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   187961篇
  免费   10492篇
  国内免费   162篇
林业   7941篇
农学   5919篇
基础科学   1193篇
  23221篇
综合类   30842篇
农作物   7223篇
水产渔业   9276篇
畜牧兽医   98682篇
园艺   2409篇
植物保护   11909篇
  2019年   1858篇
  2018年   2805篇
  2017年   3130篇
  2016年   2865篇
  2015年   2474篇
  2014年   3125篇
  2013年   7394篇
  2012年   5552篇
  2011年   6677篇
  2010年   4480篇
  2009年   4559篇
  2008年   6632篇
  2007年   6347篇
  2006年   5977篇
  2005年   5496篇
  2004年   5318篇
  2003年   5437篇
  2002年   4980篇
  2001年   6090篇
  2000年   5997篇
  1999年   4782篇
  1998年   1923篇
  1997年   1897篇
  1996年   1794篇
  1995年   2110篇
  1994年   1771篇
  1993年   1852篇
  1992年   3784篇
  1991年   4023篇
  1990年   3934篇
  1989年   3995篇
  1988年   3605篇
  1987年   3635篇
  1986年   3624篇
  1985年   3456篇
  1984年   2767篇
  1983年   2486篇
  1982年   1688篇
  1979年   2506篇
  1978年   1964篇
  1977年   1702篇
  1976年   1603篇
  1975年   1748篇
  1974年   2135篇
  1973年   2210篇
  1972年   2153篇
  1971年   1971篇
  1970年   2002篇
  1969年   1874篇
  1967年   1635篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
Systemic and pulmonary antibody responses of calves to Pasteurella haemolytica were evaluated by measuring immunoglobulin production in blood for 9 days and in pulmonary lavage fluid for 7 days after intrapulmonary inoculation. Clinical signs, pulmonary lesions, pulmonary and systemic inflammatory response, and amount of antigen in lavage fluid were used to evaluate the response of calves to challenge with P haemolytica. The pulmonary response consisted of production of IgG, IgE, and IgM antibodies to P haemolytica antigens and a 17- to 68-fold increase of cells in lavage fluid 8 hours after inoculation, with a gradual decrease toward normal. Antibodies of the IgM isotype to P haemolytica were demonstrated as early as 8 hours through 7 days after inoculation in 3 of 3 calves. Of the anti-P haemolytica isotypes, IgM was found in the highest concentration. In all of the inoculated calves, IgE was found 1 to 2 days after inoculation, and IgG was found in 2 of 3 inoculated calves from day 1 through 7 after inoculation. Detection of IgG correlated with smaller pulmonary lesions. Immunoglobulin A was not detected in lavage fluid. Serum was evaluated for IgG and IgM antibody response to P haemolytica. Specific IgM was detectable 5 days after inoculation, and IgG was detectable 7 days after inoculation. Pasteurella haemolytica antigens were not detected in serum or plasma. A transient increase in neutrophil count was found 8 hours after inoculation, with return to baseline values by 24 hours after inoculation.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
72.
A technique is described for biopsy of the bovine udder, employing sedation and local anaesthesia. Tissue samples of approximately 5 g were obtained by electrocautery from two quarters of the udder of a cow laterally recumbent. Care was taken to ensure complete haemostasis which was achieved by electrocoagulation and ligation. Postoperative recovery was rapid, and loss of yield was no greater in biopsied glands than in control glands of the same cow. Yield from all quarters returned to preoperative levels within 48 h.  相似文献   
73.
29 samples of commonly used surfaces were tested for their water characteristics (litre weight, water capacity, water binding, water evaporation) and their contribution to airborne fungal spores (dust formation, dust setting). The results are discussed in comparison to the literature with regard to the environment. The results are: 1. Any surface--no matter of what material--eventually causes air pollution with fungal spores and dust. 2. Correct watering prevents air pollution by any surface. 3. Artificial products have no advantage over natural materials in the parameters tested. 4. The question of proper disposal of old surface material has to be clarified before purchase. The results show that a mixture of sand and wood shavings should be recommended as a surface for indoor arenas, especially in regard to environmental protection and proper disposal.  相似文献   
74.
75.
Restriction endonuclease analysis and DNA hybridization revealed five ovine strains of Mycobacterium paratuberculosis from South Africa had identical DNA patterns to an ovine strain from Canada. Genetically this strain type has features in common with the two major groups of M. paratuberculosis.  相似文献   
76.
Abstract. A two year field experiment was carried out in a semiarid Mediterranean area in order to evaluate, the effect on soil erosion of adding different urban organic wastes: a stabilized municipal waste (compost), an unstabilized municipal waste, and an aerobic sewage sludge. All the treatments significantly reduced soil erosion, compared to the control soil. The soil amended with compost was the most effective treatment, reducing soil loss by 94% and runoff by 54%.  相似文献   
77.
Salivary or secreted carbonic anhydrase (CA), which constitutes a new class of CA, designated CA-VI, was isolated. Swine CA-VI purified from swine saliva by inhibitor-affinity chromatography and ion exchange chromatography had a specific activity of 5,468 units/mg. The molecular weight was 250,000, as determined by gel filtration under non-denaturing conditions, and the subunit molecular weight was found to be 37,000 by sodium dodecyl sulfate polyacrylamide gel electrophoresis, indicating that swine CA-VI consists of 7 subunits. The treatment of the enzyme with endo-N-acetylglucosaminidase F reduced its subunit molecular weight from 37,000 to 35,000 and 32,000. We raised a rabbit antibody against purfied swine CA-VI. Double immunodiffusion showed that anti-swine CA-VI serum reacted with swine CA-VI and swine saliva, but not with hemolysate (containing CA-I and CA-Il) or muscle extracts (containing CA-III). The concentration of CA-VI in swine saliva, measured using single radial immunodiffusion, was 0.027 +/- 0.017 mg/mg total protein.  相似文献   
78.
Twelve different mating types among the Hampshire and Landrace breeds were used to determine direct, maternal, heterosis, and recombination effects for performance and carcass traits. Mating types used were two purebred, two F1, two F2, two F3, and four backcross. Carcass data were collected on 238 barrows and 262 gilts over four replications. Traits measured were length (LENG), 10th rib off midline backfat (BF10), longissimus muscle area (LMA), and dressing percentage (DRS%). Average backfat (AVBF) was calculated as the mean of three midline fat depths measured opposite the first rib, last rib, and last lumbar vertebra. The model used to evaluate the carcass traits included main effects of mating type, farrowing season, and sex and included slaughter weight as a covariate. The performance traits of ADG, feed efficiency (FE), daily feed consumption (DFC), lean gain per day (LNGN), and lean efficiency (LNEF) were measured on a pen basis. Comparisons of reciprocal F1 crosses showed that carcasses from pigs sired by Hampshire boars were leaner and had more LMA than those sired by Landrace boars. Heterosis percentages were significant for AVBF (7.2%; P less than .01), BF10 (8.8%; P less than .01), DRS% (1.5%; P less than .01), ADG (11.5%; P less than .01), DFC (10.2%; P less than .01), LNGN (10.6%; P less than .01), and LNEF (6.0%; P less than .05). Epistatic recombination losses in the offspring were significant for LENG (3.6 cm; P less than .05) and approached significance for BF10 (6.1 mm; P less than .10).  相似文献   
79.
Abstract. In field and laboratory experiments the conditioner‘Agri-SC’has shown improvements in the structure of loamy sand soils in east Shropshire, UK. It resulted in statistically significant decreases in soil bulk density values and increases in soil porosity and aggregate stability. Further experiments are in progress on both loamy sand and silt loam soils.  相似文献   
80.
A shrubby plant, abundant in east Kenya, Gynandropsis gynandra (L.) Brig., was shown to exhibit repellent and acaricidal properties to larvae, nymphs and adult Rhipicephalus appendiculatus and Amblyomma variegatum ticks. All stages of ticks avoided the leaves of the plant and a high percentage of the ticks which were continuously exposed to its leaves died; surviving ticks were weak and inactive but regained activity when exposed to fresh air. The effectiveness of the plant's leaves as a repellent and acaricide was most pronounced on nymphs and least pronounced on adults. Field investigations indicated that ticks were not found up to 2-5 m from the plant in areas where the plant was predominant. The potential of using the plant for tick control within an integrated tick management system in the resource-poor farming context in Africa was highlighted.  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