全文获取类型
收费全文 | 1337篇 |
免费 | 73篇 |
国内免费 | 74篇 |
专业分类
林业 | 42篇 |
农学 | 66篇 |
基础科学 | 40篇 |
395篇 | |
综合类 | 446篇 |
农作物 | 73篇 |
水产渔业 | 98篇 |
畜牧兽医 | 276篇 |
园艺 | 20篇 |
植物保护 | 28篇 |
出版年
2024年 | 7篇 |
2023年 | 34篇 |
2022年 | 35篇 |
2021年 | 53篇 |
2020年 | 38篇 |
2019年 | 44篇 |
2018年 | 44篇 |
2017年 | 59篇 |
2016年 | 59篇 |
2015年 | 44篇 |
2014年 | 48篇 |
2013年 | 104篇 |
2012年 | 136篇 |
2011年 | 72篇 |
2010年 | 56篇 |
2009年 | 86篇 |
2008年 | 53篇 |
2007年 | 69篇 |
2006年 | 49篇 |
2005年 | 49篇 |
2004年 | 27篇 |
2003年 | 38篇 |
2002年 | 39篇 |
2001年 | 25篇 |
2000年 | 26篇 |
1999年 | 16篇 |
1998年 | 21篇 |
1997年 | 17篇 |
1996年 | 15篇 |
1995年 | 24篇 |
1994年 | 18篇 |
1993年 | 13篇 |
1992年 | 14篇 |
1991年 | 10篇 |
1990年 | 9篇 |
1989年 | 8篇 |
1988年 | 5篇 |
1987年 | 9篇 |
1986年 | 5篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1980年 | 1篇 |
1975年 | 1篇 |
排序方式: 共有1484条查询结果,搜索用时 296 毫秒
1.
N. Fenner D. J. Dowrick M. A. Lock C. R. Rafarel & C. Freeman 《Soil Use and Management》2006,22(3):267-273
The temperature dependence of chemical reaction rates and microbial metabolism mean that temperature is a key factor regulating soil trace gas emissions and hydrochemistry. Here we evaluated a novel approach for studying the thermal response of soils, by examining the effects of temperature on gas emissions and hydrochemistry in (a) peat and (b) soil from a Sitka spruce plantation. A thermal gradient was applied along an aluminium bar, allowing soil to be incubated contemporaneously from 2 to 18 °C. The approach demonstrated clear differences in the biogeochemical responses of the two soil types to warming. The peat showed no significant emission of CH4 at temperatures below 6 °C, while above 6 °C, a marked increase in the rate of release was apparent up to 15 °C (Q10 = 2.5) with emissions being similar between 15 and 18 °C. Conversely, CH4 emissions from the forest soil did not respond to warming. Nitrate availability in the peat decreased by 90% between 2 and 18 °C (P < 0.01), whereas concentrations in the forest soil did not respond. Sulphate availability in the peat decreased significantly with warming (60%, P < 0.01), while the forest soil showed the opposite response (a 30% increase, P < 0.01). Conventionally, thermal responses are studied by incubating individual soil samples at different temperatures, involving lengthy preparation and facilities to incubate samples at different temperatures simultaneously. Data collected on a given thermal response is usually limited and thus interpolated or extrapolated. The thermal gradient method overcomes these problems, is simple and flexible, and can be adapted for a wide range of sample types (not confined to soil). Such apparatus may prove useful in the optimization of management practices to mitigate the effects of climate change, as thermal responses will differ depending on land use and soil type. 相似文献
2.
探讨了汽车无级变速器控制的关键技术之一———汽车起步离合器的控制技术,并针对汽车起步时条件的多样性以及驾驶员意图的不确定性,研究了适用于复杂条件的模糊控制算法.在离合器的滑磨阶段,把动态跟踪发动机转速的油压作为控制目标,离合器结合的控制便转化为一闭环控制.结果表明汽车能在任意工况下平顺快速起步.因此,本研究为控制CVT(Continuously Variable Transmission)起步离合器这种时变、非线性系统提出了一种简单可行的解决方法. 相似文献
3.
[目的]为了解烟草试验站土壤中微量元素的空间变异性,进而为大田试验布局提供依据。[方法]用地统计学方法,分析彭水试验站的耕层土壤养分的空间变异特征。[结果]该站内土壤有效铁、有效铜、有效钙的含量丰富;有效锌和有效锰为中等含量水平,有效硫和有效镁含量从低至高均有分布。各中微量元素间具有显著的相关性。所有元素均可用指数模型和球状模型进行较好拟合;有效锌、有效镁和有效硫由于受结构性因素影响而呈现强空间自相关性;其余元素的空间自相关性由结构性因素和随机因素共同作用而呈现中等空间相关性。Kriging插值显示,东部及东北部较高,中部和西南部区域含量较低;有效铁、有效钙和有效硫的空间分布的规律性差。[结论]该试验站土壤中微量元素含量存在较大的空间变异性。在安排大田试验时,要充分认识这种变异性,合理布局大田试验。 相似文献
4.
Erin L Deters Allison J VanDerWal Katherine R VanValin Stephanie L Hansen 《Journal of animal science》2021,99(6)
To assess the relative bioavailability of bis-glycinate bound Cu, 90 Angus-cross steers (265 ± 21 kg) were blocked by body weight (BW) to pens with GrowSafe bunks and randomly assigned to dietary treatments (14 to 18 steers/treatment): 0 mg supplemental Cu/kg dry matter (DM; CON), 5 or 10 mg supplemental Cu/kg DM as Cu sulfate (CS5; CS10) or bis-glycinate bound Cu (GLY5; GLY10). Steers received a high antagonist growing diet (analyzed 4.9 mg Cu/kg DM, 0.48% S, and 5.3 mg Mo/kg DM). Steers were weighed at the beginning (days 1 and 0) and end (days 125 and 126) of the trial to determine average daily gain (ADG) and gain:feed (G:F). Blood was collected from all steers on days 0, 28, 56, 84, and 126. Liver samples were collected on days −3 or −2 and day 123 or 124. Data were analyzed using ProcMixed of SAS (experimental unit = steer; fixed effect = treatment and block). Plasma Cu was analyzed as repeated measures (repeated effect = day). Plasma and liver Cu concentrations were regressed against total Cu intake using ProcGLM to calculate relative bioavailability of GLY. Final BW and overall ADG were greatest for CS5 and CS10 and least for CON and GLY5 (P = 0.01). Overall, DMI was not affected by treatment (P = 0.14), but overall G:F tended to be greatest for CS5, CS10, and GLY5 and least for CON (P = 0.08). Total and supplemental Cu intake was greatest for steers supplemented either source at 10 mg Cu/kg DM and least for CON (P < 0.01). However, total and supplemental Cu intake was greater for CS5 than GLY5 (P < 0.01). Final liver Cu concentrations were greatest for CS10, least for CON, CS5, and CS10, and intermediate for GLY10 (P < 0.01). Final plasma Cu was greatest for steers supplemented either source at 10 mg Cu/kg DM (P < 0.01). Relative bioavailability of GLY was 82% compared to CS based on liver Cu (P < 0.01) but did not differ based on plasma Cu (P = 0.60). The lesser bioavailability of GLY relative to CS could be due to a high concentration of dietary antagonists and lower solubility of GLY (68.9% relative to CS) in pH conditions (5.2) similar to the ruminal pH of a beef animal consuming a high concentrate diet. Future studies should examine the effects of bis-glycinate bound Cu fed in blended combination with inorganic Cu sulfate to determine the most effective blend of sources for feedlot cattle experiencing varying amounts of dietary Cu antagonists. 相似文献
5.
Mahmoud Alagawany Shaaban S. Elnesr Mayada R. Farag Ruchi Tiwari Mohd. Iqbal Yatoo Kumaragurubaran Karthik Izabela Michalak Kuldeep Dhama 《The Veterinary quarterly》2021,41(1):1
Nutraceuticals have gained immense importance in poultry science recently considering the nutritional and beneficial health effects of their constituents. Besides providing nutritional requirements to birds, nutraceuticals have beneficial pharmacological effects, for example, they help in establishing normal physiological health status, prevent diseases and thereby improve production performance. Nutraceuticals include amino acids, vitamins, minerals, enzymes, etc. which are important for preventing oxidative stress, regulating the immune response and maintaining normal physiological, biochemical and homeostatic mechanisms. Nutraceuticals help in supplying nutrients in balanced amounts for supporting the optimal growth performance in modern poultry flocks, and as a dietary supplement can reduce the use of antibiotics. The application of antibiotic growth enhancers in poultry leads to the propagation of antibiotic-resistant microbes and drug residues; therefore, they have been restricted in many countries. Thus, there is a demand for natural feed additives that lead to the same growth enhancement without affecting the health. Nutraceuticals substances have an essential role in the development of the animals’ normal physiological functions and in protecting them against infectious diseases. In this review, the uses of amino acids, vitamins and minerals as well as their mode of action in growth promotion and elevation of immune system are discussed. 相似文献
6.
不同养殖模式翘嘴鲌肌肉营养品质比较 总被引:1,自引:0,他引:1
为评价池塘与大湖不同养殖模式翘嘴鲌(Culter alburnus)肌肉营养品质差异,试验分析了商品翘嘴鲌肌肉常规营养成分、氨基酸、脂肪酸、矿物元素含量等营养指标。结果表明:大湖翘嘴鲌肌肉水分含量显著低于池塘翘嘴鲌(P<0.05),粗蛋白质含量高于池塘翘嘴鲌,差异不显著(P>0.05)。池塘与大湖翘嘴鲌肌肉,除了赖氨酸和组氨酸以外(P<0.05),绝大部分氨基酸含量均无显著性差异(P>0.05)。大湖翘嘴鲌肌肉的总氨基酸、必需氨基酸和鲜味氨基酸含量分别较池塘翘嘴鲌高出2.72%、2.87%和2.50%。根据氨基酸评分(AAS)和化学评分(CS)结果,池塘与大湖翘嘴鲌肌肉中第一限制性氨基酸均为蛋氨酸+胱氨酸,赖氨酸相对含量最高,必需氨基酸指数(EAAI)分别为68.81和69.62。池塘翘嘴鲌的棕榈油酸(C16:1)含量显著高于大湖翘嘴鲌(P<0.05),而大湖翘嘴鲌肌肉的单不饱和脂肪酸(MUFAs)、高不饱和脂肪酸(HUFAs)、必需脂肪酸(EFA)、EPA+DHA含量则高于池塘翘嘴鲌(P>0.05)。试验检测了池塘与大湖翘嘴鲌肌肉11种矿物元素,其中镁有显著差异(P<0.05),重金属元素(铬、镉、铅)含量均在限量范围以内。综上所述,大湖养殖翘嘴鲌营养品质优于池塘养殖。 相似文献
7.
8.
褚元娟 《农业装备与车辆工程》2007,(3):33-36
轮胎印迹是轮胎在地面上滑动,摩擦生热,使轮胎橡胶产生塑性变形,并产生橡胶微粒,粘附于地面上形成的。事故现场的轮胎印迹分为三种:胎印、擦印和拖印。其中轮胎拖印是事故分析过程中最有价值的资料之一。它不仅反映了事故前后车辆的运动轨迹、轮胎状态、碰撞点的位置和制动措施,而且还可以根据印迹长度和形态分析车辆碰撞分离时刻的速度。 相似文献
9.
中微量元素对番茄产量和品质影响的整合分析 总被引:3,自引:0,他引:3
基于已有文献数据讨论了施用中微量元素对番茄单果质量、产量和品质影响的程度,以期为科学施用中微量元素肥料提供依据。研究数据来源于中国知网、万方数据和Web of science,以不施用中微量元素为对照,选择反应比作为效应值,通过Meta分析方法综合分析施用中微量元素对番茄产量和品质的作用效果,并分亚组进行讨论分析。整合分析发现,施用中微量元素能够显著提高番茄果实单果质量和产量,提高比率分别为5.8%、10.2%;显著提高了果实中维生素C和番茄红素含量,提高比率分别为26.3%、69.4%;显著降低了果实中硝酸盐含量达6.7%。综上,施用中微量元素能够显著提高番茄果实单果质量、产量和维生素C含量、番茄红素含量等品质指标,叶面喷施效果优于土壤施用,微量元素效果优于中量元素,且中微量元素施用应以平衡施肥为前提。 相似文献
10.
《Soil Science and Plant Nutrition》2013,59(1)
pp. 875–880 The trace-element composition of kernel in pickled Japanese apricot (Prunus mume Sieb. et Zucc.) was determined using an inductively coupled plasma optical emission spectrometer in order to distinguish between Japanese products and Chinese products. Strontium and barium concentrations in the kernels of Chinese products were 10 or more times those of the Japanese ones. When based on 8.0 mg kg?1 of strontium concentration in kernel, 93.2% of sample was distinguished as Japanese products or Chinese ones. Applying principal component analysis using 9 elements (Mn, Zn, Fe, Ni, Ba, Sr, Cu, Co, Cr), the pickled Japanese apricots tend to separate into two countries. Linear discriminant analysis (LDA) using 9 elements allowed a reasonable classification of pickled Japanese apricots according to the country of production. The result of the analysis of K-nearest neighbors (KNN) was better than that of LDA. 相似文献