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   196篇
  免费   5篇
林业   16篇
农学   2篇
  46篇
综合类   22篇
农作物   5篇
水产渔业   58篇
畜牧兽医   46篇
园艺   2篇
植物保护   4篇
  2020年   2篇
  2019年   2篇
  2018年   3篇
  2017年   4篇
  2016年   3篇
  2015年   3篇
  2014年   7篇
  2013年   9篇
  2012年   11篇
  2011年   14篇
  2010年   4篇
  2009年   10篇
  2008年   11篇
  2007年   11篇
  2006年   8篇
  2005年   8篇
  2004年   10篇
  2003年   8篇
  2002年   7篇
  2001年   4篇
  2000年   4篇
  1998年   2篇
  1997年   3篇
  1996年   2篇
  1995年   3篇
  1994年   3篇
  1993年   4篇
  1991年   1篇
  1990年   1篇
  1989年   2篇
  1988年   2篇
  1987年   3篇
  1986年   4篇
  1985年   2篇
  1984年   3篇
  1983年   1篇
  1982年   1篇
  1980年   2篇
  1979年   1篇
  1978年   1篇
  1977年   1篇
  1976年   1篇
  1973年   1篇
  1971年   1篇
  1968年   1篇
  1966年   2篇
  1953年   1篇
  1939年   1篇
  1924年   4篇
  1918年   1篇
排序方式: 共有201条查询结果,搜索用时 20 毫秒
1.
2.
Gynogenetic and androgenetic brown trout (Salmo trutta Linnaeus 1758) haploids (Hs) and doubled haploids (DHs) were produced in the present research. Haploid development was induced by radiation‐induced genetic inactivation of spermatozoa (gynogenesis) or eggs (androgenesis) before insemination. To provide DHs, gynogenetic and androgenetic haploid zygotes were subjected to the high pressure shock to suppress the first mitotic cleavage. Among haploids, gynogenetic embryos were showing lower mortality when compared to the androgenetic embryos; however, most of them die before the first feeding stage. Gynogenetic doubled haploids provided in the course of the brown trout eggs activation performed by homologous and heterologous sperm (rainbow trout) were developing equally showing hatching rates of 14.76 ± 2.4% and 16.14 ± 2.90% and the survival rates at the first feeding stage of 10.48 ± 3.48% and 12.78 ± 2.18%, respectively. Significantly, lower survival rate was observed among androgenetic progenies from the diploid groups with only few specimens that survived to the first feeding stage. Cytogenetic survey showed that among embryos from the diploid variants of the research, only gynogenetic individuals possessed doubled sets of chromosomes. Thus, it is reasonable to assume that radiation employed for the genetic inactivation of the brown trout eggs misaligned mechanism responsible for the cell divisions and might have delayed or even arrested the first mitotic cleavage in the androgenetic brown trout zygotes. Moreover, protocol for the radiation‐induced inactivation of the paternal and maternal genome should be adjusted as some of the cytogenetically surveyed gynogenetic and androgenetic embryos exhibited fragments of the irradiated chromosomes.  相似文献   
3.

Purpose  

The purpose of this hydropedological work is to investigate the feasibility of a method to calculate permeability of soil peds on the matrix pore scale resolution. This paper focuses on imaging of the intra-aggregate microstructure and, based on the three-dimensional (3D) images, quantification of the pore network connectivity and permeability tensor. Finally, lattice Boltzmann equation (LBE) simulations of Navier–Stokes flow in the thus derived pore network allow to compute the heterogeneous 3D flow velocity field.  相似文献   
4.
Wild bananas are abundant in tropical areas and many ecologists have observed that the succession process is quicker following increased disturbance. This study was conducted to analyze animal-seed interactions and their effects on the seed fate of a wild banana species (Musa acuminata) in tropical southern Yunnan (China) through experiments considering spatial (site and habitat) and temporal (seasons) variation. The largest proportion of fruits (81%) was removed by frugivorous seed dispersers, especially by bats at nighttime. Only 13% of the fruits were removed by climbing seed predators (different species of rats). In the exclosure treatment, rodents accounted for a significantly higher total artificially exposed seed removal number than ants, but with spatial and temporal differences. The highest seed predation rate by rodents (70%) was found in forest with wild banana stands, corresponding with the highest rodent diversity (species numbers and abundance) among the habitat types. In contrast, the seed removal number by ants (57%) was highest in the open land habitats, but there was no close correlation with ant diversity. Seed removal numbers by ants were significantly higher in the dry compared to the rainy season, but rodent activity showed no differences between seasons. The overall results suggest that the largest proportion of seeds produced by wild banana are primarily dispersed by bats. Primary seed dispersal by bats at nighttime is essential for wild banana seeds to escape seed predation.  相似文献   
5.
In the literature, studies of Chlamydia infection in birds have usually been confined to the search for Chlamydia (C., formerly Chlamydophila) psittaci, so that little is known about the presence of other chlamydial agents. In the present study, cloacal swabs and faeces samples of urban pigeons have been examined by real-time PCR, DNA microarray assays and partial ompA sequencing. Whilst C. psittaci was the predominant chlamydial agent in this pigeon population (75.8% of all Chlamydiaceae positives), the combined use of highly specific and sensitive molecular assays facilitated the detection of atypical serovars of C. psittaci, as well as other species of Chlamydia, such as C. abortus. Detection of C. pecorum and C. trachomatis from an avian host is reported here for the first time. Rather unexpectedly, 19.5% of all Chlamydiaceae-positive cases turned out to be infected with non-classified organisms. The considerable prevalence of these novel agents raises the question of their epidemiological importance and possible role as pathogens. Future surveys in domestic and wild birds will have to take the extended variety of chlamydial organisms into account.  相似文献   
6.
The aim of the present study was to analyse the occurrence of chlamydiae in several mammalian host species. Clinical samples that previously tested positive in a Chlamydiaceae-specific real-time PCR were retested using six species-specific real-time PCR assays to identify the chlamydial species involved. Chlamydophila (Cp.) abortus was the agent most frequently found in cattle, sheep, horses, goats, and pigs. Detection in cattle of Cp. psittaci (11% of samples) and Chlamydia (C.) suis (9%), as well as Cp. psittaci in a goat sample was somewhat unexpected. DNA of two different chlamydiae was identified in 56 (12.7%) of 440 samples tested. Cp. felis was the predominant species found in cats, while in guinea pigs and rabbits only Cp. caviae was detected. Interestingly, the latter two pathogens were also identified in samples from dogs. The data show that mixed chlamydial infections are not rare and suggest an extended host range of individual species.  相似文献   
7.
A hypothesis has been presented and tested that bicarbonate (HCO3) and nitrate (NO3) are the most important anions inducing iron (Fe) chlorosis because these anions increase the pH of leaf apoplast which in turn depresses ferric‐iron [Fe(III]) reduction, and hence, the uptake of Fe into the symplasm. Experiments with young sunflower (Helianthus annuus) plants showed that nutrition with NO3 as the sole nitrogen (N) source induced chlorosis whereas ammonium nitrate (NH4NO3) did not. Monohydrogen carbonate (bicarbonate) also favoured the development of chlorosis. The degree of chlorosis was not related to the Fe concentration in the leaves. Both anion species, NO3 and HCO3, increased the pH of the leaf apoplast which was measured by means of the fluorescence dye 5‐carboxyfluorescein. A highly significant negative correlation between leaf apoplast pH and chlorophyll concentration in the leaves (r = ‐0.97) was found. Ferric‐Fe reduction in the apoplast—measured by means of ferrocene—provided evidence that a low leaf apoplast pH, obtained with ammonium (NH4) supply, favoured the reduction of Fe(III) as compared with a higher leaf apoplast pH obtained with NO3 supply. These results support the hypothesis tested.  相似文献   
8.
In 2010, two independent cases of cutaneous diphtheria caused by toxigenic C. ulcerans were identified in Germany. Both patients had intense occupational contact with pigs. Diagnostic work‐up comprising biochemical differentiation, rpoB sequencing, matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOF) analysis, real‐time tox PCR and Elek test as well as public health measures including an intensified source tracing involving 83 asymptomatic pigs of an associated pig farm are presented.  相似文献   
9.
Diets based on fish meal and soyabean meal were fed to grass carp fry, the average body weight of each individual being 0.4 g. During the 70 days of the experiment, the best results were obtained with a diet containing 40% fish meal, when growth increment, protein efficiency ratio and apparent net protein utilization amounted to 209, 1.26 and 20.3, respectively. There was a deficiency in sulphuric amino acids in both fish meal- and soyabean meal-containing diets, and this was particularly marked in the latter. A significant decrease in available lysine was found in the diets containing soyabean meal. The main reason for the retardation in fish growth and the reduction of protein utilization when soyabean meal content is increased in the diets, seemed to be that the destruction of anti-nutritional factors by the heat processing of the meal was only partial.  相似文献   
10.
Abstract.— Despite a large economical value of the ornamental fish trade, little information is available on the dietary ascorbic acid requirements of these species. Ascorbic acid requirements of fishes of the cichlid family appear to vary widely. Juvenile angelfish, a widely produced ornamental cichlid, were maintained on diets containing graded levels of ascorbyl monophosphate. Liver ascorbic acid concentrations after 96 d of feeding were significantly reduced in groups receiving 120 mg or less ascorbic acid equivalents/kg diet. However, no differences in growth or mortality between groups were found, and no external signs of ascorbic acid deficiency were observed, indicating a high resistance of this species against prolonged ascorbic acid deficiency. Based on the long possible life span of angelfish in the aquarium, we proposed a conservative dietary ascorbic acid requirement of 360 mg/ kg diet, necessary to maintain maximum tissue storage of this vitamin.  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