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   63605篇
  免费   2897篇
  国内免费   43篇
林业   2603篇
农学   1674篇
基础科学   434篇
  6345篇
综合类   14934篇
农作物   2498篇
水产渔业   2441篇
畜牧兽医   31459篇
园艺   677篇
植物保护   3480篇
  2018年   548篇
  2017年   579篇
  2016年   563篇
  2014年   578篇
  2013年   1915篇
  2012年   1269篇
  2011年   1539篇
  2010年   969篇
  2009年   919篇
  2008年   1488篇
  2007年   1457篇
  2006年   1448篇
  2005年   1441篇
  2004年   1371篇
  2003年   1446篇
  2002年   1399篇
  2001年   1503篇
  2000年   1479篇
  1999年   1221篇
  1997年   547篇
  1995年   602篇
  1994年   582篇
  1993年   572篇
  1992年   1316篇
  1991年   1402篇
  1990年   1454篇
  1989年   1490篇
  1988年   1404篇
  1987年   1359篇
  1986年   1400篇
  1985年   1376篇
  1984年   1154篇
  1983年   1008篇
  1982年   719篇
  1981年   690篇
  1980年   647篇
  1979年   1127篇
  1978年   916篇
  1977年   819篇
  1976年   768篇
  1975年   862篇
  1974年   1129篇
  1973年   1064篇
  1972年   1129篇
  1971年   1082篇
  1970年   1022篇
  1969年   875篇
  1968年   711篇
  1967年   844篇
  1966年   693篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
122.
The objective of this study was to investigate epidemiological risk factors for porcine non-specific colitis (NSC). Forty-seven Scottish pig farms, with and without a clinical history of diarrhoea during the growing period (15-40 kg), were selected. The study included farm visits, clinical inspection of pigs, completion of farm management questionnaires, pathological tests into the cause of the diarrhoea and analysis of the non-starch polysaccharide (NSP) content of feeds. The results from 17 farms designated as NSC and 10 control farms suggest dietary associations with NSC. Farms with NSC fed diets with significantly higher levels of NSPs, especially containing the sugars glucose, arabinose and xylose. Few management factors were identified, although the high prevalence of infectious colitis reduced the power of the study.  相似文献   
123.
124.
The performance of a Didcot-type thermometer screen on an Automatic Weather Station and a conventional Stevenson screen were compared, both with each other and against an aspirated Assmann psychrometer. Maximum temperature in the Didcot screen exceeded the conventional screen maximum by up to 1 K on calm, sunny days, and Didcot minimum temperatures were as much as 1.5 K less than the screen minima on still, clear nights; mean temperature discrepancies between screens were + 0.3 and − 0.4 K for maxima and minima, respectively. The apparent psychrometric constant for the Didcot screen was 1.0 mbar K−1 which increased to 1.2 mbar K−1 when u < 2 m s−1 in contrast to the value of 0.799 mbar K−1 frequently used for the Stevenson screen.  相似文献   
125.
Carbon kinetics of milk formation in Holstein cows in late lactation   总被引:3,自引:0,他引:3  
Carbon transfer to milk in Holstein cows in late lactation was measured by introducing changes in the natural stable carbon isotope composition of the feed. Six Holstein cows in mid-lactation were placed on a diet naturally low in 13C (-25.0% vs Pee Dee belemnite [PDB] an international carbon isotope standard), based on alfalfa-barley, and six others were placed on a diet naturally enriched in 13C (-11.5% vs PDB), based on corn. After a 7-wk equilibration period on these diets, three cows were switched from alfalfa-barley to corn, and three were switched from corn to alfalfa-barley. The three other cows in each group served as controls. 13C/12C ratios were measured in daily morning milk samples during the week before and for 6 wk after the changes in diet. After the diets had been switched, milk isotope ratios rapidly approached the isotopic composition of the new diet, indicating rapid transfer of dietary carbon into milk. The data were consistent with a model whereby milk was synthesized from a single precursor pool that responded rapidly to dietary perturbation. The milk precursor pool had a half-life of approximately .9 d and had a mass of approximately 7 kg of carbon, which was renewed daily by the entry of 5 kg of digestible dietary carbon.  相似文献   
126.
Letters     
  相似文献   
127.
128.
ABSTRACT Production of cacao in tropical America has been severely affected by fungal pathogens causing diseases known as witches' broom (WB, caused by Moniliophthora perniciosa), frosty pod (FP, caused by M. roreri) and black pod (BP, caused by Phytophthora spp.). BP is pan-tropical and causes losses in all producing areas. WB is found in South America and parts of the Caribbean, while FP is found in Central America and parts of South America. Together, these diseases were responsible for over 700 million US dollars in losses in 2001 (4). Commercial cacao production in West Africa and South Asia are not yet affected by WB and FP, but cacao grown in these regions is susceptible to both. With the goal of providing new disease resistant cultivars the USDA-ARS and Mars, Inc. have developed a marker assisted selection (MAS) program. Quantitative trait loci have been identified for resistance to WB, FP, and BP. The potential usefulness of these markers in identifying resistant individuals has been confirmed in an experimental F(1) family in Ecuador.  相似文献   
129.
130.
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