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   1263篇
  免费   15篇
  国内免费   1篇
林业   97篇
农学   30篇
基础科学   9篇
  238篇
综合类   215篇
农作物   33篇
水产渔业   108篇
畜牧兽医   441篇
园艺   33篇
植物保护   75篇
  2024年   10篇
  2023年   10篇
  2022年   9篇
  2021年   19篇
  2020年   36篇
  2019年   32篇
  2018年   32篇
  2017年   27篇
  2016年   30篇
  2015年   25篇
  2014年   41篇
  2013年   59篇
  2012年   73篇
  2011年   113篇
  2010年   60篇
  2009年   44篇
  2008年   85篇
  2007年   80篇
  2006年   81篇
  2005年   67篇
  2004年   60篇
  2003年   65篇
  2002年   57篇
  2001年   11篇
  2000年   6篇
  1999年   16篇
  1998年   19篇
  1997年   9篇
  1996年   5篇
  1995年   6篇
  1994年   9篇
  1993年   14篇
  1992年   7篇
  1991年   7篇
  1990年   8篇
  1989年   2篇
  1988年   5篇
  1987年   3篇
  1985年   4篇
  1984年   5篇
  1983年   4篇
  1981年   2篇
  1980年   3篇
  1975年   2篇
  1974年   3篇
  1971年   2篇
  1916年   2篇
  1896年   1篇
  1885年   1篇
  1883年   2篇
排序方式: 共有1279条查询结果,搜索用时 15 毫秒
1.
It is currently unclear whether observed pelagic ecosystem responses to ocean warming, such as a mid-1970s change in the eastern North Pacific, depart from typical ocean variability. We report variations in planktonic foraminifera from varved sediments off southern California spanning the past 1400 years. Increasing abundances of tropical/subtropical species throughout the 20th century reflect a warming trend superimposed on decadal-scale fluctuations. Decreasing abundances of temperate/subpolar species in the late 20th century indicate a deep, penetrative warming not observed in previous centuries. These results imply that 20th-century warming, apparently anthropogenic, has already affected lower trophic levels of the California Current.  相似文献   
2.
    
This paper explores land use conflicts between non-farm neighbors and farmers to illustrate the usefulness of the concepts of interdependence, rules, and property rights when doing rural development. Recognizing interdependence and its implications helps economic analysis focus on and understand the types of rules and institutions having the most influence on economic behavior, and thus identify policy alternatives. The resolution of land uses conflicts, for example, unavoidably changes the bundle of rights associated with land, andinfluences who can impose costs of whom; it makes a difference if a large farm has the right to produce odors, flies, or noise that reduces their neighbors' abilities to enjoy theneighbors' own land, or if instead, neighbors have the right to use their property without experiencing farm-produced odors, flies or noise the farm may be unable to use its own land for agriculture without being inconvenienced.  相似文献   
3.
    
In a 2 X 2 factorial study, a broiler starter ration was amended for vitamin A (control, C; deficient, A) and probiotic status (-, P) to investigate their modulatory effects onthe host immune system. Birds were inoculated orally with Eimeria acervulina (EA) oocysts, and disease susceptibility was evaluated by assessment of fecal oocyst shedding. Humoral and local cellular mediated immunity were assessed by evaluation of antibody and cytokine (interferon-gamma [IFN-gamma] and interleukin-2 [IL-2]) levels in sera and intestinal secretions on a 3-day interval after inoculation. Fecal oocyst shedding was highest (P < 0.05) in A- birds, followed by AP, C-, and CP birds. Feeding the probiotic reduced shed oocysts by 20% in A fed birds and by 26% in C fed birds. Intestinal IFN-gamma was relatively constant in all treatment groups except for A-, where it declined steadily and was lower (P < 0.05) from day 6 on. Serum IFN-gamma levels fluctuated within each treatment and over time were not revealing. Intestinal IL-2 was highest in CP birds at 3 and 9 days postinfection (DPI) and lowest in A- birds at 3, 9, and 12 DPI (P < 0.05); no difference between treatments was found at 6 DPI (P > 0.05). Eimeria-specific intestinal antibody (Ab) level was constant (P > 0.05) in C- birds but increased with time (P < 0.05) in A-, AP, and CP birds. Serum Ab levels were also constant in A- and CP birds but increased (P < 0.05) in C- and AP birds after 6 DPI. The data demonstrate for the first time a probiotic-enhanced immunity in vitamin A deficient birds. This study is also the first to demonstrate the probiotic effect on local cell-mediated immunity of chickens, best manifested by apparent lower intestinal invasion and development by EA, on the basis of higher IL-2 secretion and lower EA oocyst production.  相似文献   
4.
    
Major complaints, nuisance suits, and vandalism threaten the ability of farms in suburbanizing areas to survive as development occurs around them. This is a particular concern in one county in southeastern Pennsylvania, an area of rapid population growth but also the nation's most productive mushroom producing area. This study examines mushroom growers' indications that on-farm composting is a major source of complaints from nonfarming neighbors. The study found that the size of the farm and the number of homes nearby were significantly more important factors than composting in explaining which mushroom farms receive complaints.  相似文献   
5.
6.
Grapefruit growers in the tropics require information about existing and new citrus cultivars with high productivity potential. The objective of this study was to determine the growth, yield, and fruit quality performance of seven pigmented and four white grapefruit cultivars under the dry tropic conditions of Colima, Mexico. The trees were budded on sour orange (Citrus aurantium L.) rootstock and planted at a distance of 8 × 4 m. ‘Oroblanco’ and ‘Marsh Gardner’ white-fleshed grapefruit cultivars and ‘Chandler’, a pink-fleshed pummelo, were the largest trees with the greatest height (5.0–5.6 m), canopy diameter (6.2–6.3 m), trunk diameter (21.9–23.3 cm), and canopy volume (109–123 m3). Lower height (4.3–4.8 m) and canopy volume (73–96 m3), but with similar canopy diameter to the previously mentioned cultivars, were recorded for the remaining pigmented cultivars. ‘Chandler’ pummelo and four pigmented grapefruit cultivars (‘Shambar’, ‘Río Red’, ‘Ray Ruby’, and ‘Redblush #3’) had yearly productions of 34.8, 34.9, 34.1, 32.7, and 30.6 ton ha−1, respectively. The most productive white grapefruit cultivar was ‘Marsh Gardner’ (30.5 ton ha−1). Grapefruit cultivars having the largest fruit size showed a higher inverse relationship between fruit weight and yield than those with small fruit. Most genotypes had higher values of fruit weight, juice content, and maturity index than those required by the local market. The most promising grapefruit cultivars based on their acceptable growth, yield superior to 30 ton ha−1, and acceptable fruit color were ‘Río Red’, ‘Shambar’, ‘Ray Ruby’, and ‘Redblush #3’.  相似文献   
7.
A nutritional evaluation of an Irish dairy herd indicated gross overfeeding of late lactation cows, over-conditioning of cows at parturition and a high rate of body condition loss in early lactation. Metabolisable-energy based nutritional modelling software was used to guide recommended dietary changes to prevent excessive condition gain in late lactation. Immediately following the implementation of the changes there was an unexpected reduction in performance affecting both milk yield and protein concentration. An investigation into the poor performance revealed underestimation of peak milk yield; over-estimation of maize silage quality; a large difference in the concentrate being fed compared to the concentrate recommended, and failure of the blend of concentrate ingredients to maintain the intended proportions in the in-parlour feeding system. The estimated maximum cumulative effect of these errors was to cause undersupply of energy and protein in the recommended diet of 16% and 3% respectively to cows in early lactation. Use of a net-energy nutritional model would have indicated a requirement for a higher energy supply in this case. This report highlights the challenges in obtaining accurate on-farm data for use in dairy cow nutritional models.  相似文献   
8.
    
An important driver behind introductions for aquaculture of alien fish species into Pacific Island Countries and Territories (PICTs) is a lack of knowledge about domestication suitability and specific culture requirements of indigenous taxa. Introductions may be appropriate in some circumstances, but in other circumstances, the associated risks may outweigh the benefits, so greater understanding of indigenous species' aquaculture potential is important. This review summarises literature for indigenous freshwater food fish species from Papua New Guinea, Fiji, Vanuatu, the Solomon Islands, Samoa and Tonga, and evaluates their aquaculture potential for food security and/or small-scale livelihoods. A species selection criteria incorporating economic, social, biological and environmental spheres was used to score 62 candidate species. Tilapia (Oreochromis mossambicus and O. niloticus) now established in PICTs were evaluated for comparison. Results show that 13 species belonging to the families Mugilidae (Mullets), Terapontidae (Grunters), Kuhliidae (Flagtails) and Scatophagidae (Scats) have the highest culture potential according to selection criteria. These feed at a relatively low trophic level (are herbivores/detritivores), have comparatively fast growth rates and overall possess characteristics most amenable for small-scale, inland aquaculture. The four top-ranked candidates are all mountain mullets Cestraeus spp., followed by Nile tilapia (Oreochromis niloticus). Lower ranked candidates include three other mullets (Planiliza melinoptera, P. subviridis and Mugil cephalus) and rock flagtail Kuhlia rupestris. Importantly, many species remain data deficient in aspects of their reproductive biology or culture performance. Species profiles and ranked priority species by country are provided with logistical, technological and environmental assessments of country capacities to culture each species.  相似文献   
9.
提摩西·克里根(Timothy Corrigan)被国外多家媒体称为当今时尚先锋,他一直致力于创造一个融合了浓郁的欧式典雅风格与加州的休闲舒适氛围的居住环境。他在洛杉矶和巴黎都设有办事处,负责世界各地的建筑项目。除了提供设计服务之外,  相似文献   
10.
  总被引:3,自引:0,他引:3  
Atmospheric black carbon (BC) warms Earth's climate, and its reduction has been targeted for near-term climate change mitigation. Models that include forcing by BC assume internal mixing with non-BC aerosol components that enhance BC absorption, often by a factor of ~2; such model estimates have yet to be clearly validated through atmospheric observations. Here, direct in situ measurements of BC absorption enhancements (E(abs)) and mixing state are reported for two California regions. The observed E(abs) is small-6% on average at 532 nm-and increases weakly with photochemical aging. The E(abs) is less than predicted from observationally constrained theoretical calculations, suggesting that many climate models may overestimate warming by BC. These ambient observations stand in contrast to laboratory measurements that show substantial E(abs) for BC are possible.  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