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   741篇
  免费   57篇
  国内免费   1篇
林业   61篇
农学   30篇
基础科学   3篇
  124篇
综合类   65篇
农作物   40篇
水产渔业   106篇
畜牧兽医   312篇
园艺   15篇
植物保护   43篇
  2023年   9篇
  2022年   8篇
  2021年   22篇
  2020年   43篇
  2019年   37篇
  2018年   28篇
  2017年   27篇
  2016年   28篇
  2015年   34篇
  2014年   30篇
  2013年   37篇
  2012年   65篇
  2011年   62篇
  2010年   41篇
  2009年   34篇
  2008年   32篇
  2007年   45篇
  2006年   21篇
  2005年   22篇
  2004年   32篇
  2003年   25篇
  2002年   23篇
  2001年   9篇
  2000年   11篇
  1999年   10篇
  1998年   4篇
  1997年   4篇
  1996年   6篇
  1995年   2篇
  1993年   3篇
  1992年   8篇
  1991年   1篇
  1990年   3篇
  1989年   2篇
  1988年   5篇
  1987年   1篇
  1986年   4篇
  1985年   1篇
  1984年   3篇
  1983年   1篇
  1982年   2篇
  1981年   1篇
  1979年   2篇
  1978年   1篇
  1977年   1篇
  1975年   1篇
  1972年   1篇
  1965年   3篇
  1963年   2篇
  1957年   1篇
排序方式: 共有799条查询结果,搜索用时 15 毫秒
1.
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   
2.
Forest fragmentation caused by human activities has many implications for natural landscapes, such as habitat reduction and the loss of biodiversity. This study investigated the temporal fragmentation process of forest remnants in a strongly agro-industrialised region in southern Brazil over 25 years. The studied watershed area hosts two important typologies of the Atlantic Forest biome as well as Quaternary remnants of the Brazilian Savanna biome, which are considered hotspots of biodiversity and reflect the intense process of forest fragmentation caused by Brazilian urban and agro-industrial development. Thus, studies encompassing multitemporal scales are paramount to understanding changes in forest patterns and are fundamental for trend predictions of landscape dynamics. To perform the calculation of the mean normalised difference vegetation index, Landsat satellite images from 1991 to 2016 were processed using SPRING® software. Subsequently, FRAGSTATS® software was used to calculate landscape metrics. A reduction in the number of forest fragments since 1991 was observed, with a maximum amount of 5 243 fragments in 1993 that declined to 4 015 fragments in 2016. Although the number of fragments in the watershed decreased, the mean area increased by 72.9% and the mean of the shape index increased from 1.3 in 1991 to 1.5 in 2016. In addition, there was a 64.7% increase in the edge density and a reduction of 35.6 m in the isolation between the nearest neighbours. The degree of isolation of the fragments underwent a process of expansion and reduction when compared to 1991, presenting results that support the hypothesis that the Atlantic Forest is in a process of stabilisation and forest restoration.  相似文献   
3.
Euphytica - The hard-shelled character of ordinary cultivated Tartary buckwheat has become a factor influencing its taste and nutritional efficacy. However, the local variety, Rice-Tartary, can...  相似文献   
4.
The use of spent coffee grounds (SCG) as organic amendment is a triple solution: a reuse of this bio-residue (6 millions of tons per year), an increase in soil organic carbon (SCG contain 82% of carbohydrates and 13% of proteins) and a decrease in CO2 emissions into the atmosphere. Thus, we investigate the effect of SCG on soil and plants in an in vitro assay. The variables considered were SCG dose (2.5 and 10%), two agricultural soils (Calcisol and Luvisol), and four cultivation times (15, 30, 45 and 60 days). The chemical and physicochemical soil properties, SEM images and growth parameters were analyzed. The highest dose of SCG increased organic carbon, total N and available K and P by 286%, 188%, 45%, and 9%, respectively, while decreasing lettuce growth by 233% compared to control. The SEM study revealed that SCG particles are incorporated into soil aggregates and microorganisms grow over them, which is also corroborated by a 10-times increase of soil respiration rate. The behavior of both soils differs for some SCG effects. In conclusion, SCG have a great potential to increase soil fertility and further investigations are needed in order to improve the use of SCG as amendment.  相似文献   
5.
This study was conducted on 78 13-month-old crossbred beef heifers that weighed 215 kg in Southern Rio Grande do Sul (RS) State, Brazil. We evaluated the performance of beef heifers that were reared in a pasture system that received a mineral supplement energy-type protein with added sodium lasalocid (LAS). The heifers were randomly and uniformly divided into 2 groups, with 39 animals in each group. One group of animals received a mineral supplement energy-type protein without sodium lasalocid (CON), and the other group received a mineral supplement energy-type protein with added LAS. The mean feed intake, the body weight (BW), the average daily gain (ADG), the body condition score (BCS), and ovarian cyclicity were recorded, and economic parameters were calculated. No differences in supplement intake were observed between the groups, which ensures adequate intake of the other components of the mineral mixture, which are part of the nutritional requirements for the production process. Similarly, no difference in the ADG was observed between treatments. We observed that the heifers in the LAS group had a higher BW gain (51 kg) that the CON heifers (40 kg; P?<?0.05). In addition, LAS-supplemented heifers had a higher BCS (3.53) than CON heifers (3.38) at the end of the experiment (P?<?0.05). The heifers supplemented with LAS had a higher profitability than the CON heifers, even with the higher cost of the supplement containing LAS; this effect was due to the higher live BW at the end of the study. We concluded that the administration of a mineral supplement energy-type protein with added LAS has beneficial effects on beef heifers in terms of production and economic feasibility.  相似文献   
6.
In this paper, a systematic methodology is shown for the scaling‐up of Nannochloropsis gaditana production for aquaculture uses. First, an adequate culture medium was developed, prepared using fertilizers instead of pure chemicals. Subsequently, the performance of N. gaditana was modelled as a function of average irradiance; this model being validated in continuous culture experiments. The model was used to determine the optimal dilution rate as well as the expected biomass concentration and productivity at optimal conditions. Finally, outdoor experiments were performed to confirm the model's validity and to determine optimal conditions at real production step. Biomass productivity values of up to 0.08 g L?1 day?1 were obtained at an optimal dilution rate of 0.25 per day in 0.2 m wide flat‐panel reactors using culture medium containing 0.4 g/L NO?3 and 0.034 g/L·PO4?3. Fish trials with Atlantic Salmon demonstrated that the inclusion of produced biomass into fed increase the final weight up to 5%, thus confirming the adequacy of the biomass produced for aquaculture uses. The growth model and the scaling‐up strategy proposed here are necessary to develop real industrial‐scale processes capable of supplying microalgal biomass to the aquaculture markets, which in turn require a guaranteed supply and quality of the raw materials provided.  相似文献   
7.
The objectives of this study were to evaluate the reproductive cycle, morphological changes of ovary and mobilization of energy reserves in Nile tilapia reared with biofloc technology (BFT). In general, the growth and reproductive performance were highly similar between BFT and Control system (clear water). Difference between the systems was found in the hepatosomatic index (using mixed‐effects models), which suggested that BFT can alter the energy mobilization in the post‐spawning period. The absolute and relative fecundity, fertilization rate, number of larvae produced per female, gonadosomatic index, proportion of oogenesis cells, number of post‐ovulatory and atretic follicles were similar between the two systems. We also did not detect a reduction in the reproductive cycle length in Nile tilapia reared in BFT. Because there was no evidence of the negative effects of BFT on Nile tilapia reproduction, we concluded that BFT might be used for breeder stocking of this species.  相似文献   
8.
Beyer  Nicole  Kirsch  Felix  Gabriel  Doreen  Westphal  Catrin 《Landscape Ecology》2021,36(9):2657-2671
Landscape Ecology - Pollinator declines and functional homogenization of farmland insect communities have been reported. Mass-flowering crops (MFC) can support pollinators by providing floral...  相似文献   
9.
Increased atmospheric carbon dioxide (CO2) is a consequence of recent anthropogenic environmental changes, and few studies have evaluated its effects on tropical grasses used in Brazilian pastures, the main feed source for major part of ruminant livestock. This study evaluated forage production, chemical composition, in vitro total gas production and organic matter degradability of Brachiaria brizantha under contrasting CO2 atmospheric conditions in a free air carbon dioxide enrichment (FACE) facility. The forage plants were sown in each of the 12 octagonal rings of the FACE facility: six under ambient atmospheric CO2 concentration of approximately 390 μmol/mol, hereafter referred to as control (CON) plots, and other six rings enriched with pure CO2 flux to achieve a target CO2 concentration of 550 μmol/mol, hereafter called elevated CO2 (eCO2) plots. Soil samples were collected to determine carbon and nitrogen concentrations. After seventy days of sowing, a standardization cutting was performed and then at regular intervals of 21 days the forage was harvested (ten harvest dates) and forwarded to laboratorial analyses. Forage above‐ground biomass production (dry matter (DM): 6,143 vs. 6,554 kg/ha), as well as morphological characteristics (leaves: 71% vs. 68%; stem: 28% vs. 31%), chemical composition (crude protein: 162.9 vs. 161.8; neutral detergent fibre: 663.8 vs. 664.3; acid detergent fibre: 369.5 vs. 381; lignin: 60.1 vs. 64.1 g/kg DM; total C: 45.9 vs. 45.9; total N: 2.8 vs. 2.8; total S: 0.2% vs. 0.2%), organic matter in vitro degradability (573.5 vs. 585.3 g/kg), methane (5.7 vs. 4.3 ml/g DM) and total gas (128.3 vs. 94.5 ml/g DM) production did not differ significantly between CON and eCO2 treatments (p > .05). The results indicated that at least under short‐term enrichment, B. brizantha was not affected by eCO2.  相似文献   
10.
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