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   135758篇
  免费   7797篇
  国内免费   70篇
林业   5581篇
农学   4646篇
基础科学   862篇
  17499篇
综合类   19018篇
农作物   5089篇
水产渔业   7095篇
畜牧兽医   73002篇
园艺   1803篇
植物保护   9030篇
  2020年   1172篇
  2019年   1411篇
  2018年   2232篇
  2017年   2493篇
  2016年   2299篇
  2015年   2001篇
  2014年   2452篇
  2013年   5736篇
  2012年   4352篇
  2011年   5169篇
  2010年   3503篇
  2009年   3548篇
  2008年   5154篇
  2007年   4972篇
  2006年   4642篇
  2005年   4268篇
  2004年   4113篇
  2003年   4180篇
  2002年   3810篇
  2001年   4773篇
  2000年   4645篇
  1999年   3748篇
  1998年   1473篇
  1997年   1453篇
  1996年   1341篇
  1995年   1599篇
  1994年   1327篇
  1993年   1359篇
  1992年   2762篇
  1991年   2902篇
  1990年   2869篇
  1989年   2898篇
  1988年   2615篇
  1987年   2592篇
  1986年   2539篇
  1985年   2402篇
  1984年   1960篇
  1983年   1718篇
  1982年   1155篇
  1979年   1651篇
  1978年   1285篇
  1977年   1136篇
  1976年   1100篇
  1975年   1165篇
  1974年   1361篇
  1973年   1452篇
  1972年   1379篇
  1971年   1198篇
  1970年   1251篇
  1969年   1146篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
Pericardial tissue was obtained from 14 dogs with idiopathic pericarditis, and from three dogs with pericardial effusion associated with neoplastic disease, for histopathological assessment and characterisation of infiltrating leucocytes by immunohistochemistry. The major pathological change was extensive pericardial fibrosis which was generally accompanied by a mixed inflammatory response that was of greatest intensity at the cardiac surface of the tissue. Perivascular lymphoplasmacytic aggregates were present at the pleural surface and within the fibrosed pericardium. There were no features that clearly distinguished the samples from dogs with neoplastic disease from dogs with idiopathic pericarditis. The pericardial infiltrates were dominated by MAC 387+ monocyte-macrophages and plasma cells expressing immunoglobulin (Ig)A or IgG. CD3+ T lymphocytes and major histocompatibility complex (MHC) class II+ macrophages were less common, although the perivascular aggregates were mixtures of T and B lymphocytes and a proportion of fibroblasts expressed MHC class II. There was no vascular pathology or deposition of immunoglobulin or complement within vessel walls. These findings are consistent with an immune response dominated by humoral effector mechanisms (Th2 immunity) but do not clearly support a primary immune-mediated pathogenesis for idiopathic pericarditis.  相似文献   
972.
Mature weight and rate of maturing were estimated in 177 Angus, Brahman, and reciprocal-cross cows grazing bermudagrass or endophyte-infected tall fescue over a 4-yr period to evaluate genotype x environment interactions. Data were collected every 28 d until cows were approximately 18 mo of age and then at prebreeding, postcalving, and weaning of calf. All cows with weight data to at least 42 mo of age were included in the analysis. Mature weight and rate of maturing were estimated using the three-parameter growth curve model described by Brody (1945). Data were pooled over year and analyzed by the general linear model procedure of SAS. Included in the models for mature weight and rate of maturing were the independent variables of genotype, environment, and genotype x environment interaction. There was a genotype x environment interaction (P < 0.01) for mature body weight (BW) but not for rate of maturing. Angus cows grazing fescue pastures had greater (P < 0.01) mean mature BW than Angus x Brahman cows grazing bermudagrass (611 +/- 17 vs 546 +/- 16 kg). Angus x Brahman cows grazing bermudagrass had lower (P < 0.05) mean mature BW than Brahman x Angus cows grazing bermudagrass or endophyte-infected fescue and Brahman cows grazing bermudagrass (546 +/- 16 vs 624 +/- 19, 614 +/- 22 and 598 +/- 20 kg, respectively). Brahman cows grazing endophyte-infected fescue had smaller (P < 0.05) mean mature BW than all genotype x forage combinations except for Angus x Brahman cows grazing bermudagrass. Angus cows had a smaller (P < 0.05) mean rate of maturing than Angus x Brahman and Brahman x Angus cows (0.039 +/- 0.002 vs 0.054 +/- 0.002 and 0.049 +/- 0.002%/mo, respectively), respectively, and Angus x Brahman cows had a larger (P < 0.05) mean rate of maturing than Brahman x Angus and Brahman cows (0.054 +/- 0.002 vs 0.049 +/- 0.002 and 0.041 +/- 0.002 %/mo, respectively). There was a direct breed x forage interaction (P < 0.05) for mature BW. These data suggest that the choice of breed type is important in maintaining a crossbreeding program, in that mature BW and rate of maturing are critical to the matching of animal requirements to available production resources.  相似文献   
973.
A health problem of an animal or a group of animals represents a potential stress situation for the animal keeper. In this article, the transactional stress model, which was developed in social psychology was applied on the animal keeper and shows, how he perceives the influence of animal care. If veterinary help is considered relevant, it may lead to an improved coping, which affects the health of the animal keeper and the animal as well. This viewpoint leads to a cycle, in which cause, success, coping and utility are closely linked together.  相似文献   
974.
1. In two separate experiments, the possibility of detecting embryonic development in chicken eggs was assessed using the same spectrophotometric method used to detect blood in Table eggs, using a combination of two wavelengths (577 and 610 nm) of the transmission spectrum. 2. In the first experiment, during the first 10 d of incubation, transmission spectra of 30 Hisex White eggs and 30 Hybro eggs were measured daily. 3. In the second experiment, 292 Hisex White eggs were incubated. Seven groups were randomly assigned. Six received an injection of sodium azide (NaN3) at different times during incubation in order to stop embryonic development, and during the first 12 d of incubation the transmission spectrum was measured daily. The acoustic resonance analysis method was also used on a group of uninjected eggs. 4. In the first experiment, it was possible to detect embryonic development from 120 h of incubation onwards in fertile eggs. In the second experiment changes in light transmission due to embryonic development were detected from 108 h of incubation. Detection of embryonic development using the acoustic resonance analysis method in the second experiment was possible only from 120 h of incubation. 5. It was concluded that the detection of embryonic development using visible light transmission is not directly linked with the formation of blood, but with the formation of sub-embryonic fluid, which takes place from 72 h of incubation onwards. This fluid makes the yolk sac translucent so that absorption of light at 577 nm can be detected.  相似文献   
975.
The conduct of randomized controlled trials in livestock with production, health, and food-safety outcomes presents unique challenges that might not be adequately reported in trial reports. The objective of this project was to modify the CONSORT (Consolidated Standards of Reporting Trials) statement to reflect the unique aspects of reporting these livestock trials. A 2-day consensus meeting was held on November 18–19, 2008 in Chicago, IL, to achieve the objective. Before the meeting, a Web-based survey was conducted to identify issues for discussion. The 24 attendees were biostatisticians, epidemiologists, food-safety researchers, livestock production specialists, journal editors, assistant editors, and associate editors. Before the meeting, the attendees completed a Web-based survey indicating which CONSORT statement items would need to be modified to address unique issues for livestock trials. The consensus meeting resulted in the production of the REFLECT (Reporting Guidelines for Randomized Control Trials) statement for livestock and food safety and 22-item checklist. Fourteen items were modified from the CONSORT checklist, and an additional subitem was proposed to address challenge trials. The REFLECT statement proposes new terminology, more consistent with common usage in livestock production, to describe study subjects. Evidence was not always available to support modification to or inclusion of an item. The use of the REFLECT statement, which addresses issues unique to livestock trials, should improve the quality of reporting and design for trials reporting production, health, and food-safety outcomes.  相似文献   
976.
Paramphistomiasis is a disease of domestic and wild ruminants caused by some digenetic trematodes of the superfamily Paramphistomoidea. Human infections with certain species of paramphistomes have been reported. Given their ubiquity and their abundance within hosts, it seems likely that the importance of these worms is underestimated globally. Based on the reported difficulties of morphological identification of paramphistomes, the development of alternative approaches to better delineate species is needed. Paramphistome samples examined during this study were cercariae, rediae from snail hosts and adult flukes obtained from slaughterhouses from Kenya, Tanzania, Egypt and Nepal. The results confirm that ITS2 is a good molecular marker for paramphistome identification and can be used to determine the affiliation of samples among the different taxa of Paramphistomoidea. Our 22 samples include eight species of paramphistomes. Ten samples of paramphistome larvae (cercariae or rediae) yielded seven species and the 12 samples of adult flukes from bovines yielded four, only one of which was not also represented among the larvae. We found three cases in which the sequences for adult worms and larvae matched, thus revealing the hosts involved locally in those particular life cycles. We interpret our specimens to include five species from Kenya, two from Nepal, two from Tanzania and one from Egypt. Bulinus forskalii was found infected with three species of paramphistomes. The results of the present study will help construct a framework to facilitate identification and eventually control of this underestimated cosmopolitan group of digeneans.  相似文献   
977.
The ecology of avian influenza (AI) viruses in wild aquatic birds of Asia is poorly understood, especially for the H5N1 high pathogenicity AI (HPAI) viruses. From March 2006 through November 2008, 20 AI viruses were isolated in the Crimea region of Ukraine with an overall frequency of virus recovery of 3.3%. All the viruses were isolated from three species of dabbling ducks: mallard (Anas platyrhynchos), wigeon (Anas penelope), and garganey (Anas querquedula), making the frequency of virus recovery for dabbling ducks 6.3%. The viruses were predominantly isolated during the fall sampling period. All viruses were genetically and antigenically characterized. No H5N1 HPAI viruses were isolated, but other HA and NA subtypes were identified including H3N1 (2), H3N6 (3), H3N8 (4), H4N6 (6), H5N2 (3), H7N8 (1), and H10N6 (1) subtypes. All isolates were of low pathogenicity, as determined by the intravenous pathogenicity index of 0.00. For H5N2 and H7N8 isolates, the HA gene was sequenced and the phylogenetic analysis revealed possible ecologic connections of the Crimea region with AI viruses from Siberia and Europe. No influenza A isolates were recovered from other Anseriformes (diving ducks [two species of pochards] and graylag geese), Columbiformes (collared doves), Gruiformes (coot), and Galliformes (gray partridges).  相似文献   
978.
A nation-wide survey was carried out in 370 randomly chosen French commercial broiler chicken flocks from October 2005 to September 2006 to determine Salmonella spp. prevalence and to identify risk factors for contamination, at the end of the rearing period. The Salmonella status of the flocks was assessed from five faecal samples (litter swabs) analysed by classical bacteriological methods. A flock with at least one contaminated sample was classified as a Salmonella-positive flock. The apparent prevalence of Salmonella was 8.6% (95% CI: 5.7, 11.5%). The most prevalent serovar was S. hadar followed by S. anatum and S. mbandaka. Logistic regression methods were used to analyse the associations between husbandry practices, farm characteristics, general hygiene and the Salmonella status of the sample. The risk for Salmonella contamination of the flock at the end of the rearing period increased when neighbours helped in the placement of day-old chicks. On the contrary, the risk decreased when mobile equipment was dismantled before cleaning and disinfection, when the farm had a specific container for dead-bird disposal and when acetic acid was added to the drinking water.  相似文献   
979.
980.
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