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   4620篇
  免费   213篇
  国内免费   4篇
林业   280篇
农学   113篇
基础科学   27篇
  767篇
综合类   746篇
农作物   233篇
水产渔业   402篇
畜牧兽医   1843篇
园艺   144篇
植物保护   282篇
  2023年   40篇
  2022年   51篇
  2021年   99篇
  2020年   116篇
  2019年   126篇
  2018年   93篇
  2017年   112篇
  2016年   113篇
  2015年   79篇
  2014年   125篇
  2013年   177篇
  2012年   246篇
  2011年   327篇
  2010年   162篇
  2009年   147篇
  2008年   263篇
  2007年   283篇
  2006年   299篇
  2005年   278篇
  2004年   274篇
  2003年   252篇
  2002年   241篇
  2001年   50篇
  2000年   44篇
  1999年   51篇
  1998年   63篇
  1997年   53篇
  1996年   54篇
  1995年   40篇
  1994年   38篇
  1993年   39篇
  1992年   32篇
  1991年   32篇
  1990年   36篇
  1989年   32篇
  1988年   23篇
  1987年   20篇
  1986年   20篇
  1985年   29篇
  1984年   21篇
  1983年   27篇
  1982年   25篇
  1981年   22篇
  1980年   20篇
  1978年   18篇
  1977年   11篇
  1976年   8篇
  1975年   19篇
  1974年   16篇
  1967年   8篇
排序方式: 共有4837条查询结果,搜索用时 31 毫秒
51.
The almond moth Cadra cautella (Walker), a key pest of storage facilities, is difficult to manage using synthetic chemicals. Pheromone-based management methods remain a high priority due to advantages over conventional management practices, which typically use insecticides. Cadra cautella females release a blend of pheromone including (Z, E)-9,12-tetradecadienyl acetate (ZETA) and (Z)-9-tetradecadien-1-yl acetate (ZTA). The effect of these components on mating of C. cautella and how response varies with the population density and sex ratio remain unknown. In this study, the mating status of C. cautella was studied inside mating cages under different ratios of ZETA and ZTA diluted in hexane and at different population sizes either with equal or unequal sex ratio. The lowest percentage of mated females (highest mating disruption [MD] effects), corresponding to roughly 12.5%, was produced by a 5:1 and 3.3:1 ratio of ZETA:ZTA. Populations with equal sex ratio showed the lowest percentage of mated females, at 20% and 12.5% under lower and higher density, respectively. The next lowest percentage of mated females was produced when the sex ratio was set to 1: 2 and 2:1 male:female, with just 25% and 22.5% of moths mated, respectively. This study shows that mating status of C. cautella is influenced by ZETA:ZTA ratio, sex ratio, and population size. This current knowledge would have useful implications for mating disruption programs.  相似文献   
52.
A new cytotoxic thiodepsipeptide, verrucosamide (1), was isolated along with the known, related cyclic peptide thiocoraline, from the extract of a marine-derived actinomycete, a Verrucosispora sp., our strain CNX-026. The new peptide, which is composed of two rare seven-membered 1,4-thiazepane rings, was elucidated by a combination of spectral methods and the absolute configuration was determined by a single X-ray diffraction study. Verrucosamide (1) showed moderate cytotoxicity and selectivity in the NCI 60 cell line bioassay. The most susceptible cell lines were MDA-MB-468 breast carcinoma with an LD50 of 1.26 µM, and COLO 205 colon adenocarcinoma with an LD50 of 1.4 µM. Also isolated along with verrucosamide were three small 3-hydroxy(alkoxy)-quinaldic acid derivatives that appear to be products of the same biosynthetic pathway.  相似文献   
53.
54.
Context

Human appropriation of net primary productivity (HANPP) is employed as a measure of human pressures on biodiversity, though largely at global and national scales rather than landscape to regional scales where many conservation decisions take place. Though gaining in familiarity, HANPP is not widely utilized by conservation professionals.

Objectives

This study, encompassing the US side of the Great Lakes basin, examines how regional distributions of HANPP relate to landscape-based biodiversity proxy metrics used by conservation professionals. Our objectives were (1) to quantify the HANPP of managed lands at the county scale; and (2) to assess spatial patterns of HANPP in comparison to landscape diversity and local habitat connectedness to determine if the metric can provide useful information to conservation professionals.

Methods

We aggregated forest and cropland NPP data between 2005 and 2015 and coupled it with previously published potential vegetation maps to quantify the HANPP of each county in the study region. We mapped the outputs at 500 m resolution to analyze spatial relationships between HANPP and landscape metrics of biodiversity potential.

Results

Area-weighted HANPP across our study region averaged 45% of NPP, down to 4.9% in forest-dominated counties. Greater HANPP correlated with reduced landscape diversity (p?<?0.001, r2?=?0.28) and reduced local habitat connectedness (p?<?0.001, r2?=?0.36).

Conclusion

HANPP could be used as an additional tool for conservation professionals during regional-scale land use planning or conservation decision-making, particularly in mixed-use landscapes that both support important biodiversity and have high levels of primary production harvest.

  相似文献   
55.
Landscape Ecology - Functional landscape connectivity is vital for the conservation of wildlife species. Landscape connectivity models often overlook factors such as mortality and asymmetry in...  相似文献   
56.
Precision Agriculture - Golf course superintendent’s knowledge of variability may be an overlooked and underutilized tool for precision turfgrass management (PTM). This case study used a...  相似文献   
57.
Two aborted Chester White pig fetuses were presented to a veterinary diagnostic laboratory in Illinois. Postmortem examination identified no gross abnormalities. Histologic evaluation revealed multifocal necrosis of chorionic epithelial cells, coalescing areas of mineralization in the placenta, and focal accumulations of viable and degenerate neutrophils in the lung. Intra- and extracellular acid-fast bacilli were identified in the lesions in both the placenta and lungs. Bacterial culture of stomach contents yielded heavy growth of Mycobacterium fortuitum, a rapidly growing nontuberculous mycobacterium (NTM), which was further confirmed through whole-genome sequencing. NTM are opportunistic pathogens commonly found in the soil and in contaminated water supplies. In animals, M. fortuitum is typically introduced through cutaneous wounds leading to infections limited to the skin, with systemic infection being uncommon. To our knowledge, abortion caused by M. fortuitum has not been reported previously.  相似文献   
58.
Analysis of the chemical components from the culture broth of the marine bacterium Saccharomonospora sp. CNQ-490 has yielded three novel compounds: saccharobisindole (1), neoasterric methyl ester (2), and 7-chloro-4(1H)-quinolone (3), in addition to acremonidine E (4), pinselin (5), penicitrinon A (6), and penicitrinon E (7). The chemical structures of the three novel compounds were elucidated by the interpretation of 1D, 2D nuclear magnetic resonance (NMR), and high-resolution mass spectrometry (HRMS) data. Compound 2 generated weak inhibition activity against Bacillus subtilis KCTC2441 and Staphylococcus aureus KCTC1927 at concentrations of 32 μg/mL and 64 μg/mL, respectively, whereas compounds 1 and 3 did not have any observable effects. In addition, compound 2 displayed weak anti-quorum sensing (QS) effects against S. aureus KCTC1927 and Micrococcus luteus SCO560.  相似文献   
59.
动物精子的性别可通过流式精子分选仪和DNA标识进行鉴定,而利用鉴定性别的精子(即性控精子)并借助人工授精技术或其它授精技术产生的后代,在过去5年中估计已多达30000个(其中大多数是母牛)。有关文献资料证明,能有效地区分X精子和Y精子的唯一标记物是精子染色体中的DNA。众所周知,目前世界各地采用的方法是Beltsville精子分选技术,该技术根据X精子群和Y精子群中DNA相对含量上的差异,用荧光染液(Hoechst33342染液)标记精子,随后利用流式细胞仪分选经荧光标记的精子,从而达到分离精子的目的。目前,X精子或Y精子正常的生产速度是每小时1500万个,该项技术已在家畜、实验动物和动物园动物中应用,如果将该技术应用在人上,在预选后代性别比例上可达90%~95%的成功率。因动物品种不同,性控精子在动物体内的授精部位也不相同。常规的人工授精技术、宫内授精技术、输卵管内授精技术、用于胚胎移植的体外受精(in-vitro fertilization,IVF)技术或子宫角深部授精技术均能有效地使动物怀孕,至于利用哪种技术进行性控精子的精则取决于动物品种。尽管所有动物都能获得高纯度的分选精子,但是在实际生产中利用低剂量精子还难以让母猪怀孕。子宫角深部授精技术每次授精0.5~1.00亿个性控精子已能产生可喜的效果:利用特制的输精管,输入常规人工授精所需精子量的五十分之一的性控精子,足以使动物怀孕。性控精子通过常规的授精技术能够被猪成功利用前,还需重新设计输精管,同时输精的次数和每次授精时精子数量也需作进一步的研究。分选精子的低温保存技术已被牛人工授精普遍采用。尽管已能利用冷冻的性别分选精子生产小猪,但是性别分选精子经冷冻和解冻处理后在常规生产中的应用还未达到最理想的效果。本文将讨论猪精子性别分选的最新研究成果及其发展趋势,并重点探讨将性别分选技术应用于养猪生产中必须对其进行必要的技术开发。  相似文献   
60.
We examined the dominant patterns of variability in the fish fauna of the southern California Current based on a principal component (PC) analysis of the California Cooperative Oceanic Fisheries Investigations ichthyoplankton data set, 1951–2008. Eighty‐six taxa were analyzed, including all ecologically dominant fish species, both exploited and unexploited. The first three PCs accounted for 20.5, 12.4 and 6.8% of the variance of the data, respectively (total: 39.7%). Each was dominated by taxa from particular adult or larval habitats. PC 1 predominantly represented the coherent response of 24 mesopelagic taxa from 10 families and was most highly correlated with long‐term trends in midwater oxygen levels. PC 2 was dominated by six of the seven most abundant ichthyoplankton taxa in the region, predominantly California Current endemics including key pelagic species (northern anchovy, Pacific sardine and Pacific hake), rockfishes (genus Sebastes) and two midwater taxa. It was correlated primarily with sea surface temperature and exhibited a significant declining trend. PC 3 was dominated by coastal and reef‐associated fishes with predominantly southerly affinities. It was positively correlated with sea surface temperature and sea level height, a proxy for diminished flow of the California Current. The taxa dominating PCs 2 and 3 mostly spatially co‐occur as ichthyoplankton. These results suggest that fish assemblages in the California Current are predominantly influenced by environmental forcing of their ocean habitats as adults or larvae, or both.  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