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   296224篇
  免费   14724篇
  国内免费   9424篇
林业   19768篇
农学   19148篇
基础科学   8505篇
  38102篇
综合类   65319篇
农作物   14076篇
水产渔业   14233篇
畜牧兽医   113900篇
园艺   6824篇
植物保护   20497篇
  2022年   2919篇
  2021年   4425篇
  2020年   4189篇
  2019年   4527篇
  2018年   6713篇
  2017年   7874篇
  2016年   6269篇
  2015年   6161篇
  2014年   6520篇
  2013年   11979篇
  2012年   11470篇
  2011年   14200篇
  2010年   11758篇
  2009年   10525篇
  2008年   12357篇
  2007年   12510篇
  2006年   10628篇
  2005年   9741篇
  2004年   7397篇
  2003年   7382篇
  2002年   6905篇
  2001年   7895篇
  2000年   7775篇
  1999年   6810篇
  1998年   3750篇
  1997年   3532篇
  1996年   3269篇
  1995年   3541篇
  1994年   3157篇
  1993年   3155篇
  1992年   5014篇
  1991年   5047篇
  1990年   4899篇
  1989年   4842篇
  1988年   4342篇
  1987年   4335篇
  1986年   4367篇
  1985年   4101篇
  1984年   3435篇
  1983年   3082篇
  1982年   2194篇
  1979年   3006篇
  1978年   2496篇
  1977年   2139篇
  1974年   2659篇
  1973年   2731篇
  1972年   2684篇
  1971年   2434篇
  1970年   2304篇
  1969年   2109篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
951.
Stomachs of 271 horses and ponies from 2 sources were evaluated for the presence of Gasterophilus intestinalis and G nasalis larvae, through 2 overlapping 12-month periods of bot fly activity in southern Texas. Equids (n = 140) from one source had nearly 96% of their stomachs infected, whereas equids (n = 131) from another source had 44% of their stomachs infected by Gasterophilus spp. Seasonal dynamics of the bot infection indicated the highest average number of bot larvae per infected stomach occurred in the winter and spring. The smallest average number of bots per infected stomach occurred in the fall. Higher percentages of early (2nd instar plus young 3rd instar) larvae of G intestinalis were observed in the fall in equids from both sources. The late (more fully developed older 3rd instar) G intestinalis larvae outnumbered the early larvae in the winter, spring, and summer. Seasonal variation in numbers and development status of the larvae was consistent with the largest period of adult bot fly activity occurring during the fall. The 2 species of bots had different predilection sites of attachment. Gasterophilus intestinalis larvae concentrated in the nonglandular portions of the stomach near the margo plicatus on the cranial (parietal) surface of the stomach and in the most dorsal extent of the saccus cecus. Gasterophilus nasalis larvae attached almost exclusively in the first ampulla of the duodenum. Predilection sites for both Gasterophilus spp occurred in dorsally positioned areas in the alimentary tract favoring increased availability of oxygen.  相似文献   
952.
Two caudolateral approaches to the canine scapulohumeral joint that do not require myotomy, tenotomy, or osteotomy were used to expose the humeral heads of both scapulohumeral joints in 10 clinically normal dogs. One approach (method 1) was used on the left shoulder of 5 dogs and on the right shoulder of the other 5 dogs. The other approach (method 2) was used on the opposite joints. The amount of humeral head articular cartilage that could be exposed with each approach was measured. The amount of articular cartilage exposed with method 1 was significantly greater than that exposed with method 2 (P less than 0.0001). Method 1 also provided for exposure of the caudomedial aspect of the scapulohumeral joint. Method 1 was used unilaterally in 20 dogs with osteochondritis dissecans. This approach provided excellent exposure of the humeral head and osteochondritis dissecans lesion. Of the 16 dogs that were available for follow-up evaluation, 1 developed a seroma after surgery. Other complications were not observed or reported.  相似文献   
953.
954.
955.
Multiple hereditary ocular anomalies in a herd of cattle   总被引:1,自引:0,他引:1  
A Brahman x Santa Gertrudis herd of cows bred to a Hereford bull was evaluated because of a 3-year history of several calves born with congenital blindness. Multiple congenital ocular anomalies in 2 calves included microphthalmos, microcornea, microcoria, heterochromia iridis, microlentia, cataracts, retinal dysplasia, retinal detachment, anterior segment dysgenesis, acorea, and proliferation of the anterior neuroectoderm. On the basis of the lack of environmental factors and persistence of an intermittent problem when breeding to a single bull, a genetic defect was diagnosed as the probable cause. Dominant inheritance with varied expressivity may have best explained the lack of obvious signs in the bull, with emergence of various anterior and posterior segment defects in offspring from unrelated cows.  相似文献   
956.
Three experiments were conducted to test the hypothesis that methionine, isoleucine, valine or nitrogen either singly or in combination are limiting in an 11% crude protein, lysine-tryptophan-threonine-supplemented, corn-soybean meal diet for growing pigs. A 16% crude protein diet was used as a positive control in each experiment and all amino acid additions were made, at a minimum, to equal requirements. Average initial weights were 21.3 kg, 23.0 kg and 19.1 kg in Exp. 1, 2 and 3, respectively. The experiments averaged 4 wk in duration. In Exp. 1 and 2, neither the addition of glutamic acid as a source of nitrogen, nor methionine to the 11% diet resulted in any improvement (P greater than .20) in rate or efficiency of growth. Addition of the combination of isoleucine and valine to the 11% diet resulted in an increase (P less than .05) in both growth rate and feed efficiency to a level not different (P greater than .20) from that of the pigs consuming the positive control diet. The addition of valine to the 11% crude protein diet with supplemental lysine, tryptophan and threonine (Exp. 3) caused an improvement in daily gain (P less than .05) but not feed efficiency (P greater than .10). Isoleucine addition tended to reduce pig performance. The results of these experiments suggest that an 11% crude protein, corn-soybean meal diet fortified with lysine, tryptophan and threonine is not limiting in sulfur amino acids or nitrogen. Valine may be the only limiting amino acid.  相似文献   
957.
Clinical and laboratory findings are presented from two cases of canine IgA multiple myeloma. One of these animals presented with typical clinical and pathological changes including haematological abnormalities and skeletal lesions with secondary invasion of other organs. In contrast, the primary focus of disease in the second dog was the anterior alimentary tract and there were no skeletal changes. In this respect this second case was similar to the condition in man known as alpha chain disease. The two paraproteins differed in electrophoretic mobility.  相似文献   
958.
利用投影寻踪回归技术进行草地产量预报的研究   总被引:3,自引:0,他引:3  
李建龙 《草地学报》1995,3(1):65-74
本文利用1991~1992年在新疆阜康县不同类型草地上观测的牧草产量、环境因子和卫星遥感资料运用投影寻踪回归(PPR)技术,探讨了草地产量预报的原理与方法,建立了产量预报综合模型,其预报精度达到83.5%以上,并克服传统预报方法的一些不足,科学地提示其内在关系,进而对影响产量预报精度和产量形成的因素进行分析与实验验证。因而研究结果表明利用PPR技术基本实现了科学预报草地产量的目的。  相似文献   
959.
Positron emission tomography (PET) is often used to stage and monitor human cancer and has recently been used in a similar fashion in veterinary medicine. The most commonly used radiopharmaceutical is 2‐Deoxy‐2‐[18F]‐Fluoro‐d ‐glucose (18F‐FDG), which is concentrated and trapped within cells that use glucose as their energy substrate. We characterized the normal distribution of 18F‐FDG in 10 healthy Hispaniolan Amazon parrots (Amazona ventralis) by performing whole body PET scans at steady state, 60 min after injection. Significant variability was found in the intestinal activity. Avian species are known to reflux fluid and electrolytes from their cloaca into their colon. To evaluate reflux as the cause of variability in intestinal distribution of 18F‐FDG, dynamic PET scans were performed on the coelomic cavity of six Hispaniolan Amazon parrots from time 0 to 60 min postinjection of radiotracer. Reflux of radioactive material from the cloaca into the colon occurred in all birds to varying degrees and occurred before 60 min. To evaluate the intestinal tract of clinical avian patients, dynamic scans must be performed starting immediately after injection so that increased radioactivity due to metabolism or hypermetabolic lesions such as cancer can be differentiated from increased radioactivity due to reflux of fluid from the cloaca.  相似文献   
960.
牛副结核检疫中的非特异性干扰   总被引:2,自引:0,他引:2  
给犊牛接种各种分枝杆菌,然后以副结核PPD进行变态反应试验,观察各分枝杆菌感染对副结核变态反应的干扰;从各种分枝杆菌感染牛群采取血清进行副结核ELISA检测,以观察各分枝杆菌血清对牛副结核ELISA的干扰。结果:多种分枝杆菌,特别是鸟胞内分枝杆菌、瘰疬分枝杆菌的感染可干扰副结核变态反应;部分结核牛血清干扰副结核ELISA的检测  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