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   3709篇
  免费   193篇
  国内免费   370篇
林业   413篇
农学   651篇
基础科学   258篇
  608篇
综合类   1055篇
农作物   212篇
水产渔业   125篇
畜牧兽医   447篇
园艺   105篇
植物保护   398篇
  2024年   12篇
  2023年   51篇
  2022年   163篇
  2021年   197篇
  2020年   169篇
  2019年   141篇
  2018年   115篇
  2017年   167篇
  2016年   143篇
  2015年   201篇
  2014年   159篇
  2013年   207篇
  2012年   188篇
  2011年   239篇
  2010年   186篇
  2009年   209篇
  2008年   189篇
  2007年   212篇
  2006年   203篇
  2005年   160篇
  2004年   86篇
  2003年   72篇
  2002年   59篇
  2001年   70篇
  2000年   80篇
  1999年   87篇
  1998年   69篇
  1997年   62篇
  1996年   66篇
  1995年   58篇
  1994年   37篇
  1993年   38篇
  1992年   35篇
  1991年   33篇
  1990年   31篇
  1989年   24篇
  1988年   23篇
  1987年   13篇
  1986年   4篇
  1985年   5篇
  1984年   1篇
  1983年   2篇
  1982年   2篇
  1981年   1篇
  1980年   1篇
  1979年   2篇
排序方式: 共有4272条查询结果,搜索用时 156 毫秒
71.
提出了基于非线性势函数的一种几何分类方法,设计了利用电势理论来改造决策面判别函数和利用非线性势函数对手写数字进行分类的算法。以该算法为核心构造的分类器可改善针对不同人写字习惯的分类效果。用VC 实现了该算法,结果表明该算法能减小分类时的错误率,对手写体数字的识别有较好的效果,识别率能达到97%。  相似文献   
72.
A growth experiment was conducted to investigate the effects of replacement of fish meal (FM) by meat and bone meal (MBM) in diets on the growth and body composition of large yellow croaker (Pseudosciaena crocea). Six isonitrogenous (43% crude protein) and isoenergetic (20 kJ g− 1) diets replacing 0, 15, 30, 45, 60 and 75% FM protein by MBM protein were formulated. Each diet was randomly allocated to triplicate groups of fish in sea floating cages (1.0 × 1.0 × 1.5 m), and each cage was stocked with 180 fish (initial average weight of 1.88 ± 0.02 g). Fish were fed twice daily (05:00 and 17:30) to apparent satiation for 8 weeks. The water temperature ranged from 26.5 to 32.5 °C, salinity from 32 to 36‰, and dissolved oxygen content was approximately 7 mg l− 1 during the experimental period. Survival decreased with increasing dietary MBM and the survival in the fish fed the diet with 75% protein from MBM was significantly lower than other groups (P < 0.05). There were no significant differences in specific growth rate (SGR) among the fish fed the diets with 0 (the control group), 15, 30 and 45% protein from MBM. However, SGR in the fish fed the diets with 60 and 75% protein from MBM were significantly lower than other groups (P < 0.05). No significant differences in feeding rate were observed among dietary treatments. The digestibility experiment showed that the apparent digestibility coefficients (ADC) of dry matter, protein, lipid and energy of MBM were significantly lower compared with those of FM (P < 0.05). Essential amino acid index was found to be correlated positively with SGR in the present study, suggesting that essential amino acid balance was important. Body composition analysis showed that the carcass protein and essential amino acids were not significantly affected by dietary MBM. The lipid and n-3 highly unsaturated fatty acid (n-3 HUFA) in fish muscle, however, significantly decreased with increasing dietary MBM. These results showed that 45% of FM protein could be replaced by MBM protein in diets of large yellow croaker without significantly reducing growth. It was suggested that the reduced growth with higher MBM was due to lower digestibility and imbalance of essential amino acids.  相似文献   
73.
The study assessed the effects of surgically implanted dummy ultrasonic transmitters on physiological response of bighead carp Hypophthalmichthys nobilis in April 2011. Before the surgery, 15 blood samples were extracted randomly from 195 bighead carp samples, and then the rest of the fish were divided into three groups: (1) control group, handing but no tagging, (2) sham group, surgical procedure without implantation of transmitter and (3) surgery group, surgical implantation of transmitters. In 3 h, 24 h, 7 days and 14 days after surgery, 15 fish were extracted randomly from the three groups, respectively, for sampling. Then the plasma samples were analyzed, and physiological measures of stress response (cortisol, glucose), tissue damage [alanine aminotransferase (ALT) and aspartate aminotransferase (AST)] and nutritional status [total protein, globulin, albumin, triglyceride, cholesterol, and alkaline phosphatase (ALP)] were compared. The result showed that there was no significant difference between sham and surgery groups in 3 h, 24 h, 7 days and 14 days after surgery. When compared to the control group, there were significant increases in concentrations of plasma cortisol, glucose, ALT, AST, total protein and globulin of sham and surgery groups in 3 h after surgery. After 24 h, the levels of plasma cortisol, ALT, AST, total protein, globulin and ALP were elevated in both sham and surgery groups, whereas the levels of plasma glucose had declined to normal level and plasma albumin, cholesterol and triglyceride were significantly decreased in both sham and surgery groups. After 7 days, the levels of plasma glucose, albumin and cholesterol continued to decline, while the level of plasma ALT, globulin and ALP had declined but still remained higher for sham and surgery groups than control group; however, the plasma total protein level had returned to normal. After 14 days, there was no significant difference between the three groups. The above results showed that surgical implantation of ultrasonic transmitters had indeed caused significantly negative effects on the physiological response of bighead carp. However, all the negative influences on stress response, tissue damage and nutritional status had disappeared in 14 days after surgery, and the presence of transmitters had no significant negative impact. Therefore, the bighead carp should be temporary cultured at least 14 days for the recovery of physiological response and then released into the free water for the ultrasonic telemetry research.  相似文献   
74.
牙鲆人工育苗试验   总被引:5,自引:1,他引:5  
许文军 《海洋渔业》2003,25(1):21-23,30
2000年12月20日至2001年2月15日在日本静冈栽培中心进行牙鲆人工育苗试验。4.3万粒受精卵经50多h后,共计孵出仔鱼2.9万尾。在水温17-18℃,pH8.0-8.2,海水比重1.021的条件下,经55d培育,共育出平均全长25mm的鱼苗1.6万尾,育苗成活率的55%。  相似文献   
75.
This study was designed to examine the effects of glycinin on growth, digestive ability, immune responses, antioxidant capacity and gene expression levels of golden crucian carp. Golden crucian carp were fed diets containing glycinin at 0, 30, 60, 90 and 120 g/kg, respectively, for 8 weeks. Body weight, weight gain percentage, specific growth rate and feed efficiency ratio were negatively related to the content of glycinin in diet. Activities of protease, acid phosphatase, alkaline phosphatase, lysozyme in hepatopancreas, and activities of catalase, glutathione peroxidase, superoxide dismutase, and total antioxidant capacity in the proximal intestine, mid intestine, distal intestine and hepatopancreas were negatively related to the content of glycinin in diet, whereas malondialdehyde in proximal intestine, mid intestine, distal intestine and hepatopancreas increased directly with the content of glycinin in diet. Furthermore, the relative expressions of TNF‐α and IL‐1β in proximal intestine, mid intestine and distal intestine increased directly with the content of glycinin in diet, whereas the relative expressions of TNF‐α and IL‐1β in hepatopancreas were negatively related to the content of glycinin in diets. The relative expressions of IL‐10 in proximal intestine, mid intestine, distal intestine and hepatopancreas all were negatively related to the content of glycinin in diets. In conclusion, reductions in growth, immunity and antioxidant capacity, intestine inflammation with dysfunction of digestive system occurred in golden crucian carp that fed a diet containing glycinin at 30 g/kg or higher after 8 weeks.  相似文献   
76.
用实时荧光定量PCR技术检测了高密度精养池塘中日本囊对虾体内及浮游动物白斑综合症病毒携带量的动态变化,检测结果显示,两口池塘的日本囊对虾苗种均携带白斑综合症病毒,达2.43×10~5~9.42×10~5 IU/mg;浮游动物也携带微量白斑综合症病毒,达6.78×10~2~8.02×10~2 IU/mg。在整个养殖过程中,日本囊对虾多个组织均携带白斑综合症病毒,平均病毒量为鳃肌肉肝胰腺胃;各组织病毒携带量的变化趋势不一致,肌肉、肝胰腺、胃等呈现明显的先降后升趋势,最低降至1.78×10~3 IU/mg,鳃白斑综合症病毒携带量的下降幅度较小,最低为7.29×10~4 IU/mg。浮游动物的病毒量在白斑综合症爆发前波动不明显,发病时急剧升高。综合认为,苗种携带是白斑综合症病毒的主要来源;当养殖进入中后期,底质变差、水温降至病毒适宜复制的温度时,易爆发白斑综合症。  相似文献   
77.
为了解流溪河光倒刺鲃国家级水产种质资源保护区鱼类群落结构,分析不同河段、不同季节鱼类群落多样性的变化,于2017年12月-2018年10月对保护区上、中、下游的鱼类开展了每个季度一次的调查。调查结果显示,共采集鱼类57种,隶属于5目14科50属,其中鲤形目43种、鲈形目6种、鲇形目5种,合鳃鱼目2种,鳉形目1种;计算57种鱼类的相对重要性指数,结果显示,尼罗罗非鱼、越南■及鲤鱼为2017-2018年度优势种;Shannon-Wiener多样性指数、Margalef丰富度指数和Pielou均匀度指数计算结果显示,下游鱼类群落的多样性及均匀度最高,夏季鱼类群落的多样性、丰富度及均匀度最高;各监测点之间的种类相似性系数显示,保护区上中游、上下游之间鱼类种类组成为中等不相似,中下游种类组成为中等相似,春夏、春冬、夏冬及夏秋季节鱼类种类组成为中等不相似,春秋、秋冬季节为中等相似;ABC分析结果显示,春、夏及冬季鱼类群落处于稳定状态,秋季处于中度干扰状态。  相似文献   
78.
将3种不同规格的三疣梭子蟹[平均体质量分别为(101.26±5.36)g、(62.37±4.10)g和(24.37±2.10)g]各10只饲养于2.6m×1.8m×1m的室内水泥池单元格内,水深40cm,重复3组。每日17:00投喂鲜活的带鱼块,通过摄像机观察不同规格三疣梭子蟹的竞食行为,每组重复3次。结果表明:(1)不同规格三疣梭子蟹发现、抱持食物所需的时间相近,无显著差异(P0.05);(2)个体间竞食激烈,(101.26±5.36)g组蟹抢食的比率最高,抢夺(62.37±4.10)g组或(24.37±2.10)g组蟹所持有的食物;(101.26±5.36)g组蟹抢食的次数显著多于(62.37±4.10)g组或(24.37±2.10)g组蟹的抢食次数(P0.05),(24.37±2.10)g组蟹抢食的次数最少;(24.37±2.10)g组蟹被(101.26±5.36)g、(62.37±4.10)g、(24.37±2.10)g组蟹抢食的次数及总次数最高,显著多于(101.26±5.36)g组蟹(P0.05),(62.37±4.10)g组蟹次之,(101.26±5.36)g组蟹最少,后两组蟹无显著差异(P0.05),表现出明显的"大欺小"现象;(3)(101.26±5.36)g组蟹的食物占有率(45%)显著高于(24.37±2.10)g组蟹(近20%)(P0.05),而(62.37±4.10)g组蟹占有的食物数量介于(101.26±5.36)g组和(24.37±2.10)g组之间,(101.26±5.36)g组蟹啃食完食物所用的时间仅约为(24.37±2.10)g组蟹的1/5倍。  相似文献   
79.
 ‘北红’葡萄新品种由‘玫瑰香’与‘山葡萄’杂交育成。浆果在北京地区9月底成熟。果粒圆形,蓝黑色,果粒质量1.57g,果穗质量160.0 g,浆果可溶性固形物含量23.8 %~ 27.0 %,含酸量0.89~1.26 %。早果性及丰产性强。抗寒、抗病能力特强。酿成的酒深宝石红色,酒体平衡,酒质上等。  相似文献   
80.
上世纪九十年代中期洛宁发展的苹果园大多果园采用了3×5米中密栽培,有部分矮化果园采用了3×4m,矮化树短枝型品种多为2×4m或2×3m,亩栽44—100株极为普遍。但是在生产上多数还沿有过去乔化稀植的传统整形与修剪方法,造成部分树生长过旺,条子太多,养分浪费较大,树冠交叉郁闭,通风透光不好,上强下弱,果品质量差,落叶病及虫害难以防治,致密植园既不能早产也不能丰产。我们对密植园在修剪中存在的问题进行了调研,探索出了一套解决问题的办法,在实践上取得了良好效果。  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