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   2438篇
  免费   140篇
  国内免费   4篇
林业   71篇
农学   61篇
基础科学   14篇
  258篇
综合类   446篇
农作物   69篇
水产渔业   88篇
畜牧兽医   1414篇
园艺   39篇
植物保护   122篇
  2020年   32篇
  2019年   31篇
  2018年   36篇
  2017年   36篇
  2016年   47篇
  2015年   25篇
  2014年   47篇
  2013年   101篇
  2012年   95篇
  2011年   85篇
  2010年   65篇
  2009年   66篇
  2008年   103篇
  2007年   77篇
  2006年   83篇
  2005年   83篇
  2004年   82篇
  2003年   92篇
  2002年   70篇
  2001年   82篇
  2000年   65篇
  1999年   62篇
  1998年   24篇
  1997年   25篇
  1996年   25篇
  1995年   20篇
  1994年   18篇
  1993年   27篇
  1992年   60篇
  1991年   58篇
  1990年   54篇
  1989年   29篇
  1988年   53篇
  1987年   42篇
  1986年   54篇
  1985年   53篇
  1984年   33篇
  1983年   29篇
  1979年   34篇
  1978年   17篇
  1977年   19篇
  1976年   24篇
  1975年   22篇
  1974年   17篇
  1973年   31篇
  1972年   22篇
  1971年   20篇
  1970年   17篇
  1969年   24篇
  1967年   17篇
排序方式: 共有2582条查询结果,搜索用时 671 毫秒
991.
ObjectiveTo compare the effects of tramadol alone, or in combination with dipyrone or meloxicam, on postoperative pain and analgesia requirement after unilateral mastectomy with or without ovariohysterectomy in dogs.Study designProspective, randomized, clinical study.AnimalsTwenty seven bitches undergoing unilateral mastectomy with or without ovariohysterectomy.MethodsAnesthesia was induced with propofol and maintained with isoflurane and a constant rate infusion of morphine. Before the end of surgery, dogs were randomly assigned to receive intravenous tramadol alone (3 mg kg?1, group T), combined with dipyrone (30 mg kg?1, group TD) or meloxicam (0.2 mg kg?1, group TM). Dogs received additional doses of tramadol (groups T and TM) or tramadol with dipyrone (group TD) at 8 and 16 hours after extubation. Postoperative pain was assessed by a blinded observer before anesthesia (baseline) and at 1, 2, 3, 4, 6, 8, 12, 16 and 24 hours after extubation using a visual analog scale (VAS) and a modified Glasgow scale. Rescue analgesia (morphine, 0.5 mg kg?1) was administered if the Glasgow pain score was >3.5.ResultsThere were no significant differences among groups in pain scores evaluated by the VAS or the Glasgow scale. In groups T, TD and TM, pain scores were significantly higher than at baseline for 6, 8 and 2 hours, respectively. Rescue analgesia was administered to 3/9, 2/9 and 1/9 dogs in groups T, TD and TM, respectively (p > 0.05) [Correction added on 15 August 2013, after first online publication: ‘T, TM and TD’ was changed to ‘T, TD and TM’.].Conclusions and clinical relevanceUnder the conditions of this study, tramadol alone or in combination with dypyrone or meloxicam provided effective analgesia for 24 hours in most dogs after unilateral mastectomy with or without ovariohysterectomy. Further evaluation of combination therapies is needed in larger groups of dogs.  相似文献   
992.
Landscape Ecology - We undertook reconstructions of historical ponderosa pine forest structure and fire regimes across an entire landscape to expand understanding of spatial variability in forest...  相似文献   
993.
A summer application of nitrogen was used to induce the production of “ strong ” flowers for comparison with the " normal ” flowers produced on trees of Worcester Pearmain given only a spring application of nitrogen.

The flowers were given various pollination treatments to provide material for an assessment of flower quality. These included the following : auto and alio self-pollination and cross-pollination with diploid apple or pear at anthesis, cross-pollinations and self-pollinations at additional daily intervals subsequent to anthesis.

The stigmas of strong flowers remained receptive for a longer period than those of normal flowers.

After cross-pollination with a diploid apple, fertilization took place in 6-7 days. After self-pollination a typical incompatibility reaction was apparent. Pear pollen tubes showed an intermediate response.

Egg sacs in strong flowers continued to enlarge after those of corresponding normal flowers had ceased to grow. The fertilized ovules and embryo sacs of strong flowers showed a more rapid acceleration in growth rate than those of normal flowers.

Cell division in unfertilized ovules also continued for a longer period in strong flowers and appeared to be correlated with the behaviour of the egg sac.

Ovule longevity provided the most striking difference between the two flower types. The ovules of strong flowers remained capable of fertilization for almost twice as long as those of normal flowers.

Abnormalities of several types were seen in ovules. The most common was the production of secondary egg sacs, a feature of strong flowers which had been self-pollinated. Three types of secondary sac, micropylar, lateral and chalazal are described and illustrated.

The effects of rate of pollen tube growth, ovule longevity and the period of stigma receptivity in limiting the effective pollination period, are discussed. It is suggested that some variations in fruit set may be due to differences in flower quality. The use of summer or early autumn nitrogen is advocated as a method of improving blossom quality in certain varieties.  相似文献   
994.
Summary

Grapevines, cv. Thompson Seedless were treated with the plant growth regulator paclobutrazol in the spring of 1984. The treatments consisted of four soil application rates (including a zero control) of paclobutrazol on two phenological dates (budbreak and bloom). Vegetative growth, as measured by pruning weight, vine yield and fruit maturity characteristics, were determined in 1984 and in the next three growing seasons. There was no significant difference in vine yield between treatments at harvest in 1984; however, the fruit of vines receiving paclobutrazol at budbreak had accumulated less sugar at that time. Paclobutrazol application significantly (P < 0.01 ) reduced pruning weights in 1984 for all vines receiving paclobutrazol. Only the highest rate of paclobutrazol significantly reduced pruning weights in 1985. The number of clusters per vine and yield were significantly less for the treated vines than for the control ones in 1985 and 1986. Pruning weights in 1986 and yield in 1987 showed no significant differences between treatments. The results indicate that a single soil application of paclobutrazol was effective in reducing vegetative growth of Thompson Seedless grapevines for a period of two years if the rate of application was greater than 1.1 kg a. i. ha-1. However, the adverse effect of this growth regulator on vine yield indicate that it would be of little practical use when applied via the soil to this seedless cultivar.  相似文献   
995.

Microbiotic crusts occur extensively in rangeland soils . Developed by filaments of cyanobacteria and algae , and thalli of lichen and moss entanglement of soil particles , they create a physical discontinuity in the surface profile with greater concen trations of clay , silt , and potentially hydrophobic organic matter. These conditions potentially contribute to variability in soil hydrology of arid land and should be considered in the development of hydrologic and erosion models. However, there is limited manipulative research examining the functional relationships between soil and microbiotic crusts . We investigated the influence of cyanobacterial-dominated microbiotic crust on measured hydraulic conductivity (K) in a sandy loam soil at a southeastern Utah site . Using a tension infiltrometer , we determined K under three surface treatments: undisturbed , chemically killed (representing dead microphytes within the crust), and removed (scalped) microbiotic crusts. We applied treatments to spatially interspersed intact surface soils within shrub interspaces . Microbiotic crusts at this site and in this stage of successional development had no discernible influence on K. This finding supports results from research conducted in a variety of soils from sandy to silt dominated with a range of microbiotic development . Because this research was site and time specific , and because the role of microbiotic crusts in the environment continues to be debated , additional research is warranted to deter mine how stage of development of microbiotic crust influences soil hydrology .  相似文献   
996.
997.
998.
Immunodetection of allergens in dark chocolate is complicated by interference from the chocolate components. The objectives of this study were to establish reference materials for detecting multiple allergens in dark chocolate and to determine the accuracy and precision of allergen detection by enzyme-linked immunosorbent assay (ELISA) before and after chocolate processing. Defatted peanut flour, whole egg powder, and spray-dried milk were added to melted chocolate at seven incurred levels and tempered for 4 h. Allergen concentrations were measured using commercial ELISA kits. Tempering decreased the detection of casein and β-lactoglobulin (BLG), but had no significant effect on the detection of peanut and egg. Total coefficients of variation were higher in tempered than untempered chocolate for casein and BLG, but total and analytical CVs were comparable for peanut and egg. These findings indicate that processing has a greater effect on recovery and variability of casein and BLG than peanut and egg detection in a dark chocolate matrix.  相似文献   
999.
Among the major food allergies, peanut, egg, and milk are the most common. The immunochemical detection of food allergens depends on various factors, such as the food matrix and processing method, which can affect allergen conformation and extractability. This study aimed to (1) develop matrix-specific incurred reference materials for allergen testing, (2) determine whether multiple allergens in the same model food can be simultaneously detected, and (3) establish the effect of processing on reference material stability and allergen detection. Defatted peanut flour, whole egg powder, and spray-dried milk were added to cookie dough at seven incurred levels before baking. Allergens were measured using five commercial enzyme-linked immunosorbent assay (ELISA) kits. All kits showed decreased recovery of all allergens after baking. Analytical coefficients of variation for most kits increased with baking time, but decreased with incurred allergen level. Thus, food processing negatively affects the recovery and variability of peanut, egg, and milk detection in a sugar cookie matrix when using immunochemical methods.  相似文献   
1000.
During rice milling, the bran and germ are successively removed from the caryopsis (kernel). Because bran and germ contain large quantities of lipid, the amount of lipid remaining on the kernel surface may be used as a method for the assessment of milling quality. Bulk samples of rice pureline varieties and an experimental hybrid were milled for 0, 10, 20, 30, and 40 s. Scanning electron microscopy (SEM) revealed that brown rice kernels had large contours of linear protuberances and depressions running lengthwise along the kernel surface. The protuberances were abraded successively during milling, but varying amounts of material remained in the depressions. Light microscopy combined with the lipid-specific probes Nile Blue A or Sudan Black B demonstrated that the material in the depressions observed with SEM was lipid. Sections of whole, milled rice kernels, prepared using a modified sectioning technique and stained with Nile Blue A, showed that portions of the embryo remain after milling and that lipid is located on or near the surface of the kernel. Differences in quantity and distribution of residual lipid as milling duration increased were documented photographically to indicate the extent to which the bran and embryo components were removed during milling. This paper provides proof of concept that residual lipid is a robust measure of the degree of milling.  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