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   441篇
  免费   18篇
林业   83篇
农学   12篇
基础科学   1篇
  164篇
综合类   44篇
农作物   14篇
水产渔业   5篇
畜牧兽医   95篇
园艺   3篇
植物保护   38篇
  2023年   2篇
  2022年   3篇
  2021年   5篇
  2020年   9篇
  2019年   11篇
  2018年   5篇
  2017年   8篇
  2016年   9篇
  2015年   5篇
  2014年   16篇
  2013年   35篇
  2012年   18篇
  2011年   30篇
  2010年   15篇
  2009年   16篇
  2008年   34篇
  2007年   22篇
  2006年   25篇
  2005年   24篇
  2004年   15篇
  2003年   20篇
  2002年   14篇
  2001年   6篇
  2000年   3篇
  1999年   10篇
  1998年   4篇
  1997年   3篇
  1996年   3篇
  1995年   6篇
  1993年   8篇
  1991年   2篇
  1989年   4篇
  1988年   5篇
  1987年   4篇
  1986年   3篇
  1984年   3篇
  1980年   2篇
  1977年   2篇
  1976年   5篇
  1973年   2篇
  1970年   2篇
  1969年   2篇
  1966年   4篇
  1965年   4篇
  1953年   2篇
  1951年   2篇
  1949年   2篇
  1932年   5篇
  1928年   2篇
  1926年   1篇
排序方式: 共有459条查询结果,搜索用时 15 毫秒
181.
The object of the present study was to determine the influence of zeolite A, a calcium binder from the group of the aluminosilicate, on feed intake, macro and trace element metabolism as well as the milk yield in the following lactation in dairy cows. 46 cows were allotted to 2 groups (A--control group and B--experimental group). They were fed a total mixed ration (TMR) ad libitum 2 weeks before calving. Additionally the cows in group B received 90 g zeolite A/kg dry matter (DM). The individually feed intake was registered daily. The serum was analysed for Ca, Mg, and Pi (inorganic phosphate), Fe, FFA (free fatty acid) and beta-HB (hydroxybutyrate) and the plasma for the trace elements Cu, Zn, and Mn. After calving the milk yield (FCM) and the milk composition (fat, protein, lactose and urea) were analysed. Feed intake of group B, amounting to 6.2 +/- 1.3 kg DM/d was around 48% lower as compared to 12.0 +/- 1.4 kg DM/d for group A. The zeolite addition into the TMR showed a stabilizing effect on the average Ca concentration in the serum around calving. This effect led to a significantly lower Mg concentration on the day of calving and 1 day post partum. The Pi concentration was significantly lower already after the 1st week of zeolite supplementation and on the day of calving as compared to group A. There was no essential effect of zeolite A on the trace element concentration. The depression of feed intake for group B led to a significant increase of FFA one week after beginning zeolite supplementation and of beta-HB around calving. The feed intake post partum as well as the milk yield were not affected by zeolite supplementation. Because decreased feed intake of group B after zeolite supplementation and the occurred hypophosphatemia, it is not acceptable to use zeolite A in the proved dose for preventing milk fever.  相似文献   
182.
Call feeding for pregnant sows is a novel modular extension of a conventional electronic feeder (PigTek INTEC MAC) communicating via ISOagriNET. The call feeding module (CFM) assigns individual calls to each animal of a group supplied by one feeder and trains them to associate that call with feed access. Afterwards it actively calls sows to the feeder in a variable sequence in order to minimize queuing and thereby reducing aggression, stress and injuries associated with feeding. In this paper, we describe the automatic training procedures, the principal technical design and implementation details that make call feeding applicable in practice. The automatic training consists of an initial Pavlovian conditioning during standard electronic feeding and a subsequent operant conditioning. During Pavlovian conditioning the sows may enter the feeder whenever they have remaining feed allowance. An individual acoustic signal is then played immediately before the feed is dispensed. In the operant training the sow learns by experience that she can enter the feeder only after the individual acoustic signal has been presented. The training modes and their durations are individual to each sow’s learning success. Undersupply with feed due to incomplete learning is prevented by the training routines without manual intervention by controlling automatically the proportion of operant and Pavlovian conditioning. Thus, introducing call feeding in an existing herd is possible. The implementation is further able to estimate roughly the social hierarchy and incorporates this in the calculation of the call sequence in order to attenuate feeding associated aggressions. It helps to provide the animals with a positive anticipation of safe feeding and thereby offers a suitable way to improve welfare and health of pregnant sows. Being automatically controlled, it is easy to apply and has the potential to become a promising future element of precision livestock farming.  相似文献   
183.
184.
This study focuses on a detailed analysis of the errors introduced by two quasi-analytical approaches based on either Fick??s first law or a combination of Fick??s and Darcy??s laws to evaluate the vapour flux of chlorinated solvents from a source zone located in the unsaturated zone towards the atmosphere. A coupled one-dimensional numerical flow and transport model was developed and applied to three case studies characterised by different water content profiles in the vadose zone and under different levels of maximum dense nonaqueous-phase liquid vapour concentrations and vapour pressure conditions of the source zone. The steady-state concentration and pressure profiles obtained were then used in the two quasi-analytical approaches to estimate the flux towards the atmosphere. When mass fluxes due to density-driven advection become dominant and the vertical advective mass fluxes are increased due to strong pressure gradients in the soil air, the error was observed to increase when using the pure diffusion approach in the quantification of the surface flux calculated by the numerical model with increasing dimensionless Rayleigh numbers. Without taking into account the advective transport in the approach, the relative error calculated with only Fick??s law overestimates the real vapour flux when density-driven advection is dominant and underestimates it when pressure-gradient-driven advection dominates. The more advanced advective?Cdiffusive quasi-analytical approach fits reasonably well with the numerically obtained mass fluxes except near soil layer discontinuities, where the evaluation of both the concentration gradient and pressure gradient in the porous media as well as the determination of the average effective diffusion coefficients are rendered more difficult.  相似文献   
185.
Biochar (BC) application as a soil amendment has aroused much interest and was found to considerably improve soil nutrient status and crop yields on poor, tropical soils. However, information on the effect of BC on temperate soils is still insufficient, with effects expected to differ from tropical soils. We investigated the effects of BC on soil nutrient dynamics, crop yield, and quality in a greenhouse pot experiment. We compared three agricultural soils (Planosol, Cambisol, Chernozem), and BCs of three different feedstocks (wheat straw [WS], mixed woodchips [WC], vineyard pruning [VP]) slowly pyrolyzed at 525°C, of which the latter was also pyrolyzed at 400°C. The BCs were applied at two rates (1% and 3%, which would correspond to 30 and 90 t ha–1 in the field). Three crops, namely mustard (Sinapis alba L.), barley (Hordeum vulgare L.), and red clover (Trifolium pretense L.) were grown successively within one year. The investigated soil properties included pH, electrical conductivity (EC), cation‐exchange capacity (CEC), calcium‐acetate‐lactate (CAL)–extractable P (PCAL) and K (KCAL), C, N, and nitrogen‐supplying potential (NSP). The results show a pH increase in all soils. The CEC increased only on the Planosol. The C : N ratio increased at 3% application rate. Despite improving the soil nutrient status partly, yields of the first crop (mustard) and to a lesser extent of the second crop (barley) were significantly depressed through BC application (by up to 68%); the yield of clover as third crop was not affected. Only the BC from WS maintained yields in the range of the control and even increased barley yield by 6%. The initial yield reduction was accompanied by notable decreases (Cu, Fe, Mn, Zn) and increases (Mo) in micronutrient concentrations of plant tissues while nitrogen concentrations were hardly affected. The results of the pot experiment show that despite additional mineral fertilization, short‐term growth inhibition may occur when applying BC without further treatment to temperate soils.  相似文献   
186.
Eight streptophenazines (A–H) have been identified so far as products of Streptomyces strain HB202, which was isolated from the sponge Halichondria panicea from the Baltic Sea. The variation of bioactivities based on small structural changes initiated further studies on new derivatives. Three new streptophenazines (I–K) were identified after fermentation in the present study. In addition, revised molecular structures of streptophenazines C, D, F and H are proposed. Streptophenazines G and K exhibited moderate antibacterial activity against the facultative pathogenic bacterium Staphylococcus epidermidis and against Bacillus subtilis. All tested compounds (streptophenazines G, I–K) also showed moderate activities against PDE 4B.  相似文献   
187.
188.
In Vietnam as much as half of the total land area is already degraded by soil erosion and nutrient depletion. In particular, degradation due to deforestation is increasingly affecting mountainous areas in north-western Vietnam. The necessity to safeguard the farmers' livelihoods requires sustainable resource management, which firstly requires a qualitative and quantitative evaluation of resources. The objectives of the present study were to (1) identify the dominant soil types and their vulnerability using elicitation of local soil knowledge, (2) characterise the physical and chemical properties of the soils and (3) link them to the relief position and land use in order to (4) initiate sustainable soil use based on recommendations deduced from objectives (1) to (3). These objectives were achieved also by the elicitation of local knowledge. The final aim of the study was to initiate sustainable soil use based on recommendations for sustainable land use scenarios. The Chieng Khoi commune in Son La province of northern Vietnam was chosen as representative for other erosion-prone Southeast Asian sloping areas. In a participatory approach, combining local and scientific knowledge, sixteen sites were selected, representative for distinct relief positions, parent material (sand stone and silt stone), land use history, and erosion hazard. Chemical (e.g. content of organic matter, nitrogen, cation exchange capacity, base saturation, and plant available phosphorous) and physical properties (e.g. air capacity, plant available water) were used to estimate soil fertility. The predominant reference soil groups in the study area are Alisols and Luvisols, with a high diversity in respect to soil fertility. These soils are locally named ‘red soil’ and ‘black soil’, respectively. Although the main physical processes are erosion and selective sedimentation, farmers tend to underestimate their impact and causes, whereas soil quality was well-evaluated. Soils with high fertility were found on less eroded upper parts of hills and at sites, where agricultural use started only recently. Once degraded by cultivation practices, soils derived from sandstone did not recover even after more than 50 years of fallow. As a result of unsustainable land use, soils on middle and lower slopes are often affected by severe soil erosion, whereas foot slope soils suffer from accumulation of eroded infertile subsoil material as well as stagnic conditions. This study showed that unsustainable land use at upslope landscape positions has a severe impact on downslope areas. The elicitation of local knowledge facilitated the identification of such hot spots, allowing the implementation of spatially targeted conservation measures.  相似文献   
189.
190.
Medical records of seven dachshunds with congenital nasopharyngeal stenosis from abnormally thickened palatopharyngeal muscles were reviewed. The intrapharyngeal ostium in all cases consisted of only a narrow slit. Dogs were presented with various clinical signs--the most common being dyspnea, expiratory cheek puff, salivation, pharyngeal dysphagia, oral dysphagia (to a lesser extent), and macroglossia. Diagnostic procedures included direct pharyngeal inspection, pharyngeal and thoracic radiography, fluoroscopy, lingual ultrasonography, biopsies in two dogs, and a postmortem examination in one dog. Diagnoses were readily made with radiographs and visual examinations. Macroglossia was confirmed with transcutaneous ultrasonography or a transmandibular finger test.  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