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   183篇
  免费   10篇
  国内免费   8篇
林业   30篇
农学   20篇
基础科学   5篇
  20篇
综合类   46篇
农作物   21篇
水产渔业   5篇
畜牧兽医   37篇
园艺   12篇
植物保护   5篇
  2023年   4篇
  2022年   5篇
  2021年   1篇
  2020年   3篇
  2019年   2篇
  2018年   2篇
  2017年   3篇
  2016年   9篇
  2015年   3篇
  2014年   7篇
  2013年   7篇
  2012年   13篇
  2011年   20篇
  2010年   13篇
  2009年   19篇
  2008年   21篇
  2007年   12篇
  2006年   15篇
  2005年   5篇
  2004年   9篇
  2003年   3篇
  2002年   4篇
  2001年   3篇
  2000年   1篇
  1999年   1篇
  1998年   4篇
  1997年   3篇
  1996年   1篇
  1995年   3篇
  1994年   4篇
  1987年   1篇
排序方式: 共有201条查询结果,搜索用时 18 毫秒
1.
Infrared thermography has been used to help in diagnosing lameness. It is hypothesized that, if used in a routine basis, it could help in understanding musculoskeletal modifications during race training. This study aimed to evaluate thermal variation in the musculoskeletal regions of young Thoroughbred (TB) horses during their initial months of race training. Thermographic examinations were performed once every 2 weeks on 16 (10 male, 6 female) two-year-old TB racehorses, from arrival to the racetrack in June 2016, until January 2017, for a total of 16 evaluations. Thermographic imaging was performed using the appropriate protocol. Temperature (°C) was measured at the dorsal and palmar/plantar aspects of specific regions of interest (fetlock, metacarpal, metatarsal, carpal, tarsal, thoracolumbar, sacroiliac spine, and both hips). Initially, we found a thermal balance and all regions demonstrated a positive correlation with one another. However, a significant difference was noted between the left and right sides as training progressed. Four horses were withdrawn from the study after 50% of evaluations because of metacarpal conditions associated with training. Thermographic examination revealed changes before the clinical manifestation of these conditions. In conclusion, this study demonstrated that infrared thermography is an image technique that can facilitate understanding of musculoskeletal system modifications to race training and should be further investigated as a predictive tool to anticipate the occurrence of lesions.  相似文献   
2.
Dangasuk  O.G.  Panetsos  K.P. 《New Forests》2004,27(3):269-284
Four populations representing the entire natural distribution range of Pinus brutia (Ten.) in Crete (Greece) were sampled to determine: (1) Altitudinal variation within populations of Lassithi, Vorizia, Koustagerako, and Samaria, and; (2) Longitudinal variation among the four populations, based on morphological and anatomical differences in cone, needle and seed characteristics. Samples were collected from 10 trees per altitude, at three altitudes in each population. A total of 12 morphological and anatomical traits from needles, cones and seeds were analyzed to assess the altitudinal and longitudinal variations of the species in the island. The number of teeth per cm showed significant (P<0.01) differences between altitudes in Samaria, Vorizia and Lassithi, while needle length, width and the number of resin canals showed significant differences (P<0.01) between altitudes in Koustagerako. Cone width showed patterns of variation along the altitudinal gradient in Koustagerako, Samaria and Vorizia. Most traits had their highest means at the middle altitudes (650masl) in all the four populations. Overall, no clear altitudinal differentiation was detected, probably due to high rate of gene flow. Longitudinal variability showed a clear pattern of differentiation along the east–west climatic gradient. Needle length, width, sheath length, number of teeth per cm and the number of resin canals were the most useful traits for determining the climatic gradient clines.  相似文献   
3.
Fady  B.  Ducci  F.  Aleta  N.  Becquey  J.  Diaz Vazquez  R.  Fernandez Lopez  F.  Jay-Allemand  C.  Lefèvre  F.  Ninot  A.  Panetsos  K.  Paris  P.  Pisanelli  A.  Rumpf  H. 《New Forests》2003,25(3):211-225
Adaptive and wood quality trait data were collected and analyzed on commercially available Juglans regia and J. regia×J. nigra provenances and progenies planted across Europe in a multi-site network. A total of 19 seed sources, replicated 35 times per site, were planted at 13 sites from 5 European countries, encompassing the potential distribution area of timber production plantation sites. The following traits were evaluated: survival, height, diameter at breast height, stem form, apical dominance, vegetative budbreak, along with biotic and abiotic damage. Mean values were significantly different both among provenances/progenies and sites. Most common damage was late spring and early autumn frost. Bud break ranking was significantly correlated with provenance and progeny origin. Although J. regia is fast growing, southern European early budbreak plant material should not be planted under most middle European conditions where late spring frost can be expected, as it has a significant negative impact on architectural (and thus wood quality) traits. Hybrid J. regia×J. nigra progeny performed better than J. regia provenances/progenies for most traits measured. Differences were significantly in favor of hybrids at sites with medium to low fertility, although some locally selected seed sources tended to perform as well as hybrids on high fertility sites.  相似文献   
4.
The genetic variation in seed weight, seed number per kg and seedling traits was compared among eight Acacia senegal provenances originating from the clay plain (east) and sand plains (west) of the gum belt in Sudan. The main objective of this study was to identify germplasm sources of A. senegal that have a good seed germination capacity and seedling traits suitable for reforestation in the clay-soil part of the dryland gum belt in the Blue Nile region in Sudan. A specific objective was to tentatively explore the adaptive strategy of A. senegal populations. Seventeen-week-old seedlings were planted in the field at spacing of 3 m × 3 m, giving 100 trees per plot and replication; within a randomized complete block design with four replications. The experimental site was in the clay plain region. Seed variables showed significant differences. Clay plain provenances showed considerable variation in seed weight and seed number. They had the smallest seed weight but the highest seed number, while the sand (western) provenances had the largest seed weight but lowest seed number. Seedling branch number, root length, root to shoot ratio and shoot dry weight differed significantly among the provenances 12 weeks after germination. Clay provenances had the highest branch number and shoot dry weight but the shortest roots and lowest root to shoot ratio. This was interpreted as showing better adaptation to the site in these local provenances in comparison to those originating from the western sandy soil regions. High positive correlations were observed between seedling variables, such as root nodule and branch numbers; this could be used for early selection. The variation was greater between provenance groups than within them, suggesting that especially selection among groups would yield genetic gain.  相似文献   
5.
Shade is one of the important limiting factors in intercropped agro-ecosystems. Objective of this work is to analyze the role of eight foliar traits of juvenile plants of five different species of Jasminum grown under coconut shade. The results reveal the shade tolerant J. pubescence to produce more number of nodes and longer shoots in all the three seasons studied. J. grandiflorum produced longer leaves and internodal distance than other species tested. Winter season shoots were longer with more number of nodes and longer internodal distances than rainy season. Results are discussed in relation to the adaptive behavior of plants to shade.  相似文献   
6.
Using CROPGRO-Chickpea model (revised version), we investigated the impacts of climate change on the productivity of chickpea (Cicer arietinum L.) at selected sites in South Asia (Hisar, Indore and Nandhyal in India and Zaloke in Myanmar) and East Africa (Debre Zeit in Ethiopia, Kabete in Kenya and Ukiriguru in Tanzania). We also investigated the potential benefits of incorporating drought and heat tolerance traits in chickpea using the chickpea model and the virtual cultivars approach. As compared to the baseline climate, the climate change by 2050 (including CO2) increased the yield of chickpea by 17% both at Hisar and Indore, 18% at Zaloke, 25% at Debre Zeit and 18% at Kabete; whereas the yields decreased by 16% at Nandhyal and 7% at Ukiriguru. The yield benefit due to increased CO2 by 2050 ranged from 7 to 20% across sites as compared to the yields under current atmospheric CO2 concentration; while the changes in temperature and rainfall had either positive or negative impact on yield at the sites. Yield potential traits (maximum leaf photosynthesis rate, partitioning of daily growth to pods and seed-filling duration each increased by 10%) increased the yield of virtual cultivars up to 12%. Yield benefit due to drought tolerance across sites was up to 22% under both baseline and climate change scenarios. Heat tolerance increased the yield of chickpea up to 9% at Hisar and Indore under baseline climate, and up to 13% at Hisar, Indore, Nandhyal and Ukiriguru under climate change. At other sites (Zaloke, Debre Zeit and Kabete) the incorporation of heat tolerance under climate change had no beneficial effect on yield. Considering varied crop responses to each plant trait across sites, this study was useful in prioritizing the plant traits for location-specific breeding of chickpea cultivars for higher yields under climate change at the selected sites in South Asia and East Africa.  相似文献   
7.
We investigated the effect of soil microclimate on the structure and functioning of soil microbial communities in a Mediterranean Holm-oak forest subjected to 10 years of partial rain exclusion manipulations, simulating average drought conditions expected in Mediterranean areas for the following decades. We applied a high throughput DNA pyrosequencing technique coupled to parallel measurements of microbial respiration (RH) and temperature sensitivity of microbial respiration (Q10). Some consistent changes in the structure of bacterial communities suggest a slow process of community shifts parallel to the trend towards oligotrophy in response to long-term droughts. However, the structure of bacterial communities was mainly determined by short-term environmental fluctuations associated with sampling date (winter, spring and summer) rather than long-term (10 years) shifts in baseline precipitation. Moreover, long-term drought did not exert any chronic effect on the functioning of soil microbial communities (RH and Q10), emphasizing the functional stability of these communities to this long-term but mild shifts in water availability. We hypothesize that the particular conditions of the Mediterranean climate with strong seasonal shifts in both temperature and soil water availability but also characterized by very extreme environmental conditions during summer, was acting as a strong force in community assembling, selecting phenotypes adapted to the semiarid conditions characterizing Mediterranean ecosystems. Relations of climate with the phylogenetic structure and overall diversity of the communities as well as the distribution of the individual responses of different lineages (genera) to climate confirmed our hypotheses, evidencing communities dominated by thermotolerant and drought-tolerant phenotypes.  相似文献   
8.
The impact of climate change on maize yields in the United States and China   总被引:1,自引:0,他引:1  
This study analyzes the impacts of climate change on maize yields using an econometric model that incorporates climate, economic, and technology variables. The major finding is climate change will not universally cause negative impacts of maize yields in the United States and China. The results of a simulation of climate change on maize yields over the period 2008-2030 show that a combination of changes in temperature and precipitation can either bring positive or negative effects on maize yields. Furthermore, variation in regional climatic and economic conditions makes the impacts of climatic change on maize yields substantially different in different regions. In this research, the impacts of climate change on maize yields are not simply examined by climate factors. Economic and technology adaptation effects on maize yields are also incorporated. Thus, even with significant changes in climate conditions that alter the maize crop’s growing environment and affect crop yields, a decrease in maize supply due to a decrease in maize yields would lead to an increase in the maize price, which in turn would induce farmers to add more investments in production inputs to raise yields. Thus, the decrease in actual yields may not be as dramatic as predicted in only climate factor considered cases. In this research, findings gained from the study can be used for early-staged policymaking decisions and advanced problem prevention programs. To ensure the continuous increase in maize yields in the future, further studies and research, as well as efficient environmental policies and actions are required.  相似文献   
9.
Environment-driven genetic differentiation among populations is a common feature among forest trees, and an understanding of how populations have adapted to their home site conditions is essential for management and conservation practices. In Scotland, 84 native Scots pine (Pinus sylvestris L.) woodlands are recognised by the Forestry Commission and they occupy highly diverse environments from the maritime west coast to continental sites in eastern Scotland. However, it is not known whether adaptations to local environments along sharp temperature and rainfall gradients have occurred in different populations and as a result, the seed transfer guidelines of the species are based only on data from isozymes and monoterpenes. In this study of an outdoor common-garden trial, we used chlorophyll fluorescence to examine whether seedlings from 32 open-pollinated families and eight populations from sites experiencing contrasting annual temperature regimes differed in their response to variation in natural outdoor temperatures between September 2009 and May 2010. In addition, growth initiation in spring was recorded. Photochemical capacity at photosystem II Fv/Fm showed a distinct seasonal trend and remained at relatively high levels (∼0.7) until November. Following a period of over 2 weeks with temperatures below or close to 0 °C, Fv/Fm started decreasing towards its minimum values recorded in early March when population means varied between 0.35 and 0.45. By early May and along with rising temperatures, photochemical capacity had recovered to the same level as observed in early November. Populations were found to respond differently to the cold period starting in December. The largest drop in photochemical capacity was observed in seedlings from a low-altitude population located in the maritime western Scotland, while in seedlings from higher-altitude locations in the cooler eastern Scotland, the response was smaller. In March, the recovery of photochemical capacity was slowest in seedlings from the mildest and coolest sites. Evidence of adaptive genetic differentiation was also found in spring phenology. Initiation of shoot elongation and needle flush were earlier in families from higher altitudes (cooler areas), but population differences were not significant at the α = 0.05 level. These results suggest that adaptation to the spatially heterogeneous environment in Scotland has taken place in Scots pine and that in order to minimise the risk of planting maladapted seed stock, the patterns of environmental and adaptive genetic variation should be taken into account in the management of genetic resources in this species.  相似文献   
10.
This paper reviews the knowledge on effects of climate change on agricultural productivity in Europe and the consequences for policy and research. Warming is expected to lead to a northward expansion of suitable cropping areas and a reduction of the growing period of determinate crops (e.g. cereals), but an increase for indeterminate crops (e.g. root crops). Increasing atmospheric CO2 concentrations will directly enhance plant productivity and also increase resource use efficiencies.

In northern areas climate change may produce positive effects on agriculture through introduction of new crop species and varieties, higher crop production and expansion of suitable areas for crop cultivation. Disadvantages may be an increase in the need for plant protection, the risk of nutrient leaching and the turnover of soil organic matter. In southern areas the disadvantages will predominate. The possible increase in water shortage and extreme weather events may cause lower harvestable yields, higher yield variability and a reduction in suitable areas for traditional crops. These effects may reinforce the current trends of intensification of agriculture in northern and western Europe and extensification in the Mediterranean and southeastern parts of Europe.

Policy will have to support the adaptation of European agriculture to climate change by encouraging the flexibility of land use, crop production, farming systems etc. In doing so, it is necessary to consider the multifunctional role of agriculture, and to strike a variable balance between economic, environmental and social functions in different European regions. Policy will also need to be concerned with agricultural strategies to mitigate climate change through a reduction in emissions of methane and nitrous oxide, an increase in carbon sequestration in agricultural soils and the growing of energy crops to substitute fossil energy use. The policies to support adaptation and mitigation to climate change will need to be linked closely to the development of agri-environmental schemes in the European Union Common Agricultural Policy.

Research will have further to deal with the effect on secondary factors of agricultural production, on the quality of crop and animal production, of changes in frequency of isolated and extreme weather events on agricultural production, and the interaction with the surrounding natural ecosystems. There is also a need to study combined effects of adaptation and mitigation strategies, and include assessments of the consequences on current efforts in agricultural policy to develop a sustainable agriculture that also preserves environmental and social values in the rural society.  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