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   926篇
  免费   34篇
林业   473篇
农学   8篇
  106篇
综合类   56篇
农作物   16篇
水产渔业   30篇
畜牧兽医   196篇
园艺   24篇
植物保护   51篇
  2022年   10篇
  2021年   7篇
  2020年   8篇
  2019年   4篇
  2018年   11篇
  2017年   23篇
  2016年   24篇
  2015年   16篇
  2014年   23篇
  2013年   23篇
  2012年   39篇
  2011年   46篇
  2010年   20篇
  2009年   23篇
  2008年   34篇
  2007年   36篇
  2006年   42篇
  2005年   26篇
  2004年   30篇
  2003年   23篇
  2002年   19篇
  2001年   9篇
  2000年   4篇
  1999年   5篇
  1995年   4篇
  1987年   5篇
  1948年   30篇
  1944年   5篇
  1943年   13篇
  1942年   49篇
  1941年   32篇
  1940年   36篇
  1939年   45篇
  1938年   27篇
  1937年   21篇
  1936年   14篇
  1935年   35篇
  1934年   7篇
  1931年   14篇
  1930年   7篇
  1927年   5篇
  1926年   6篇
  1925年   4篇
  1919年   3篇
  1913年   3篇
  1912年   4篇
  1911年   3篇
  1910年   3篇
  1902年   3篇
  1901年   5篇
排序方式: 共有960条查询结果,搜索用时 15 毫秒
31.
Random amplified polymorphic DNA fingerprinting was performed on single females of root-knot nematodes, Meloidogyne spp., using a new procedure for DNA isolation. One-fourth of the total DNA amount isolated from a single female proved to be sufficient as a template in a polymerase chain reaction. Electrophoretic patterns of the amplified fragments were reproducible between replicates from a single female or sister females from the same progeny, and identical to those obtained with genomic DNA purified from a large number of nematodes. Moreover, a comparative analysis over three successive generations showed stability of the amplification patterns, thus demonstrating the utility of this procedure for epidemiological and ecological studies on root-knot nematodes.  相似文献   
32.
During the summer 1996, twelve of twenty-eight leek plants located in a garden near eské Budjovice, South Bohemia exhibited symptoms typical of diseases associated with phytoplasmas. In summer 1998 similar symptoms were detected in leek plants in a field used for seed production located in Romagna, North Italy. In both cases the plants were established in the spring of the previous year. Plants showed flower abnormalities: stamen elongation, anther sterility, pistil proliferation, as well as poor, if any, seed production. Phytoplasma-like structures were detected by scanning and transmission electron microscopy in phloem sieve elements in the Czech diseased plants, but not in healthy ones. Nested-PCR amplifications of extracted DNA with phytoplasma-specific oligonucleotide primer pairs confirmed the presence of phytoplasmas in these plants at low concentrations. Restriction fragment length polymorphism analyses of amplified ribosomal sequences allowed the identification of detected phytoplasmas: all the samples from the Czech Republic contained aster yellows related phytoplasmas (16SrI-B) while in the Italian samples aster yellows related phytoplasmas (16SrI-B) together with stolbur related phytoplasmas (16SrXII-A) were identified. This is the first report of detection and identification of a phytoplasma disease of leek in the Czech Republic and Italy.  相似文献   
33.
Seven sexually mature boars were studied in two-hour intervals for 8 to 32 hours, as to the variation of testosterone levels in the blood plasma. Blood samples were obtained by means of a cannula inserted in the v. cava cranialis. The hormone level under study showed marked fluctuation. The coefficient of variation of testosterone concentration ranged from 12.7% to 35.2% in the individual animals and for the whole part of the study it was 33.0%. It appeared impossible to derive seriously any regular periodicity of testosterone concentration in boar blood in the 24-hour period from the analysis of the variation of the levels of the hormone in individual animals during the period under study. Further, the study revealed a marked fluctuation of testosterone levels in the blood plasma of twelve sexually mature boars in the course of several days' study. The coefficient of variation of testosterone concentration in individual animals ranged from 18.1% to 106.5%, reaching 77.0% for the whole part of study. The analysis of variance showed statistically significant differences of hormonal levels in individual boars in which the fluctuation of testosterone was studied in the 8- to 32-hour period as well as in several days' period. This proved the role of the animals' individuality in determining the concentration of testosterone in their blood. The discussion concerns the importance of these findings for the evaluation of the role of basal testosterone levels in the blood in classifying the incretion function of the gonads and in the diagnosis of incretion hypogonadism of boars.  相似文献   
34.
Strawflower (Helichrysum bracteatum) with symptoms resembling those associated to phytoplasma infection were observed in several areas in the Czech Republic during the period 1994–2001. Plants with leaf bronzing, reddening and necrosis, proliferation of secondary shoots, flower abnormalities and dwarfing died in advanced stages of the disease. The disease incidence ranged from 2% to 70% and caused significant loss to the flower and seed production. Transmission electron microscopy showed phytoplasmas in sieve cells of affected plants, but not in healthy ones. Association of phytoplasmas with the disease was confirmed by polymerase chain reaction using phytoplasma universal ribosomal primers R16F2n/R16R2. An amplification product of the expected size (1.2 kb) was observed in all samples of the symptomatic strawflowers. The restriction profiles obtained following separate digestion with three endonucleases (AluI, HhaI, MseI) showed that phytoplasmas infecting strawflowers from different localities in the Czech Republic were uniform and undistinguishable from aster yellows (subgroup 16SrI-B). Sequence analysis of 1771 bp of the ribosomal operon amplified with primers P1/U3, R16F2n/R2 and 16R758/P7 indicated that the closest related phytoplasmas were those associated with 'Rehmannia glutinosa var. purpurea', both originating from Bohemia. This is the first report on the occurrence of a phytoplasma-associated disease of strawflower in the Czech Republic.  相似文献   
35.
OBJECTIVE: To compare renal clearance of technetium Tc 99m pentetate with plasma clearance by use of a glomerular filtration rate technique in pigs from 3 to 24 weeks of age. Animals: 24 female pigs. Procedure: At the time of investigation, 5 pigs were 3 weeks old, 6 pigs were 6 weeks old, 8 pigs were 12 weeks old, and 5 pigs were 24 weeks old. Plasma clearance of technetium Tc 99m pentetate was measured by the use of a single injection technique followed by collection of multiple blood samples until 5 hours after the injection. Simultaneously, urine was collected through a urinary catheter, and the renal clearance of technetiumTc 99m pentetate was calculated. Plasma clearance of technetium Tc 99m pentetate was correlated with the renal clearance (r = 0.95). Plasma clearance was higher than renal clearance at all ages (mean, 5.8%), indicating extrarenal clearance of technetium Tc 99m pentetate or methodologic errors. Volume of distribution increased with increasing age but decreased as a fraction of body weight. CONCLUSIONS: Plasma clearance of technetium Tc 99m pentetate estimates renal clearance with acceptable precision when using single injection technique and multiple biood samples in pigs from 3 to 24 weeks of age.  相似文献   
36.
In mammals, thyrotropin, or thyroid-stimulating hormone (TSH), assay is used for the diagnosis of primary hypothyroidism. Hypothyroidism is the most common type of thyroid disorder in cattle. The aim of this study was to develop and validate, under physiologic and pathologic conditions, a radioimmunoassay (RIA) for bovine TSH (bTSH). Double RIA was performed with purified bTSH and specific bovine antiserum. Laboratory validation included research of minimal detection limit, accuracy, and reproducibility. The physiologic validation included a thyrotropin-releasing hormone (TRH) challenge performed on euthyroid cows and a follow-up of bTSH concentration over a 24-hour period. Furthermore, bTSH concentration was assayed in a large population of healthy dairy and beef cows to define reference interval. The pathologic validation was made by assaying bTSH and thyroid hormones on healthy and goitrous newborn calves. The minimum detection limit (MDL) for bTSH assay was 1.3 microU/ml. The recovery was 101% to 106%. The intra- and interassay coefficients of variation (CVs) ranged from 5% to 11% and 11% to 15%, respectively. The RIA covered the whole range of physiologic bTSH values, as shown by bTSH values induced by TRH-challenge. A pulsatile secretion of bTSH was observed, accompanied by a diurnal variation with lower night values than day values. Reference intervals of bTSH ranged from 1.3 to 13.0 microU/ml for beef and dairy breeds. Finally, bTSH easily discriminated goitrous newborn calves from healthy ones, leading to the definition of a cutoff value of 35 microU/ml. The bTSH assay positively reacted to physiologic and pathologic conditions. The accuracy and precision of the RIA were satisfying.  相似文献   
37.
In a two years lasting trial the N-release of organic N-fertilizers should be analysed under covered fallow plots during the vegetation period. The following fertilizers were proved: cattle manure from two locations, “Vinasse” (product resulting from the converting of sugar-beet molasses), “Haarmehlpellets” (pellets of grounded pig bristles), “Rizinusschrot” (grist of the seed of Ricinius communis), “Rapsschrot” (coarse colza meal), “Phytopearls” (residue of maize converting), “Maltaflor” (malt germs and “Vinasse”), “Agrobiosol” (biomass from Penicillium) and “Horndünger” (horn parings in different fractions). In comparison with these organic fertilizers the mineral N-fertilizer calcium-ammonium-nitrate was used. With the exception of the control all variants got an annual N-fertilization of 90?kg?N/ha. The N-mineralisation of the soil caused by the organic substance, which was in the soil before the trial started, amounted to 84?kg?N/ha in the first and to 59?kg?N/ha in the second year of experience. In both years the N-mineralisation was higher in the second half of the vegetation period (July–November) than in the first half (March–July). In the first year the amount of N-release was clearly less than the given quantity of the N-fertilizer with the exception of “Agrobiosol”. Especially the two manure-variants showed N-immobilisation. “Agrobiosol” showed the highest N-release of the tested organic N-fertilizers. In the second year the N-release both of the two manure-variants and of “Rapsschrot”, “Phytoperls”, “Maltaflor”, “Agrobiosol” and “Horndünger” increased clearly. It was between 51 and 101% of the N-quantity given in the second year. N-release was particularly high by “Agrobiosol”, “Horndünger” and “Rapsschrot”. This increase could be traced to the mineralisation of the remained organic substance given in the first year. The variants “Haarmehlpellets” and “Rizinusschrot” had the same N-release as in the first year. The application of “Vinasse” showed a clear decrease. When “Rapsschrot”, “Phytoperls”, “Maltaflor”, “Agrobiosol”, “Horndünger” or manure are given for many years, one can consequently count on an increase of the annual N-mineralisation. Considering the results of both years of the trial the organic fertilizers “Agrobiosol”, “Horndünger” and “Maltaflor” can be classified as quickly effectiveN-fertilizers, “Haarmehlpellets”, “Rizinusschrot” and “Phytopearls” as moderately effective and “Rapsschrot” and “Vinasse” as slowly effective ones.  相似文献   
38.
    
Ohne Zusammenfassung  相似文献   
39.
40.
A long-term study was carried out in 11 dairy herds in the Khon Kaen province of northeast Thailand between August 2001 and November 2004. The objective was to investigate seroprevalence dynamics of Neospora caninum infection in the herds and to demonstrate patterns of seroconversion in individual cattle. Each herd was visited once a year, in total four times, and sera from cattle > 3 months of age and farm dogs as well as a sample from the bulk milk were collected. All samples were analysed for presence of specific antibodies by an N. caninum iscom ELISA. The overall percentage of antibody-positive cattle was constant and varied only between 10 and 13% over the 4 years, but the variation in within-herd seroprevalence between herds was substantial. Two herds had > or = 20% seropositive animals at all samplings and consistently high bulk milk OD, whereas two herds had no seropositive animal at the last two samplings and low bulk milk OD. Five herds had a decreasing trend of within-herd seroprevalence, whereas the remaining six herds had a higher portion of test-positive individuals at the end of the study. A total of 424 individuals were sampled more than once; 344 (81%) and 32 (8%) were consistently antibody-negative and antibody-positive, respectively. The proportions of animals that changed from being seronegative to seropositive and from being seropositive to seronegative between the years were 3.9-4.6% and 19-39%, respectively. Apparent vertical and horizontal transmission rates were 58% (95% CI; 44-71%) and 5% (95% CI; 3-7%), respectively. In conclusion, the overall percentage of N. caninum antibody-positive cattle was constant over the years, but the within-herd seroprevalence varied substantially between the herds. Seroconversions were likely to occur in individual cattle although most animals had consistent serological status throughout the study.  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