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   277560篇
  免费   15909篇
  国内免费   632篇
林业   24101篇
农学   14159篇
基础科学   2979篇
  43962篇
综合类   27698篇
农作物   17188篇
水产渔业   17496篇
畜牧兽医   115742篇
园艺   6388篇
植物保护   24388篇
  2021年   2355篇
  2020年   2922篇
  2019年   3689篇
  2018年   4331篇
  2017年   4707篇
  2016年   4998篇
  2015年   4440篇
  2014年   5843篇
  2013年   16406篇
  2012年   7516篇
  2011年   9609篇
  2010年   7918篇
  2009年   8396篇
  2008年   8967篇
  2007年   8181篇
  2006年   8501篇
  2005年   7706篇
  2004年   7519篇
  2003年   7239篇
  2002年   6392篇
  2001年   7318篇
  2000年   6971篇
  1999年   6207篇
  1998年   3953篇
  1997年   3998篇
  1996年   3873篇
  1995年   4579篇
  1994年   3977篇
  1993年   3641篇
  1992年   4961篇
  1991年   5222篇
  1990年   4989篇
  1989年   5004篇
  1988年   4447篇
  1987年   4479篇
  1986年   4343篇
  1985年   4522篇
  1984年   4108篇
  1983年   3819篇
  1982年   3058篇
  1981年   2916篇
  1980年   2865篇
  1979年   3492篇
  1978年   3043篇
  1977年   2816篇
  1976年   2630篇
  1975年   2502篇
  1974年   2671篇
  1973年   2753篇
  1972年   2447篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
A large, firm, multi-cystic mammary gland mass grew slowly over 4 y in a 12-y-old, female Finn–Shetland cross sheep. A diagnosis of epithelial malignancy was suspected following fine-needle aspiration cytology at 30 mo after initial observation. The sheep was euthanized when the flock was downsized 18 mo later. A field postmortem examination revealed a large mammary mass, but an absence of metastases to internal organs. Imprint cytology of the mammary tissue supported a benign proliferative process. Histologically, mammary tissue was obliterated by cystic, tubular, and papillary adenomatous arrangements of mammary epithelium, with an anaplastic component, consistent with mammary carcinoma arising in an adenoma. IHC showed strong nuclear positivity to the antibody against progesterone receptor and minimal positivity to the antibody against estrogen receptor alpha expression. Intrinsic subtyping for basal or luminal epithelial origin was attempted through adaptation of companion animal IHC classification panels; high- and low-molecular-weight cytokeratins (CK5, CK8, CK18) failed to stain, but p63 expression for basal epithelium was positive.  相似文献   
92.
93.
Increasing soil carbon (C) in arable soils is an important strategy to achieve sustainable yields and mitigate climate change. We investigated changes in soil organic and inorganic carbon (SOC and SIC) under conservation agriculture (CA) in a calcareous soil of the eastern Indo-Gangetic Plains of India. The treatments were as follows: conventional-till rice and wheat (CT-CT), CT rice and zero-till wheat (CT-ZT), ZT direct seeded rice (DSR) and CT wheat (ZT-CT), ZTDSR and ZT wheat without crop residue retention (ZT-ZT), ZT-ZT with residue (ZT-ZT+R), and DSR and wheat both on permanent beds with residue (PB-PB+R). The ZT-ZT+R had the highest total SOC in both 0–15 and 15–30 cm soil layers (20% and 40% higher (p < .05) than CT-CT, respectively), whereas total SIC decreased by 11% and 15% in the respective layers under ZT-ZT+R compared with CT-CT. Non-labile SOC was the largest pool, followed by very labile, labile and less labile SOC. The benefits of ZT and residue retention were greatest for very labile SOC, which showed a significant (p < .05) increase (~50%) under ZT-ZT+R compared with CT-CT. The ZT-ZT+R sequestered ~2 Mg ha−1 total SOC in the 0–15 cm soil layer in 6 years, where CT registered significant losses. Thus, the adoption of CA should be recommended in calcareous soils, for C sequestration, and also as a reclamation technique.  相似文献   
94.
A 3-month-old colt foal presented to the Philip Leverhulme Equine Hospital for investigation of progressive neurological signs. Diagnostic investigation included cerebrospinal fluid collection, which was performed under general anaesthesia. During this procedure, severe bradycardia which progressed to asystole occurred. Initial resuscitation was successful; however, the foal had clinical signs consistent with cerebral hypoxia post-resuscitation and was euthanased the following day due to deterioration of neurological function. Asystole was presumed due to a Cushing-type reflex as a result of changes in intracranial pressure during the sampling procedure.  相似文献   
95.
To evaluate the utility of random-effects linear modeling for herd-level evaluation of trace mineral status, we performed a retrospective analysis of the results for trace mineral testing of bovine liver samples submitted to the Michigan State University Veterinary Diagnostic Laboratory between 2011 and 2017. Our aim was to examine random-effects models for their potential utility in improving interpretation with minimal sample numbers. The database consisted of 1,658 animals distributed among 121 herds. Minerals were assayed by inductively coupled plasma–mass spectroscopy, and included cobalt, copper, iron, molybdenum, manganese, selenium, and zinc. Intraclass correlation coefficients for each mineral were significantly different (p < 0.001) from zero and ranged from 0.38 for manganese to 0.82 for selenium, indicating that the strength of herd effects, which are presumably related to diet, vary greatly by mineral. Analysis of the distribution and standard errors of best linear unbiased predictor (BLUP) values suggested that testing 5–10 animals per herd could place herds within 10 percentile units across the population of herds with 70–95% confidence, the confidence level varying among minerals. Herd means were generally similar to BLUPs, suggesting that means could be reasonably compared to BLUPs with respect to the distributions reported here. However, caution in interpreting means relative to BLUPs should be exercised when animal numbers are small, the standard errors of the means are large, and/or the values are near the extremes of the distribution.  相似文献   
96.
Farmers are the key agents who manage land and water. Agriculture Green Development (AGD) requires a transformation in farming from high resource consumption and environmental cost to sustainable intensification with high productivity, high resource use efficiency and low environmental risk. This paper analyzes the public policy challenge of AGD and makes the case for a location-sensitive policy mix made up of regulation, advice provision, voluntarism and targeted incentives. The public agricultural extension service in China is a key resource, but one that requires reorientation and reform with the aim of better balancing high farm productivity with environmental protection.  相似文献   
97.
Eurasian Soil Science - The dynamics of some soil properties and carbon stocks in the main components of postagrogenic ecosystems were studied during natural reforestation in dependence on the...  相似文献   
98.
Eurasian Soil Science - The results are presented of modeling the “contour water flooding” biotechnology aimed at cleaning heavily petroleum-polluted agrosoddy-podzolic soils (Albic...  相似文献   
99.
Eurasian Soil Science - Chernozemic soils (Voronezh oblast, Russia) are affected by the endogenous hydrogen fluid. The chemical properties of ordinary chernozems (Haplic Chernozems) are shown to be...  相似文献   
100.
Eurasian Soil Science - Data on concentrations and distribution of tritium in soils of the “Atomic” Lake excavation explosion area of Semipalatinsk Nuclear Test Site (Kazakhstan, East...  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