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   153460篇
  免费   7822篇
  国内免费   70篇
林业   9191篇
农学   5910篇
基础科学   1004篇
  21087篇
综合类   20191篇
农作物   7149篇
水产渔业   8884篇
畜牧兽医   74246篇
园艺   2750篇
植物保护   10940篇
  2020年   1194篇
  2019年   1432篇
  2018年   4925篇
  2017年   5199篇
  2016年   3471篇
  2015年   2089篇
  2014年   2488篇
  2013年   5757篇
  2012年   5186篇
  2011年   7432篇
  2010年   5783篇
  2009年   5045篇
  2008年   6645篇
  2007年   6857篇
  2006年   4681篇
  2005年   4387篇
  2004年   4231篇
  2003年   4365篇
  2002年   3904篇
  2001年   4779篇
  2000年   4690篇
  1999年   3749篇
  1998年   1474篇
  1997年   1454篇
  1996年   1343篇
  1995年   1600篇
  1994年   1328篇
  1993年   1377篇
  1992年   2790篇
  1991年   2901篇
  1990年   2867篇
  1989年   2912篇
  1988年   2622篇
  1987年   2593篇
  1986年   2540篇
  1985年   2400篇
  1984年   1956篇
  1983年   1716篇
  1982年   1154篇
  1979年   1649篇
  1978年   1284篇
  1977年   1149篇
  1976年   1100篇
  1975年   1164篇
  1974年   1357篇
  1973年   1448篇
  1972年   1379篇
  1971年   1196篇
  1970年   1247篇
  1969年   1148篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
151.
There is only limited information on sheep interferon available. Recent publications have reported on: 1. an interferon (IFN) alpha subtype, which is secreted by the fetal trophectoderm into the lumen of the uterus between the 10th and 21st day of gestation. It was therefore named ovine trophoblast protein (oTP-1), and is responsible for signalling pregnancy to the ewe via high affinity receptors in the endometrium. It is thought that oTP-1 acts by directly influencing prostaglandin metabolism. 2. the role of lentivirus-induced interferon (LV-IFN) in the pathogenesis of Maedi/Visna. The results indicate that LV-IFN limits viral replication and therefore contributes to virus persistence and is also responsible for a chronic inflammatory process. 3. the mitogen- or antigen-dependent induction of ovine interferon gamma (IFN gamma) and its characterization.  相似文献   
152.
153.
Rational perioperative management improves the success rate of abdominal surgery. Important aspects of management are discussed, including principles of fluid therapy, nutrition, intravenous catheterization, antimicrobial prophylaxis, and the use of nonsteroidal anti-inflammatory medication. Current advances in the area of immunotherapy are mentioned.  相似文献   
154.
Management of the lactating sow influences milk production and subsequent reproduction through changes in nutrient intake. The management goal during lactation is to maximize feed intake. Decreasing the effective environmental temperature, increasing the nutrient density of the lactation diet, maintaining fresh adequate supplies of feed and water, and preventing excess weight gain during the prior gestation period will increase nutrient intake during lactation. Effective environmental temperature of the lactating sow can be maintained in the thermoneutral zone by using drip cooling, increased ventilation rates and flooring materials with superior conductive properties. Sow parity, genetics, litter size, and disease level will also influence feed intake. Management practices must account for these factors and, thus, should be tailored to individual farm situations to ensure adequate nutrient intake and prevent aberrations in subsequent reproductive performance.  相似文献   
155.
The merits of integrated aquaculture farming (IAF) as a strategy for bolstering income for the rural poor have been recognized, but the economic efficiency and sustainability are yet to be determined. The effects of one species on another and the economic benefits, net of government support, have not been critically evaluated. The concept of a foodniche for alternative species in an aquatic environment has not been properly tested in an economic setting. Thus, this report reflects the results of applying the technique of mixed integer linear programming to determine the optimum combination of species within the context of a foodniche. The objective also included maximizing revenues, with minimal external input requirements, for a representative small Malaysian wet-rice farm. It was found that the foodniche concept permitted flexibility in stocking rates, and that farmers could actually stock species at the exact rate that was optimal for profit, instead of standard fixed rates. Also, government subsidies for pond construction were essential for IAF success. The net revenues from IAF were low without interest-free loans for pond construction.  相似文献   
156.
157.
Application of new procedures in the sphere of the control of sexual functions requires an extension of present knowledge of postparturient endocrinium or endogenic factors comprised in postparturient physiology of sexual activity. According to recent data, oxytocin, besides its uterotonic and luteolytic activity, acts as an ovarian factor in the local intrafollicular regulation of stereidogenesis and as a modulator of uterine secretion of prostaglandines. Based on present knowledge of oxytocin effects, this study was aimed at investigation of the influence of repeated carbetocin (Depotocin inj. Spofa) administration on the dynamics of changes in thyroxine (T4), triiodothyronine (T3), 17 beta-estradiol (E2), progesterone (P4) concentrations and their mutual correlations from the 36th hour till the 51st day after parturition. Simultaneous study of a possible delayed influence of applied carbetocin on conception of ewes after oestrus evocation on day 51 after lambing was carried out. Nineteen ewes of the Slovak Merino breed, lambed in the first decade of February, were assigned to the experimental (n = 9) and to the control group (n = 10). Experimental ewes were subjected to repeated postparturient carbetocin treatment at the dose 0.07 mg per animal. The first dose was applied i. m. in 24 hours, and the second in 72 hours after parturition. On day 51 oestrus was induced in nine ewes of each group by combined treatment with chlorsuperlutin (Agelin, vaginal pessaries, Spofa) and PMSG (500 I.U./animal). On the day of PMSG application ewes were housed together with rams for the period of the next six days. Samples of blood were taken 24 hours before parturition (-1st day), up to 36 h after parturition and on days 4, 7, 14, 17, 21, 25, 34, 42 and 51 after parturition. Concentrations of T4, T3, E2 and P4 were determined by commercial kits RIA-test-T4; RIA-test-T3; RIA-test-ESTRA and RIA-test-PROG (URVJT Kosice). Animal of the control group showed variations of T4 concentrations (Tab. I, Fig. 1) at the level of original values (59.4 +/- 9.69 nmol.l-1) up to the 21st day with the exception of temporary drop on day 4 and rise on day 7, insignificant compared to the -1st day. T4 concentrations of the control group displayed an intermittent increasing trend with the statistically insignificant peak after 36 h and on day 17, compared to the -1st day. After the 21st day controls revealed a sustained moderate increase while the experimental ewes displayed a decline of its concentrations until the 51st day.(ABSTRACT TRUNCATED AT 400 WORDS)  相似文献   
158.
Bei 72 einzeln gehaltenen Legehennen wurde die Fähigkeit zur selektiven Lysinaufnahme überprüft. 2 unterschiedliche Alleinfutter auf der Basis von Federmehl (A) oder Maiskleber (B), aber mit dem gleichen defizitären Lysingehalt (0,36%), wurden während einer zweiwöchigen Lernphase jeweils an eine Hälfte der Tiere verfuttert. Anschließend wurden beide Vorbehandlungen in je 4 Gruppen geteilt, und eine der Mangelrationen und eine mit Lysin bedarfsgerecht supplementierte Ration (A+ bzw. B+, 0,64%) in folgenden Kombinationen zur Selbstauswahl über 14 Tage angeboten: A/A+, A/B+, B/A+ und B/B+. Über den prozentualen Rationsanteil am Gesamtfutterverzehr wurde die selektierte Lysinaufnahme ermittelt. Mit beiden Mangelrationen wurde eine Anorexie und in Folge eine Leistungsdepression ausgelöst, die jedoch bei B stärker ausgeprägt war. Die lysinangereicherte Ration wurde in fast allen Fällen innerhalb weniger Tage signifikant und stetig zunehmend bevorzugt, was für einen erlernten Lysinappetit bzw. -hunger spricht. Gleichzeitig wurde eine deutlichere Präferenz für die Federmehlration beobachtet, die bei Gruppe A/B+ nach B sogar zu einer Ablehnung der ergänzten Maiskleberration führte. Vertrautheit bzw. Neuartigkeit und sensorische Qualitäten einer Ration spielten neben den physiologischen Effekten eine bedeutende Rolle bei der Selbstauswahl. Die komplexen Wechselbeziehungen zwischen Futter, hedonistischen Komponenten und Erfahrung für die Entwicklung eines spezifischen Hungers wurden diskutiert.  相似文献   
159.
Plasma cortisol and corticosterone levels were measured in macaws, African Grey Parrots, Amazon Parrots and budgerigars using a RIA after preliminary cleaning. As in other avian species, the plasma contains predominantly corticosterone.  相似文献   
160.
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