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   543篇
  免费   28篇
林业   49篇
农学   16篇
基础科学   6篇
  171篇
综合类   46篇
农作物   39篇
水产渔业   35篇
畜牧兽医   142篇
园艺   18篇
植物保护   49篇
  2023年   6篇
  2022年   5篇
  2021年   19篇
  2020年   16篇
  2019年   16篇
  2018年   19篇
  2017年   16篇
  2016年   16篇
  2015年   13篇
  2014年   19篇
  2013年   26篇
  2012年   45篇
  2011年   50篇
  2010年   21篇
  2009年   24篇
  2008年   29篇
  2007年   34篇
  2006年   29篇
  2005年   27篇
  2004年   16篇
  2003年   16篇
  2002年   17篇
  2001年   5篇
  2000年   3篇
  1999年   7篇
  1998年   4篇
  1997年   2篇
  1996年   4篇
  1995年   3篇
  1994年   3篇
  1993年   5篇
  1990年   2篇
  1988年   3篇
  1986年   4篇
  1985年   6篇
  1984年   5篇
  1982年   2篇
  1981年   2篇
  1972年   2篇
  1966年   2篇
  1963年   3篇
  1959年   1篇
  1956年   2篇
  1955年   2篇
  1954年   1篇
  1936年   2篇
  1934年   1篇
  1932年   1篇
  1931年   1篇
  1925年   1篇
排序方式: 共有571条查询结果,搜索用时 15 毫秒
531.
532.
533.
OBJECTIVE: To determine the effects of short-term IV administration of hydrocortisone or equine growth hormone (eGH) or long-term IM administration of eGH to horses on tissue sensitivity to exogenous insulin. ANIMALS: 5 Standardbreds and 4 Dutch Warmblood horses. PROCEDURE: The euglycemic-hyperinsulinemic clamp technique was used to examine sensitivity of peripheral tissues to exogenous insulin 24 hours after administration of a single dose of hydrocortisone (0.06 mg/kg), eGH (20 microg/kg), or saline (0.9% NaCl) solution and after long-term administration (11 to 15 days) of eGH to horses. The amounts of metabolized glucose (M) and plasma insulin concentration (I) were determined. RESULTS: Values for M and the M-to-I ratio were significantly higher 24 hours after administration of a single dose of hydrocortisone than after single-dose administration of eGH or saline solution. After long-term administration of eGH, basal I concentration was increased and the mean M-to-I ratio was 22% lower, compared with values for horses treated with saline solution. CONCLUSIONS AND CLINICAL RELEVANCE: Increases in M and the M-to-I ratio after a single dose of hydrocortisone imply that short-term hydrocortisone treatment increases glucose use by, and insulin sensitivity of, peripheral tissues. Assuming a single dose of hydrocortisone improves sensitivity of peripheral tissues to insulin, it may be an interesting candidate for use in reducing insulin resistance in peripheral tissues of horses with several disease states. In contrast, long-term administration of eGH decreased tissue sensitivity to exogenous insulin associated with hyperinsulinemia. Therefore, increased concentrations of growth hormone may contribute to insulin resistance in horses with various disease states.  相似文献   
534.
The ability of plant-derived smoke to act as a germination cue in many species has led to widespread interest in this aspect of seed biology. Recently, 3-methyl-2H-furo[2,3-c]pyran-2-one was identified as the main germination cue from smoke. Here, we report on the formation of this compound from reactions of sugars with amino acids. Heating proteins or amino acids with sugars at 180 degrees C for 30 min produces water soluble extracts that promote germination. High-performance liquid chromatography indicated that the active compound(s) derived from these reactions coeluted with the active fraction from a smoke solution. Gas chromatography-mass spectroscopy showed that the active constituent is identical to the germination cue from plant-derived smoke. The results presented in this paper provide evidence for the formation of the major germination cue found in smoke from ubiquitously occurring organic compounds.  相似文献   
535.
ABSTRACT The genus Cercospora is one of the largest genera of hyphomycetes. Cercospora apii sensu lato is the oldest name for a large complex of morphologically indistinguishable Cercospora spp. occurring on a wide host range. There are currently 659 recognized Cercospora spp., and names of another 281 morphologically identical species are included in the synonymy of C. apii sensu lato. Two of the species that belong to the C. apii complex, C. apii and C. beticola, cause Cercospora leaf spot on Apium graveolens (celery) and Beta vulgaris (sugar beet), respectively. In the present study, multilocus sequence data, amplified fragment length polymorphism analysis, and cultural characteristics were used as additional features to characterize morphologically similar Cercospora strains occurring on celery and sugar beet. From the data obtained, it is shown that C. apii and C. beticola, although morphologically similar and able to cross-infect each others' hosts, are distinct functional species that should be retained as separate entities. Furthermore, a third, as yet undescribed species of Cercospora was detected in celery fields in Korea and Venezuela, suggesting that additional undescribed species also may be found to cause Cercospora leaf spot on celery. A polymerase chain reactionbased diagnostic protocol distinguishes all three Cercospora spp.  相似文献   
536.
Hepatogenous photosensitization in sheep is an important problem in various parts of the world. Most photosensitization diseases are associated with ingestion of plant or fungal toxins. The lily, Narthecium ossifragum, has long been associated with photosensitization in lambs in western Norway (Ender 1955, Flåøyen 1993) and in the northern regions of the British Isles (Ford 1964).  相似文献   
537.
There has been a trend toward intensification of shrimp farming in the U.S. FiCteen simulated farms were used to evaluate economies of scale and to compare three Penaeus vannclmei commercial production strategies: semi-intensive, intensive, and very-intensive. Large economies of scale were associated with each production strategy. Over the range of farm sizes considered, investment cost per hectare decreased approximately 50% and production cost decreased approximately 25%. Farms' returns were measured with Internal Rate of Return (IRR). When investment was greater than $0.75 million, the intensive strategy provided slightly better returns to the investor than semi-intensive or very-intensive strategies. At investment levels less than $0.75 million, the semi-intensive strategy provided the highest IRR.  相似文献   
538.
539.
为研究烯丙孕素、马绒毛膜促性腺激素(eCG)和人绒毛膜促性腺激素(hCG)或促性腺激素释放激素类似物(GnRH-A)D-Phe6-LHRH在促使诊断为未孕的高胎龄母猪同步发情同步排卵以使其重返预定的人工授精方案中的作用,特进行了一项现场研究。试验选用531头在人工授精后第21~35天期间经超声波检查确诊为未孕的母猪,并随机分入3个试验组:⑴试验Ⅰ组(EGⅠ):每头母猪按16mg/d的剂量连续15d口服烯丙孕素以阻止滤泡的生长,停服24h后肌肉注射1000IU马绒毛膜促性腺激素以刺激卵泡生长,78~80h后再肌肉注射500IU人绒毛膜促性腺激素以诱导排卵;⑵试验Ⅱ组(EGⅡ):除将烯丙孕素的剂量由16mg改为20mg和马绒毛膜促性腺激素剂量由1000IU改为800IU以外,其它与试验Ⅰ组相似;⑶试验Ⅲ组(EGⅢ):除用50μg促性腺激素释放激素类似物D-Phe6-LHRH替代500IU人绒毛膜促性腺激素来诱导排卵外,其它均与试验Ⅱ组相似。试验母猪分别在注射eCG/D-Phe6-LHRH后16h和40h各进行一次人工授精,并利用超声波扫描仪检查卵巢情况以确诊激素处理是否获得成功。记录母猪的受孕率、分娩率、总产仔数和活产仔数,并与同步发情首次配种的母猪比较。试验母猪在激素处理前卵巢结构各不相同,烯丙孕素的两种口服剂量都能使80%以上的母猪卵巢卵泡生长得到阻止,不过口服16mg/d的烯丙孕素会提高多囊卵巢变性的发生率。4%~18%的母猪经烯丙孕素处理后其卵巢中仍存在黄体,大多数母猪在2次人工授精之间或第2次人工授精之后排卵(P<0.05)。注射促性腺激素释放激素类似物(GnRH-A)D-Phe6-LHRH的母猪排卵往往早于注射人绒毛膜促性腺激素的母猪。被诊断为未怀孕的母猪受孕率和分娩率比初次交配母猪低25%(P<0.05)。在诊断为未孕的母猪中,注射促性腺激素释放激素类似物(GnRH-A)D-Phe6-LHRH的母猪受孕率较高(P<0.05),但在总产仔数和活产仔数上各试验组间无显著差异。总之,给诊断为未孕的母猪口服20mg烯丙孕素,然后肌注800IU马绒毛膜促性腺激素和50μg促性腺激素释放激素类似物D-Phe6-LHRH是诱使其同步发情和同步排卵的最佳方法,至于母猪口服烯丙孕素的时间是否一个较长的18d间隔还有待进一步的研究。  相似文献   
540.
OBJECTIVE: To compare the effects of administration of 2 volumes of a calcium solution (calcium oxide and calcium gluconate) on plasma ionized calcium concentration (PICaC) and clinical recovery from naturally occurring hypocalcemia (NOHC; milk fever) in lactating dairy cows. ANIMALS: 123 cows with NOHC (PICaC < 0.95 mmol/L [3.81 mg/dL]) and 20 clinically normal control cows. PROCEDURES: Affected cows were treated IV once or repeatedly with 450 (n = 56) or 750 mL (67) of calcium solution (1.65 g of calcium/100 mL) until clinical recovery was achieved. The PICaC was assessed 48 hours after the first treatment or after the treatment that achieved clinical recovery. Biochemical recovery was defined as PICaC > or = 0.95 mmol/L. Plasma from control cows was used for PICaC reference range determination. Plasma samples from both groups were assessed after storage for 20 days at 20 degrees C. RESULTS: The PICaC reference range derived from blood collected in tubes containing lithium heparin was 1.02 to 1.29 mmol/L (4.09 to 5.17 mg/dL). Following storage, plasma samples were suitable for PICaC assessment. All cows treated with > or = 1 volume of 450 and 750 mL of calcium solution recovered clinically; however, 31 of 83 (37%) evaluated cows were not biochemically recovered at 48 hours following treatment. Only cows with PICaC < 0.48 mmol/L (1.92 mg/dL) before the first treatment had to be treated > or = 3 times. CONCLUSIONS AND CLINICAL RELEVANCE: Results did not support the need to increase the administered volume of calcium solution from 450 to 750 mL for treatment of NOHC in dairy cows.  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