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   232054篇
  免费   12731篇
  国内免费   148篇
林业   9389篇
农学   7427篇
基础科学   1537篇
  28625篇
综合类   38123篇
农作物   8793篇
水产渔业   11786篇
畜牧兽医   120208篇
园艺   3082篇
植物保护   15963篇
  2019年   2096篇
  2018年   3241篇
  2017年   3557篇
  2016年   3346篇
  2015年   2869篇
  2014年   3555篇
  2013年   9174篇
  2012年   6462篇
  2011年   7980篇
  2010年   5269篇
  2009年   5265篇
  2008年   7869篇
  2007年   7535篇
  2006年   7203篇
  2005年   6606篇
  2004年   6499篇
  2003年   6539篇
  2002年   6172篇
  2001年   7442篇
  2000年   7209篇
  1999年   5845篇
  1998年   2376篇
  1997年   2413篇
  1996年   2213篇
  1995年   2676篇
  1994年   2395篇
  1993年   2358篇
  1992年   4772篇
  1991年   4998篇
  1990年   5066篇
  1989年   5077篇
  1988年   4651篇
  1987年   4716篇
  1986年   4730篇
  1985年   4553篇
  1984年   3798篇
  1983年   3376篇
  1982年   2341篇
  1981年   2163篇
  1979年   3373篇
  1978年   2762篇
  1977年   2388篇
  1976年   2222篇
  1975年   2361篇
  1974年   2983篇
  1973年   3049篇
  1972年   2943篇
  1971年   2589篇
  1970年   2574篇
  1969年   2410篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
241.
Abstract. Usual residue-management options are to remove the residue, use it as mulch with or without undercutting or to incorporate it into the soil. While the role of surface mulch in evaporation has been widely studied, the information on the effect on evaporation of mulch with undercutting or residue incorporated into soil, particularly in relation to soil type and evaporativity (Eo) is lacking. We studied the effect of wheat straw used in various ways on the course of evaporation loss from soil columns with three soils at Ludhiana, India and one soil at Bushland, Texas, USA, under two Eo's Energy-limited evaporation rates under mulch (Eom) followed the soil-specific relation Eom/Eo= a e(bRes+cEo), where Res is residue rate t/ha and a, b and c are constants; Eo, is expressed in mm/d. In an effort to model the total evaporation (CE) during the energy-limited stage ‘U’ was obtained from appropriate CE versus time curves and (CE-U) was regressed over (t - ti)0.5 to obtain the slope ‘α’ (Ritchie 1972) for the soil-limited evaporation stage. The observed ‘U’ was independent of mulch rate and Eo but was strongly affected by soil type, Values of ‘α’ decreased with increase in mulch rate and decrease in Eo and coarseness of soil. The otherwise short lived benefit of evaporation reduction with mulch per se, which peaked after a few days was maintained when residue was mixed with soil at the stage when evaporation reduction reached a maximum; this benefit continued for several weeks. Cumulative evaporation values computed from ‘U’ and ‘α’ agreed closely with the observed values under straw mulch for loamy sand and clay loam soils and for ‘undercut’ and ‘residue mixed’ treatments on all soils regardless of Eo, and for all situations under small Eo. However, for sandy loam and silt loam soils under Eo of 10 mm/d, the modified square root of the time function of Jalota et al. (1988) gave a better fit.  相似文献   
242.
The determination of differentiation and transplantation antigens will be of growing importance in immune diagnosis for individual animals as well as for breeding purposes in populations. Differentiation antigens characterize subsets of cell populations and indicate their functional capacity while transplantation antigens represent markers of individuals of a species. Occurrence and significance of these antigenic systems are briefly reviewed.  相似文献   
243.
The objective of this study was to assess the effect of dietary copper and/or vitamin E supplementation on composition and oxidation of M. longissimus in Iberian pigs and to compare it with meat samples from pigs produced extensively and fed with acorn and grass. Grass had the highest α-tocopherol content (> 150 mg/kg DM), while acorns had the highest copper concentration (78.1 mg/kg DM). Dietary treatment did not affect copper composition in muscle and no interactions were detected between copper and α-tocopherol. The α-tocopherol content in muscle from pigs fed diets supplemented with α-tocopheryl acetate (100 mg/kg feed) was 1.5 times greater (p < 0.0001) than those from pigs receiving a basel diet. M. longisssimus dorsi from pigs fed extensively had a higher concentration of α-tocopherol than those fed in confinement with the basel level of α-tocopheryl acetate, but lower values than pigs fed supplemented levels. Total n -3 fatty acids (p < 0.02) and hematin (p < 0.0001) concentrations were significantly higher in muscle from pigs fed extensively than when fed in confinement. Muscle homogenates from Iberian pigs fed in extensive conditions showed significantly (p < 0.02) higher susceptibility to oxidation than those from pigs fed mixed diets. Dietary α-tocopheryl acetate supplementation (100 mg/kg feed) significantly (p < 0.05) reduced lipid oxidation of muscle, while dietary copper concentration did not modify susceptibility to lipid oxidation.  相似文献   
244.
The reagent carrier Reflotron-Creatinine was tested for its suitability for whole blood and plasma samples of dogs and cats. It showed a very good correlation between the results of Reflotron-Creatinine and the reference method. Using plasma the results of both methods were extremely close. Using whole blood with a hematocrit of up to 50% good results could be achieved. If the hematocrit exceeded 50% large deviations could result.  相似文献   
245.
Respiratory diagnostic instruments previously available only to referral centers should be converted into miniature, portable, wireless medical devices for use in practice. Internet and wireless connections of these instruments should expedite the process of seeking second opinions and in general information sharing. We are moving into a world of greater objectivity that should free us from speculation in some areas, yet open new avenues for research and innovation in others.  相似文献   
246.
Abstract— In a blinded trial to determine the efficacy of potentiated sulfa drugs in the treatment of canine pyoderma, 50 dogs with superficial pyoderma were randomly assigned to receive either once or twice daily trimethoprim-sulfadiazine (30mg.kg-1) or once daily sulfadimethoxine-ormetoprim (55mg.kg-1 on the first day, then 27.5 mg.kg-1 thereafter). Dogs were evaluated prior to antibiotic therapy. Clinical efficacy of the antibiotic was assessed after 3 weeks of antibiotic therapy and, if needed, after 6 weeks of antibiotic therapy. Forty-five dogs completed the study; 43 dogs were used in the statistical analysis. The percentage of dogs that were cured of their pyoderma with once daily trimethoprim-sulfadiazine was 38.5% by 3 weeks and 75.9% by 6 weeks. With twice daily trimethoprim-sulfadiazine, 57.1 and 78.6% of dogs were cured by 3 and 6 weeks, respectively. With once daily sulfadimethoxine-ormetoprim, 75.0 and 100% of dogs were cured by 3 and 6 weeks, respectively. This difference was not statistically significant, possibly due to the low sample size. Few adverse effects were noted. Résumé— Dans une étude en double aveugle pour apprécier l'efficacité des sulfamides potentialisés dans le traitement des pyodermites canines, cinquante chiens présentant une pyodermite superficielle ont reçu, au hasard, soit deux fois ou une fois par jour du triméthoprime-sulfadiazine (30 mg.kg-1) ou une fois par jour du sulfadiméthoxine-ormétoprime (55mg.kg-1 le premier jour, puis 27.5 mg.kg-1). Les chiens ont étéévalués avant le traitement. L'efficacité clinique de l'antibiotique a été appréciée après 3 semaines de traitement et, si besion est, après 6 semaines. Le pourcentage de chiens guéris avec une prise quotidienne de triméthoprime-sulfadiazine était de 33.5%à 3 semaines et 75.9%à 6 semaines. Avec des prises biquotidiennes de triméthoprime-sulfadiazine 57.1% et 78.6% des chiens étaint guéris à 3 et 6 semaines. Avec une prise quotidienne de sulfadiméthoxine-ormétoprime, 75.0% et 100.0% des chiens étaient guéris à 3 et 6 semaines. Cette différence n'était pais statistiquement significative, probablement à cause de la taille trop faible de l'échantilon. Très peu d'effects secondaires ont été notés. Zusammenfassung— Für eine Blindstudie zur Bestimmung der Wirksamkeit von potenzierten Sulfonamidpräparaten bei der Behandlung kaniner Pyrodermie wurden 50 Hunde mit oberflächlicher Pyodermie zufällig ausgewählt, um entweder einmal oder zweimal täglich Trimethoprim-Sulfadiazin (30mg/kg) oder einmal täglich Sulfadimethoxin-Ormetoprim zu erhalten (55 mg/kg am ersten Tag, danach 27,5 mg/kg). Die Hunde wurden vor der antibotischen Therapie untersucht. Die klinische Wirksamkeit der Antibiose wurde nach 3 Wochen antibiotischer Therapie und, wenn nötig, nach 6 Wochen antibiotischer Therapie ausgewertet. 45 Hunde durchliefen die Studie vollständig; 43 Hunde fanden in die statistische Analyse Eingang. Der Prozentsatz der Hunde, deren Pyrodermie durch einmal tägliches Trimethoprim-Sulfadiazin geheilt wurde, lag bei 38,5% nach 3 Wochen und bei 75,9% nach 6 Wochen. Bei zweimaliger Trimethoprim-Sulfadiazin-Verabreichung wurden 57,1% und 78,6% der Hunde nach 3 bzw. 6 Wochen geheilt. Mit einer einmal täglichen Verabreichung von. Sulfadimethoxin-Ormetoprim wurden 75,0% und 100% der Hunde nach 3 bzw. 6 Wochen geheilt. Dieser Unterschied war statistisch nicht signifikant, möglicherweise aufgrund der geringen Fallzahlen. Es wurden nur wenige Nebenwirkungen festgestellt. Resumen A cincuenta perros que presentaban pioderma superficial se les evaluó para la eficacia de antibioticos potenciados de tipo sulfamídico en el tratamiento del pioderma canino, por medio de la administración de una a dos veces diarias de trimetoprim-sulfadiazina (30 mg/kg), o sulfadimetoxinaormetoprim una vez al día (55 mg/kg en al primer día, y 27.5 mg/kg después de la primera dosis). Los animales se examinaron antes del tratamiento con antibióticos. La eficacia clínica del tratamiento se evaluó a las tres o seis semanas en los casos necesarios. Cuarenta y cinco de los perros completaron el estudio; 43 se utilizaron en analisis estadísticos. El porcentaje de perros curados con el tratamiento diario a base de trimetoprim-sulfadiazina fué del 38.5% a las 3 semanas, y 75.9% a las 6 semanas. De los animales tratados dos veces al día con trimetoprim-sulfadiazina, el porcentaje de cura fue del 57.1% y 78.6%, a las 3 y 6 semanas respectivamente. Con el tratamiento a base de sulfadimetoxina-ormetoprim una vez al día, 75.0% y 100% de los perros fueron curados a las tres y seis semanas. La diferencia no se encontró estadísticamente significativa, posiblemente debido al reducido número de perros evaluados. Se observaron posas reacciones adversas a la medicación.  相似文献   
247.
Field experiments were conducted at Tamil Nadu Rice Research Institute, Aduthurai, India, during the wet seasons of 1992 and 1993 to study the effect of full and partial substitution of fertiliser N with green manure N (Sesbania rostrata) on nitrogen uptake, yield attributes and yield of rice. The experiment consisted of eight treatments with two levels of N (100 and 200 kg ha−1) and three sources of N application viz., fertilizer, integrated (1:1 fertilizer and green manure N) and green manure N compared to the recommended practice (150 kg fertilizer plus 6.25 t ha−1 (72 kg N) green manure) and a no N control. Nitrogen application markedly increased the N uptake. Combined use of the two N sources at 200 and 222 kg N ha−1 and of single fertilizer N at 200 kg N ha−1 recorded the maximum N uptake, increased the yield attributes such as number of panicles per unit area, weight per panicle, number of total and filled grains per panicle and test weight. At 200 kg N ha−1 full substitution of N by green manure reduced the grain yield but only partial substitution of N by green manure resulted in almost similar yield as single fertilizer N. Thus 200 kg N ha−1 applied in equal proportions of fertilizer and green manure N can be recommended for medium duration rice cultivars.  相似文献   
248.
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   
249.
250.
The clinical signs and laboratory changes of brodifacoum (BDF) intoxicated dogs and their response to vitamin K1 treatment were examined. Brodifacoum, a second-generation anticoagulant rodenticide, was fed to four dogs for 3 consecutive days producing a cumulative dose of 1.1 mg BDF/kg body weight. Clinical observations of the animals were made daily throughout the study. Monitored laboratory parameters included: one-stage prothrombin time (OSPT), activated partial thromboplastin time (APTT), activated coagulation time (ACT), complete blood counts, thrombocyte counts, and serum chemistry values. Response to vitamin K1 therapy was evaluated clinically and by laboratory tests. Serum BDF concentrations were monitored. Inappetence and hemorrhagic tendencies were exhibited by day 5 postrodenticide exposure. One-stage prothrombin time, APTT, and ACT were 25% greater than time zero values at 24, 24, and 72 hours postdosing, respectively. All laboratory parameters returned to normal within 48 hours of initiating vitamin K1 therapy (0.83 mg/kg orally, TID for 5 days). Serum brodifacoum concentrations were highest (1065-1215 ng/mL) during the 3 days after BDF dosing and were detectable (3.0-7.5 ng/mL) until day 24 postexposure. A mean BDF elimination half-life of 6 +/- 4 days was observed.  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