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   9004篇
  免费   599篇
  国内免费   713篇
林业   743篇
农学   665篇
基础科学   445篇
  859篇
综合类   3661篇
农作物   271篇
水产渔业   530篇
畜牧兽医   2559篇
园艺   278篇
植物保护   305篇
  2024年   48篇
  2023年   171篇
  2022年   321篇
  2021年   389篇
  2020年   297篇
  2019年   462篇
  2018年   209篇
  2017年   383篇
  2016年   443篇
  2015年   357篇
  2014年   459篇
  2013年   513篇
  2012年   688篇
  2011年   666篇
  2010年   530篇
  2009年   510篇
  2008年   499篇
  2007年   571篇
  2006年   495篇
  2005年   406篇
  2004年   276篇
  2003年   219篇
  2002年   179篇
  2001年   159篇
  2000年   139篇
  1999年   105篇
  1998年   96篇
  1997年   95篇
  1996年   84篇
  1995年   84篇
  1994年   79篇
  1993年   65篇
  1992年   57篇
  1991年   69篇
  1990年   50篇
  1989年   57篇
  1988年   25篇
  1987年   16篇
  1986年   10篇
  1985年   4篇
  1984年   2篇
  1983年   3篇
  1982年   2篇
  1981年   2篇
  1980年   4篇
  1979年   2篇
  1976年   2篇
  1975年   2篇
  1974年   4篇
  1956年   6篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
951.
AIM: To determine the effect of increasing molybdenum (Mo) intakes on serum and liver copper (Cu) concentrations and growth rates of grazing red deer (Cervus elaphus).

METHODS: Molybdenum- and Cu-amended fertilisers were applied to six 1.1-ha paddocks in a 3 × 2 design. Three levels of Mo were applied on two paddocks at each level in mid April (designated Day 1); levels were: none (control), 0.5 (medium) and 1.0 (high) kg Mo/ha as sodium molybdate. In late May (Day 39), two levels of Cu (none and 3.0 kg Cu/ha, as copper sulphate) were applied to each of the three levels of Mo-treated paddocks. Pasture Mo, Cu and sulphur (S) concentrations were measured at about fortnightly intervals. In late June (Day 74), ten 6-month-old red deer hinds were placed on the six experimental pastures, and serum and liver Cu concentrations were monitored at about monthly intervals for 102 days. The hinds were weighed on four occasions during the trial.

RESULTS: Mean pasture Mo concentrations on Day 56 were 2, 4.6 and 11.3 mg/kg dry matter (DM) for the untreated control, medium and high Mo-treated pastures, respectively. Pasture Cu concentration was 95 mg/kg DM on Day 59, 53 mg/kg DM on Day 90, and 9 mg/kg DM by Day 153. Mean S concentration in pasture was 3.3 (range 3.03–3.45) g/kg DM. Copper application to pasture had no significant effect on serum and liver Cu concentrations in deer so data were pooled within Mo treatment. Mean initial (Day 74) serum Cu concentration was 9.2 µmol/L. In the deer grazing the control Mo pasture, this increased to 10.3 µmol/L on Day 112, before decreasing to 6.4 µmol/L on Day 176. In deer grazing the medium and high Mo-treated pastures, mean serum Cu concentrations were 3.8 and 3.9 µmol/L, respectively, on Day 112, and 2.5 and 3.3 µmol/L, respectively, on Day 176. Mean initial (Day 74) liver Cu concentration was 131 µmol/kg fresh tissue. In the deer grazing the control Mo pasture, this declined to 120 and 52 µmol/kg on Days 112 and Day 176, respectively. In deer grazing the medium and high Motreated pastures, liver Cu concentrations decreased to 55 and 52 µmol/kg fresh tissue, respectively, on Day 112, and 21 and 20 µmol/kg fresh tissue, respectively, on Day 176. Mean serum and liver Cu concentrations were not significantly different between deer grazing the medium and high Mo-treated pastures, and were lower (serum p=0.003, liver p<0.001) in those groups than in deer grazing the untreated control pastures. No clinical signs of Cu deficiency associated with lameness were observed. Deer grazing pastures that had Mo concentrations >10 mg/kg DM had lower (p=0.002) growth rates (100 vs 130 g/day) than those on pastures containing <2.4 mg Mo/kg DM.

CONCLUSION: Increasing pasture Mo concentrations from 2 mg/kg DM to ≥4.6 mg/kg DM significantly reduced serum and liver Cu concentrations in grazing deer. Reduced growth rate was observed at pasture Mo concentrations >10 mg/kg DM.  相似文献   
952.
A small tumour was found at the periphery of the lung during routine necropsy of a 13-year-old Romney ewe. The tumour consisted of vacuolated, cuboidal cells which lined alveolar spaces without destroying them. Electron microscopy revealed numerous microvilli on the free surface of the cells and irregularly shaped osmiophilic inclusions within the vacuoles. No viral particles were found and the tumour was diagnosed as a well differentiated Type II pneumocyte adenocarcinoma of unknown aetiology.  相似文献   
953.
CASE HISTORY: A 14-year-old neutered male Bearded Collie was presented with a history of recurrent, intermittent urinary incontinence of 7 years duration.

CLINICAL FINDINGS: A large, firm, non-painful mass was found in the mid-abdominal region on palpation. Ultrasonography of the mass revealed a compartmentalised structure with mixed echogenicity, and which did not appear to be associated with any of the abdominal organs. Ultrasound-guided fine needle aspirates contained several clusters of epithelial cells with cytological features of hepatocytes. At exploratory laparotomy, the mass was found in the gastrosplenic ligament within the greater omentum.

PATHOLOGICAL FINDINGS AND DIAGNOSIS: Histopathologically, the mass consisted of sheets of hepatocytes, but without the characteristic hepatic architecture. The cells showed moderate variation in nuclear size and were sometimes binucle- ate. A diagnosis of hepatocellular carcinoma (HCC) in the mesentery was made.

CLINICAL RELEVANCE: The presence of ectopic hepatic tissue has been reported rarely in man and cats, but not in the dog. Neoplastic transformation of ectopic hepatic tissue is seen in man. This is the first report of the presentation, clinical findings and treatment of a dog with ectopic HCC.  相似文献   
954.
955.
将模糊控制引入到径向基函数(RBF)神经网络中,采用模糊控制与RBF神经网络相结合的方式建立预测模型,对河北顺平地区种植的桃树进行病虫害发生预测.仿真结果表明,该预测模型相对误差小,并解决了人工神经网络缺乏处理不确定性和模糊信息能力的缺点,预测预报及时、准确.  相似文献   
956.
Skp2被称为S期激酶相关蛋白,属于F-box蛋白家族成员,在蛋白泛素化降解过程中可作为SCFSkp2复合物的重要成分起识别底物蛋白的作用,主要通过降解细胞周期调节蛋白介导细胞周期调控和细胞增殖.为更全面地了解Skp2的研究进展及开展其在动物科学与医学领域的研究,从Skp2基因的结构、功能、与人类肿瘤致病机理、转录调控和启动子方面的关系进行了综述.  相似文献   
957.
为了解柞蚕丝素解酒保肝功能食品的解酒防醉效果,进行了小鼠防醉和解酒试验,观察柞蚕丝素解酒保肝功能食品对小鼠醉酒时间和睡眠时间的影响。结果表明:柞蚕丝素解酒保肝功能食品可显著延长小鼠对酒精的耐受时间,缩短醉酒的持续时间。  相似文献   
958.
孙炳双  杨肖肖  杨维 《安徽农业科学》2013,(7):3063-3066,3131
应用模糊评判理论,以2010和2011年双台子河监测数据为研究对象,选择DO、CODCr、NH3-N、TP、TN、BOD5、氯化物7个评价因子,建立模糊评价模型,确定模糊关系矩阵,选择熵权赋权法求各评价因子权重,以最大隶属原理为评价类别,提出一种基于熵权赋权的方法,利用模糊变换对双台子河水质进行评价与分类。评价结果表明,从权重分析,主要污染因子为BOD5、TN、CODCr、TP,次要因子依次为NH3-N、DO、氯化物;由模糊向量大小分析,水质类别为Ⅳ、Ⅴ类;由模糊向量平均值分析,污染状况下游>中游>上游;并将评价结果与综合标识指数法对比,结果显示两种评价结果近乎一致,但模糊评判使评价结果更直观反映河流水质状况,评价结果更加合理准确。  相似文献   
959.
以海河教育园区一期规划为背景,对园区的生态功能区做了大致的划分:生态休闲区、生态文化区、生态经济区。在对各功能区培育模式进行说明的基础上,对各功能区形成的模式和建设的方向做了认真的思考,对形成模式发展中的生态问题和遇到的困难进行了探讨。  相似文献   
960.
基于3S技术,采用我国陆地生态系统的服务价值测算方法,利用1990、2000与2011年卫星遥感图,分析了焉耆盆地土地利用变化对生态系统服务价值与结构的影响。研究结果显示:①焉耆盆地在1990~2011年间土地利用变化很明显;②生态服务价值从1990年的952 881万元增加到2011年的1 012 946万元,生态服务价值增加了6.30%,达60 065万元。生态服务总价值的增加主要是由水体和耕地总面积的增加而导致的;③各生态服务功能对区域总生态服务价值贡献率由高到低依次为废物处理、水源涵养、气候调节、娱乐休闲、生物多样性保护、土壤形成、食物生产、气体调节和原材料。研究区生态服务价值对生态服务功能价值指数是缺乏弹性的,研究结果是可靠的。  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