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   90253篇
  免费   5553篇
  国内免费   46篇
林业   3755篇
农学   3475篇
基础科学   632篇
  11315篇
综合类   13582篇
农作物   3399篇
水产渔业   4957篇
畜牧兽医   47907篇
园艺   1186篇
植物保护   5644篇
  2020年   742篇
  2019年   965篇
  2018年   1592篇
  2017年   1850篇
  2016年   1673篇
  2015年   1417篇
  2014年   1732篇
  2013年   3745篇
  2012年   2956篇
  2011年   3613篇
  2010年   2346篇
  2009年   2348篇
  2008年   3501篇
  2007年   3233篇
  2006年   3167篇
  2005年   2716篇
  2004年   2613篇
  2003年   2625篇
  2002年   2407篇
  2001年   3396篇
  2000年   3309篇
  1999年   2562篇
  1998年   986篇
  1997年   1056篇
  1996年   890篇
  1995年   1025篇
  1994年   897篇
  1993年   896篇
  1992年   1849篇
  1991年   1921篇
  1990年   1862篇
  1989年   1864篇
  1988年   1611篇
  1987年   1597篇
  1986年   1629篇
  1985年   1529篇
  1984年   1226篇
  1983年   1035篇
  1982年   715篇
  1979年   1061篇
  1978年   817篇
  1975年   819篇
  1974年   898篇
  1973年   864篇
  1972年   806篇
  1971年   795篇
  1970年   800篇
  1969年   781篇
  1968年   708篇
  1967年   734篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
221.
The effects of canopy cover, seeding depth, and soil moisture on emergence of Centaurea maculosa Lam. and C. diffusa Lam. were studied. Canopy cover had no effect on ermergence rate (seedlings/day/100 seeds) of either species. Seeds of both species emerged faster when placed on the soil surface. Emergence rate decreased as seeding depth increased. Both species required more than 55% initial soil moisture to initiate emergence, with 65–70% being optimum. Percentage emergence followed exactly the same trend as emergence rate. C. maculosa had better emergence characteristics over a wider range of conditions than C. diffusa, possibly being some of the reasons for the former having a wider geographical distribution in the United States and Canada.  相似文献   
222.
Summary The study reported in the present paper discusses the clinical and histological picture of bovine demodecosis and the morphology of Demodex mites as seen in four cows suffering from generalized demodecosis. There were no clinical signs of other skin affections. Changes in both the number and the appearance of visible skin lesions were seen and related to the level of nutrition and the exposure to sunshine of the cattle. Histological sections of some skin nodules showed the presence of mite colonies in the hair follicles. Only adults were seen in the sebaceous glands. Microscopical study of the morphology of the mites revealed the presence of two types of demodicids in the skin lesions and three types from epilated eyelashes. Morphological criteria are presented to aid in identification of species and of life stages.  相似文献   
223.
224.
225.
226.
The development of fuels from biomass can lead naturally to dispersed facilities that incorporate food or materials production (or both) with fuel production, forming adaptive systems that can be modified to meet evolving needs and constraints. The technology that is appropriate to each system needs to be worked out, taking into account associated food and materials opportunities in order to decrease the ultimate cost of energy delivered to the consumer. I analyze possible systems based on sugarcane, corn, and guayule.  相似文献   
227.
Partially purified alkaline phosphatase (ALP) from canine intestine, liver, and bone were injected into rabbits to elicit anti-canine intestinal, hepatic, and osseous ALP antibodies, respectively. The antibody formed a soluble enzyme-antienzyme complex when directly interacted with the ALP antigen. In order to form an insoluble complex, it was then necessary to interact the initial soluble complex with the goat anti-rabbit gamma-globulin antibody in the 2nd step. Antiintestinal ALP antibody was highly specific and did not cross react with canine hepatic, osseous, splenic, and renal ALP. Antiliver and antibone ALP antibodies, on the other hand, did cross react with hepatic, osseous, splenic, and renal ALP, but not with the intestinal ALP.  相似文献   
228.
229.
230.
1. The α‐tocopherol (AT) content of abdominal fat and fat stability increased as a function of the duration of α‐tocopheryl acetate (ATA) feeding in broilers fed on diets containing either 40 g acidulated soyabean‐oil soapstock (ASS) or tallow/kg. A linear relationship was observed between AT content of the adipose tissue and its stability with both types of fat supplements.

2. The stabilities of carcass fat and meat of 9‐week‐old broilers fed on diets containing ASS with 300 mg ethoxyquin (EQ)/kg or diets containing tallow with 20 mg ATA/kg from weeks 5 to 9 were comparable with those obtained by feeding these antioxidants and fats throughout the growth period.

3. ATA added to a diet containing ASS, and EQ, added to a diet containing tallow did not improve meat stability despite such an effect on adipose tissue when fed for 9 weeks. The results emphasise the importance of using the dietary antioxidant appropriate to the fat supplement to improve carcass stability.  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