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   282944篇
  免费   16130篇
  国内免费   682篇
林业   24343篇
农学   14225篇
基础科学   2987篇
  45815篇
综合类   28150篇
农作物   17664篇
水产渔业   17762篇
畜牧兽医   117418篇
园艺   6413篇
植物保护   24979篇
  2021年   2564篇
  2020年   2940篇
  2019年   3727篇
  2018年   4396篇
  2017年   4776篇
  2016年   5051篇
  2015年   4480篇
  2014年   5881篇
  2013年   16472篇
  2012年   7633篇
  2011年   9767篇
  2010年   7998篇
  2009年   8457篇
  2008年   9083篇
  2007年   8280篇
  2006年   8577篇
  2005年   7766篇
  2004年   7573篇
  2003年   7448篇
  2002年   6629篇
  2001年   7605篇
  2000年   7293篇
  1999年   6543篇
  1998年   4263篇
  1997年   4313篇
  1996年   4045篇
  1995年   4603篇
  1994年   4009篇
  1993年   3699篇
  1992年   5025篇
  1991年   5262篇
  1990年   5074篇
  1989年   5079篇
  1988年   4478篇
  1987年   4518篇
  1986年   4375篇
  1985年   4536篇
  1984年   4126篇
  1983年   3831篇
  1982年   3062篇
  1981年   2923篇
  1980年   2877篇
  1979年   3522篇
  1978年   3059篇
  1977年   2838篇
  1976年   2662篇
  1975年   2537篇
  1974年   2708篇
  1973年   2800篇
  1972年   2481篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
994.
Sodium selenite was mixed in a commercial pig food so as to increase the original selenium content (0.126 ppm) by 1.2 ppm. The supplemented food was given to 7 pigs during 78 days. A control group of as many pigs was given the non-supplemented commercial food. The pigs were killed, starting 4 hours after the last selenium-supplemented food had been given and then with intervals during the next 14 days. Small but significant increases of tissue selenium were noticed in liver and striated muscle as compared with the control animals. The kidneys showed no increase.  相似文献   
995.
1. Methods for quantitative determination of proteolytic, lipolytic and haemolytic activities and egg yolk factors in semen are described.2. Comparative quantitative determinations of these enzyme activities in the semen of goat, ram, bull, boar, dog and horse have been carried out.3. Special interest attaches to the egg yolk turbidity factor which is found in goat semen and, in small concentrations, also in boar semen.4. As each species appears to have its specific enzyme pattern, regular enzymatic control does not seem to have great significance.  相似文献   
996.
Uptake of I131-labelled triiodothyronine by erythrocytes (EU test) as a test of thyroid function has been studied in cattle. The normal EU value for 141 Swedish Red-and-white cows was 6.99 ± 0.95 (range 4.45–10.23) at an incubation time of one hour. Statistically significantly higher values were obtained for newly-calved cows. The EU values for calves are significantly higher but by 12 to 18 months of age decline to the same level as for older cows.The principle of the test is described and the results obtained compared with those for human beings. The EU test is suitable for screening thyroid function for large groups of animals and for studying associations between thyroid activity and different production qualities.  相似文献   
997.
998.
Summary. Seeds of Polygonam persiaria were collected from seven localities during 1962. In 1963 plants of these seven populations were grown in the field and sampled at regular intervals over the period 21st June to 13th August. At all samplings there were differences between populations in total dry weight, in leaf area index and in total nitrogen content, and populations also differed in the date of flowering. The populations also differed significantly in relative growth rate, leaf area ratio, net assimilation rate and relative nitrogen absorption rate. The variations in net assimilation rate did not appear to be closely associated with variations in leaf area index. Certain growth attributes were correlated with the latitude of origin, and it is tentatively suggested that the populations may represent genetically distinct latitudinal ecotypes.
Études sur des espèces de mauvaises herbes du genre Polygonum L.  相似文献   
999.
1000.
To evaluate the suitability of the analytical methods used in determining food safety, a new metrological characteristic "MQS" is suggested. MQS is defined as the absolute minimum quantity in micrograms of a substance that can be determined in a test solution (solubilized test portion). MQS accounts for 2 factors: (a) the necessity for a reliable determination of ML (maximum permitted level, i.e., regulatory tolerance), and (b) the optimum quantity of test portion of a food product to be analyzed, and thus assists in evaluating the suitability of a method to assure food safety. The MQS of 8 toxic elements in any food are As, 3; Cd, 0.5; Cu, 20; Fe, 50; Hg, 0.2; Pb, 4; Sn, 200; Zn, 100 micrograms. To characterize the applicability of any given method, the specific minimum limit of determination, MQSM, must be established for each method. The method in question may be used to control food safety only if MQSM is less than MQS. MQSM values are given for the common polarographic and colorimetric methods for determining these elements.  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