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   50053篇
  免费   2741篇
  国内免费   33篇
林业   1718篇
农学   1669篇
基础科学   256篇
  5908篇
综合类   8695篇
农作物   1557篇
水产渔业   2773篇
畜牧兽医   26753篇
园艺   583篇
植物保护   2915篇
  2019年   426篇
  2018年   784篇
  2017年   831篇
  2016年   748篇
  2015年   617篇
  2014年   764篇
  2013年   1757篇
  2012年   1368篇
  2011年   1727篇
  2010年   1175篇
  2009年   1110篇
  2008年   1710篇
  2007年   1618篇
  2006年   1510篇
  2005年   1407篇
  2004年   1419篇
  2003年   1398篇
  2002年   1204篇
  2001年   1923篇
  2000年   1998篇
  1999年   1519篇
  1998年   578篇
  1997年   503篇
  1996年   451篇
  1995年   618篇
  1994年   530篇
  1993年   465篇
  1992年   1165篇
  1991年   1228篇
  1990年   1198篇
  1989年   1107篇
  1988年   1063篇
  1987年   1090篇
  1986年   1052篇
  1985年   976篇
  1984年   812篇
  1983年   697篇
  1982年   425篇
  1979年   681篇
  1978年   548篇
  1977年   484篇
  1976年   442篇
  1975年   476篇
  1974年   559篇
  1973年   541篇
  1972年   547篇
  1971年   463篇
  1970年   465篇
  1969年   488篇
  1967年   412篇
排序方式: 共有10000条查询结果,搜索用时 16 毫秒
21.
22.
A sample of ergot from meadow grass collected in the south-western part of Norway was examined for alkaloids and dosed orally to four lambs about 10 months old. Sclerotia were dosed at a rate of 0.12-0.75 g per kg body weight per day for 4-27 days. The concentration of total alkaloids in the sclerotia was estimated to be about 0.4%. Ergosine, ergocristine, ergotamine and the three corresponding isomers were identified by thin-layer chromatography. Of the total alkaloids, ergosine and ergocristine constituted 35% each, ergotamine 15% and the isomers 5% each. Three lambs dosed outdoors in cold and wet weather all became ill, with anorexia and increased pulse rate. No symptoms nor post mortem lesions were seen in the single lamb kept indoors. On post mortem examination of the three lambs showing symptoms, hyperemic mucosae, haemorrhages, erosions and necroses in the digestive tract, most pronounced in abomasum and cecum, were demonstrated. In two lambs, slight oedema was found distally in some of the limbs and tail, but no sign of gangrene was observed.  相似文献   
23.
Although porcine adenoviruses (PAdV) are present in the swine populations worldwide, they usually do not cause any disease, or the infection is only manifested in a mild diarrhoea or respiratory signs. The importance of adenoviruses, however, is constantly growing as there is a possibility of developing them into viral vector vaccines against more significant swine pathogens. A short summary of the well-established facts of porcine adenoviruses is given and recent developments of the genetic analysis of these viruses are discussed in detail. The possibilities of vector development and examples of vector vaccines already reported in the literature are mentioned.  相似文献   
24.
A flexed dorsoplantar radiographic view of the talocrural joint was a useful additional view to diagnose abnormalities of the lateral trochlear ridge of the talus of 2 dogs. This view outlined the subchondral bone of both trochlear ridges of the talus and the apposing cochlea tibiae of the distal portion of the tibia. The tarsus was flexed at the level of the talocrural joint, and an x-ray beam was centered on the joint. With this additional view, fractures of the lateral trochlear ridge were readily diagnosed. This view would help to demonstrate osteochondral lesions of the lateral trochlear ridge.  相似文献   
25.
The purpose of this review is to outline and interpret the effects of nutrition, physiological state, sex, genetics, and age on composition of the fat-free dry matter of birds, mammals, and fishes in order to test the hypothesis that the protein:ash ratio in the body is constant. Calculations from a broad array of published data revealed that nutritional factors, including mineral, protein and amino acid nutriture, frequency and level of feeding, and energy intake and nonnutritional factors, including physiological state, sex, genetic background, and age of animal, may each alter the ratio of protein:ash in the body. All or part of the response may be associated with changes in accretion of body protein and(or) skeletal development and mineralization imposed by these nutritional and nonnutritional factors. In most cases of amino acid or protein deficiency, the protein:ash ratio was decreased relative to the control because the deposition of protein in muscle and viscera was reduced more than the growth and mineralization of the skeleton. In other cases, the protein:ash ratio was increased relative to a control because the dietary Ca or P intake or absorption limited skeletal mineralization. More knowledge is needed of how skeletal development and protein:ash ratio in the body is regulated, especially under the influence of nutritional, ontogenetic, and physiological variables among different genotypes of mammals, birds, and fishes. Documentation that the protein:ash ratio in the whole body of representative mammals, birds, and fishes is not constant, but is subject to manipulation, has important physiological and economic implications for developing food animals for the efficient production of lean meat.  相似文献   
26.
27.
Ruminally fistulated lambs were used to measure the effect of sodium hydroxide treatment on rate of passage and rate of ruminal fiber digestion. Corncobs were raised to 60% moisture and treated to a final concentration of 0, 2.5, 5.0, 7.5 and 10.0% NaOH on a dry matter basis. The diets fed contained 80% cobs and 20% supplement, giving a complete mixed diet containing 0, 2.0, 4.0, 6.0 and 8.0% NaOH (dry basis). Chromic oxide was used as an external marker, and rumen samples were collected at 3, 6, 12, 24, 48 and 96 hr post-dosing. As level of NaOH treatment increased, rate of passage increased linearly (P less than .05). Mean ruminal retention time decreased from 32.4 hr for the control diet to 20.7 hr for the 8% NaOH diet. When rate of passage was regressed against NaOH level, the slope of the line was .142%/hr per unit NaOH, with r2 = .733. In another trial, sodium concentrations equal to the 4% NaOH diet were obtained by the addition of 7.3% NaCl to the basal diet. Rates of passage for lambs fed the diet containing NaCl were faster (P less than .05) than for lambs fed the control diet but slower (P less than .05) than for lambs on the 4% NaOH diet. These data suggest that both sodium intake and NaOH treatment affect rate of passage and are additive. Nylon bags containing .15 g cotton were used to measure the rate of ruminal fiber digestion in lambs fed the five NaOH-treated diets. Bags were removed from the rumen after 12, 24, 36 and 48 hr, and the loss in weight were used to estimate ruminal digestion. As level of NaOH increased, the rate of ruminal cotton digestion decreased linearly (P less than .05). When rate of cotton digestion was regressed against NaOH level, the slope of the line was -.488%/hr per unit NaOH, with r2 = .934.  相似文献   
28.
29.
OBJECTIVE: To determine the clinical usefulness of ultrasonography for diagnosis of meniscal pathology in dogs. STUDY DESIGN: Double-blind prospective study. ANIMALS: Dogs (n=10) with lameness localized to the stifle. MATERIALS AND METHODS: Sonographic examination of each affected stifle was performed by 1 ultrasonographer unaware of specific historical and clinical data for the dog. Sonographic findings were recorded, but not reviewed until study completion. Arthroscopic examination of the affected stifle was performed within 48 hours of ultrasonography by 1 surgeon unaware of ultrasonographic findings. Arthroscopic findings were recorded, but not reviewed until study completion. Two investigators compared the ultrasonographic and arthroscopic findings at study completion to determine the sensitivity, specificity, positive predictive value, and negative predictive value. RESULTS: Two of the 10 dogs had bilateral stifle evaluation. Twenty-four lateral and medial menisci, with normal and abnormal findings, were examined. The sensitivity and specificity for ultrasonographic diagnosis were 90.0% and 92.9%, respectively; positive and negative predictive values were 90.0% and 92.9%, respectively. CONCLUSIONS: Ultrasonography is highly specific and sensitive for diagnosing bucket handle tears of the medial meniscus in dogs. CLINICAL RELEVANCE: Ultrasonographic evaluation of menisci in dogs is a noninvasive method for accurately and efficiently diagnosing pathology, determining the need for surgical intervention, and providing comprehensive information to clients.  相似文献   
30.
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