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   289篇
  免费   26篇
林业   3篇
农学   7篇
基础科学   2篇
  17篇
综合类   35篇
农作物   15篇
水产渔业   12篇
畜牧兽医   202篇
园艺   6篇
植物保护   16篇
  2023年   2篇
  2021年   6篇
  2020年   3篇
  2019年   3篇
  2018年   7篇
  2017年   4篇
  2016年   11篇
  2014年   4篇
  2013年   10篇
  2012年   11篇
  2011年   6篇
  2010年   2篇
  2009年   6篇
  2008年   14篇
  2007年   21篇
  2006年   12篇
  2005年   10篇
  2004年   16篇
  2003年   15篇
  2002年   8篇
  2001年   6篇
  2000年   8篇
  1999年   8篇
  1998年   4篇
  1997年   4篇
  1996年   6篇
  1995年   3篇
  1994年   3篇
  1993年   6篇
  1992年   4篇
  1991年   6篇
  1990年   9篇
  1989年   5篇
  1988年   2篇
  1987年   6篇
  1986年   4篇
  1985年   5篇
  1984年   6篇
  1982年   3篇
  1981年   3篇
  1979年   5篇
  1974年   2篇
  1973年   2篇
  1972年   4篇
  1970年   7篇
  1967年   2篇
  1966年   2篇
  1908年   1篇
  1901年   1篇
  1883年   1篇
排序方式: 共有315条查询结果,搜索用时 171 毫秒
61.
62.
N-Acetylglutamate (NAG) and N-acetylaspartate (NAA) are amino acid derivatives with reported activities in a number of biological processes. However, there is no published information on the presence of either substance in foodstuffs. We developed a method for extracting and quantifying NAG and NAA from soybean seeds and maize grain using ultra performance liquid chromatography-electrospray ionization tandem mass spectrometry (UPLC-ESI-MS/MS). The lower limit of quantification for both NAG and NAA was 1 ng/mL. The method was then utilized to quantify NAG and NAA in other foodstuffs (fruits, vegetables, meats, grains, milk, coffee, tea, cocoa, and others). Both NAG and NAA were present in all of the materials analyzed. The highest concentration of NAG was found in cocoa powder. The highest concentration of NAA was found in roasted coffee beans. Both NAG and NAA were found at quantifiable concentrations in all foods tested indicating that these two acetylated amino acids are common components of the human diet.  相似文献   
63.
Rural veterinary services in Western Australia: Part B. Rural practice   总被引:1,自引:1,他引:0  
OBJECTIVE: To determine the current status of rural veterinary services in Western Australia. PROCEDURE: A questionnaire was sent to all eligible rural practitioners registered in 2006 and the replies were transferred to Microsoft Excel for analysis. RESULTS: Of the rural practitioners invited to participate in the survey replies were received from 67%. There were equal numbers of females and males. Their mean age was 44 years. Ninety per cent of respondents considered knowledge gained as an undergraduate was sufficient to equip them for practice, but only 60% considered their practical skills adequate. Thirteen per cent of those in rural practices in 2005 had left by 2006. Eighty-nine per cent of respondents were in mixed practice, the balance in specific species practice, such as equine, large animal and production animal consultancy. The majority of rural practitioners relied on servicing companion animals for their viability; 7% earned their income from servicing production animals only. Seventy per cent utilised merchandising and the sale of pet foods to supplement the income received from the traditional veterinary services and 34% found it necessary to earn an independent income. A quarter considered that rural practice did not have a future. CONCLUSION: The majority of rural practitioners in Western Australia depend on companion animals, not production animals, to remain viable, with very few operating production animal services. Poor remuneration is a major reason why veterinarians leave rural practice, and many find it necessary to supplement their income or develop an independent income.  相似文献   
64.
When body size varies greatly, drug disposition can best be described as an allometric function of body weight. Therefore, the allometry of standard metabolic rate (SMR; 3/4 power) and body surface area (BSA; 2/3 power) have been advocated as surrogate markers for the prediction of key pharmacokinetic parameters. The goal of the present study was to examine the allometric basis of pharmacokinetic scaling within a species, green iguanas. Enrofloxacin was administered intravenously to 20 green iguanas (322-3824 g), and noncompartmental analysis was used to calculate standard pharmacokinetic parameters, which were log(10) transformed and regressed against log(10) body weight. The slopes of significant regressions were compared with the values of unity, 3/4, and 2/3. The slope of enrofloxacin total body clearance (Cl) was also compared with the slopes relating SMR and renal Cl of (99m)Tc-diethylenetriamine penta-acetic acid ((99m)DTPA) to body weight in iguanas. Enrofloxacin Cl depended allometrically on body weight with the power of 0.32. The slope of enrofloxacin Cl was significantly less than those of SMR, Cl of (99m)DTPA, and the 2/3 value. Therefore, the relationship between enrofloxacin Cl and body weight does not directly depend on the allometry of BSA, SMR, or renal Cl of (99m)DTPA in iguanas.  相似文献   
65.
66.

Context

The relationship between biodiversity and ecosystem functioning (BEF) has been a central topic in ecology for more than 20 years. While experimental and theoretical studies have produced much knowledge of how biodiversity affects ecosystem functioning, it remains poorly understood how habitat fragmentation affects the BEF relationship.

Objectives

To develop a framework that connects habitat fragmentation to the BEF relationship from a landscape perspective.

Methods

We reviewed the literature on habitat fragmentation, BEF, and related fields, and developed a framework to analyze how habitat fragmentation affects the BEF relationship through altering biodiversity, environmental conditions, and both, based on the pattern-process-scale perspective in landscape ecology.

Results

Our synthesis of the literature suggests that habitat fragmentation can alter BEF relationship through several processes. First, habitat fragmentation causes the non-random loss of species that make major contributions to ecosystem functioning (decreasing sampling effect), and reduces mutualistic interactions (decreasing complementarity effects) regardless of the changes in species richness. Second, environmental conditions within patches and ecological flows among patches vary significantly with the degree of fragmentation, which potentially contributes to and modulates the BEF relationship.

Conclusions

Habitat fragmentation can affect the BEF relationship directly by altering community composition, as well as indirectly by changing environmental conditions within and among habitat patches on both local and landscape levels. The BEF relationship obtained from small plots and over short time periods may not fully represent that in real landscapes that are fragmented, dynamic, and continuously influenced by myriad human activities on different scales in time and space.
  相似文献   
67.
Spatial and temporal availability of pasture vegetation and cattle grazing behaviour in three seasons (intermediate season [IM], peak season [PS] and dry season [DS]) in the Guinea Savanna agroecological zone of Ghana was examined. The frequency of occurrence of three species groups (grass, legume and forbs) and biomass yield was estimated in 1 m2 plots. The grazing behaviour of 18 cows from six herds was monitored over three days per animal. Season affected the prevalence of species groups with grasses being most dominant in all seasons. Most grazing lands were heavily grazed in IM and DS but moderately grazed in PS. Whilst harvested rice fields represented the most frequently grazed land in DS, fallowed hard pans were used in PS and IM. Biomass yield was highest in DS owing to the contribution of crop residues from harvested farmlands. Carrying capacity was linked to biomass production and averaged 0.55 in DS, 0.38 in PR and 0.21 in IM. Animals spent less time outside the kraal in PS than IM and DS. Walking time was higher in DS than in IM and PS. Less than 20% of the time on pasture was spent on resting, watering and social interaction across seasons.  相似文献   
68.
Greyface (Border Leicester × Scottish Blackface) ewes, mated in October, were housed in winter and turned out after lambing in late March to one of four treatments replicated three times on perennial ryegrass (Lolium perenne)-dominated pastures. On three of the treatments the stocking rate was 12·5 ewes plus lambs ha?1 annum?1 (SR12·5), whereas on the fourth it was 15·0 ewes plus lambs ha?1 annum?1 (SR15·0). There were two levels of applied fertilizer nitrogen (N), 152 kg N ha?1 annum?1 (N152) and 205 kg N ha?1 annum?1 (N205). Three patterns of nitrogen application were used: predominantly in spring (E), predominantly in autumn (L), and mainly in spring and autumn (E + L). Grazed sward surface height was controlled within the range 3·5–5·5 cm during spring and summer, and supplementary feed was offered when sward height was below 3·5 cm. The effects on animal performance, yield of silage and requirement for supplementary feed were measured over 3 years (1986–88). Management of the sward height within the specified limits resulted in similar levels of individual animal performance for all treatments, but treatment SR15·0N205E + L produced a significantly (P < 0·05) greater output of lamb per hectare. There was no evidence to suggest that, by increasing the amount of N fertilizer applied in the autumn, ewe reproductive performance was increased. Neither the treatment with the lower level of N applied in the spring (SR12·5N152L) nor the high stocking rate treatment (SR15·0N205E + L) was self-sufficient in winter feed (silage) production. SR15·0N205E + L also required more supplementary feed in both the lactation (38·1 kg ewe?1) and the mating (9·1 kg ewe?1) periods. SR12·5N152L required the second greatest amount of supplementary feed during lactation (36·1 kg ewe?1), whereas SR12·5N205E + L required the least supplementary feed over both periods (27·8 kg and 4·8 kg ewe?1). Taking all treatments together, there were significant (P < 0·05) differences between years in lamb weaning weight (kg), lamb output (kg ha?1), yield of silage (kg ewe?1) and supplementary feed required (kg ewe?1), arising mainly from a lower level of herbage production in one year. Treatment SR12·5N152E generated the highest gross margin per hectare.  相似文献   
69.
Stay-green maize genotypes have been associated with tolerance to biotic and abiotic stresses, including tolerance to drought, and to stalk and root lodging, but there is limited information on its inheritance. Thus, this research was conducted to study the inheritance of the stay-green trait using both conventional analysis and QTL mapping of the Design III in a tropical maize population developed from two inbred lines genetically divergent for this trait. Two-hundred and fifty F2 plants were genotyped with 177 microsatellite markers, and their backcrossed progenies to both parental inbreds were evaluated at three locations. Ten plants per plot were assessed 120 days after sowing and the plot means scores for stay-green, adjusted for days to silking emergence, were used for analysis. The additive variance was larger than the dominance variance, the genetic by location interaction variance presented a high magnitude, and the heritability coefficient on a plant-basis a low magnitude. Seventeen QTL were mapped, most of them were clustered on four chromosomes and accounted for by 73.08 % of the genetic variance. About half of the QTL interacted with location, and the average level of dominance was partial dominance. The additive effects were larger than the dominance effects; the latter were not unidirectional, so that heterosis could not be exploited in crosses. Procedures for marker-assisted selection to increase the level of stay-green are discussed and an approach is suggested for using both stable and non-stable QTL in a marker-assisted backcross program.  相似文献   
70.
Genomic characterization using nonradioactive probes, polymerase chain reaction with degenerate primers for whitefly transmitted geminiviruses and nucleotide sequencing were used to describe a new bipartite geminivirus, associated with dwarfing and leaf curling of tomatoes and peppers in Jamaica. Partial DNA-A and DNA-B clones were obtained. DNA sequence analysis showed that tomato and pepper samples have a similar geminivirus associated with them. Nucleotide sequence identity > 92% between the common regions of DNA-A and DNA-B confirmed the bipartite nature of the Jamaican geminivirus isolates. Nucleotide sequence comparisons of DNA-A and DNA-B with those of geminiviruses representing the major phylogenetic groups of Western Hemisphere geminiviruses showed the greatest similarity to potato yellow mosaic virus and members of the Abutilon mosaic virus cluster of geminiviruses. This new virus is given the name tomato dwarf leaf curl virus (TDLCV) because of the dwarfing and leaf curling symptoms associated with infected tomato plants. Polymerase chain reaction and Southern hybridization showed mixed infections of TDLCV with tomato yellow leaf curl virus from Israel in 16% of the field samples of tomatoes and peppers.  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