用十折交叉方法进行精度检验,对检验结果进行对比分析。【结果】3种KNN模型的估测结果均高于传统的线性模型,并且在3种KNN模型中,FW-KNN算法效果最好,决定系数达到0.69,为3种模型中最高;3种KNN模型中,本研究优化欧氏距离KNN模型的估测精度最高,其均方根误差为30.3%,相比于传统KNN模型的均方根误差降低了5.1%,相比于DW-KNN模型降低了3.3%。【结论】采用DW-KNN蓄积量估测结果明显优于其他两种模型,说明通过特征与蓄积量的相关性优化样本间的距离是一种可行的KNN优化方法。 相似文献
3.
5.
The immunohistochemical detection of Mycoplasma bovis and bovine viral diarrhea virus in tissues of feedlot cattle with chronic, unresponsive respiratory disease and/or arthritis.
下载免费PDF全文
![点击此处可从《The Canadian veterinary journal. La revue veterinaire canadienne》网站下载免费的PDF全文](/ch/ext_images/free.gif)
D M Haines K M Martin E G Clark G K Jim E D Janzen 《The Canadian veterinary journal. La revue veterinaire canadienne》2001,42(11):857-860
The purpose of this study was to determine the frequency of selected pathogens in the tissues of a group of feedlot cattle with chronic disease (most often respiratory disease and/or arthritis). Samples of lung and joint tissues from 49 feedlot animals that had failed to respond to antibiotic therapy were tested by immunohistochemical staining for the antigens of Mycoplasma bovis, Haemophilus somnus, Pasteurella (Mannheimia) hemolytica, and bovine viral diarrhea virus (BVDV). Mycoplasma bovis was demonstrated in over 80% of cases, including in 45% of joints and 71% of lungs tested. Mycoplasma bovis was the only bacterial pathogen identified in the joints. Haemophilus somnus and Pasteurella (Mannheimia) haemolytica were found in 14% and 23% of cases, respectively, and were confined to the lungs in all instances. Infection with BVDV was demonstrated in over 40% of cases. Mycoplasma bovis and BVDV were the most common pathogens persisting in the tissues of these animals that had failed to respond to antibiotic therapy. 相似文献
6.
7.
8.
9.
Pharmacokinetic and Phase I Evaluation of Carboplatin in Dogs 总被引:1,自引:1,他引:0
Rodney L. Page DVM MS Margaret C. McEntee DVM Steven L. George PhD Patrick L. Williams PhD Greta L. Heidner DVM Carol A. Novotney DVM Jim E. Riviere DVM PhD Mark W. Dewhirst DVM PhD Donald E. Thrall DVM PhD 《Journal of veterinary internal medicine / American College of Veterinary Internal Medicine》1993,7(4):235-240
Thirty dogs with spontaneously occurring malignant neoplasms were treated monthly with carboplatin (CBDCA) given as a 30-minute intravenous infusion in a dose escalation study. Twenty-eight dogs were considered evaluable for toxicity. The maximally tolerated dose of CBDCA was conceptually defined as that dose, determined by logistic regression analyses of toxicity data, resulting in a 50% incidence of moderate toxicity (MOD50) or a 5% incidence of severe toxicity (SEV5). Each designated maximally tolerated dose was calculated for the first course of treatment only and for the first and second courses of treatment combined to estimate cumulative drug toxicity. The MOD50 and SEV5 for the first treatment course were 340 and 278 mg/M2, respectively. MOD50 and SEV5 values for the first plus second treatment courses were 327 and 231 mg/M2, respectively. The nadir of neutrophil and platelet counts occurred approximately 14 days after treatment. The mean neutrophil and platelet values for all dogs experiencing myelosuppression during the first two treatment courses were 1541/μL and 62,600/μL, respectively. Nonparametric pharmacokinetic analysis of plasma CBDCA values suggested that half-life (T1/2), area-under-the-curve and total body clearance (CLb) were not dose dependent. Volume of distribution (VDss) significantly increased with dose only between 100 and 150 mg/M2, not between 150 and 300 mg/M2. Dose-independent serum CBDCA pharmacokinetic disposition indicates that detailed investigation of tissue CBDCA distribution would be warranted and may identify novel dosing strategies that could improve the therapeutic index of CBDCA by minimizing toxicity. (Journal of Veterinary Internal Medicine 1993; 7:235–240. Copyright © 1993 by the American College of Veterinary Internal Medicine.) 相似文献
10.
河北省平原农区肉羊规模舍饲的前景分析 总被引:2,自引:0,他引:2
本文指出了当前河北省平原农区肉羊产业的发展面临瓶颈局面,理性分析了造成这一尴尬局面的原因,并提出了发展河北省平原农区肉羊规模舍饲是打破这一瓶颈局面的突破口。 相似文献