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   150304篇
  免费   8135篇
  国内免费   94篇
林业   6815篇
农学   5194篇
基础科学   933篇
  17610篇
综合类   26495篇
农作物   5859篇
水产渔业   7452篇
畜牧兽医   77011篇
园艺   1903篇
植物保护   9261篇
  2018年   2157篇
  2017年   2419篇
  2016年   2233篇
  2015年   1913篇
  2014年   2370篇
  2013年   5701篇
  2012年   4067篇
  2011年   5010篇
  2010年   3301篇
  2009年   3237篇
  2008年   4897篇
  2007年   4709篇
  2006年   4508篇
  2005年   4014篇
  2004年   3853篇
  2003年   3929篇
  2002年   3656篇
  2001年   4861篇
  2000年   4810篇
  1999年   3837篇
  1998年   1529篇
  1997年   1579篇
  1995年   1660篇
  1994年   1401篇
  1993年   1438篇
  1992年   2980篇
  1991年   3100篇
  1990年   3139篇
  1989年   3104篇
  1988年   2833篇
  1987年   2778篇
  1986年   2850篇
  1985年   2749篇
  1984年   2249篇
  1983年   1989篇
  1982年   1367篇
  1979年   2137篇
  1978年   1674篇
  1977年   1397篇
  1976年   1401篇
  1975年   1588篇
  1974年   1932篇
  1973年   1951篇
  1972年   1907篇
  1971年   1837篇
  1970年   1754篇
  1969年   1669篇
  1968年   1477篇
  1967年   1543篇
  1966年   1384篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Thyme ( Thymus vulgaris L.) plants were spaced at 15, 30 or 45 cm distances in a clay-loamy soil. They received different levels of nitrogen and phosphorus, besides a constant level of potassium. The results showed, that the wider spacing promoted the growth and production of herb and oil per plant, however dense cultivation significantly increased the yields of herb and oil per unit area. In all cases, the applied fertilization treatments significantly increased the productivity per unit area. Dense cultivation accompanied with higher levels of fertilization proved to be very usefull. The essential oil content was not influenced by either the plant spacing or fertilization treatments applied in this study.  相似文献   
92.
93.
Goats from 28 states were tested for antibodies to caprine arthritis-encephalitis virus. Of 3,790 goats, 1,175 (31%) tested positive, and of 196 herds tested, 143 (73%) had 1 or more seropositive members. This prevalence, based on serum samples from all goats in the participating herds, was lower than most rates reported in other studies. Such studies were based on fewer samples, incomplete sampling of herds, or smaller geographic base. Prevalence was highest in western Pacific and northern plains regions, increased with age to 3 years, was highest among goats on family-owned farms, and was lowest in the Angora breed. Differences in prevalence was not related to gender or size of herd.  相似文献   
94.
Four mares fed a low fiber, high soluble carbohydrate diet were used in a crossover design to evaluate the effects of dietary sodium bicarbonate (NaHCO3) supplementation during daily low-intensity submaximal working conditions. Mares were fed the diet at 1.7 times the maintenance energy requirement for mature horses at work. The horses tolerated the diet well and had no clinical abnormalities. Resting venous blood bicarbonate (HCO3), standard HCO3, and base excess (BE) concentrations significantly (P less than 0.05) increased with NaHCO3 supplementation, but no significant changes in resting venous blood pH or carbon dioxide tension (PCO2) were recorded. Venous blood HCO3, standard HCO3, BE, hemoglobin, and heart rate were significantly (P less than 0.05) increased and plasma lactate concentration was significantly (P less than 0.05) decreased in the control horses and in the horses given the NaHCO3 supplement during low-intensity submaximal exercise. There were no significant changes in venous blood pH, PCO2, or plasma protein concentration with exercise. Venous blood HCO3, standard HCO3, and BE concentrations were significantly (P less than 0.05) greater during submaximal exercise in horses given the NaHCO3 supplement. There were no significant differences in plasma lactate or total protein concentrations, blood pH, PCO2, or hemoglobin concentration between the 2 groups during exercise.  相似文献   
95.
96.
97.
Thirty-two crossbred cattle (steers = 17; heifers = 15) exhibiting an ultrasound fat thickness at the 12 to 13th rib region of at least 10 mm were selected from a slaughter shift at a commercial packing plant. After splitting, alternating sides of each carcass were trimmed of 1) subcutaneous fat in excess of 6.4 mm; 2) all kidney, pelvic, and heart fat; and 3) all cod or udder fat and fat in the flank region. Both sides of each carcass were fabricated into subprimals (final trim level of 6.4 mm) according to normal industry procedures. Effect of hot-fat trimming, yield grade (3, 4, and 5), and gender on hot-fat trim, fabrication fat trim, major subprimal, and total subprimal yield of untrimmed and trimmed carcasses were determined. Higher numerical yield grade (YG) corresponded with higher (P less than .05) percentages of hot-fat trim. Hot-fat trimming increased (P less than .05) the difference in fabrication fat trim between steers and heifers and between YG 3 and YG 5. Steers and heifers differed (P less than .05) in percentage of major subprimals and total subprimals when processed conventionally, whereas hot-fat trimming eliminated this difference (P less than .05). Untrimmed YG 3 carcasses had 3.1 and 5.0% higher major subprimal yield (P less than .05) than untrimmed YG 4 and YG 5 carcasses, respectively, whereas hot-fat trimming reduced this difference to 2.5% for YG 4 and to 3.7% for YG 5.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
98.
Effects of atherosclerosis on the mean value and daily variation of arterial pressure were studied in 12 Watanabe-heritable hyperlipidemic (WHHL) rabbits aged 12 to 35 months and 25 normal Japanese white rabbits aged 6 to 30 months. A pressure catheter was inserted through the left subclavian artery under pentobarbital anesthesia. A few days after the catheterization, the mean arterial pressure (MAP) of the rabbits, which were active and in a good state of appetite, was recorded by an analogue-to-digital converter every second for about 6 hrs and stored in a computer. The mean (M) and standard deviation (SD) in the WHHL rabbit, calculated from each successive MAP record, ranged widely from 85.8 to 131.4 mmHg and 5.6 to 12.6 mmHg, respectively. There was no significant correlation between M and SD in the WHHL rabbit. M and variance (V) of MAP in the WHHL rabbit were significantly higher than those in the normal rabbit. M did not show any significant change with increasing ages, whereas SD increased significantly with aging in the WHHL rabbit. Concentrations of serum total cholesterol and triglyceride in the WHHL rabbit were 475 and 328 mg/dl, which were about nine and seven times as high as those in the normal rabbit, respectively. Macroscopic and histopathological examinations of the aorta revealed development and spread of sclerotic lesions with aging in the WHHL rabbit. We can conclude that development of atherosclerosis with aging in the WHHL rabbit causes malfunction of the baroreceptors, which contributes to hypertension and lability of arterial pressure.  相似文献   
99.
100.
This study examines LEIBNIZ' idea of Veterinary medicine in a biographical context. It is based on material from the Leibniz-Archives of the Nieders?chsische Landesbibliothek Hannover, primary sources as well as the correspondence between LEIBNIZ and F. HOFFMANN and B. RAMAZZINI. Critical analysis of LEIBNIZ' proposal to establish a medical administrative authority and an analysis of further sources corroborate the view of LEIBNIZ as a progressive thinker who included the epidemiology of veterinary medicine (the preventive approach) in his conception. In this way he conceived of veterinary medicine in scientific terms which is going to be the relevant approach today.  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