全文获取类型
收费全文 | 38128篇 |
免费 | 2302篇 |
国内免费 | 281篇 |
专业分类
林业 | 1576篇 |
农学 | 1412篇 |
基础科学 | 324篇 |
5481篇 | |
综合类 | 5649篇 |
农作物 | 1486篇 |
水产渔业 | 1796篇 |
畜牧兽医 | 19857篇 |
园艺 | 457篇 |
植物保护 | 2673篇 |
出版年
2021年 | 363篇 |
2020年 | 423篇 |
2019年 | 469篇 |
2018年 | 785篇 |
2017年 | 829篇 |
2016年 | 747篇 |
2015年 | 626篇 |
2014年 | 782篇 |
2013年 | 1828篇 |
2012年 | 1309篇 |
2011年 | 1528篇 |
2010年 | 1055篇 |
2009年 | 1089篇 |
2008年 | 1477篇 |
2007年 | 1327篇 |
2006年 | 1253篇 |
2005年 | 1077篇 |
2004年 | 1035篇 |
2003年 | 1028篇 |
2002年 | 910篇 |
2001年 | 1244篇 |
2000年 | 1230篇 |
1999年 | 927篇 |
1998年 | 387篇 |
1997年 | 418篇 |
1996年 | 341篇 |
1995年 | 391篇 |
1994年 | 341篇 |
1992年 | 656篇 |
1991年 | 780篇 |
1990年 | 685篇 |
1989年 | 720篇 |
1988年 | 661篇 |
1987年 | 623篇 |
1986年 | 666篇 |
1985年 | 591篇 |
1984年 | 462篇 |
1983年 | 412篇 |
1979年 | 553篇 |
1978年 | 431篇 |
1977年 | 358篇 |
1976年 | 365篇 |
1975年 | 396篇 |
1974年 | 459篇 |
1973年 | 474篇 |
1972年 | 447篇 |
1970年 | 366篇 |
1969年 | 400篇 |
1968年 | 357篇 |
1967年 | 359篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
861.
L. Song D.‐W. Zhang F.‐M. Li X.‐W. Fan Q. Ma N. C. Turner 《Journal of Agronomy and Crop Science》2010,196(5):323-335
Competition for water generates a classic aspect of the tragedy of the commons, the ‘race for fish’, where crops must allocate more resource to acquisition of the limiting resource than is optimal for crop yield allocation. A pot experiment using a simple additive (target–neighbour) design was conducted to examine the above‐ground and below‐ground growth of three spring wheat (Triticum aestivum L.) cultivars when grown alone and in mixtures at three levels of water availability. The effects of competition and water availability were compared by observing patterns of growth, biomass allocation and below‐ground outcomes. Competitive interactions were investigated among cultivars ‘HST’, ‘GY602’ and ‘LC8275’, target plant of each cultivar grown without neighbouring plants are referred to herein as control plant and one target plant of each cultivar sown surrounded either by same or another cultivar as intra‐ or inter‐cultivar competition. Competitive ability was assessed as the response ratio (lnRR) between the target plant surrounded by six other plants and the target plant in isolation. Our results showed that the cultivar ‘HST’, released over a century ago, produced a higher biomass and grain yield than the more recently released cultivars ‘LC8275’ and ‘GY602’ when grown as isolated plants with sufficient water supply. However, competition for resources from neighbours led to target plant biomass and grain yield being significantly reduced relative to controls in all three cultivars, particularly in ‘HST’. When subjected to intra‐cultivar competition, the two recently released cultivars ‘LC8275’ and ‘GY602’ had higher grain yields and water use efficiency for grain than ‘HST’ in all three water regimes. The landrace ‘HST’ had better and significantly linear relationships between biomass and biomass allocation, root length and specific root length, whereas the recent and modern cultivars had much more water‐related species‐specific changes in root morphology and allocation patterns. These results suggest that crop traits that influence competitive ability, such as biomass allocation to roots and root plasticity in response to drought have changed in modern wheat cultivars because of breeding and selection. 相似文献
862.
Objective To compare the efficacy of the non-steroidal antiinflammatory drugs, diclofenac sodium and flunixin meglumine as adjuncts to the antibiotic treatment of bovine respiratory disease (BRD). Procedure We randomly allocated 80 Holstein calves with BRD to three groups. All the calves received a dose of 2.5 mg/kg tulathromycin by single subcutaneous injection and two of the groups received, in addition, either 2.5 mg/kg diclofenac sodium as a single intramuscular injection (diclofenac group, n = 30) or 2.2 mg/kg flunixin meglumine as an intravenous injection on the first three consecutive days after tulathromycin administration (flunixin group, n = 30). All calves were given a clinical score prior to initial treatment (day 0) and after treatment (days 1, 2, 3, 7 and 14) by observing appetite, demeanour, rectal temperature, the rate and type of respiration, presence or absence of coughing, and nasal discharge. Results During the first 48 h, improvement of adverse signs of respiratory disease, such as pyrexia and elevated respiratory rate, and of a high clinical index score was significant in the two adjunct groups compared with the calves receiving antibiotic alone. The reduction in pyrexia was greatest in the diclofenac group. There were no statically significant differences between treatment groups with regard to eventual perceived recovery from respiratory disease in 14 days. Conclusion In this trial, a single intramuscular dose of diclofenac sodium was equally effective as three intravenous injections of flunixin meglumine given on consecutive days as adjunctive therapy for BRD. 相似文献
863.
864.
865.
Miyazono S, Aycock JN, Miranda LE, Tietjen TE. Assemblage patterns of fish functional groups relative to habitat connectivity and conditions in floodplain lakes.Ecology of Freshwater Fish 2010: 19: 578–585. © 2010 John Wiley & Sons A/S Abstract – We evaluated the influences of habitat connectivity and local environmental factors on the distribution and abundance patterns of fish functional groups in 17 floodplain lakes in the Yazoo River Basin, USA. The results of univariate and multivariate analyses showed that species–environmental relationships varied with the functional groups. Species richness and assemblage structure of periodic strategists showed strong and positive correlations with habitat connectivity. Densities of most equilibrium and opportunistic strategists decreased with habitat connectivity. Densities of certain equilibrium and opportunistic strategists increased with turbidity. Forested wetlands around the lakes were positively related to the densities of periodic and equilibrium strategists. These results suggest that decreases in habitat connectivity, forested wetland buffers and water quality resulting from environmental manipulations may cause local extinction of certain fish taxa and accelerate the dominance of tolerant fishes in floodplain lakes. 相似文献
866.
867.
D.S. Hendrie B.Sc. B.Sc. N.D.A. N.D.D. 《New Zealand veterinary journal》2013,61(5):123-125
Extract Mr. President, Ladies and Gentlemen, I can assure you that I speak with complete sincerity when I say I regard it as a very great honour indeed to be asked to open this conference. I value the invitation to do so for my own sake (today's will be a memory to cherish when I have returned to Britain) and, not less, for the implied compliment to the High Commissioner for the United Kingdom (whose adviser I am) and to the British Ministry of Agriculture who employ me. It is also particularly valuable to me, in my work, to get among a body of people who are so very much part and parcel of the farming industry of the Dominion. And, not least, my wife and I are delighted to be able to spend a weekend in such pleasant surroundings and in such congenial company. This is indeed the art of “conferring” brought to the ultimate degree of luxury! 相似文献
868.
Abstract Rainfall variability is a major determinant of system dynamics and profitability of livestock enterprises in arid and semi‐arid environments. Range managers consequently require detailed information on the financial and ecological implications of various stocking strategies in order to formulate viable management systems. Data collected over seven seasons (1986–1993), from a series of extensive grazing trials in the semi‐arid savanna of KwaZulu‐Natal with cattle stocked at three rates (0.17, 0.23 and 0.30 LSU ha?1) were used to develop a bioeconomic stocking model (LOWBEEF). The model comprised two biological sub‐models (BEEF and GRASS), and an integrated economic component. The BEEF sub‐model related seasonal live mass gain to stocking rate and rainfall. The GRASS sub‐model related residual herbage at the end of summer to summer stocking intensity, range condition (indexed as the sum of proportions of three key forage species, Themeda triandra, Panicum maximum and P. coloratum) and rainfall. The period over which supplementary feeding would be required to maintain cattle mass was related to residual summer herbage mass. The biological sub‐models were linked to an economic component model (ECON) to reflect the influence of various environmental and economic parameters on profitability. 相似文献
869.
870.
N. A. Savelyeva E. E. Belova N. E. Pervova A. A. Kolontsov 《Moscow University Soil Science Bulletin》2013,68(4):192-194
The effect of lead and cadmium ions on the fluctuating asymmetry of the side leaflets’ central veins of the common bean (Phaseolus vulgaris L.) was estimated. The impact of heavy metals on the parameter with the lead or cadmium content in the ground in the concentration of 650 mg/kg or 10 mg/kg respectively was not revealed. 相似文献