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   279170篇
  免费   16010篇
  国内免费   687篇
林业   24143篇
农学   14133篇
基础科学   2998篇
  45152篇
综合类   30207篇
农作物   17454篇
水产渔业   17504篇
畜牧兽医   113002篇
园艺   6425篇
植物保护   24849篇
  2021年   2496篇
  2020年   2934篇
  2019年   3756篇
  2018年   4325篇
  2017年   4753篇
  2016年   4970篇
  2015年   4404篇
  2014年   5771篇
  2013年   16347篇
  2012年   7352篇
  2011年   9607篇
  2010年   7883篇
  2009年   8416篇
  2008年   8980篇
  2007年   8037篇
  2006年   8384篇
  2005年   7691篇
  2004年   7457篇
  2003年   7257篇
  2002年   6458篇
  2001年   7039篇
  2000年   6831篇
  1999年   6129篇
  1998年   4239篇
  1997年   4296篇
  1996年   4007篇
  1995年   4662篇
  1994年   4018篇
  1993年   3712篇
  1992年   4820篇
  1991年   4997篇
  1990年   4700篇
  1989年   4687篇
  1988年   4143篇
  1987年   4174篇
  1986年   4175篇
  1985年   4423篇
  1984年   3995篇
  1983年   3708篇
  1982年   3018篇
  1981年   2876篇
  1980年   2875篇
  1979年   3557篇
  1978年   3120篇
  1977年   2851篇
  1976年   2733篇
  1975年   2652篇
  1974年   2835篇
  1973年   2812篇
  1972年   2460篇
排序方式: 共有10000条查询结果,搜索用时 234 毫秒
991.
992.
The merits of integrated aquaculture farming (IAF) as a strategy for bolstering income for the rural poor have been recognized, but the economic efficiency and sustainability are yet to be determined. The effects of one species on another and the economic benefits, net of government support, have not been critically evaluated. The concept of a foodniche for alternative species in an aquatic environment has not been properly tested in an economic setting. Thus, this report reflects the results of applying the technique of mixed integer linear programming to determine the optimum combination of species within the context of a foodniche. The objective also included maximizing revenues, with minimal external input requirements, for a representative small Malaysian wet-rice farm. It was found that the foodniche concept permitted flexibility in stocking rates, and that farmers could actually stock species at the exact rate that was optimal for profit, instead of standard fixed rates. Also, government subsidies for pond construction were essential for IAF success. The net revenues from IAF were low without interest-free loans for pond construction.  相似文献   
993.
Erwinia amylovora has been spreading in the Czech Republic since 1986 regardless of emergency phytosanitary measures which have been taken. Its spread follows the predominant wind direction (west to east). The infested area now covers two thirds of the area of the country. Wild Crataegus spp. are the most important and widespread host plants. E. amylovora is still regulated as a quarantine pest, and areas free from fireblight are subject to special phytosanitary measures.  相似文献   
994.
995.
Long‐term experiments are a classical case of repeated measurements. Traits are measured on the same experimental unit over many years so that correlations arise between the observations made on the same plot in consecutive years. This paper describes the analysis of a three‐crop‐rotation long‐term experiment. We analysed the yields of the crops and the organic carbon content in the topsoil over 30 consecutive years. Several variance–covariance approaches are discussed and the trait‐specific best fit is interpreted. Mixed models are used to describe the structure of the experiment. Both yields and soil organic carbon show a more or less pronounced variance heterogeneity. Especially for yields, the heterogeneity of cycles and years is dominant. The consideration of correlations results in a better model fit in all cases.  相似文献   
996.
997.
Annual sex hormonal profiles, gonad development and age determination of 18 (13–14 kg body weight) and three (145–226 kg body weight) Mekong giant catfish (MGC) (Pangasianodon gigas, Chevey) reared in earthen ponds in Chiang Mai and from the Mekong River in Chiang Rai, Thailand, respectively, were investigated. The hormonal profiles were determined from blood samples of the fish by electrochemiluminescence immunoassay during May to August in 2000. The highest testosterone levels of 0.06 ng mL?1 in both sexes and the highest oestradiol of 47.8 and 14.23 pg mL?1 in females and males, respectively, were observed in May. The gonadosomatic index was found to be 0.07 for males and 0.38 for females from ponds in comparison with 2.27 for males and 8.29 for females from the Mekong River. Higher development stages of spermatocytes and oocytes of the cultured fish in May than in February and November were demonstrated. No mature germ cells were obtained from either the males or females, indicating no sexual maturity. The average age of fish was determined from the annual rings of the cross‐section of the pectoral fins by a stereomicroscope. The average age of fish in earthen pond determined from the annual ring was 8 years, which agreed with the actual culture records, while those from the Mekong River were 21 years. This information will be beneficial for the breeding programmes and conservation of the MGC.  相似文献   
998.
999.
Field experiments were conducted to characterize the demography of Abutilon theophrasti and Setaria faberi in a conventionally managed 2‐year (maize/soya bean) rotation, and in 3‐year (maize/soya bean/triticale + red clover) and 4‐year (maize/soya bean/triticale + lucerne/lucerne) rotations managed with 72% and 79% lower herbicide inputs respectively. Rates of weed seedling recruitment, seedling survival and adult plant fecundity were determined for populations in each phase of each rotation and used to calculate annual rates of weed population change, Δ. In both years of the study, Δ for A. theophrasti populations declined or remained stable in all three rotation systems. Despite greater rates of seedling survival and fecundity in maize and soya bean in the 3‐ and 4‐year rotations, increases in Δ for A. theophrasti populations were prevented in these systems because of low fecundity in triticale and low seedling survival and fecundity in lucerne. For Setaria faberi populations, Δ remained stable in the 2‐year rotation, increased in the 3‐year rotation in both years, and increased in the 4‐year rotation in 1 year. The results of this study indicate that when herbicide use is reduced, rotations that include triticale and lucerne can facilitate the suppression of A. theophrasti. Rotations that include lucerne can contribute to restraining S. faberi population growth, given adequate levels of seedling mortality in this crop.  相似文献   
1000.
A. Wjtowicz 《EPPO Bulletin》2003,33(3):515-516
A computer system Agrogis has been developed to record information on pest infestation of plants in different regions of Poland. Data can be entered manually or from disks, and can be displayed in the form of charts and maps. Agrogis will be made available on the Internet.  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