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   987篇
  免费   58篇
  国内免费   2篇
林业   78篇
农学   96篇
基础科学   15篇
  289篇
综合类   64篇
农作物   118篇
水产渔业   71篇
畜牧兽医   224篇
园艺   16篇
植物保护   76篇
  2023年   16篇
  2022年   25篇
  2021年   33篇
  2020年   52篇
  2019年   56篇
  2018年   59篇
  2017年   77篇
  2016年   57篇
  2015年   30篇
  2014年   38篇
  2013年   106篇
  2012年   61篇
  2011年   72篇
  2010年   43篇
  2009年   40篇
  2008年   44篇
  2007年   24篇
  2006年   16篇
  2005年   14篇
  2004年   14篇
  2003年   17篇
  2002年   17篇
  2001年   15篇
  2000年   15篇
  1999年   8篇
  1998年   6篇
  1997年   4篇
  1996年   7篇
  1995年   3篇
  1994年   5篇
  1993年   5篇
  1992年   5篇
  1991年   7篇
  1990年   9篇
  1989年   5篇
  1988年   6篇
  1987年   3篇
  1986年   4篇
  1985年   5篇
  1980年   6篇
  1978年   2篇
  1977年   2篇
  1975年   1篇
  1973年   1篇
  1971年   2篇
  1970年   1篇
  1968年   1篇
  1960年   1篇
  1959年   3篇
  1957年   2篇
排序方式: 共有1047条查询结果,搜索用时 62 毫秒
141.
142.
143.
Nuclear magnetic resonance ‐based metabolomics was applied to study effects of egg aging on ovarian fluid metabolites in rainbow trout, Oncorhynchus mykiss. The eggs of three females were pooled and then assigned to three plastic vials for 18 days in vitro storage at 4°C. Ovarian fluid samples were taken 0, 6, 12 and 18 days after storage. Three groups of metabolites including amino acids, osmolytes and energy metabolites were found to change during storage period. The glucose levels of ovarian fluid showed significant decreases on days 12 and 18 after storage (< .05). For acetoacetate and acetate, significant increases were observed, respectively, on days 12 and 18 after storage (< .05). The creatine levels of ovarian fluid increased significantly on days 12 and 18 after storage (< .05). Lactate levels in ovarian fluid elevated during storage period (< .05). Glycerol levels showed a significant increase on days 12 and 18 after storage (< .05). The values of osmolytes in ovarian fluid (e.g., betaine, taurine, trimethylamine‐N‐oxide, N,N‐dimethylglycine) showed a decreasing trend on day 12 which continued until the end of storage on day 18 (< .05). Almost all amino acids elevated on days 12 and 18 after storage (< .05). After an apparent elevation in isoleucine levels on days 6 and 12, this amino acid decreased on day 18 after storage (< .05). The osmolytes might act as antioxidant against free‐radicals produced as a result of over‐ripening. Glucose can be used as energy resource for eggs and bacteria during ova storage. Also, change pattern of amino acids indicate hydrolysis of proteins as the time of storage increases.  相似文献   
144.
Parasitoids are characterized by a defined range of hosts, either more specialist or generalist. Under natural conditions, females may encounter different host species on the same plant or in the same location. In this case, their preference for one host could influence their choice. However, the presence of less suitable hosts may also affect their choice and, in some cases, may reduce their interest in a patch where both preferred and less preferred hosts are available. The aim of the present study was to test the consequences of the simultaneous presence of three cereal aphids (Sitobion avenae Fabricius, Metopolophium dirhodum Walker, and Rhopalosiphum padi Linnaeus) on the parasitism by two of their parasitoids, Aphidius ervi Haliday and Praon volucre Haliday. Firstly, in the no-choice experiment, A. ervi parasitized on S. avenae at a significantly higher rate as compared to M. dirhodum, whereas no parasitism on R. padi was observed. P. volucre parasitized the three species of cereal aphids with a significant preference for S. avenae. Interestingly, when two or three host species were offered simultaneously in the same quantity to pairs of parasitoids, the level of parasitism was less than that observed for one host species alone. This observation exhibits a distractive effect on non-host species, from the defense mechanism of a non-suitable host or from the perception of bad quality patches. These results raise the question of the practical application of inundative release of parasitoids for biocontrol when several hosts are available simultaneously.  相似文献   
145.
To reduce reliance on scientific fertilizer due to rapid increase of fertilizer prices and environmental constraint, it necessary to improve crop productivity and soil fertility on sustainable basses. Utilization of “summer gap” through biochar and legumes have pleasant effects on improving crop productivity and soil fertility on long term basses. Two years’ field experiments were conducted on wheat and maize crops with “summer gap” utilization with legumes and biochar at research farm of agronomy, the University of Agriculture Peshawar during 2011–2013. Wheat-maize-wheat cropping system was followed with the adjustment of legumes in “summer gap” (land available after wheat harvest till maize sowing). Legumes i.?e. mung bean, cowpea and Sesbania with a fallow were adjusted in the “summer gap” with and without biochar application. Biochar was included at the rate of 0 and 50?t?ha-1 with four N levels of 0, 90, 120, 150 and 0, 60, 90, 120?kg?ha-1 to subsequent maize and wheat crops, respectively. In legumes’ experiment, biochar increased fresh and dry fodder yield in cowpea and Sesbania, grain and biological yields in mung bean. In maize experiments, biochar improved grain yield. Nitrogen application increased grain and biological yields. In wheat experiments with increasing nitrogen level enhanced biological and grain yields. It is concluded that use of biochar and legumes in “summer gap” improve overall farm productivity and soil fertility on sustainable basses.  相似文献   
146.
As the use of high performance textiles has grown, the need for chemical finishes to provide the fabric properties required in the special applications has grown accordingly. In this project, a series of water dispersible polyurethanes dispersion (CS-PUs) with multipurpose performance profile was developed using isophorone diisocyanate (IPDI), polyethylene glycol (PEG), 2,2-dimethylol propionic acid (DMPA) and chitosan (CS) for textile applications. In two step synthesis process, NCO functional PU prepolymers prepared by reacting IPDI, PEG, and DMPA were extended with varying molar quantities of chitosan followed by structural characterization through FTIR. The prepared CS-PU dispersions were applied onto the dyed and printed poly-cotton blend fabrics. The performance behavior of the treated fabric in terms of crease recovery, tear strength, tensile strength, and antibacterial properties was evaluated by applying standard test methods. These investigations show that the CS-PU dispersions can be applied as antibacterial textile finishes with significant improvement in the physical and mechanical properties of poly-cotton fabrics.  相似文献   
147.

Background

Fungal endophytes are the living symbionts which cause no apparent damage to the host tissue. The distribution pattern of these endophytes within a host plant is mediated by environmental factors. This study was carried out to explore the fungal endophyte community and their distribution pattern in Asparagus racemosus and Hemidesmus indicus growing in the study area.

Results

Foliar endophytes were isolated for 2 years from A. racemosus and H. indicus at four different seasons (June–August, September–November, December–February, March–May). A total of 5400 (675/season/year) leaf segments harbored 38 fungal species belonging to 17 genera, 12 miscellaneous mycelia sterile from 968 isolates and 13 had yeast like growth. In A. racemosus, Acremonium strictum and Phomopsis sp.1, were dominant with overall relative colonization densities (RCD) of 7.11% and 5.44% respectively, followed by Colletotrichum sp.3 and Colletotrichum sp.1 of 4.89% and 4.83% respectively. In H. indicus the dominant species was A. strictum having higher overall RCD of 5.06%, followed by Fusarium moniliforme and Colletotrichum sp.2 with RCD of 3.83% and 3%, respectively. Further the overall colonization and isolation rates were higher during the wet periods (September–November) in both A. racemosus (92.22% and 95.11%) and H. indicus (82% and 77.11%).

Conclusion

Study samples treated with 0.2% HgCl2 and 75% EtOH for 30 s and 1 min, respectively, confirmed most favorable method of isolation of the endophytes. Owing to high mean isolation and colonization rates, September–November season proved to be the optimal season for endophyte isolation in both the study plants. Assessing the bioactive potential of these endophytes, may lead to the isolation of novel natural products and metabolites.
  相似文献   
148.
Agricultural sector is a major source of employment and export earnings for Pakistan. Exports of Pakistan are highly concentrated in agricultural products including cotton, rice, sugar and fruit. This study analyzed comparative and competitive advantages of the main agricultural products of Pakistan, i.e. cotton, rice and tangerines. Consumption patterns of top exporting countries in each category were also analyzed. Results revealed that Pakistan had strong competitiveness for export of these commodities. Strong potential for growth in the agricultural sector of Pakistan was found and it could be optimized to raise export earnings and meet the objectives of the strategic trade policy framework(STPF) 2015-2018 of Pakistan.  相似文献   
149.
Common ragweed invasion poses serious risks to human health, biodiversity and agricultural production throughout the world. Although ragweed has an enormous potential to expand its range in Turkey, studies on ragweed have only concentrated on the Black Sea region. An exploratory survey was conducted to observe the level of ragweed occurrence on the nodes of predecided 10 × 10 km grids in the Thrace region of Turkey. Ragweed populations were observed in 44 out of 129 sites in a wide range of habitats, including roadsides, pastures, agricultural fields and non‐agricultural areas. The highest levels of ragweed infestation were recorded in the agricultural fields, followed by the roadsides. The most‐invaded crop was sunflower. The sunflower fields had the highest ragweed coverage as well and invasion even resulted in crop failure alongside the field borders. A spatial analysis of the data indicated that ragweed exhibited an aggregated pattern over the Thrace region. Ragweed distribution was not associated with the characteristics of the soils, which had a varying range of pH, electrical conductivity and texture. The results revealed that ragweed invasion can arise as a serious weed problem, interfering with the sustainability of sunflower production in Thrace and other sunflower‐producing regions of the country. A high degree of ragweed infestation also will contribute to the pollen level counts for the most crowded cities in the region, posing a serious danger to human health.  相似文献   
150.
Evaluation of various fungicides against needle blight disease of pine was carried out at Sheri Kashmir University of Agriculture Sciences and Technology of Kashmir(India) during the year 2008 and 2009. Application of various fungicides were evaluated in vivo at different concentrations. However in both, mist-chamber and poly-chamber, the Blue pine(Pinus wallichiana Jackson)seedlings treated with hexaconazole 5EC(@ 0.03 %) or carbendazim 50WP(@ 0.1 %) depicted significantly less disease incidence and intensity. The seedlings treated with mancozeb 75WP(@ 0.3 %) and chlorothalonil 75WP(@0.3 %) exhibited less disease incidence and intensity.Increase in relative humidity from 60 to 100 % significantly enhanced needle blight disease incidence and intensity. In field trial the fungitoxicants used either as single spray or protectant followed by systemic fungitoxicant spray significantly reduced disease incidence in Blue pine compared to check. The mean disease incidence in fungitoxicant treated plants varied from 11.82 to 25.51 %as compared to 36.03 % in control.  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