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   253835篇
  免费   15297篇
  国内免费   657篇
林业   23749篇
农学   14071篇
基础科学   2973篇
  43243篇
综合类   22338篇
农作物   17354篇
水产渔业   16967篇
畜牧兽医   98934篇
园艺   6291篇
植物保护   23869篇
  2021年   2479篇
  2020年   2888篇
  2019年   3676篇
  2018年   4103篇
  2017年   4520篇
  2016年   4893篇
  2015年   4389篇
  2014年   5739篇
  2013年   16274篇
  2012年   6495篇
  2011年   8509篇
  2010年   7504篇
  2009年   8030篇
  2008年   7886篇
  2007年   6904篇
  2006年   7428篇
  2005年   6516篇
  2004年   6406篇
  2003年   6173篇
  2002年   5495篇
  2001年   6472篇
  2000年   6124篇
  1999年   5638篇
  1998年   4158篇
  1997年   4239篇
  1996年   3957篇
  1995年   4510篇
  1994年   3901篇
  1993年   3585篇
  1992年   4396篇
  1991年   4519篇
  1990年   4236篇
  1989年   4221篇
  1988年   3633篇
  1987年   3691篇
  1986年   3596篇
  1985年   3914篇
  1984年   3592篇
  1983年   3366篇
  1982年   2838篇
  1981年   2747篇
  1980年   2706篇
  1979年   3071篇
  1978年   2801篇
  1977年   2555篇
  1976年   2421篇
  1975年   2333篇
  1974年   2436篇
  1973年   2361篇
  1971年   2075篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
271.
Abstract  To evaluate the suitability of angling groundbaits as a fish feed and the potential for nutrient loss, the macronutrient digestibility of four groundbaits was investigated in carp, Cyprinus carpio L., using a sieving method of faeces collection and chromic oxide. Apparent digestibility coefficients (ADCs) of protein and lipids were >80% for all angling baits. ADCs of nitrogen-free extracts and phosphorus (P) differed significantly between the test diets, probably because of divergent feed compositions and ingredient treatments. The generally high digestibility estimates indicated that fish feeding on angling groundbait will benefit from this food source. The differences in nutrient digestibility suggested that type of groundbait and ingredient used will have a major effect on potential eutrophication and fish production caused by groundbaiting. The principle of groundbaits to protect the environment should be to minimise the P-content of the groundaits and maximising P-digestibility and P-retention efficiency.  相似文献   
272.
Paratuberculosis is a chronic granulomatous enteritis and lymphadenitis in ruminants caused by Mycobacterium avium subsp. paratuberuclosis . To investigate the efficacy of assays for early diagnosis, experimental infections were performed in goat kids. Oral doses of M. a. paratuberculosis were administered to goat kids over extended periods and the resulting infection was followed for 12–24 months. Complementary experiments were performed on goat kids vaccinated against M. a. paratuberculosis . Immunological, pathological and morphological examinations were undertaken in both these experimental systems. The analysis of results from these experiment series provided novel insights into the influence of the biology of gut-associated lymphoid tissue on the progress of disease, into the character of the tissue granuloma and its dominance of the local tissue response and into the relationship between the local and systemic immune responses.  相似文献   
273.
Abstract –  Factors affecting long-term variation in brown trout, Salmo trutta L., stocking success were examined in a large lake, Lake Oulujärvi, in central Finland. Brown trout were stocked in spring (late May to early June) in 1974–1991 and in summer (late June to early July) in 1992–2001. The biomass of the vendace, Coregonus albula (L.), population (prey) at release time had the largest positive effect on stocking success within both periods: biomass of adult vendace in spring and both 0+ and adult vendace in summer. Increasing the size of stocked fish had a positive effect if the vendace available at release were only adults. The increasing trend of predator-catch-per-unit-effort (CPUE) [combined CPUE of northern pike Esox lucius L., burbot Lota lota (L.), and pike-perch Stizostedion lucioperca (L.)] through the study period and its negative effect on trout stocking success suggested an increasing effect of predation within the entire time series.  相似文献   
274.
At a site in Kent, perennial ryegrass (Lolium perenne) variety S24 and Italian ryegrass (L. multiflorum) variety RVP, wheat varieties Armada and Norman, and the original mixed grass ley were grown in small plots during 1982–84. Two toxic pesticides (phorate and aldicarb) were applied to half the total number of plots three times each year to eliminate soil invertebrate populations. Fertilizer was also applied to most plots. The yields of the crops, grown with and without pesticide, and the effects of the fertilizer were compared. Grass herbage yield was measured on three occasions during the summers of 1983 and 1984. Wheat grain yields were also determined in 1983. During the first year significant differences were not apparent in grass dry matter yield between pesticide-treated and non-treated plots, but significant differences were found in the second year. The perennial ryegrass was more susceptible to pest damage than the Italian ryegrass or the grass ley. Grass yields varied between cuts and in relation to variety and pesticide treatment, yields tending to be greater in untreated plots. Fertilizer treatment greatly increased grass dry matter yields, particularly with the Italian ryegrass. The effects of pesticide treatment on both wheat varieties varied although some yield enhancement was evident. Invertebrate animal populations in pesticide and fertilizer-treated plots were also assessed in autumn 1982, spring and autumn 1983 and spring 1984. In contrast to pesticide treatment, fertilizer treatment had little effect on soil invertebrate populations. Nematode populations were reduced at each sampling occasion by the pesticide treatment. Slug populations were initially unaffected but were subsequently reduced. Leatherjackets, by far the most abundant pest in both grass and wheat plots, were markedly affected by pesticides on all sampling occasions. Generally, fewer soil-dwelling dipterous larvae were recovered in spring than in autumn. Stem-boring dipterous larvae were virtually absent.  相似文献   
275.
276.
277.
278.
279.
280.
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