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   532篇
  免费   35篇
林业   52篇
农学   26篇
  101篇
综合类   28篇
农作物   22篇
水产渔业   69篇
畜牧兽医   208篇
园艺   30篇
植物保护   31篇
  2024年   1篇
  2023年   6篇
  2022年   17篇
  2021年   21篇
  2020年   28篇
  2019年   36篇
  2018年   28篇
  2017年   21篇
  2016年   36篇
  2015年   24篇
  2014年   19篇
  2013年   44篇
  2012年   38篇
  2011年   39篇
  2010年   24篇
  2009年   23篇
  2008年   24篇
  2007年   30篇
  2006年   26篇
  2005年   10篇
  2004年   11篇
  2003年   12篇
  2002年   10篇
  2001年   5篇
  2000年   5篇
  1999年   3篇
  1998年   1篇
  1997年   3篇
  1995年   2篇
  1994年   2篇
  1993年   1篇
  1990年   1篇
  1989年   2篇
  1988年   1篇
  1986年   2篇
  1984年   1篇
  1983年   1篇
  1981年   1篇
  1980年   1篇
  1979年   1篇
  1977年   1篇
  1974年   1篇
  1973年   1篇
  1972年   2篇
  1971年   1篇
排序方式: 共有567条查询结果,搜索用时 664 毫秒
71.
The aim of this work was to investigate differences in soil chemistry and understory composition between recent forests (sites afforested in the last 170 years) and ancient forests growing on non-acidic soils. The study was carried out on hardwood forests at moderate elevation (400–600 m asl) in the Jura Mountains (N.E. France) on four main pedological substrates with different characteristics. The floristic composition of 127 stands from recent forests (n = 65) or ancient forests (n = 62) was surveyed. Some functional traits and the Ellenberg indicator values of the surveyed species were recorded. In addition, the topsoil from 30 stands was analysed. The composition of the flora was analysed by Detrended Correspondence Analysis and the species which were typical of one class of forest age were identified using a chi-square (χ2) test. The difference between forest classes for plant traits, their indicator values, or soil chemistry was tested using the generalized linear model and Bonferroni t-tests (or Kruskall–Wallis tests). The floristic composition of the ancient forests was significantly different from that of the recent forests and was characterized by a high occurrence of shrub species in recent forests. These differences were associated with higher specific leaf area, low-range seeds dispersal, and some life forms like geophytes. There was no clear difference in soil chemistry between the two classes of forests, except for δ15N values. The weakness of the difference in the soil between ancient and recent forests suggested that changes in soil chemistry caused by a former agricultural land use were not responsible for the differences in understory composition recorded. The differences in functional traits between the two forest classes supported this conclusion. We finally concluded that (i) past land use modifies the vegetation composition of current forests, even on neutral soils and that (ii) in our context, biological filters were probably responsible for these changes.  相似文献   
72.
We synthesized nano-scaled periodic ripple patterns on silicon and titanium dioxide (TiO(2)) surfaces by xenon ion irradiation, and performed adsorption experiments with human plasma fibrinogen (HPF) on such surfaces as a function of the ripple wavelength. Atomic force microscopy showed the adsorption of HPF in mostly globular conformation on crystalline and amorphous flat Si surfaces as well as on nano-structured Si with long ripple wavelengths. For short ripple wavelengths the proteins seem to adsorb in a stretched formation and align across or along the ripples. In contrast to that, the proteins adsorb in a globular assembly on flat and long-wavelength rippled TiO(2), but no adsorbed proteins could be observed on TiO(2) with short ripple wavelengths due to a decrease of the adsorption energy caused by surface curvature. Consequently, the adsorption behavior of HPF can be tuned on biomedically interesting materials by introducing a nano-sized morphology while not modifying the stoichiometry/chemistry.  相似文献   
73.
The aim of this work was to evaluate the spermatic parameters of jundia, Rhamdia quelen, semen stored for short periods under different temperatures. Fifteen males were used in a time factorial experimental design (13 × 3 × 3 × 3). The sperm was stored at: 15; 25 and 35°C and activated in water at: 15; 25 and 35°C each, respectively, within storage periods of: 0; 2; 4; 6; 8; 10; 12; 16; 20; 24; 32; 40 and 48 h after collection. The treatments were performed in triplicates and in sequential protocols every 50 h. The motility parameters were evaluated using Pearson's correlation analysis and the significant parameters (< 0.05) were summarized using principal component analysis forming two predicted groups. Group 01 composed the following parameters: curvilinear velocity, wobble and linearity; group 02 comprised average path velocity, straight line velocity, beating cross frequency and motility rate. Effects (< 0.05) of treatments were observed only in group 02, with interaction between time and temperature of exposure and between exposure time and water temperature. Superior results of spermatic parameters of group 02 were observed right after collection. However, the exposure and spermatic activation at 15°C assured significant results 48 h after collection when compared with 25 and 35°C.  相似文献   
74.
This study evaluated the effectiveness of chemical control for eradicating eucalyptus sprouts using images obtained with Unmanned Aerial Vehicle (UAV). The study was carried out in eucalyptus plantations in Itabela, BA, in replanting areas during pre-planting application of herbicide. Aerial images obtained by a UAV were used to evaluate the effectiveness of the herbicide application for sprouts control. After the images were acquired, they were processed to calculate the normalized difference vegetation index (NDVI) and submitted to a supervised classification to quantify the percentage reduction of sprout green matter. The percentage data were submitted to analysis of variance and the means were compared by Tukey test at 5%. The differences, both visual and average percentages were observed only during the evaluation period with no evident effect of the treatments on sprout control. The images provided by the UAV allowed to monitor and identify visually the plots where biomass reduced or increased and to evaluate the effectiveness of chemical control of eucalyptus sprouts, indicating areas where it was nonexistent, partial or total.  相似文献   
75.
The use of silvopastoral systems (SPS) can be a good alternative to reduce the environmental impacts of livestock breeding in Brazil. Despite the advantages offered by public policies, many producers hesitate to use this system. One of the reasons is the lack of information on health and productivity of cattle raised under these conditions. The experiment reported here was designed to compare the behavior of infection by gastrointestinal nematodes and weight gain of beef cattle raised in a SPS and a conventional pasture system. We monitored the number of eggs per gram of feces, the prevalent nematode genus, data on climate, forage availability, weight gain and packed cell volume (PCV) of the animals bred in the two systems. The infection by nematodes was significantly higher in the cattle raised in the SPS (p < 0.05). The coprocultures revealed the presence of nematodes of the genera Haemonchus, Cooperia, Oesophagostomum and Trichostrongylus, in both systems, but the mean infestation rates of Haemonchus and Cooperia were higher in the SPS (p < 0.05). The average of PCV values did not differ between the cattle in the two systems. The individual weight gain and stocking rate in the period did not vary between the systems (p > 0.05). Despite the higher prevalence of nematodes in the SPS, no negative impact was detected on the animals’ weight gain and health. The results of this experiment indicate that under the conditions studied, there is no need to alter the parasite management to assure good productive performance of cattle.  相似文献   
76.
We evaluated the effects of macronutrient and micronutrient omission, and of liming, on Jatropha curcas (JC) in greenhouse study. The experimental design was a factorial combination of 2 liming treatments (nil or 4.5 t. ha?1 of dolomitic lime) x 7 combinations of fertility status, with five replications. The concentrations and uptake of nutrients by JC plants follow the order: nitrogen > potassium > calcium ≥ Magnesium > sulfur > phosphorus > iron > boron > manganese > zinc > copper (N > K > Ca ≥ Mg > S > P > Fe > B > Mn > Zn > Cu), but the growth of JC plants was negatively affected mainly by omission of macronutrients, in the following order: P > N = K = S, on limed soil. Phosphorus is the most critical element for development. JC is responsive to micronutrients only when applied together with macronutrients. Potassium chloride is not recommended as K+ source for JC crop. Lime is recommended to improve growth and nutritional status of JC plants.  相似文献   
77.
Tropical Animal Health and Production - A study was conducted with the objective to characterize the morphology of Socorro Island Merino sheep. A total of 67 sheep, 62 females, 26 purebred and 36...  相似文献   
78.
In plant breeding, correlations between the statistics of stability and adaptability of popcorn cultivars are not yet well understood. Therefore, the objectives of the present experiment was to investigate the correlations between sdi2 \sigma_{\rm di}^{2} and bi \beta_{\rm i} from Eberhart and Russell, ωi from Wricke, \textS\texti(1) {\text{S}}_{\text{i}}^{(1)} , \textS\texti(2) {\text{S}}_{\text{i}}^{(2)} and \textS\texti(3) {\text{S}}_{\text{i}}^{(3)} from Huehn, Pi from Lin and Binns and the rank-sum from Kang, and indicate the most reliable method for selecting popcorn cultivars. These statistics were estimated by data of crop yield from 19 Brazilian genotypes under 21 environments and popping expansion under 16 environments. The ωi, \textS\texti(1) {\text{S}}_{\text{i}}^{(1)} , \textS\texti(2) {\text{S}}_{\text{i}}^{(2)} , \textS\texti(3) {\text{S}}_{\text{i}}^{(3)} and sdi2 \sigma_{\rm di}^{2} were positively and significantly correlated indicating that just one in these five statistics is sufficient for selecting stable genotypes although they were not correlated with the means of crop yield and popping expansion. The bi \beta_{\rm i} was negatively and significantly correlated with Pi for crop yield indicating that the most adaptable genotypes tend to have the lowest estimates of Pi. Although Pi was not correlated with ωi, \textS\texti(1) {\text{S}}_{\text{i}}^{(1)} , \textS\texti(2) {\text{S}}_{\text{i}}^{(2)} , \textS\texti(3) {\text{S}}_{\text{i}}^{(3)} , or sdi2 \sigma_{\rm di}^{2} statistics, it displayed positive correlation with the Index 1 (crop yield and popping expansion +  \textS\texti(1) {\text{S}}_{\text{i}}^{(1)} rank) and Index 2 (crop yield and popping expansion + Wi) indicating that superior popcorn genotypes are also stable. Finally, both Pi and the rank-sum are useful statistics in breeding programmes where crop yield, popping expansion and stability are essential traits for selecting genotypes.  相似文献   
79.
A Web-based system is presented, integrating spatial information from remote sensing images, GPS measurements and inventory data. Monitoring, research and management of the grape production at Rio Grande do Sul State, Brazil (624,000 metric tons in 2006) can be done through a system entirely based in open-source codes. Information from three different sources are integrated: Information on production comes from the State’s Viticultural Inventory, detailing the regional yearly grape production; positional information comes from field GPS measurements of vine parcels and the system’s capabilities of making maps; imagery information comes from aerial or satellite images. Further capabilities on image classification leads to the identification of vine areas; this allows crossing this data with information provided by owners. This system allows both the monitoring of grape production for administrative purposes and investigations either on regional land and soil cover, or in other applications derived from image classification. The system concept can be readily extended to other applications in land use monitoring.  相似文献   
80.
In Israel four European pear cultivars are grown: ‘Spadona’ is the main cultivar and ‘Coscia’, ‘Gentile’ and ‘Spadochina’ are its pollinators. However, molecular S-genotyping revealed that ‘Spadona’ is semi-compatible with its three pollinators. This explains, at least in part, the relatively low pear yield in Israel. The Syrian pear (Pyrus syriaca) grows wild in Israel and blooms intensively, overlapping the blooming of the cultivated European pears. Cross-fertilization between Syrian pear and ‘Spadona’ was shown to be efficient suggesting that Syrian pear might be a potent pollinator for ‘Spadona’. Twenty-six Syrian pear seedlings, from different sites in north-east Israel were S-genotyped identifying 11 that are fully compatible with the four European pear varieties cultivated in Israel. By this screening, 24 different S-RNases were cloned; ten of them are new, whereas the other fourteen had been identified previously. In addition, seedlings of two wild pear species were also S-genotyped. Two seedlings from Pyrus betulifolia and one from Pyrus korshinskii were found to be genetically compatible with the four European pear cultivars. From these seedlings four S-RNases were cloned, two are new, one had been cloned previously and one was identical to an S-RNase allele cloned from Syrian pear in this work.  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