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   2873篇
  免费   289篇
  国内免费   1篇
林业   236篇
农学   121篇
基础科学   27篇
  620篇
综合类   102篇
农作物   166篇
水产渔业   428篇
畜牧兽医   1163篇
园艺   65篇
植物保护   235篇
  2024年   7篇
  2023年   42篇
  2022年   51篇
  2021年   123篇
  2020年   132篇
  2019年   159篇
  2018年   158篇
  2017年   171篇
  2016年   170篇
  2015年   122篇
  2014年   155篇
  2013年   174篇
  2012年   222篇
  2011年   236篇
  2010年   120篇
  2009年   111篇
  2008年   154篇
  2007年   131篇
  2006年   122篇
  2005年   82篇
  2004年   76篇
  2003年   64篇
  2002年   71篇
  2001年   50篇
  2000年   35篇
  1999年   25篇
  1998年   20篇
  1997年   29篇
  1996年   18篇
  1995年   18篇
  1994年   12篇
  1993年   10篇
  1992年   11篇
  1991年   15篇
  1990年   17篇
  1989年   20篇
  1988年   7篇
  1985年   7篇
  1984年   2篇
  1983年   1篇
  1982年   1篇
  1980年   1篇
  1978年   2篇
  1977年   1篇
  1975年   1篇
  1970年   1篇
  1934年   2篇
  1930年   1篇
  1926年   1篇
  1925年   1篇
排序方式: 共有3163条查询结果,搜索用时 15 毫秒
31.
The two most common haemotropic Mycoplasma of cats, Mycoplasma haemofelis and 'Candidatus Mycoplasma haemominutum' have been identified using molecular techniques in all continents, except Antarctica. We report the first molecular characterization in South America of a dual infection with M haemofelis and 'Candidatus Mycoplasma haemominutum' in three domestic cats. The 16S ribosomal RNA gene was amplified in three anaemic cats in which haemoplasma organisms were seen attached to the erythrocytes in the peripheral blood smear. Bands of the expected size for M haemofelis and 'Candidatus Mycoplasma haemominutum' were observed in all three cats. The 393 bp segment of one of the amplicons had a similarity value of 100% to M haemofelis, whereas the other amplicon, a 192 bp segment, was 100% similar to 'Candidatus Mycoplasma haemominutum'. After diagnosis, two cats received blood transfusion and they were all treated with doxycycline. All three cats recovered uneventfully.  相似文献   
32.
A single-step duplex polymerase chain reaction (PCR) technique and traditional microscopic examination of haemolymph smears were used to detect Babesia bigemina and/or Babesia bovis infection in engorged female ticks of Boophilus microplus recovered from calves raised in an endemic area of the State of Minas Gerais, Brazil. In the PCR amplification of tick-derived DNA, pairs of oligonucleotide primers specific for a 278-bp sequence from B. bigemina and for a 350-bp sequence from B. bovis were used conjointly. The microscopic examination of haemolymph revealed that 16.7% of the engorged ticks were infected with Babesia spp., although no significant differences (rho > 0.05) were found in the infection rate of ticks collected from calves of different age groups. PCR analysis showed that 77.8% of the engorged ticks whose haemolymph contained sporokinetes were infected with B. bigemina, 7.8% with B. bovis and 14.4% with both protozoan species. However, the PCR assay further revealed that, amongst the engorged female ticks whose haemolymph was apparently negative for the presence of sporokinetes, 15.6% were infected with B. bigemina, 2.2% with B. bovis and 10.0% with both species. The duplex PCR method is thus more efficient and sensitive than the microscopic assay and also permits facile identification of the protozoa species present in engorged female ticks.  相似文献   
33.
34.
Fungicides are indispensable for high-quality crops, but the rapid emergence and evolution of fungicide resistance have become the most important issues in modern agriculture. Hence, the sustainability and profitability of agricultural production have been challenged due to the limited number of fungicide chemical classes. Resistance to site-specific fungicides has principally been linked to target and non-target site mechanisms. These mechanisms change the structure or expression level, affecting fungicide efficacy and resulting in different and varying resistance levels. This review provides background information about fungicide resistance mechanisms and their implications for developing anti-resistance strategies in plant pathogens. Here, our purpose was to review changes at the target and non-target sites of quinone outside inhibitor (QoI) fungicides, methyl-benzimidazole carbamate (MBC) fungicides, demethylation inhibitor (DMI) fungicides, and succinate dehydrogenase inhibitor (SDHI) fungicides and to evaluate if they may also be associated with a fitness cost on crop pathogen populations. The current knowledge suggests that understanding fungicide resistance mechanisms can facilitate resistance monitoring and assist in developing anti-resistance strategies and new fungicide molecules to help solve this issue. © 2023 Society of Chemical Industry.  相似文献   
35.
36.
The objective of this retrospective study was to assess the effect of receiving a single (n = 50,285) or double (n = 4392) artificial insemination (AI), 12 h apart, within a timed artificial insemination protocol on pregnancy per AI (P/AI) in nulliparous heifers (inseminated with either sex-sorted or conventional semen) and pluriparous Holstein cows in a commercial dairy herd. Also, this study aimed to investigate the relationship between temperature-humidity index (THI) and time of the first AI and fertility. Fertility of cows receiving two AI with normothermia (THI <68) was higher (p < .05) than cows receiving a single AI (42.9% vs. 36.4%). P/AI of cows receiving two AI with severe heat stress (THI >85) was higher (p < .05) than cows receiving a single AI (21.0% vs. 12.6%). Regardless of heat stress conditions, applying the first AI in the morning increased (p < .05) P/AI in cows with double AI than in cows whose first AI occurred in the afternoon (38.4 vs. 33.3%). With moderate heat stress, and sexed-sorted semen, P/AI to timed AI was higher (65.0 vs. 51.9%; p < .05) in heifers receiving double AI than those serviced once. It was concluded that double AI, 12 h apart, enhanced fertility at timed AI than herd mates with a single AI, particularly with heat stress at breeding.  相似文献   
37.
This study was carried out to evaluate the advantage of preselecting SNP markers using Markov blanket algorithm regarding the accuracy of genomic prediction for carcass and meat quality traits in Nellore cattle. This study considered 3675, 3680, 3660 and 524 records of rib eye area (REA), back fat thickness (BF), rump fat (RF), and Warner–Bratzler shear force (WBSF), respectively, from the Nellore Brazil Breeding Program. The animals have been genotyped using low-density SNP panel (30 k), and subsequently imputed for arrays with 777 k SNPs. Four Bayesian specifications of genomic regression models, namely Bayes A, Bayes B, Bayes Cπ and Bayesian Ridge Regression methods were compared in terms of prediction accuracy using a five folds cross-validation. Prediction accuracy for REA, BF and RF was all similar using the Bayesian Alphabet models, ranging from 0.75 to 0.95. For WBSF, the predictive ability was higher using Bayes B (0.47) than other methods (0.39 to 0.42). Although the prediction accuracies using Markov blanket of SNP markers were lower than those using all SNPs, for WBSF the relative gain was lower than 13%. With a subset of informative SNPs markers, identified using Markov blanket, probably, is possible to capture a large proportion of the genetic variance for WBSF. The development of low-density and customized arrays using Markov blanket might be cost-effective to perform a genomic selection for this trait, increasing the number of evaluated animals, improving the management decisions based on genomic information and applying genomic selection on a large scale.  相似文献   
38.
In most cases, when calculating soil water availability, only thewater content is considered. The effect of salinity on the wiltingpoint is neglected. The objective of this work is to use asimulation model (CERES-maize) in order to predict cornyields as a function of water salinity under severalenvironmental, agrotechnical, and plant characteristics. A modelis presented in which the wilting point is a function of the soilsalt content. At high salinity, the water content at wilting pointis higher than at low salinity, resulting in an insufficient amountof available water and, therefore, a reduced yield. The modelwas used to simulate several theoretical and experimentalsituations for forage corn and grain corn. Simulation resultsshowed that nitrogen fertilization increases the salinity thresholdvalue and the yield sensitivity (rate of yield reduction per unitof salinity). The also showed that forage corn is more sensitiveto salinity than grain corn. If the soil is not leached, a heaviersoil texture has a higher salinity threshold value. On the otherhand, if the soil is leached, the soil texture has no influence onthe salinity threshold value and the yield is less sensitive tosalinity in sandy soils. The determination coefficient (r2= 0.75) indicated that the results of the simulations were in goodagreement with the field data.  相似文献   
39.
An outbreak of a new and severe disease was observed in Eucalyptus plantations of Bahia state, Brazil. An Ascomycota fungus has been frequently associated with the main symptoms of the disease namely leaf spot, branch cankers, shoot blight, defoliation, and dieback. Based on morphological characteristics, phylogenetic analysis (ITS and TEF-1α genes), and pathogenicity test on Eucalyptus plants, Pseudoplagiostoma eucalypti was identified as the causal agent of the disease. Although P. eucalytpi has been known from in Brazil since 1998, this is the first report of it causing severe disease and die-back on Eucalyptus spp. and we also record new symptoms associated with the pathogen.  相似文献   
40.
Factors shaping individual behavior remain poorly understood for European cyprinids. We examined the effects of extrinsic (temperature and year) and intrinsic factors (body size and shoal size) on the daily activity of chub Squalius torgalensis, a critically endangered cyprinid found in the Torgal stream, Portugal. We quantified chub behaviors while snorkeling, using instantaneous sampling of focal individuals, in spring 2009 and 2010. We used Principal Component Analysis to describe individual behavior and assessed competing models relating individual behavior to explanatory variables using Akaike's Information Criterion. Year had a strong influence on chub behavior, with individuals spending more time drift foraging and orienting in 2009, whereas in 2010 they spent more time benthic foraging and cruising. Shoal size did not influence behaviors in 2009, but in 2010 individuals in small shoals foraged benthically and searched with greater frequency than those in large shoals. Individuals increased cruising and decreased searching as they grew in length, but the opposite trend was observed as temperature increased. Our results indicate that both extrinsic and intrinsic factors influence daily activity of chub, with individual behaviors varying between years, and depending upon shoal size, body size and temperature. This suggests that chub display high levels of individual behavioral flexibility, which may be important for fish inhabiting harsh and highly variable Mediterranean streams. Conservation efforts for this species will benefit by integrating behavioral information into management strategies.  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