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   28770篇
  免费   1376篇
  国内免费   740篇
林业   2502篇
农学   1422篇
基础科学   521篇
  5556篇
综合类   3900篇
农作物   1699篇
水产渔业   2327篇
畜牧兽医   9858篇
园艺   974篇
植物保护   2127篇
  2023年   277篇
  2022年   572篇
  2021年   794篇
  2020年   742篇
  2019年   759篇
  2018年   989篇
  2017年   1133篇
  2016年   1088篇
  2015年   1136篇
  2014年   1175篇
  2013年   1573篇
  2012年   2087篇
  2011年   2013篇
  2010年   1323篇
  2009年   1169篇
  2008年   1389篇
  2007年   1299篇
  2006年   1029篇
  2005年   1589篇
  2004年   1329篇
  2003年   1035篇
  2002年   606篇
  2001年   477篇
  2000年   383篇
  1999年   412篇
  1998年   99篇
  1992年   195篇
  1991年   204篇
  1990年   221篇
  1989年   206篇
  1988年   218篇
  1987年   166篇
  1986年   195篇
  1985年   177篇
  1984年   134篇
  1983年   120篇
  1979年   178篇
  1978年   137篇
  1977年   121篇
  1976年   92篇
  1975年   147篇
  1974年   156篇
  1973年   166篇
  1972年   173篇
  1971年   146篇
  1970年   96篇
  1969年   115篇
  1968年   122篇
  1967年   134篇
  1966年   111篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
To test the hypothesis that development of a BTV-specific IgE response plays a role in clinical disease manifestation, the humoral immune response of cattle to inactivated and virulent BTV was studied. Three calves received three sensitizing immunizations of inactivated BTV, 3 weeks apart. The BTV-sensitized animals, two non-sensitized BTV-seropositive and 4 BTV-seronegative control cattle. were challenge-exposed with BTV-11, UC8 strain. All cattle inoculated with inactivated BTV developed group-specific non-neutralizing and serotype-specific neutralizing antibodies. The development of post-challenge-exposure neutralizing antibody titers was inversely correlated with protective immunity. None of the BTV-challenged animals showed clinical disease. The levels of IgE were greatest in the sensitized calves after virus challenge in comparison with control groups. The sequential development, specificity and intensity of virus protein-specific humoral responses were evaluated using immunostaining. After challenge exposure of BTV-sensitized and non-sensitized cattle, total and IgE antibodies reacted consistently within BTV structural proteins VP2, VP5 and VP7. Although no correlation was found between clinical disease and IgE, results add support to the hypothesis that IgE may be involved in the pathogenesis of clinical disease, since infection with BTV causes an increase in serum IgE levels. However, these results suggest that the levels of virus-specific reactivity may be an important factor in determining whether or not clinical disease manifestation occurs.  相似文献   
62.
An indirect fluorescent antibody (IFA) test is usually performed to detect antibodies in dogs naturally infected by Ehrlichia canis. In this work, results obtained using three different E. canis strains as antigen (a commercial antigen, the E. canis Oklahoma strain and the E. canis Madrid strain) were compared. One hundred and forty-nine serum samples obtained from dogs living in the centre of Spain were analysed. When qualitative results were evaluated, identical results were detected in 87.2% of samples for the three antigens tested. When comparing antibody titre results, differences between the Madrid strain and the commercial antigen, and between the Madrid and Oklahoma strains were statistically significant (P < 0.0001). No differences were found when comparing the Oklahoma strain with the commercial antigen (P = 0.562). Subtle intra-laboratory variations shown in this study suggest a higher sensitivity of the IFA test when an autochthonous strain is used as antigen.  相似文献   
63.
Near-infrared (NIR) spectroscopy is a rapid, non-destructive and accurate technique for analyzing a wide variety of samples, thus, the growing interest of using this technique in soil science. The objective of this study was to evaluate the potential of NIR spectroscopy to predict organic carbon (OC), total nitrogen (TN), available phosphorus (P) and available potassium (K) in the soil. NIR spectra from 20 cm3 of soil samples were acquired on the range of 750 to 2500 nm in diffuse reflectance mode, resolution of 16 cm?1 and 64 scans. Eight models of calibration/validation were constructed. Calibration and validation models showed that the predictive potential of NIR varied with the specific soil property (OC, TN, P and K) under evaluation and according to the methodology employed in the model construction (cross-validation or test set). Good prediction models were obtained for OC and TN content based on the statistical parameters. Test set methodology was able to predict soil OC, TN, P, and K better than cross-validation methodology.  相似文献   
64.
Conservation management conflicts frequently arise when an overpopulation of a protected organism has negative effects on other valuable elements in the same ecosystem. We studied the interactions between a colony of protected tree-nesting wading birds and a remnant population of centenarian cork oaks that was part of the formerly dominant forests in the Doñana Biological Reserve (SW Spain). A significant increase in the tree mortality rates has been recorded in areas that are yearly influenced by the bird colony.We analysed a cohort of surviving trees using a gradient of nesting bird influence. Tree-nesting history, bird isotopic signature (δ15N), tree health-related parameters (defoliation, δ13C and leaf surface coverage by faeces) and several soil variables were evaluated. Bird influence was related to increased soil salinity. This increase correlated to increased water-use efficiency for the leaves and to crown defoliation, suggesting that the heavily occupied trees are under higher stress and in poorer health condition than the unoccupied ones. We tested structural equations models (SEM) that were based on hypothesised bird effects on the health of the trees. Soil-mediated effects of the nesting birds best explained the symptoms of the declining health of the trees, whereas the percent of leaves’ surface that was covered by faeces did not improve the fitted SEM model.For the reserve’s managers, a challenging trade-off exists between preserving the relict trees, which have a high genetic diversity and a key ecological role in these savannah-like ecosystems, and maintaining the current nesting area for these protected, but expanding, wading birds.  相似文献   
65.
Mycoplasmas were isolated from yak calves in Mongolia, the 1st experience worldwide with this species. They were also isolated from pathologically affected sheep and goat lambs. The disease was experimentally reproduced in yak calves as well as in sheep and goat lambs. Clinical manifestations were identical with those recordable from animals with spontaneous outbreak of the disease. This had been for the 1 time ever that a therapy was tested on animals with mycoplasmosis, using water with addition of oligodynamic silver. Something between 95 and 100% of all patients were clinically cured. The experimental vaccine involved was found to offer protection against mycoplasmosis to sheep and goat lambs. Damage association with mycoplasmosis was found to occur 1st in liver, spleen, and lymph nodes and to be subsequently proliferated to lung and other organs. Respiratory mycoplasmosis is the common definition used at present but is not in conformity with the pathological processes involved, as the major role is played by liver damage. Mycoplasmosis in these 3 species, therefore, should better be named mycoplasmosis bovi hepatica.  相似文献   
66.
This work has evaluated the effects of thermally dried (TDS) or composted (CDS) dewatered sewage sludge on β-glucosidase activity, total (TCH) and extractable (ECH) carbohydrate content, microbial biomass carbon and basal respiration of soils from limestone quarries under laboratory conditions. Two doses (low and high) of the dewatered sludge (DS) or of the respective TDS or CDS were applied to a clayey and a sandy soil, both coming from working quarries. The soil mixtures and the controls (soils with no added sludge) were incubated for 9 months at 25°C and 30% of field capacity. The addition of sludge increased all the studied soil parameters, and the increase depended on the amount of sludge. Except in the case of TCH and ECH, the enhancing effect decreased with time, but at the end of incubation, parameters of the treated soils were higher than those of the control. The rank order of the initial stimulating effect was soil–TDS ≥ soil–DS ≥ soil–CDS, and probably, this order depended on the proportion of stable organic matter, which was the lowest in the TDS. Values of metabolic quotient (qCO2) were higher at the lower dose, and they did not change during incubation in the CDS-treated soils. Both TCH and ECH were the parameters with the greatest significant sludge and dose effects. Basal respiration, microbial biomass carbon and β-glucosidase activity were the best measured parameters in distinguishing the long-term effects of the three sludge types over the soils.  相似文献   
67.
Background and Goal  A number of global events have generated intense scientific scrutiny and public concern of polychlorinated dibenzo-p-dioxins (dioxin). DIoxins have been associated with a range of adverse health effects. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is recognized as among the most dangerous of the dioxin compounds, and was a contaminant found In one of the herbicides used for vegetation control during the Vietnam conflict: Agent Orange. As a result of purging spray systems and leaking drums of Agent Orange concentrate, TCDD contamination occurred in the soils surrounding Hardstand 7 at Eglin Air Force Base, Florida. This research uses a multimedia model to estimate the TCDD concentrations in surface soil around Hardstand 7 for a 70-year time Interval beginning with observed surface soil concentrations 1984. Methods  Hardstand 7 is a nearly 40-m circular concrete and asphalt aircraft parking area. The hardstand was used as a staging area for spraying equipment used to disseminate herbicide In a test area called C-52A within EAFB. Concentrated herbicide was also stored in 208-L barrels for use with the equipment, and later, for disposal. In 1984, a field investigation characterized the extent of TCDD contamination in surface soils around Hardstand 7 using a radial sampling protocol. The 1984 observed concentrations and locations was used in a multimedia model, CalTOX, as an initial source term concentrations and locations to estimate expected concentrations during the subsequent 70-years. Results  The results indicate that more than 94% of the TCDD observed in surface soils 1984 will remain after a 70-year period. Access restrictions and remediation activities at the site eliminate bar verification of the CalTOX estimates. Conclusions. TCDD is highly persistent in the soil medium and natural attenuation may not produce a significant decrease In soil concentrations. Recommendation  Active remediation actions may be required to prevent exposure to TCDD contamination surface soils. Verifying CalTOX concentration estimates an Important step that should be performed, however, the, model provides an easy to use tool to estimate TCDD surface soil contamination at herbicide storage or dispersion staging sites.  相似文献   
68.
The objective of this study was to determine the effects of various agglomerated phosphate blends on the quality of vacuum-tumbled catfish fillets. Catfish fillets were tumbled with a brine solution at 15% over initial, raw weight prior to tray-packing and storage at 4°C for 10 days. Fillets were evaluated for protein exudate, tumbling yield, color, pH, cooking loss, tenderness, purge loss, and shelf life. A specific blend of agglomerated sodium phosphates (AGSP) that contains mono-, tri-, and polyphosphates had significantly less protein (p < 0.05) exudate and significantly higher pH (p < 0.05) than other treatments. All phosphate treatments significantly increased (p < 0.05) tenderness and significantly decreased (p < 0.05) purge loss, but agglomerated phosphate blends significantly decreased (p < 0.05) cooking loss and yellowness. Psychrotrophic plate counts for all phosphate treatments were similar to the control at each storage time. All phosphate treatments improved the yield and quality of catfish fillets, but the use of AGSP may optimize quality attributes.  相似文献   
69.
Tropical Animal Health and Production - To reduce costs, lactating cows are often fed a higher proportion of roughage and alternative feeds; however, such protocols may not lead to optimal milk...  相似文献   
70.
The successful application of postharvest regulated deficit irrigation (RDI) over ten consecutive years (from season 1999/2000 to season 2007/2008) confirms the sustainability of this strategy for producing ‘Algerie’ loquat. Postharvest RDI consisting in a reduction of watering (between 45 and 80% depending on the season) from early June until the end of August, improved loquat profitability by increasing fruit value and by reducing water consumption with respect to fully irrigated trees (control). The increase in fruit value in RDI trees was due to a consistent improvement in harvest earliness as a result of an earlier blooming. Water savings of around 20% did not diminish yield nor fruit quality. Water use efficiency in RDI trees rose by over 30%. Water productivity reached 9.5 € m−3 of water applied in RDI trees versus 6.6 € m−3 in control trees. The most noticeable effect of RDI on vegetative growth was a significant and progressive decline in trunk growth. The canopy volume seems to be strongly influenced by pruning and no significant effects were detected in this parameter. Our results confirm the suitability of RDI in loquat and the economic benefits of saving water during the summer.  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