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   333篇
  免费   21篇
  国内免费   1篇
林业   21篇
农学   18篇
基础科学   6篇
  45篇
综合类   49篇
农作物   12篇
水产渔业   10篇
畜牧兽医   184篇
园艺   3篇
植物保护   7篇
  2022年   3篇
  2021年   8篇
  2020年   7篇
  2018年   8篇
  2017年   3篇
  2016年   3篇
  2015年   5篇
  2014年   7篇
  2013年   13篇
  2012年   13篇
  2011年   17篇
  2010年   14篇
  2009年   7篇
  2008年   11篇
  2007年   20篇
  2006年   18篇
  2005年   8篇
  2004年   10篇
  2003年   11篇
  2002年   11篇
  2001年   6篇
  2000年   10篇
  1999年   5篇
  1998年   3篇
  1997年   5篇
  1996年   3篇
  1995年   5篇
  1994年   5篇
  1993年   5篇
  1992年   3篇
  1991年   6篇
  1989年   4篇
  1988年   4篇
  1987年   3篇
  1986年   11篇
  1985年   6篇
  1984年   6篇
  1983年   4篇
  1981年   3篇
  1980年   3篇
  1976年   3篇
  1975年   3篇
  1974年   9篇
  1973年   3篇
  1972年   3篇
  1971年   2篇
  1970年   3篇
  1968年   2篇
  1961年   3篇
  1929年   3篇
排序方式: 共有355条查询结果,搜索用时 15 毫秒
1.
2.
Processes to develop criteria and indicators (C and I) for sustainableforest management are often expert driven. Although the long-termobjective of the Nepalese forest policy is sustainable forestmanagement, no national level C and I specifically for thishave been developed. However, the Ministry of Forests and SoilConservation awards an annual prize to successful communityforest user groups (CFUGs) based on evaluation according to10 criteria. This study explored local C and I for successfulforest management in Nepal through six meetings with CFUG members.A total of 14 local criteria and 52 indicators for awardinga prize to successful CFUGs were identified, and comparisonwith official C and I was undertaken. Most conspicuously, localC and I focus more on environmental and governance-related aspectsthan the official list, and the official list leaves much tothe discretion of the evaluator. The study concludes that developmentof a national level C and I for sustainable forest managementcould benefit from involvement of local forest users.  相似文献   
3.
RIP-X is software which was developed to simulate the rough mill cutup of lumber. It determines the yields of the current and least-cost grade mixes for both the crosscut-first system and the rip-first system. A statistical comparison of the crosscut-first and rip-first current yields is made. The least-cost grade mix is determined by a linear programming model incorporated into the software.Validation of the RIP-X linear programming model was performed by comparing its results to results from an existing model. When the parts' lengths were restricted to the existing model's constraints, the RIP-X results did not differ significantly from the existing model's results. Comparison of current available model results with the unrestricted RIP-X model, indicated that previous models have provided sub-optimal solutions because of maximum parts' length restrictions in some lumber grades.Field tests were conducted to determine the accuracy of the crosscut-first and rip-first simulations in rough mills. The yield estimates from the crosscut-first and rip-first simulations did not differ significantly from the actual rough mill yields.  相似文献   
4.
Summary Genetic variance components and heritability were estimated for cut-flower yield of gerbera in the Davis population using ordinary least squares and maximum likelihood methods. The overall estimate of narrow-sense heritability is 0.33 based on least squares (LS) and 0.31 based on maximum likelihood (ML). The results of the study indicate that (1) ML and LS provide very similar results if sample size is large enough, suggesting both are useful for plant breeding programs; (2) about one third of the variation in gerbera cut-flower yield is additive, implying selection in cut-flower will be successful; and (3) although additive variation gradually decreased, heritability remained near 0.27 suggesting there is still potential variation in the population for further selection.  相似文献   
5.
Summary The quality ranking of a sample of Gerbera cut-flowers varies greatly from one person to another. Further, there are significant differences between the rankings assigned by a panel of trained horticulturalists and a panel sampled from the general public. Therefore, the selection of a panel to evaluate the flower quality component of a breeding program is very important to the outcome. If the purpose of a program is to breed (those) flowers preferred by the general public then the selection of flowers should be done by a panel sampled from the general public. The prediction of consumer preferences by multiple linear regression techniques using eleven flower characters was not successful. Much greater predictive value was obtained by measuring the perceived information content of flowers. This indicates that flower characteristics which increase flower complexity may be preferred. Implications to the practical breeding program are discussed.  相似文献   
6.
The overall objective of this research was to find a new way to valorize rye bran, by producing a gellifier from the enzymatic solubilization of arabinoxylans (AX). The effects of three pure endo-xylanases from Aspergillus niger (Xyl-1), Talaromyces emersonii (Xyl-2), and Bacillus subtilis (Xyl-3) and of Grindamyl S100 (GS100), a commercial enzyme preparation containing a Xyl-1 type endo-xylanase, were tested on rye bran to study the solubilization of water-unextractable arabinoxylans (WUAX). Eight different extrusion-treated rye brans were also used as substrates to find the best physical treatment to facilitate enzymatic arabinoxylan (AX) solubilization. Arabinoxylans were better solubilized from the bran extruded at high temperature using Xyl-3. This enzyme was then tested in combination with pure (1,4)-beta-d-arabinoxylan arabinofuranohydrolase (AXH) and endo-beta-d-glucanase or ferulic acid esterase (FAE), from A. niger. Only beta-glucanase in combination with Xyl-3 improved the AX extraction, but it did not have a marked effect on the viscosity of the extracts. Xyl-3 was then tested on a high-shear-treated rye bran, and results were compared to those obtained with the high-temperature-extruded rye bran. The high-shear treatment did not improve the bran AX enzymatic solubilization. The combination of FAE with Xyl-1 or Xyl-3 did not improve the AX extraction from untreated and high-shear-treated rye bran. Finally, to study the gelation capacity of the enzymatically solubilized AX, the effect of the hydrogen peroxide/horseradish peroxidase (H(2)O(2)/POD) was tested on the Xyl-3 high-temperature-extruded bran extracts. Solubilized AX did not gel in the presence of the oxidizing system.  相似文献   
7.
OBJECTIVE: To validate a new food-frequency questionnaire (FFQ) for measuring the intake of fruit, vegetables and tea reported by women participating in the Norwegian Mother and Child Cohort Study (MoBa). DESIGN: Intake of fruits, vegetables and tea estimated by the FFQ was compared with urinary flavonoid excretion, plasma carotenoid concentration and intake measured by a 4-day weighed food diary (FD). The triangular method was applied to calculate FFQ validity coefficients using two independent biomarkers. SETTING AND SUBJECTS: One hundred and nineteen women participating in MoBa. RESULTS: The FFQ estimate of fruit intake was significantly correlated with urine phloretin (r = 0.33), citrus fruit/juice with urine hesperetin (r = 0.44), cooked vegetables with plasma alpha-carotene (r = 0.37), and tea with urine kaempferol (r = 0.41) (P < 0.01 for all). On average, 60% of the participants fell into the same or adjacent quintiles when classified by FFQ and biomarkers. Significant correlations between the FFQ and FD were found for fruit (r = 0.39), vegetables (r = 0.34), juices (r = 0.50) and tea (r = 0.53). The FFQ validity coefficient was 0.65 for citrus fruit/juice and 0.59 for cooked vegetables as calculated by the triangular method. CONCLUSIONS: The validation study shows that the MoBa FFQ can be used to estimate fruit, juice, vegetable and tea intake in pregnant Norwegian women, and to rank individuals within the distribution.  相似文献   
8.
The role of phospholipase A2 in the induction of drip loss from pig muscle has been investigated. In samples from porcine M. longissimus dorsi, total PLA2 activity as well as mRNA and protein levels of the group VIA iPLA2 (iPLA2-VIA) increased during the initial 4 h post-mortem period. Morphological studies of porcine muscle showed that at 4 h post-mortem, gaps had formed between muscle fibers and that the sarcolemma membrane borders appeared blurred. At the same time iPLA2-VIA protein levels were increased inside muscle fibers and at the sarcolemma. iPLA2-VIA mRNA abundance in samples from different breeds of pigs with variations in drip loss revealed no clear correlation between drip loss level and iPLA2-VIA expression. Together, these data indicate that during the post-mortem period, iPLA2-VIA expression and activity is increased at the muscle fiber membranes. PLA2 activity may affect membrane permeability and consequently the progression of drip formation in porcine muscle.  相似文献   
9.
Changes in all woodlands within study areas in Northamptonshire, Lincolnshire and Cambridgeshire between 1946 and 1972/73 are enumerated using air photographs and ground survey. These conformed to national, long-term trends from coppice to high forest management and from native to introduced species. Differences between areas are ascribed to variation in the relative importance of forestry and agriculture. The effects on wildlife and scientific interest were assessed against criteria developed from an historical approach to woodland conservation, and by examining the area of woodland available to species requiring woodland of particular physiognomy and origin. Modern forestry as an alternative to agriculture has probably favoured species which are good colonisers, catholic in their woodland requirements, and/or require conifers. Species which are poor colonisers and/or require broadleaf woodland have probably declined most in those areas where reforestation has been extensive, and have survived best where forestry has been insignificant in relation to agriculture. Since these are the species most highly valued for conservation and because they are correlated in their occurence with features of special scientific interest, modern forestry is judged to have been relatively harmful to conservation values in Eastern England during the study period. However, the net long-term effects of modern forestry on woodland conservation values are difficult to predict, for this depends partly on the fate of woods in areas devoted mainly to agriculture, which are unlikely to be managed by modern forestry methods.  相似文献   
10.
Yield and water productivity of potatoes grown in 4.32 m2 lysimeters were measured in coarse sand, loamy sand, and sandy loam and imposed to full (FI), deficit (DI), and partial root-zone drying (PRD) irrigation strategies. PRD and DI as water-saving irrigation treatments received 65% of FI after tuber bulking and lasted for 6 weeks until final harvest. Analysis across the soil textures showed that fresh yields were not significant between the irrigation treatments. However, the same analysis across the irrigation treatments revealed that the effect of soil texture was significant on the fresh yield and loamy sand produced significantly higher fresh yield than the other two soils, probably because of higher leaf area index, higher photosynthesis rates, and “stay-green” effect late in the growing season. More analysis showed that there was a significant interaction between the irrigation treatments and soil textures that the highest fresh yield was obtained under FI in loamy sand. Furthermore, analysis across the soil textures showed that water productivities, WP (kg ha−1 fresh tuber yield mm−1 ET) were not significantly different between the irrigation treatments. However, across the irrigation treatments, the soil textures were significantly different. This showed that the interaction between irrigation treatments and soil textures was significant that the highest significant WP was obtained under DI in sandy loam. While PRD and DI treatments increased WP by, respectively, 11 and 5% in coarse sand and 28 and 36% in sandy loam relative to FI, they decreased WP in loamy sand by 15 and 13%. The reduced WP in loamy sand was due to nearly 28% fresh tuber yield loss in PRD and DI relative to FI even though ET was reduced by 9 and 11% in these irrigation treatments. This study showed that different soils will affect water-saving irrigation strategies that are worth knowing for suitable agricultural water management. So, under non-limited water resources conditions, loamy sand produces the highest yield under full irrigation but water-saving irrigations (PRD and DI) are not recommended due to considerable loss (28%) in yield. However, under restricted water resources, it is recommended to apply water-saving irrigations in sandy loam and coarse sand to achieve the highest water productivity.  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