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   376篇
  免费   33篇
林业   19篇
农学   4篇
基础科学   2篇
  48篇
综合类   50篇
农作物   6篇
水产渔业   14篇
畜牧兽医   232篇
园艺   16篇
植物保护   18篇
  2021年   8篇
  2020年   13篇
  2019年   5篇
  2018年   10篇
  2017年   9篇
  2016年   7篇
  2015年   8篇
  2014年   8篇
  2013年   15篇
  2012年   17篇
  2011年   12篇
  2010年   12篇
  2009年   13篇
  2008年   21篇
  2007年   12篇
  2006年   22篇
  2005年   24篇
  2004年   16篇
  2003年   7篇
  2002年   14篇
  2001年   7篇
  2000年   11篇
  1999年   9篇
  1998年   5篇
  1997年   4篇
  1994年   6篇
  1993年   3篇
  1992年   9篇
  1991年   8篇
  1990年   6篇
  1989年   3篇
  1988年   4篇
  1985年   5篇
  1984年   4篇
  1981年   2篇
  1980年   2篇
  1979年   3篇
  1978年   3篇
  1977年   3篇
  1976年   2篇
  1975年   2篇
  1974年   5篇
  1973年   3篇
  1972年   6篇
  1971年   9篇
  1970年   2篇
  1969年   4篇
  1968年   2篇
  1967年   3篇
  1966年   2篇
排序方式: 共有409条查询结果,搜索用时 2 毫秒
401.
  • 1. Mapping of seabed habitats is increasingly being used to identify the distribution and structure of marine ecosystems and as surrogate measures of biodiversity for marine protected area (MPA) planning. In this study, the distribution of seabed habitats to the 3 nmi limit around the Kent Group of islands, south‐eastern Australia were mapped using video ground‐truthed single‐beam acoustics at the mesoscale level (10 m to 1 km) as part of an MPA planning process.
  • 2. Six distinct seabed habitat types (continuous reef, patchy reef, sand, hard sand, sparse sponge, and seagrass) were identified based primarily on visual differences in the first and second echo and a further four (low, medium and high profile reef, and sand hills) on variations in seabed profile identified in the echogram. Extensive acoustic and video transects allowed an estimate of the broad‐scale spatial distribution of seabed habitats defined at several hierarchical levels and provided information on the cover of the dominant benthic species or assemblages.
  • 3. The island group supports a range of consolidated habitats, including rocky reefs of varying profile dominated by the macroalgae Phyllospora comosa and Ecklonia radiata in depths down to around 45 m, adjacent to deeper sponge‐dominated reefs containing encrusting, erect and branching forms. Unconsolidated habitats occurred broadly through the island group, with the offshore region dominated by hard sand (sand with scallop shells and/or shell grit) and sparse sponge‐habitats (sand interspersed with low cover of sponge‐dominated assemblages). The sheltered coves were dominated by sand and seagrass habitats consisting of beds of the seagrasses Halophila australis, Zostera tasmanica and Posidonia australis, with variations in species composition, patchiness and percentage cover evident within and between coves.
  • 4. In February 2004 the Kent Group MPA was announced, covering all waters out to the 3 nmi limit containing two areas defined as a Sanctuary Zone (‘no take’) and a Habitat Protection Zone (‘restricted take’). Overall, seabed habitat mapping generated a capability to define the boundary and size of potential MPA zones within the Kent Group of islands and was an essential component of the planning process to improve the likelihood that the MPA was comprehensive, adequate and representative (CAR).
  • 5. The need to define habitats at multiple scales within a hierarchical classification scheme that are meaningful in terms of biodiversity and CAR principles and identifiable using mapping techniques is discussed.
Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   
402.
The complex anatomy of the equine skull creates a radiographic challenge. However, radiographs are a commonly required tool in diagnosing dental and sinonasal disease, as well as other disorders of the equine skull. High quality, complete radiographic studies are essential for adequate evaluation. Radiographic technique and interpretation are reviewed in this paper.  相似文献   
403.
This paper explains and offers a criticism of the technical solutions that have been proposed in recent years to address Africa's hunger problems, summarizes selected results of some of these approaches, and suggests a more useful conceptualization of African hunger for policymakers. Hunger is a problem with multifactorial causality. As such, it is not given to solution by the sequence of reductionist approaches that have been applied in recent years. Widespread adoption by African governments of ultimately unsuccessful reductionist conceptualizations of hunger has had much to do with foreign aid dependency, the general absence from central policymaking circles of senior government officials with responsibility for hunger-related policies, and political preference for centralized bureaucracy. The paper concludes with some recommendations for community-based strategies of hunger alleviation.Barrett is an assistant professor of economics at Utah State University. This paper was written while he was a doctoral candidate in the Departments of Agricultural Economics and Economics at the University of Wisconsin-Madison. Previously he worked for four years as a development economist monitoring African economies for an international institution in Washington. His research focuses on agricultural development strategies and rural poverty alleviation.  相似文献   
404.
King K  Neville C 《Science (New York, N.Y.)》1977,195(4284):1333-1335
The rate of isoleucine epimerization in fossil planktonic foraminifera is strongly species-dependent. Alloisoleucine/isoleucine ratios of two species of the same age can vary by more than a factor of 2. This finding, in combination with the known temporal and spatial variability of foraminiferal assemblages, demonstrates the critical importance of basing geochronological studies of marine sediments on monospecific samples. One rapidly epimerizing species generates a calibration curve of potentially high precision for dating sediments between the ages of about 50,000 to 400,000 years.  相似文献   
405.
406.
407.
The objectives were to examine effects of dietary Se supplementation and nutrient restriction during defined periods of gestation on maternal adaptations to pregnancy in primigravid sheep. Sixty-four pregnant Western Whiteface ewe lambs were assigned to treatments in a 2 x 4 factorial design. Treatments were dietary Se [adequate Se (ASe; 3.05 microg/kg of BW) vs. high Se (HSe; 70.4 microg/kg of BW)] fed as Se-enriched yeast, and plane of nutrition [control (C; 100% of NRC requirements) vs. restricted (R; 60% of NRC requirements]. Selenium treatments were fed throughout gestation. Plane of nutrition treatments were applied during mid (d 50 to 90) and late gestation (d 90 to 130), which resulted in 4 distinct plane of nutrition treatments [treatment: CC (control from d 50 to 130), RC (restricted from d 50 to 90, and control d 90 to 130), CR (control from d 50 to 90, and restricted from d 90 to 130), and RR (restricted from d 50 to 130)]. All of the pregnant ewes were necropsied on d 132 +/- 0.9 of gestation (length of gestation approximately 145 d). Nutrient restriction treatments decreased ewe ADG and G:F, as a result, RC and CR ewes had similar BW and maternal BW (MBW) at necropsy, whereas RR ewes were lighter than RC and CR ewes. From d 90 to 130, the HSe-CC ewes had greater ADG (Se x nutrition; P = 0.05) than did ASe-CC ewes, whereas ADG and G:F (Se x nutrition; P = 0.08) were less for HSe-RR ewes compared with ASe-RR ewes. The CR and RR treatments decreased total gravid uterus weight (P = 0.01) as well as fetal weight (P = 0.02) compared with RC and CC. High Se decreased total (g; P = 0.09) and relative heart mass (g/kg of MBW; P = 0.10), but increased total and relative mass of liver (P < or = 0.05) and perirenal fat (P < or = 0.06) compared with ASe. Total stomach complex mass was decreased (P < 0.01) by all the nutrient restriction treatments, but was reduced to a greater extent in CR and RR compared with RC. Total small intestine mass was similar between RC and CC ewes, but was markedly reduced (P < 0.01) in CR and RR ewes. The mass of the stomach complex and the small and large intestine relative to MBW was greater (P = 0.01) for RC than for CR ewes. Increased Se decreased jejunal DNA concentration (P = 0.07), total jejunal cell number (P = 0.03), and total proliferating jejunal cell number (P = 0.05) compared with ASe. These data indicate that increased dietary Se affected whole-body and organ growth of pregnant ewes, but the results differed depending on the plane of nutrition. In addition, the timing and duration of nutrient restriction relative to stage of pregnancy affected visceral organ mass in a markedly different fashion.  相似文献   
408.
Fructans are not digested in the small intestines of humans. While many health benefits have been attributed to these carbohydrates, they can cause gastrointestinal symptoms in some individuals. We measured the total fructans in 60 vegetables and 43 fruits using the Megazyme fructan assay. Vegetables with the highest quantity of fructans included garlic, artichoke, shallots, leek bulb, and onions (range, 1.2-17.4 g/100 g fw). Fruits with low, but detectable, fructans included longon, white peach, persimmon, and melon (range, 0.21-0.46 g/100 g fw). The fructan assay was modified to provide an estimate of the average chain length (degree of polymerization) for high fructan vegetables. d-Fructose can also be malabsorbed in the small intestine of humans, so the d-fructose content in some foods was measured to supplement the current food tables. Research in this area will be facilitated through the availability of more comprehensive food composition data.  相似文献   
409.
Pressures for more responsible use of anti‐microbial (AM) medicines in food animals are likely to increase from policymakers and the food industry, including retailers. To address this challenge, participatory approaches to welfare interventions and disease prevention may also be necessary alongside more conventional regulatory measures. This article describes the process of enabling groups of dairy producers to use a participatory policy making approach to develop an AM stewardship policy. The policy includes measures agreed to by all producers for more responsible use of AMs, whilst maintaining or improving dairy herd health and welfare. This process provided a unique opportunity for collaboration and dialogue between producers, veterinarians, industry and researchers. Its participatory nature encouraged comprehensive learning for all involved. This integration of science with producers’ knowledge and experience led to credible and practical recommendations designed to deliver real and lasting change in AM use. The multidisciplinary nature of this research marks a significant contribution to embedding social science skills and approaches into the veterinary sphere. As an initial step in creating better understanding of how participatory approaches with farmers can be applied in a UK context and more widely, this work serves as a pilot for promoting more responsible use of veterinary medicines in other livestock species.  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