全文获取类型
收费全文 | 7793篇 |
免费 | 398篇 |
国内免费 | 1182篇 |
专业分类
林业 | 616篇 |
农学 | 799篇 |
基础科学 | 2265篇 |
1514篇 | |
综合类 | 2259篇 |
农作物 | 239篇 |
水产渔业 | 303篇 |
畜牧兽医 | 966篇 |
园艺 | 185篇 |
植物保护 | 227篇 |
出版年
2024年 | 48篇 |
2023年 | 136篇 |
2022年 | 262篇 |
2021年 | 277篇 |
2020年 | 302篇 |
2019年 | 352篇 |
2018年 | 264篇 |
2017年 | 355篇 |
2016年 | 431篇 |
2015年 | 363篇 |
2014年 | 461篇 |
2013年 | 520篇 |
2012年 | 585篇 |
2011年 | 562篇 |
2010年 | 449篇 |
2009年 | 435篇 |
2008年 | 353篇 |
2007年 | 423篇 |
2006年 | 395篇 |
2005年 | 317篇 |
2004年 | 247篇 |
2003年 | 231篇 |
2002年 | 190篇 |
2001年 | 190篇 |
2000年 | 174篇 |
1999年 | 142篇 |
1998年 | 131篇 |
1997年 | 114篇 |
1996年 | 117篇 |
1995年 | 82篇 |
1994年 | 81篇 |
1993年 | 73篇 |
1992年 | 79篇 |
1991年 | 59篇 |
1990年 | 49篇 |
1989年 | 33篇 |
1988年 | 40篇 |
1987年 | 26篇 |
1986年 | 12篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1979年 | 2篇 |
1955年 | 1篇 |
排序方式: 共有9373条查询结果,搜索用时 15 毫秒
901.
本文探讨研究了家畜能量代谢过程中产热量的模式及影响因素。详细探讨了畜体产热量(HP)和食入代谢能(MEI)的关系模式,提出是以MEm为转折点的分段线性模式:当MEI≤MEm时,Hp/W ̄0.75=a+bMEI/W ̄0.75,Km=l-b,NEm=a;当MEI≥MEm时,Hp/W ̄=a'+b'MEI/W ̄0.75,Kp=l-b',MEm=a'/(l-b')。同时,对能量代谢的重要参数NEm、MEm、Km和Kn的求法做了改进。 相似文献
902.
Many authors suggest the need to define ‘sustainable development’in operational terms. This paper looks at the problems ofattempting to ask whether peasant farming systems are sustainable.Any attempt at sustainability assessment needs to consider issuesrelated to the selected indicators or performance criteria, spatialscale or boundaries, and temporal scale. While there is certainlya need for more rigorous analysis of sustainability issues, thereis limited outlook for an approach based on indicators. Even if themany purely technical problems associated with specific indicatorscan be surmounted, will accurate bio-physical data advance ourknowledge about sustainability? Peasant systems arepolitically-guided management systems, whose boundaries are the state,not the field or the farm. Given the dynamic nature of peasant farmingsystems, where do we draw the line in assessing sustainability?Attempts at sustainability assessment 100 years ago or even 20–30years ago would have been completely superseded by events. We drawattention to the system as a whole, to a web of interconnections,causes and effects – of varying significance over both time andspace. 相似文献
903.
抑制杨木APMP纸浆返黄技术的研究(Ⅱ)——助剂协同处理的研究 总被引:3,自引:1,他引:3
为了降低处理成本并达到较为满意的白度稳定效果 ,该文主要研究了几种助剂协同处理对黑杨APMP纸浆干 湿热和光返黄的抑制作用效果 .采用了几种较优的黑杨APMP纸浆光稳定剂 (紫外吸收剂ZW 1,ZW 2及抗氧剂抗坏血酸、次亚磷酸钠 )组合处理纸浆 .研究发现 ,抗坏血酸和其它助剂共同处理纸浆同它单独作用情形一致 ,随其用量增加对纸浆诱导返黄促进作用增大 ;用特定的助剂组合共同处理纸浆 ,可以在降低助剂总用量的前提下大幅度的改善黑杨APMP纸浆的光稳定性 ,与它们单独处理效果相比存在着协同作用效果 .ZW 1 抗坏血酸 次亚磷酸钠是该研究中最优的助剂结合 相似文献
904.
畜禽废水胁迫对芦苇叶绿素含量及抗氧化酶系统的影响 总被引:11,自引:0,他引:11
采用自然条件模拟实验,以两种浓度畜禽废水(高浓度畜禽废水原水和经一倍稀释的中浓度畜禽废水)胁迫湿地植物芦苇,研究其对芦苇叶绿素含量及其抗氧化酶系统的影响。结果表明,高浓度畜禽废水较中浓度畜禽废水对芦苇的影响更大。两种浓度畜禽废水胁迫均促使芦苇叶绿素含量下降、叶绿素a/b值上升、CAT活性下降、POD活性急速下降后逐渐上升,到处理末期时与对照基本相同、SOD活性明显上升。说明畜禽废水胁迫对芦苇的影响较小,芦苇在畜禽废水胁迫下具有较强的抗逆性和耐受性.如果将其运用于我国集约化畜禽养殖业废水的处理,将极大地缓解我国畜禽养殖业发展与环境污染之间的矛盾。 相似文献
905.
高校校办产业建立现代企业制度的思考 总被引:4,自引:0,他引:4
建立现代企业制度是高校校办产业改革的方向和自身发展的客观需要。本就高校校办产业按照现代企业制度原理实施“一校两制”、事企分开、明晰产权、转化机制等问题作了初浅探讨,并对设立相应的管理机构、清产核资、资产评估、界定产权以及改制、改组等工作提出具体操作思路。 相似文献
906.
热处理对板栗采后生理及贮藏效果的影响 总被引:6,自引:0,他引:6
将‘青扎’板栗果实进行50℃,60min热处理,研究板栗果实的贮藏效果以及贮藏过程中的生理变化,与未经热处理的栗果对照,结果表明:热处理能抑制栗果贮藏过程中的呼吸强度,POD活性,CAT活性,降低了淀粉酶活性,可溶性糖和还原糖含量,显著抑制板栗中真菌发生和蔓延,降低了果实的失重率,但Vc含量有所降低。 相似文献
907.
针对靖咸输油管道输油生产中存在的泵出口严重节流现象以及生产运行中对提高输量的要求,分析对比了各种增输措施,对不同的离心泵分别采取了抽级和换电机的方法,达到了增输降耗的目的,获得了显著的经济效益. 相似文献
908.
40 ℃持续热应激对肉鸡股动脉压的影响 总被引:1,自引:3,他引:1
研究40 ℃持续热应激对肉鸡动脉压的影响,探讨肉鸡热应激发生发展规律.将60只25日龄商品代雄性AA肉鸡随机分为对照组(21~24 ℃,相对湿度(50±5)%)和试验组((40±0.5)℃,相对湿度(70±5)%),分组后自由采食和饮水,对肉鸡的股动脉压进行动态检测.结果显示热暴露使肉鸡平均股动脉压((16.66±0.47),(10.81±1.59) kPa)、股动脉收缩压((18.61±0.48),(12.00±1.49) kPa)和股动脉舒张压((15.68±0.47),(10.21±1.66)kPa)极显著下降(P<0.01);热暴露10 h时使股动脉收缩压最大变化速率((51.36±20.27),(32.72±4.85) kPa·s-1)显著下降(P<0.05);热暴露后2和5 h使股动脉舒张压最大变化速率((54.97±25.91),(31.88±5.49)和(53.49±22.94),(31.92±5.56) kPa·s-1)显著下降(P<0.05);热暴露后10 h股动脉舒张压最大变化速率((58.65±18.49),(18.09±4.66) kPa·s-1)极显著下降(P<0.01).平均股动脉压与呼吸频率、体温极显著负相关(r=-0.623,r=-0.642;P<0.01).结果表明,40 ℃持续热暴露,使肉鸡的股动脉压极显著下降;股动脉压短时间内极显著下降可能为肉鸡死亡的主要原因之一. 相似文献
909.
Qingru Zhang Yaodi Ni Hongbin Guo Chunguang Wang 《Frontiers of Agriculture in China》2007,1(4):478-480
In order to investigate the effects of a Chinese herbal formula Heat-stress-releasing on the antioxidant function in dairy
cows, ten dairy cows were randomly divided into the control group and the experimental group, with five cows in each group.
All the cows were fed with a basal diet. The animals in the experimental group were given with 220 g of herbs per day in addition
to the basal diet. The trial was conducted for 14 days. Blood samples were taken from the vena cava at day 0, day 7, and day
15, respectively. The antioxidant statuses were examined. The results are as follows. (1) Heat-Stress-releasing formula can
significantly increase the milk yield of dairy cows under heat stress. Compared with the control group, the milk yield of
the herb-treated group increased by 14.01% (P<0.05), 14.32% (P<0.05) and 15.01% (P<0.05) in prophase, metaphase and anaphase of the test, respectively. (2) Heat-Stress-releasing formula can increase significantly
the antioxidant status of the heat stressed dairy cows. Compared with the control group, the superoxide dismutase (SOD) activity
increased by 45.93% (P<0.01) at day 7 and by 54.40% (P<0.01) at day 15. The Glutathioneperoxidase (GSH-PX) activity of the test group increased by 17.99% (P<0.05) at day 7 and 25.98% (P<0.01) at day 15. The total antioxidant capacity (T-AOC) of the test group increased by 43.64% (P<0.01) at day 7 and 46.35% (P<0.01) at day 15. The malondaldehyd (MDA) content of test group declined by 23.88% (P<0.01) at day 7 and 25.32% (P<0.01) at day 15. 相似文献
910.
In the absence of access to formal credit, informal contracts with independent investors give the small ranchers of the Lower Amazon an acceptable means through which to surmount the high investment hurdle of starting a cattle herd. These contracts – called sociedades – allow small ranchers to raise an outside investor's cattle in return for a portion of the offspring and are commonplace in the cattle production systems of the Amazon. But, notwithstanding a vast literature on cattle production in the Amazon, informal contracts have been largely overlooked. This paper presents the results of a field survey and financial analysis for informal contracts on small ranches in the Lower Amazon. In the results, we suggest that informal contracts are an important means of cattle herd start-up and herd production for small ranchers. Internal rates of returns in cattle production under these contracts are in the range of –7 to –12% for the small rancher and 12% for the investor. The net present value of the contract to the small rancher ranges from –R$1219 to –R$8599 and from R$1681 to R$8845 for the investor, for a 10-year period, depending on herd size. Financial returns contracts are sensitive to the contract design – e.g., to who pays health costs – and to beef prices. The small ranchers have a negative IRR, lose money, and bear all the risk of loss, yet persist in using this form of herd development. We surmise that this is due to the non-financial benefits of cattle ownership and the lack of access to formal credit structures. In conclusion, although outside the formal economy and apparently financially unrewarding, these contracts are an important mechanism by which the small ranchers on the Amazon Floodplains create cattle herds. 相似文献