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   2912篇
  免费   161篇
  国内免费   271篇
林业   255篇
农学   245篇
基础科学   147篇
  978篇
综合类   861篇
农作物   241篇
水产渔业   159篇
畜牧兽医   327篇
园艺   81篇
植物保护   50篇
  2024年   13篇
  2023年   52篇
  2022年   85篇
  2021年   80篇
  2020年   89篇
  2019年   99篇
  2018年   74篇
  2017年   110篇
  2016年   135篇
  2015年   111篇
  2014年   139篇
  2013年   203篇
  2012年   264篇
  2011年   210篇
  2010年   193篇
  2009年   200篇
  2008年   183篇
  2007年   195篇
  2006年   148篇
  2005年   148篇
  2004年   77篇
  2003年   97篇
  2002年   55篇
  2001年   54篇
  2000年   50篇
  1999年   43篇
  1998年   34篇
  1997年   39篇
  1996年   26篇
  1995年   26篇
  1994年   25篇
  1993年   17篇
  1992年   13篇
  1991年   9篇
  1990年   12篇
  1989年   16篇
  1988年   8篇
  1987年   5篇
  1986年   1篇
  1985年   3篇
  1984年   1篇
  1982年   1篇
  1977年   1篇
排序方式: 共有3344条查询结果,搜索用时 203 毫秒
51.
The hypothesis that increase in dietary phytin amplifies phytin binding to protein thereby reducing protein digestion, which is alleviated by phytase, was tested. A 2 × 2 factorial arrangement of dietary treatments was used to investigate the response of growing pigs to supplemental phytase (0, or 1200 units/kg) in low- or high-phytin P diets (2.2 or 3.9 g/kg). Eight crossbred barrows (28–30 kg) were canulated and assigned to crates using a double, 4 × 4 Latin Square design. Pigs were fed each of the 4 diets at 3 times metabolic BW (0.09  BW kg 0.75) for 7 d. Ileal digesta was collected for 12 h on d 6 and d 7 by attaching plastic bags to the cannula. Feed and ileal digesta were analyzed for N, energy and P. Phytase had no effect on apparent ileal digestibility (AID) of N or AAs. The AID of some AAs was higher in the high-phytin diet, which contradicts the hypothesis that higher phytin content would have a negative impact. In contrast, the AID of P was depressed by high dietary phytin (P < 0.01) and increased by phytase (P < 0.01) more so at the higher dietary phytin resulting in a phytin × phytase interaction (P < 0.01).  相似文献   
52.
High moisture corn (HMC) can undergo fermentation during storage that may improve its nutritional value for pigs. Stored HMC used in this study contained (n = 4) 75% dry matter (DM), and, on a DM basis, 63.8% starch, 0.5% mono and disaccharides, 9.9% protein, 19 mM acetic acid, 18 mM lactic acid (LA), 1.25 × 107 cfu g− 1 of LA producing bacteria, 2.88 g kg− 1 total phosphorus (P), and 1.26 g kg− 1 soluble P. The soluble P content was higher than in freshly harvested HMC (0.27 g kg− 1 DM) and indicates P release during storage. Studies were aimed at altering the nutritional value of stored HMC by controlled fermentation or steeping. Changes in LA content were not affected (P > 0.05) by addition of exogenous Lactobacillus and Bacillus bacteria. Steeping of HMC with phytase (Ronozyme) was examined at four inclusion levels (0, 500, 750, 1000 FTU kg− 1) and at either 21 °C or 37 °C. At 37 °C added phytase released virtually all phytate P within 6 h irrespective of the level (levels increased by 1.44 g P kg− 1 DM); at 21 °C, the increase was maximized at 1.2 g P kg− 1 DM after 24 h. The feeding value of HMC for pigs can be altered by steeping with exogenous phytase and does not appear to require microbial inoculants.  相似文献   
53.
内蒙古草原植被是一个极脆弱的生态系统,易受干旱等自然灾害影响。自1999年以来,全区大部分地区连续三年干旱和虫害(蝗灾),造成严重的草原退化和沙化。2002年后,内蒙古天然植被整体上有不同程度好转,草原生态环境有所改善。本文以锡林郭勒1999至2004年MODIS和TM影象数据和地面样点数据为依据,从宏观和微观两方面揭示了锡林郭勒草地植被动态规律和变化趋势,并系统分析了草地植被发生的变化原因、驱动因素和机理,阐明了锡林郭勒盟典型区草地所面临的问题,以期为我区草原植被恢复应采取的对策和措施提供依据。  相似文献   
54.
A non-ambulatory dog with tetraparesis following a pain episode that had evolved over 2 months was submitted for medical treatment and diagnosed with intervertebral disk disease at C3-C4 and dorsal extradural compression at C1-C2 and C3-C4 using myelography and computed tomography. The dog experienced ambulation recovery after 15 days of treatment with only electroacupuncture and Chinese herbal medicine, with marked improvement occurring after only 10 treatments. Six months of follow-up demonstrated that the dog was stable and had no recurrence of symptoms. Therefore, it was concluded that the combination of electroacupuncture and Chinese herbal medicine was responsible for motor rehabilitation.  相似文献   
55.
Exergy, the thermodynamic function that represents the distance of an open system from equilibrium, is proposed as an ecological indicator for summarizing the complex dynamics occurring in a disturbed community during its recovery processes. These complex dynamics can be difficult to capture by using classical indices. In this study, we sampled macrobenthos using the BACI scheme (before versus after; control versus impact) in tidal wetlands of west Chongming Island, China, an area that has been disturbed by ecological engineering measures. Exergy storage is estimated for benthic communities. The control area is proposed as dynamic reference for estimating local exergy storage of the benthic community. Three different methods were used for estimating exergy on the basis of coefficients: (a) taken for taxa groups; (b) estimated from coding genes given for broad taxonomic groups; and (c) estimated from genome size taken as close as possible to the taxonomic level of the species, providing a basis for inferring similarities. We found a decrease of local exergy content in the disturbed area 9 days after the disturbance. Subsequently, the reference exergy of the benthic community increased (i.e. in the surrounding control area) in accordance with the proposed hypothesis regarding the dynamics of exergy storage during a system's development. Estimates of local exergy arrived at using the three methods provided comparable results. Based on this, we then discuss the feasibility of using the more readily available genome size data for estimating exergy. This result may have important implications for broader application of this indicator to biological systems. Moreover, the adjacent control samples seem to represent an appropriate dynamic reference for estimating the local exergy of disturbed communities. We found that exergy was a useful indicator that integrates the processes underlying the recovery of the benthic community after disturbance.  相似文献   
56.
The objective of this study was to evaluate various equine follicle-stimulating hormone (eFSH) treatment protocols and the effect of “follicle coasting” on ovulation and embryo recovery rates in mares. Cycling mares (n = 40) were randomly assigned to one of four groups 7 days after ovulation: (1) 12.5 mg eFSH twice daily until follicles were 35 mm or larger; (2) 12.5 mg eFSH twice daily until follicles were 32 mm or larger; (3) 12.5 mg eFSH twice daily for 3.5 days followed by 12.5 mg eFSH enriched with luteinizing hormone (LH) twice daily until follicles were 35 mm or larger; and (4) 25 mg eFSH once daily until follicles were 32 mm or larger. Mares in groups 1 and 3 were injected with human chorionic gonadotropin (hCG) (2500 IU intravenously) at the end of eFSH treatment, whereas mares in groups 2 and 4 were given hCG approximately 42 and 54 hours, respectively, after the last eFSH treatment (“follicle coasting”). Nonsurgical embryo collection was performed 6.5 to 7.5 days after ovulation. Each mare experienced a nontreated estrous cycle before being reassigned to a second treatment. Ovulation rates for mares in treatment groups 1 to 4 were 3.3 ± 0.4, 4.1 ± 0.4, 3.5 ± 0.4, and 2.8 ± 0.4 (mean ± SEM; P < .05), respectively. One or more embryos were recovered from more than 80% of mares in each treatment group, and embryo recovery rate per flush was similar among treatment groups (1.9 ± 0.3, 2.6 ± 0.3, 1.9 ± 0.3 and 1.9 ± 0.3, respectively; P > .05). The overall embryo recovery rate was 2.1 ± 1.5 embryos per flush. In summary, ovulation rate was higher for mares treated with eFSH (3.4 ± 0.4) compared with non-treated controls (1.1 ± 0.2). Ovulation rate in mares in which hCG was delayed (follicle coasting) was higher (P < .05) when treatments were given twice per day versus once per day. Administration of equine luteinizing hormone (eLH) in conjunction with eFSH did not have an advantage over mares treated only with eFSH.  相似文献   
57.
N、P、K肥配施对杂交甜高梁草产量及效益的影响   总被引:6,自引:0,他引:6  
应用D-最优化设计研究N、P、K肥配施与杂交甜高粱品种甜格雷兹饲草产量及效益的关系,结果表明:N肥对草产量和效益影响最大;在较高N水平下,P、K间交互效应均利于甜格雷兹的增产增收;其最优施肥方案为N150.53kg/hm^2、P20550.82kg/hm^2、K2075.71kg/hm^2,草产量为112.09t/hm^2,获得效益20.902×10^3元/hm^2。  相似文献   
58.
建立用Agilent1100高效液相色谱法(HPLC)检测饲料样品中苏丹红含量的方法。采用C18柱(250mm×4.6mmi.d.,5μm),以乙腈和水的混合溶液为流动相,对比采用不同处理方法后,苏丹红Ⅰ、Ⅱ、Ⅲ和Ⅳ相应的平均回收率。结果表明,在0.1、0.2、0.5、1.0、2.0mg/kg5种添加浓度下,苏丹红Ⅰ、Ⅱ、Ⅲ和Ⅳ相应的回收率均高于90%。说明该方法具有较好的准确性。  相似文献   
59.
本研究旨在采用多重线性回归法测定生长猪内源磷排泄量及高粱和豆粕中磷的真消化率。试验选用6头健康大白×长白阉公猪为消化试验动物,平均体重为(23.6±1.23)kg。采用6×6拉丁方设计,设6个磷水平(0.08%、0.15%、0.23%、0.30%、0.38%、0.53%),以豆粕、葡萄糖、玉米淀粉等为基础,以高粱和豆粕为磷唯一来源,配制半纯合试验饲粮。饲粮中添加0.35%Cr2O3作为外源指示剂。试验分6个试验期,每期8d,其中6d适应期,2d收粪期。饲粮中磷表观消化率受饲粮中磷水平的影响(P<0.05),并随饲粮中磷水平的提高由14.88%增大至34.88%。在以g/kgDMI为计量单位条件下,生长猪粪磷的排出量与饲粮磷的摄入量呈线性关系(P=0.0001)。结果表明,多重线性回归法可用于生长猪内源磷排泄量和植物性饲料磷真消化率的测定;以豆粕-高粱型模型饲粮测定出生长猪内源磷排泄量为0.2940g/kgDMI,高粱中磷的真消化率为56.05%,豆粕中磷的真消化率为39.41%。  相似文献   
60.
Linking Land-use, Water Body Type and Water Quality in Southern New Zealand   总被引:3,自引:0,他引:3  
Land-use and vegetation cover have been linked to the nutrient levels (nitrogen, phosphorus) of surface waters in several countries. However, the links generally relate to streams and rivers, or to specific types of standing water, for example shallow lakes in a geologically defined region. We measured physical variables and nutrient chemistry of 45 water bodies representative of the wide range of lentic wetland environments (swamps, riverine wetlands, estuaries, reservoirs, shallow lakes, deep lakes) in Otago, New Zealand, and related these to catchment variables and land-use in order to assess the potential influence of catchment modification on water quality of these diverse wetlands. Catchment boundaries and land cover were derived from maps using ArcView GIS software. Our predictions that concentrations of nutrients and other components of water quality would correlate positively with the nature and intensity of catchment modification were confirmed in multivariate analyses. Physical and chemical measures were positively related to the extent of modification in the catchment (percentage of the catchment in pasture, planted forest, scrub and urban areas), and negatively related to lack of catchment modification (more of the catchment in bare ground, tussock grassland and indigenous forest). The strong negative correlations between nutrient concentrations, suspended sediment, water colour and the percentage of tussock cover in the catchment imply that increased conversion of the␣native tussock grassland to pastoral farming in␣Otago will increase nutrient concentrations and␣reduce water quality of the diverse lentic ecosystems.  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