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   199篇
  免费   15篇
林业   2篇
农学   9篇
基础科学   2篇
  24篇
综合类   21篇
农作物   1篇
水产渔业   3篇
畜牧兽医   145篇
园艺   4篇
植物保护   3篇
  2024年   1篇
  2023年   2篇
  2021年   2篇
  2020年   3篇
  2018年   5篇
  2017年   5篇
  2016年   4篇
  2015年   4篇
  2014年   5篇
  2013年   22篇
  2012年   5篇
  2011年   4篇
  2010年   8篇
  2009年   8篇
  2008年   4篇
  2007年   9篇
  2006年   2篇
  2005年   4篇
  2004年   2篇
  2003年   4篇
  2002年   3篇
  2001年   4篇
  2000年   3篇
  1999年   6篇
  1998年   13篇
  1997年   5篇
  1996年   10篇
  1995年   5篇
  1994年   4篇
  1993年   4篇
  1992年   9篇
  1991年   11篇
  1990年   8篇
  1989年   8篇
  1988年   7篇
  1987年   2篇
  1985年   1篇
  1983年   1篇
  1982年   1篇
  1977年   3篇
  1976年   1篇
  1975年   1篇
  1942年   1篇
排序方式: 共有214条查询结果,搜索用时 31 毫秒
81.
Spatial prioritization techniques are increasingly applied in the design of protected area networks, which are regarded as the cornerstone of nature conservation efforts. These techniques are becoming ever more sophisticated, but are still founded primarily upon biological data. A common assumption made in most spatial prioritizations is that land throughout a planning region is available for acquisition. We interviewed land managers in the Eastern Cape province, South Africa, and mapped their willingness-to-sell their land using a psychometric analytical technique. We examined the, (i) degree to which vegetation type targets are achieved across a planning region, (ii) areal and cost efficiency, and (iii) spatial configuration, of candidate protected areas identified as important for achieving conservation targets. We found that only 10 out of 48 land managers were willing-to-sell their land. Only seven, five and one of the 19 vegetation types, respectively, could achieve their conservation targets of 10%, 30% and 50% when unwilling land managers were removed from the analysis. Assuming unwilling land managers could be convinced to sell if offered a premium price, the cost of acquiring all lands was between 6.20% and 30.67% more expensive than 2006 land prices. Accounting for implementation opportunities and constraints, such as land manager willingness-to-sell, not simply identifying biological priorities, is of fundamental importance for ensuring spatial prioritizations deliver maps with the potential to usefully guide expansion of protected area networks which can be feasibly implemented.  相似文献   
82.
83.
Toxigenic strains of Pasteurella multocida were readily differentiated from non-toxigenic strains by an agarose overlay method using bovine turbinate cells or bovine lung cells. Cells which were young and densely confluent were best suited to this assay. The incubation period required to distinguish toxigenic strains was dependent on the confluence of the monolayers, which was affected by the seeding rate, cell passage level and growth time prior to overlay. The agarose overlay method correctly identified 11 of 11 reference strains of Pasteurella multocida, and visible cytotoxic changes were present in the monolayers after 48 to 65 h. Outbreaks of the enzootic form of atrophic rhinitis in 2 New South Wales piggeries were associated with the isolation of toxigenic type D strains of P. multocida.  相似文献   
84.
Triplets in red deer (Cervus elaphus)   总被引:1,自引:0,他引:1  
  相似文献   
85.
Fresh, mature, ungrazed Tribulus terrestris plant material was subjected to a standard alkaloid extraction procedure. The extract was fractionated by thin layer chromatography (TLC) and high performance liquid chromatography (HPLC). Two major alkaloid fractions were demonstrated. These fractions were identified by means of TLC, ultraviolet spectrofluorimetry (UVS) and HPLC, as the beta-carboline indoleamines harmane and norharmane. The extractable alkaloid content was determined to be 44 mg/kg dry matter. Synthetic harmane and norharmane were administered subcutaneously to sheep at a dose rate of 54 mg/kg. Both compounds caused similar nervous effects. The main effect observed was limb paresis, which in some sheep was body side blased. The clinical signs observed in the experimental sheep were consistent with those described for naturally occurring cases of Tribulus terrestris staggers. It was proposed that harmane and norharmane accumulate in tryptamine-associated neurones of the central nervous system, during months of tribulus ingestion, and gradually interact irreversibly with a specific neuronal gene DNA sequence.  相似文献   
86.
87.
Global Oscillation Network Group data reveal that the internal structure of the sun can be well represented by a calibrated standard model. However, immediately beneath the convection zone and at the edge of the energy-generating core, the sound-speed variation is somewhat smoother in the sun than it is in the model. This could be a consequence of chemical inhomogeneity that is too severe in the model, perhaps owing to inaccurate modeling of gravitational settling or to neglected macroscopic motion that may be present in the sun. Accurate knowledge of the sun's structure enables inferences to be made about the physics that controls the sun; for example, through the opacity, the equation of state, or wave motion. Those inferences can then be used elsewhere in astrophysics.  相似文献   
88.
Information on carbon (C) flows and transformations in the rhizosphere is vital for understanding soil organic matter dynamics and modelling its turnover. We followed the translocation of photosynthetically fixed C in three hill pastures that varied in their phosphorus (P) fertility, using a 14C-CO2 pulse-labelling chamber technique. Pasture shoot, root and soil samples were taken after 4h, 7 days and 35 days chase periods to examine the fluxes of 14C in the pasture plant-root-soil system. Shoot growth over 35 days amounted to 114, 179 and 182gm–2 at the low (LF), medium (MF) and high (HF) fertility pasture sites, respectively. The standing root biomass extracted from the soil did not differ significantly between sampling periods at any one level of fertility, but was significantly different across the three levels of fertility (1367, 1763 and 2406gm–2 at the LF, MF and HF pastures, respectively). The above- and below-ground partitioning of 14C was found to vary with the length of the chase period and fertility. Although most 14C (74%, 65% and 57% in the LF, MF and HF pastures, respectively) was in the shoot biomass after 4h, significant translocation to roots (23–39%) was also detected. By day 35, about 10% more 14C was partitioned below-ground in the LF pasture compared with the HF pasture. This is consistent with the hypothesis that, at limiting fertility, pasture plants allocate proportionally more resource below-ground for the acquisition of nutrients. In the LF site, with an annual assimilated C of 7064kgha–1, 2600kg was respired, 1861kg remained above-ground in the shoot and 2451kg was translocated to roots. In the HF pasture, of the 17313kgha–1 C assimilated, 7168kg was respired, 5298 remained in the shoot and 4432kg was translocated to the roots. This study provides, for the first time, data on the fluxes and quantities of C partitioned in a grazed pasture. Such data are critical for modelling C turnover and for constructing C budgets for grazed pasture ecosystems. Received: 31 July 1996  相似文献   
89.
El-Naggar  A. G.  Hedley  C. B.  Roudier  P.  Horne  D.  Clothier  B. E. 《Precision Agriculture》2021,22(4):1045-1066

Soil water content (θ) measurement is vital for accurate irrigation scheduling. Electromagnetic induction surveys can be used to map spatial variability of θ when other soil properties are uniform. However, depth-specific θ variations, essential for precision irrigation management, have been less investigated using this method. A quasi-2-dimensional inversion model, capable of inverting apparent soil electrical conductivity (ECa) data to calculate estimates of true electrical conductivity (σ) down the entire soil profile, was developed using ECa data collected by a multi-coil Dualem-421S sensor. The optimal relationships between σ and volumetric water content (θv) were established using all coil arrays of the Dualem-421S, a damping factor of 0.04, an initial model of 35 mSm?1, and with ten iterations (R2?=?0.70, bias?=?0.00 cm3cm?3, RMSE?=?0.04 cm3cm?3). These relationships were then used to derive soil profile images of these properties, and as expected, θv and σ follow similar trends down the soil profile. The derived soil profile images for θv have potential use for irrigation scheduling to two ECa-derived soil management zones under a variable rate irrigation system at this case study site. They reflect the intrinsic soil differences that occur between texture, texture transitions and drainage characteristics. The method can also be used to guide placement of soil moisture sensors for in-season monitoring of spatio-temporal variations of θv. This soil imaging method showed good potential for predicting 2D depth profiles of soil texture, moisture and drainage characteristics, and supporting soil, plant and irrigation management.

  相似文献   
90.
Six lop rabbits were presented with clinical signs of otitis media or externa. The presence of disease was confirmed by computerized tomography examination, with two rabbits suffering from bilateral disease. The rabbits were anaesthetized and underwent surgery of the affected bulla. Rabbits with bilateral disease had a minimum of 2 weeks between procedures. A single vertical incision was made over the base of the vertical canal, which was bluntly dissected free from surrounding tissue. The ventral portion of the vertical canal was removed and a lateral bulla osteotomy was performed. The mucosa at the base of the dorsal vertical canal was apposed and the aural cartilage sutured to form a blind‐ending pouch open at the pretragic incisure. Histopathological samples taken from the dorsal margin of the vertical canal yielded subtle and non‐specific changes in the six samples submitted. All rabbits were discharged within 48 hours of surgery. The cosmetic outcome was excellent with animals retaining visually normal aural anatomy. The partial ear canal ablation/lateral bulla osteotomy procedure is quick and has a good cosmetic result when performed in rabbits.  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