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   237077篇
  免费   14537篇
  国内免费   651篇
林业   23362篇
农学   13992篇
基础科学   2967篇
  41723篇
综合类   16645篇
农作物   16878篇
水产渔业   16578篇
畜牧兽医   90501篇
园艺   6275篇
植物保护   23344篇
  2021年   2436篇
  2020年   2882篇
  2019年   3660篇
  2018年   3945篇
  2017年   4261篇
  2016年   4754篇
  2015年   4329篇
  2014年   5677篇
  2013年   16298篇
  2012年   5981篇
  2011年   7880篇
  2010年   7291篇
  2009年   7829篇
  2008年   7164篇
  2007年   6255篇
  2006年   6907篇
  2005年   6124篇
  2004年   5974篇
  2003年   5713篇
  2002年   5076篇
  2001年   5687篇
  2000年   5309篇
  1999年   5052篇
  1998年   4107篇
  1997年   4017篇
  1996年   3731篇
  1995年   4384篇
  1994年   3772篇
  1993年   3485篇
  1992年   3951篇
  1991年   4246篇
  1990年   3906篇
  1989年   3843篇
  1988年   3424篇
  1987年   3465篇
  1986年   3278篇
  1985年   3632篇
  1984年   3451篇
  1983年   3214篇
  1982年   2729篇
  1981年   2646篇
  1980年   2610篇
  1979年   2936篇
  1978年   2644篇
  1977年   2455篇
  1976年   2327篇
  1975年   2140篇
  1974年   2241篇
  1973年   2194篇
  1972年   1937篇
排序方式: 共有10000条查询结果,搜索用时 453 毫秒
941.
An 8-month-old Holstein heifer was evaluated for right hind limb lameness of 3 weeks' duration. Diagnoses were osteomyelitis and fracture of the distal sesamoid bone, septic arthritis of the distal interphalangeal joint, and osteomyelitis of the third phalanx. After excision of a section of tissue from the plantar aspect of the digit and partial section of the deep digital flexor tendon, the distal sesamoid bone was excised. The third phalanx was curetted, and the surgical wound was lavaged. Twenty months after surgery, the heifer was fully weightbearing on the affected digit. Surgical exploration of the digit should be considered an alternative to claw amputation in cattle that have severe digital infections.  相似文献   
942.
Three oat (Avena saliva L.) populations (i.e., lines of descent), high grain yield (HG), high protein content (HP), and high protein yield per se (HGP), each developed by three cycles of S, recurrent selection, were evaluated for the effect of selection for groat-protein yield upon other agronomic traits. Selections making up the HG line of descent had high protein yield primarily due to high grain yield, and those selected for HP had high protein yield due to both high protein content and high grain yield. Selection in HGP was on the basis of protein yield per se. Selection caused increases in bundle weight, harvest index, vegetative growth rate, and seed number in all lines of descent. Heading date, plant height, and seed weight were unaffected, whereas groat percentage and test weight were decreased in HP and HGP. Heritabilities were high for heading date, plant height, test weight, and seed weight, moderate for harvest index and bundle weight, and low for groat percentage. Genetic variability generally declined from CO to C3 for all traits. Groat-protein yield and amount of protein per groat increased in all lines of descent. In HG, the increase in groat weight was due primarily to increases in the nonprotein fraction, with groat-protein content actually decreasing. In HGP, groat weight increased due to increases in protein and nonprotein fractions, and groat protein content remained constant. In HP, groat weight and amount of nonprotein per groat decreased, thus increasing groat-protein content. Three cycles of recurrent selection resulted in oat lines with groat-protein yields significantly higher than the highest lines from CO. Utilization of index selection may be desirable to obtain populations of greater breeding value.  相似文献   
943.
An eight-year-old beaver lost one of its upper central incisors through unknown causes. As the anterior teeth of this species continually erupt in a similar manner to rodents' teeth, the unopposed lower incisor had over-erupted perforating the palate and entered the nasal cavity. The height of the lower incisor tooth was reduced and a cobaltchromium alloy bridge was constructed which was attached to the remaining upper incisor. The occlusal surface of the bridge was of sufficient width that both incisors could contact it. It was cemented into place with composite resin cement and cross-pinned through the labial surface of the metal into the tooth substance. It is unknown how such treatment will affect the biological processes underlying the continually erupting incisor and the animal is being monitored for any adverse effects.  相似文献   
944.
Scanning electron microscopy of vascular corrosion casts of the optic nerve region in normal and glaucomatous Beagles demonstrated that the blood supply to the laminar optic nerve is derived from short posterior ciliary arteries, cilioretinal arteries, and longitudinal pial vessels. The short posterior ciliary arteries formed a ring of striated pillars around the scleral canal. The central retinal artery was not present in the dog. Differences between the casts in normal and glaucomatous dogs were not detected.  相似文献   
945.
Multiplication of the rye-specific and barley-specific pathovars secalis and translucens, respectively, of Xanthomonas campestris (earlier named Xanthomonas translucens f. sp. cerealis) was determined in two rye and two barley cultivars. Fresh bacterial cultures were suspended in boiled tap water, adjusted to about 3 × 107 cfu ml?1 and infiltrated into the mesophyll of young leaves. In the compatible combinations, a generation time of 1.7–2.1 h was determined. Significantly higher multiplication rates than in incompatible combinations were not observed before 3 or 4 days after inoculation. At this time, the first small water-soaked areas appeared. Bacteria multiplied up to the seventh day after inoculation, reaching a maximum of at least 109 cells per cm2, the same time at which the water-soaked areas had reached their maximum size. Thus, the rate of bacterial multiplication paralleled with the development of the water-soaked areas. In the incompatible combinations, nearly the same generation times were detected during the exponential growth phase. However, water-soaked areas did not appear, and the final bacterial concentration ranged from 1:1000 to 1:100 of that determined in compatible combinations. A distinct early lag phase, characterized by a decrease of the bacterial population down to 1:100 of the starting concentration, was detected in all cases.  相似文献   
946.
947.
精子及其相关精浆中所含的脂肪成分在组成和脂肪酸成分上是非常独特的。长链多不饱和脂肪酸 (即具有 2 0或 2 2个碳原子并含三个以上双键的脂肪酸 )在绝大多数主要组织所含脂肪中的含量非常有限 ,但在精子和精浆中的含量相当高 ,并具有明显的种属特异性。油脂的这种特性现在日益被认为是使受精最终成功所特别需要的。本综述根据最近一的些研究结果撰写而成 ,从精液质量和功能的一系列决定因子的角度论述了使现代种公猪精子中脂肪 /脂肪酸参数最佳化的可能性 ,可见人们对一系列参数所作的一系列重要研究 ,产生了使公猪和母猪的繁殖能力都得到提高的高度有益结果。文章还从主要的营养学特性和化学特性方面对研究结果进行了讨论 ,并与其它家畜进行了比较 ,还讨论了其对猪繁育部门可能的商业应用价值。  相似文献   
948.
An experiment comprising 19 German Landrace sows was established to evaluate the effect of folic acid supplements (10 mg/kg concentrate) on homocysteine and folic acid concentration in plasma and serum, respectively, of highly pregnant sows as compared to an unsupplemented control (basal diet contained 0.62 mg folic acid/kg concentrate). Blood samples were taken between day 75 and 110 of gestation for homocysteine analysis and on day 100 of gestation for folic acid determination. Due to the folic acid supplements serum folic acid concentration increased significantly (104 nmol/l in controls and 140 nmol/l in supplemented sows). In contrast, homocysteine concentration in the plasma was not significantly influenced by folic acid supplements (16.6 mumol/l in controls and 15.2 mumol/l in supplemented sows). Further investigations seem to be necessary to clarify the physiology of homocysteine metabolism in swine.  相似文献   
949.
Samples from the mammary tissue of 14 lactating goats (12 naturally infected and two experimentally infected) were examined for the presence of Mycoplasma agalactiae. A monoclonal antibody (5G12) was applied to formalin‐fixed, paraffin‐wax‐embedded sections and labelled by the avidin–biotin peroxidase complex (ABC) method. Histological examination of tissue sections revealed strong immunoreactivity in all animals included in the study. Mycoplasma agalactiae antigen was mainly detected in the cellular debris at the periphery of purulent exudates present within lactiferous sinuses, and lactiferous and interlobular ducts. In addition, M. agalactiae organisms appeared in the cytoplasm of the epithelium of ducts, and in infiltrating macrophages and neutrophils within the ducts, alveoli, interstitial tissue and regional lymph node sinuses. It is concluded that this monoclonal antibody‐based immunohistochemical technique is an efficient and specific method for the post‐mortem detection of M. agalactiae in cases of clinical mastitis as well as being a useful tool for the study of the route of infection and cellular types involved during mastitis caused by this organism.  相似文献   
950.
关于饲料中胆碱含量化学分析值的资料很多。饲料中的胆碱含量因饲料种类、出产国别、出产年份等的不同而不同。问题还不止于此,据发现,测定饲料中胆碱含量时所用的技术也不能给出充分可靠的结果,因而需要一种新技术。  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