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   4812篇
  免费   180篇
  国内免费   3篇
林业   245篇
农学   202篇
基础科学   36篇
  732篇
综合类   1266篇
农作物   217篇
水产渔业   304篇
畜牧兽医   1525篇
园艺   110篇
植物保护   358篇
  2023年   22篇
  2022年   21篇
  2021年   57篇
  2020年   73篇
  2019年   93篇
  2018年   90篇
  2017年   83篇
  2016年   89篇
  2015年   64篇
  2014年   84篇
  2013年   182篇
  2012年   259篇
  2011年   339篇
  2010年   187篇
  2009年   145篇
  2008年   331篇
  2007年   337篇
  2006年   376篇
  2005年   320篇
  2004年   330篇
  2003年   303篇
  2002年   302篇
  2001年   59篇
  2000年   32篇
  1999年   41篇
  1998年   35篇
  1997年   25篇
  1996年   25篇
  1995年   36篇
  1994年   30篇
  1993年   36篇
  1992年   31篇
  1991年   31篇
  1990年   29篇
  1989年   36篇
  1988年   27篇
  1987年   32篇
  1986年   21篇
  1985年   27篇
  1984年   32篇
  1983年   25篇
  1982年   23篇
  1981年   26篇
  1980年   23篇
  1979年   15篇
  1978年   17篇
  1977年   18篇
  1976年   19篇
  1975年   16篇
  1974年   24篇
排序方式: 共有4995条查询结果,搜索用时 15 毫秒
1.
Journal of Pest Science - Over the past three decades, highly increased whitefly (Bemisia tabaci) populations have been observed on the staple food crop cassava in eastern Africa and associated...  相似文献   
2.
Rising temperatures caused by climate change are likely to affect cool‐water and warm‐water fishes differently. Yet, forecasts of anticipated temperature effects on fishes of different thermal guilds are lacking, especially in freshwater ecosystems. Towards this end, we used spatially explicit, growth rate potential (GRP) models to project changes in seasonal habitat quality for a warm‐water piscivore (largemouth bass Micropterus salmoides), a cool‐water piscivore (walleye Sander vitreus) and a hybrid piscivore (saugeye S. vitreus × S. canadensis) in two Midwestern reservoirs. We assessed habitat quality for two periods (early and middle 21st century) under two realistic greenhouse gas emission scenarios (a mid‐century emissions peak and a rapid continuous increase in emissions). Largemouth bass were projected to experience enhanced or slightly reduced habitat during all seasons, and throughout the mid‐21st century. By contrast, walleye habitat was projected to decline with anticipated warming, except during the spring in the smaller of our two study reservoirs and during the fall in the larger of our two study reservoirs. Saugeye habitat was projected to either increase modestly or decline slightly during the spring and fall and declines in habitat quality and quantity that were smaller than those for walleye were identified during summer. Collectively, our findings indicate that climate warming will differentially alter habitat suitability for reservoir piscivores, favouring warm‐water species over cool‐water species. We expect these changes in habitat quality to impact the dynamics of reservoir fish populations to varying degrees necessitating the consideration of climate when making future management decisions.  相似文献   
3.
Agroforestry Systems - Increasing tree cover and managing trees better on farms in Ethiopia supports livelihoods and the environment but most tree-planting schemes promote only a few species. This...  相似文献   
4.
Abstract

A study was carried out in sandy clay loam textured soil of Mymensingh, Bangladesh to evaluate weed control efficiency of pre-emergence herbicide pyrazosulfuron-ethyl either alone or in sequential application with post-emergence herbicide in transplanted rainy season rice under non-puddled strip-tilled (NPST) field condition and also to examine the residual effect of those herbicides on germination and growth of the subsequently grown wheat crop. Five treatment combinations of pyrazosulfuron-ethyl were tested against one weedy check and one weed-free check. The study revealed that NPST rice field was mostly infested by grass and sedge weeds and herbicide treatments offered a wide range of control (above 50% to 95%) on all types of weeds. Application of pyrazosulfuron-ethyl followed by (fb) orthosulfamuron fb butachlor plus propanil provided the most effective and economic weed control over two years of the study. Moreover, micro-plot bioassay study claimed germination and growth of subsequently grown wheat were not adversely affected by herbicides that were applied in rice. Therefore, application of pyrazosulfuron-ethyl followed by post-emergence herbicide could be effective and economic to control weeds in NPST rice under rice-wheat system, but proper rate and time of application should strictly be followed.  相似文献   
5.
The effects of pH on the adsorption of silicate and phosphate, either singly or in competition, by two acid soils were investigated. Both soils adsorbed two to three times more P than Si and adsorption isotherms at pH 5.0, 5.5, 6.0 and 6.5 showed that increasing pH greatly increased Si adsorption but decreased that of P. Silicate adsorption was very low below pH 5.0, increased rapidly up to pH 9–10 before decreasing again. Adsorption of P was at a maximum at pH 2.0, decreased slowly up to pH 7.0 and then more rapidly above pH 7.0. When Si and P were added at equimolar concentrations, the presence of P decreased Si adsorption between pH 6.0 and 8.0 while the presence of Si decreased P adsorption in the pH region 6.0 and 11. Addition of calcium silicate at rates equivalent to 300, 600 and 1200 kg Si ha?1 resulted in a progressive increase in soil pH. Separate samples of soil were treated with Ca(OH)2 to give the same pH values so that the effect of Si could be identified. The highest rate of Si (1200 kg ha?1 which gave a pH of 6.5) caused a significant decrease in P adsorption (as determined by adsorption isotherms) and an increase in resin-extractable P but the lower rates had little effect. Addition of P to the soil as calcium phosphate at rates equivalent to 30, 60 and 100 kg P ha?1 all caused a decrease in Si adsorption capacity and an increase in CaCl2-extractable Si. It was concluded that the strategy of adding Si to lower P requirements in acid soils is not likely to be effective while addition of fertilizer P may well lower Si adsorption and promote Si desorption and its increased mobility.  相似文献   
6.
To identify cost-effective ways to control invasive species, we need to evaluate alternative methods. The invasion of tropical Australia by cane toads (Rhinella marina) has killed many native predators, prompting efforts to cull adult toads. We analyzed a dataset on >?17,500 toads killed by government-employed teams from 2005 to 2008, using a range of methods (incidental captures, targeted searches, traps) as well as records of toads found as road kills. The culling occurred in the northwestern part of the Northern Territory, moving west as the toad front expanded into Western Australia. Both season and method affected rates of capture and demographic attributes (sex ratio, proportion of adults) of the culled anurans, with a strong interaction between these factors. Most methods were more effective during and after the seasonal monsoons (when moist conditions facilitated anuran activity), but that seasonal variation was greater for numbers of toads found on the roads than captured in other ways. Juvenile toads and adult female toads constituted higher proportions of total captures in the Early Dry than in other times of year, reflecting seasonal breeding. Traps caught a higher proportion of adult female toads than did other methods, but overall had low capture rates. Rates of range expansion by toads were similar before, during and after the culling effort, suggesting that removing adult toads did not affect the speed of the invasion. Nonetheless, culling adult cane toads may be effective in some situations (“island” populations, extralimital translocations), and results from this program may be valuable for future toad-control efforts.  相似文献   
7.
Analysis of uppermost fully expanded leaves is useful to detect a deficiency of mineral nutrients such as phosphorus (P) and potassium (K) in soybean. Although, the leaf P or K status aids in fertilizer management, information on nutrient seasonal relationships with growth and yield traits at maturity are limited. To investigate this, soybean was grown under varying P or K nutrition under ambient and elevated CO2 concentrations. Results show significant relationships of the relative total biomass and yield‐related traits with the foliar P and K concentrations measured several times in the season across CO2 levels. However, the relationships established earlier in the season showed that the growth period between 25 and 37 d after planting (DAP), representing the beginning of flowering and pod, respectively, is the best for leaf sampling to determine the foliar P or K status. The leaf P and K status as well as the critical leaf P (CLPC) and K (CLKC) concentrations for traits such as seed yield peaked around 30 DAP (R2 stage) and tended to decline thereafter with the plant age. The CLPC and CLKC of seed yield indicate that the leaf P and K concentration of at least 2.74 mg g?1 and 19.06 mg g?1, respectively, in the uppermost fully expanded leaves are needed between 25 and 37 DAP for near‐optimum soybean yield. Moreover, the greatest impact of P and K deficiency occurred for the traits that contribute the most to the soybean yield (e.g., relative total biomass, seed yield, pod and seed numbers), while traits such as seed number per pod, seed size, and shelling percentages were the least affected and showed smaller leaf critical concentration. The CLPC or CLKC for biomass and seed yield was greater under elevated CO2 24–25 DAP but varied thereafter. These results are useful to researchers and farmers to understand the dynamics of the relationship of pre‐harvest leaf P and K status with soybean productivity at maturity, and in the determination of suitable growth stage to collect leaf samples.  相似文献   
8.
9.
Overdose and death of both target and non-target species are not uncommon when α–chloralose is used for animal control. Alpha–chloralose appears to depress the central nervous system by having an affect on the GABA-A receptor in a manner similar to ethanol. Recently central administration of oxytocin was shown to block ethanol-induced impairment in rodents. Our study investigates whether IM administration of oxytocin speeds avian recovery from α–chloralose and reduces mortality under simulated field conditions. We found that when injected twice with oxytocin (30 uL/kg) chickens entered α–chloralose narcosis more quickly and showed greater behavioral impairment. When fed the LD50 dose of α–chloralose, repeated injection with oxytocin produced no significant difference in mortality rate or time to death in comparison to the control group. The effects of peripheral oxytocin on female chicks were different than results found previously in cockerels given oxytocin centrally. Oxytocin is not a suitable antidote for α–chloralose overdose during animal capture in the field but instead accentuates α–chloralose narcosis, making capture more likely and efficient.  相似文献   
10.
Hydrogen peroxide (H2O2) is a commonly used treatment for a range of parasitic diseases of marine finfish, including amoebic gill disease (AGD). While this treatment is partially effective at reducing parasite load, H2O2 can have detrimental effects on the host under certain conditions. Treatment temperature and dose concentration are two factors that are known to influence the toxicity of H2O2; however, their impact on the outcome of AGD treatment remains unclear. Here, we investigated the effects of treatment temperature (8, 12 or 16°C) and dose concentration (750, 1,000, 1,250 mg/L) on the efficacy of H2O2 to treat AGD. We demonstrated that a 20-min bath treatment of H2O2 at all doses reduced both parasite load and gross gill score significantly. Parasite load and gross gill score were lowest in the 1,000 mg/L treatment performed at 12°C. At the high dose and temperature combinations, H2O2 caused moderate gill damage and a significant increase in the plasma concentration of electrolytes (sodium, chloride and potassium). Taken together, our study demonstrates that higher H2O2 treatment temperatures can adversely affect the host and do not improve the effectiveness of the treatment.  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