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   157篇
  免费   3篇
林业   12篇
农学   3篇
  55篇
综合类   8篇
农作物   7篇
水产渔业   4篇
畜牧兽医   67篇
植物保护   4篇
  2022年   1篇
  2020年   1篇
  2019年   3篇
  2018年   3篇
  2017年   4篇
  2016年   2篇
  2015年   6篇
  2014年   6篇
  2013年   19篇
  2012年   6篇
  2011年   11篇
  2010年   5篇
  2009年   4篇
  2008年   17篇
  2007年   11篇
  2006年   9篇
  2005年   11篇
  2004年   3篇
  2003年   6篇
  2002年   7篇
  1999年   2篇
  1998年   3篇
  1997年   1篇
  1995年   5篇
  1991年   1篇
  1982年   1篇
  1979年   1篇
  1978年   1篇
  1977年   2篇
  1975年   1篇
  1973年   1篇
  1972年   1篇
  1971年   2篇
  1968年   1篇
  1966年   1篇
  1964年   1篇
排序方式: 共有160条查询结果,搜索用时 15 毫秒
1.
During the Schmallenberg virus (SBV) epidemic, the European Food Safety Authority (EFSA) collected data on SBV occurrence across Europe in order to provide an assessment of spread and impact. By May 2013, twenty-nine countries were reporting to EFSA and twenty-two countries had reported cases of SBV. The total number of SBV herds reported was 13,846 and the number of SBV laboratory confirmed herds was 8730. The surveillance activities were based on the detection of SBV clinical cases (either adults or newborns). Malformation in newborns was the most commonly reported clinical sign of SBV-infection. All countries were able to provide the date when the first suspicion of SBV in the herd was reported and nineteen could report the location of the herd at a regional level. This allowed the spread of SBV in Europe to be measured both temporally and spatially. The number of SBV confirmed herds started to increase in December 2011 and two peaks were observed in 2012 (February and May). Confirmed herds continued to be reported in 2012 and into 2013. An increase during winter 2012 and spring 2013 was again observed, but the number of confirmed herds was lower than in the previous year. SBV spread rapidly throughout Europe from the initial area of detection. SBV was detected above the latitude of 60° North, which exceeds the northern expansion observed during the bluetongue virus serotype 8 epidemic in 2006–2009. The impact of SBV was calculated as ratio of the number of herds with at least one malformed SBV positive foetus and the total number of herds in this region. The 75th percentile of the malformations ratio in the various affected countries for the whole reporting period was below 1% and 3% for cattle and sheep herds, respectively. International data collection on emerging diseases represents a challenge as the nature of available data, data quality and the proportion of reported cases may vary widely between affected countries. Surveillance activities on emerging animal diseases are often structured only for case detection making the estimation of infection/diseases prevalence and the investigation of risk factors difficult. The impact of the disease must be determined to allow risk managers to take appropriate decisions. Simple within-herd impact indicators suitable for emerging disease outbreaks should be defined that could be measured as part of routine animal health surveillance programmes and allow for rapid and reliable impact assessment of emerging animal health diseases.  相似文献   
2.
Stand-level tree diameter growth patterns were explored for evergreen moist forests in the southern Cape, South Africa. Results of standard multiple regression analyses, involving 934 permanent sample plots with data spanning a 10-year interval, revealed that stand-level increment of canopy species in the canopy layer (>30 cm dbh) was significantly determined by inherent species-specific growth capacities (species composition of the stand), water availability, forest matrix crowding and tree condition impairment (age-related manifestations of reduced vitality indicated by signs of crown die-back, damage and stem rot). In contrast, stand-level increment of trees of canopy species in the subcanopy layer (10-20 cm dbh) was prominently shaped by light availability, as mainly determined by the degree of canopy-level disturbance (mortality rate of trees >30 cm dbh), crowding (canopy-level overhead and forest matrix crowding) and proximity to conspecific adults (within 6-8 m). In addition to species-inherent and resource factors, considerable variation in stand-level growth resulted from site-climate interactions. For 507 of the permanent sample plots, increment data was available for two consecutive 10-year intervals; permitting the analysis of spatiotemporal interactions of growth patterns (repeated measures ANOVA). In the Knysna forests higher canopy-level increment rates were associated with the moister southerly facing slope sites in comparison with the drier northerly facing and ridge sites during the first increment period. During the second increment period, increment rates on the drier, but better illuminated sites had increased disproportionately. In contrast, in the Tsitsikamma forests, higher increment rates during the second increment period were encountered on moister flat bottomland sites (with extended periods of subsoil wetness) than on the comparatively drier southerly facing slope sites (increment period × site-based water availability × forests interaction). In both forests relatively higher growth performance of subcanopy-level trees during the second increment period was associated with stands experiencing conditions of enhanced light availability. Atmospheric temperatures were higher during the second increment period (mean periodic Tmax: + 0.64 °C). The detected spatiotemporal interactions were interpreted as site × climate interactions where site-related conditions of favourable light or water availability resulted in enhanced temperature-linked growth responses during the second increment period. A metabolic performance trade-off model provided a framework for the interpretation of these complex site-climate interactions by placing the patterns of forest growth into an ecophysiological explanatory context.  相似文献   
3.
The cyclization mechanism of (E)-2,6-dimethyl-6-hydroxyocta-2,7-dienoic acid to wine lactone under acidic aqueous conditions was investigated using the two stereoselectively deuterium-labeled precursors (2E,6R,7Z)-[8-2H]-2,6-dimethyl-6-hydroxyocta-2,7-dienoic acid and (2E,7E)-(+/-)-[8-2H]-2,6-dimethyl-6-hydroxyocta-2,7-dienoic acid. A detailed analysis of the generated wine lactone isomers by enantioselective multidimensional gas chromatography (MDGC)/ion trap tandem mass spectrometry demonstrates that the formation of wine lactone proceeds via a nonenzymatic stereoselective cationic cyclization cascade that includes a 1,3-hydride shift. Usually, such mechanisms are features of cyclization reactions that are catalyzed by terpene cyclases. This nonenzymatic conversion of an acyclic precursor to a bicyclic monoterpene under relevant cationic cyclization conditions has rarely been observed and confirms recent suggestions that the precursor itself can provide the chemical functionality required for specific steps in the cyclization cascade.  相似文献   
4.
Mass flux assessment can provide information that is essential for a sustainable management of elements in agricultural soils. In this article, we present an assessment of regional-scale averages of zinc (Zn) fluxes into agricultural soils and crops of central Iran for the period 1997–2011, using available databases such as regional agricultural statistics. The basic units of the balances were 15 townships of the provinces Qom, Isfahan and Fars. Averaged over the entire study region, the net Zn input into arable soil resulting from all fertilizer inputs – Zn removal with harvested crops was 1515 g ha?1 yr?1 across the entire region, with a range of 438–3009 g ha?1 yr?1 among townships. Estimated average Zn inputs with manure, mineral fertilizers, sewage sludge and compost were 1254, 531, 19 and 7 g ha?1 yr?1, respectively. The input-to-output ratio of these fluxes ranged from 1.8 to 12.9 among townships and averaged 6.1 for the entire study area. Considering that outputs other than with crop harvests are minor, Zn stocks are rapidly building up in the soils of the study region. Uncertainties in the manure and crop removal data were the main sources of estimation uncertainty in this study.  相似文献   
5.
In October 2004, 41 goats > 2 years old from a Saanen dairy goat herd located in Purranque County, 10th Region, Chile, were sampled and tested for paratuberculosis. While collecting samples it was observed that several goats were thin and emaciated. One goat was sufficiently debilitated to warrant humane euthanasia. This animal was brought to the Veterinary School at the Universidad Austral de Chile for necropsy. The goat selected for necropsy was a 12-year-old doe. The animal showed classical clinical signs of caprine paratuberculosis: emaciation despite willingness to eat, dry and rough hair coat, and no evidence of diarrhea. Gross pathology and histopathology of the necropsied goat were consistent with paucibacillary paratuberculosis. Bacteriology, serology, and PCR confirmed the diagnosis. This is the first published report of goat paratuberculosis in Chile confirming a case of caprine paucibacillary paratuberculosis.  相似文献   
6.
The terpene biosynthesis in leaves of Vitis vinifera L. cv. Morio Muskat was studied using methyl jasmonate to induce defensive responses in vivo. The experiments demonstrated the strong activation of the de novo biosynthesis of terpenoids via the octadecanoid-signaling cascade and release of the compounds to the gas phase. Feeding experiments with [5,5-(2)H(2)]-1-deoxy-d-xylulose and [5,5-(2)H(2)]mevalonic acid lactone allowed the investigation of the dynamic allocation of resources via the mevalonic acid and 1-deoxy-d-xylulose/2-C-methyl-d-erythritol 4-phosphate (DOXP/MEP) pathway under induced conditions and after treatment with the specific inhibitors mevastatin and fosmidomycin. The experiments reveal that monoterpenes are almost exclusively synthesized via the DOXP/MEP pathway, whereas sesquiterpenes are generated via both pathways at approximately equal rates. The biosynthesis of the homoterpene (E)-4,8-dimethyl-1,3,7-nonatriene was not affected by mevastatine or fosmidomycin.  相似文献   
7.
To assess the possible degree of root competition from fruit trees which could potentially be used in agroforestry systems, fine root density of fruit trees Strychnos cocculoides Bak., Strychnos spinosa Lam. (Loganiaceae) and Vangueria infausta Burch. (Rubiaceae), as well as from the shrubby species, Grewia flava DC. (Tiliaceae) was investigated. Vangueria infausta had the highest fine root densities in both vertical and horizontal extensions. In Vangueria infausta fine root density decreased with increasing soil depth. For the other species in the 80 cm soil profile investigated, no significant changes in fine root density with soil depth were found. For Strychnos cocculoides almost no fine roots were detected in the upper soil horizon (0–20 cm). Using fine root surface area densities, exploration and exploitation indices were calculated. Vangueria infausta had the highest value of the exploration index compared to the other species. For use in agroforestry systems Vangueria infausta was estimated to be the most competitive of the investigated species, whereas Strychnos cocculoides seems to be the less competitive. Strychnos cocculoides has additionally spatial arrangements of fine roots favourable for agroforestry, slowly increasing with depth and additionally low concentrations in upper soil layers.  相似文献   
8.
9.
The objective of this study was to evaluate luteinizing hormone (LH) and luteal progesterone (P4) secretion in systemic blood and blood near the ovaries in Mangalica (M) and Landrace (L) gilts by implanting catheters into the Vena jugularis and the Vena cava caudalis via the Vena saphena, respectively. Furthermore, leptin was analyzed in jugular vein blood. Blood was collected twice daily from day 7 to day 19 of the oestrous cycle and frequently (10-min intervals for 6 h) on day 9, day 12 and day 15 in M (n=3) and L gilts (n=4). L gilts had congruent pulsatile LH secretion in both veins, but the LH concentrations in M were always below the assay sensitivity during the luteal phase. In both breeds, episodic P4 secretion was found in the jugular and caval veins, and both sampling site and breed had an influence on P4 secretion (P<0.05). The mean concentration of P4 was higher (P<0.01) in utero-ovarian blood (75.8+/-5.3 in M; 49.6+/-4.2 ng/ml in L) than in the periphery (31.3+/-2.0 in M; 21.2+/-1.8 ng/ml in L). M pigs had a lower number of corpora lutea (9.7+/-2.3 vs. 20.5+/-4.4), and analysis of the P4 secretion ratio per corpus luteum revealed an influence of breed (P<0.01). This ratio was significantly higher in M (3.8+/-0.3 and 8.7+/-0.7 ng/ml) compared with the L gilts (1.4+/-0.1 and 2.8+/-0.3 ng/ml) in the jugular and caval veins, respectively. Blood sampling from the Vena cava caudalis is potentially more precise than from the Vena jugularis for evaluation of ovarian P4 secretion. Both the higher P4 concentration and increased leptin secretion (11.3+/-0.6 vs. 3.0+/-0.1 ng/ml, P<0.05) and consequently the altered LH secretion pattern in the Mangalica may contribute to the lower fecundity of this breed.  相似文献   
10.
Determining some parameters of capillary moisture transport in porous building materials. Part 1: Deriving the material parameters Various relevant material parameters were defined and their mutual correlations pointed out. By means of simple mathematical approaches, one can calculate several characteristic data such as e.g. water absorption and water penetration coefficients, water capacity, capillary rise maximum, velocity of the rising liquid front and post-suction time.  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