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   482篇
  免费   1篇
林业   5篇
  68篇
综合类   131篇
农作物   6篇
水产渔业   1篇
畜牧兽医   260篇
园艺   1篇
植物保护   11篇
  2023年   3篇
  2012年   42篇
  2011年   43篇
  2010年   8篇
  2009年   6篇
  2008年   30篇
  2007年   29篇
  2006年   31篇
  2005年   31篇
  2004年   25篇
  2003年   24篇
  2002年   22篇
  2001年   12篇
  2000年   15篇
  1999年   19篇
  1998年   7篇
  1997年   15篇
  1996年   16篇
  1995年   4篇
  1994年   5篇
  1993年   4篇
  1992年   6篇
  1991年   4篇
  1990年   8篇
  1989年   3篇
  1987年   3篇
  1983年   3篇
  1979年   3篇
  1978年   3篇
  1973年   3篇
  1963年   2篇
  1959年   3篇
  1958年   5篇
  1957年   2篇
  1955年   3篇
  1954年   3篇
  1953年   2篇
  1946年   2篇
  1944年   2篇
  1940年   2篇
  1937年   1篇
  1936年   1篇
  1935年   2篇
  1933年   4篇
  1928年   1篇
  1927年   1篇
  1922年   1篇
  1914年   1篇
  1911年   1篇
  1902年   1篇
排序方式: 共有483条查询结果,搜索用时 15 毫秒
91.
The lunar meteorite Sayh al Uhaymir 169 consists of an impact melt breccia extremely enriched with potassium, rare earth elements, and phosphorus [thorium, 32.7 parts per million (ppm); uranium, 8.6 ppm; potassium oxide, 0.54 weight percent], and adherent regolith. The isotope systematics of the meteorite record four lunar impact events at 3909 +/- 13 million years ago (Ma), approximately 2800 Ma, approximately 200 Ma, and <0.34 Ma, and collision with Earth sometime after 9.7 +/- 1.3 thousand years ago. With these data, we can link the impact-melt breccia to Imbrium and pinpoint the source region of the meteorite to the Lalande impact crater.  相似文献   
92.
BACKGROUND: Infection with Ehrlichia canis causes a highly variable, multisystemic disease in dogs. Nevertheless, many clinicians in Rio de Janeiro, Brazil, use the presence of only thrombocytopenia to make a presumptive diagnosis of E canis infection. OBJECTIVE: The objective of this study was to determine the prevalence of E canis in thrombocytopenic dogs from Rio de Janeiro, Brazil, using polymerase chain reaction (PCR). METHODS: Following DNA extraction of whole blood samples from 226 dogs, PCR assays were done using primers for rickettsial DNA (including Ehrlichia spp, Anaplasma platys and A phagocytophilum) and using E canis-specific primers (16S rRNA gene). Dogs were grouped as thrombocytopenic and nonthrombocytopenic based on platelet counts. The null hypothesis that there was no difference in the prevalence of E canis in these groups was rejected at P<.05. RESULTS: Thirty-six (32.1%) of the thrombocytopenic dogs and 4 (3.5%) of the nonthrombocytopenic dogs were positive for rickettsial gene sequences (P<.0001). Further, 30 (26.8%) of thrombocytopenic dogs and 4 (3.5%) nonthrombocytopenic dogs were positive for E canis-specific gene sequences (P<.0001). CONCLUSIONS: Although the prevalence of E canis infection was higher in thrombocytopenic dogs, less than one third of these dogs had demonstrable E canis infection. Thus, thrombocytopenia is not specific for the detection of E canis infection and should not be used solely to establish a diagnosis of canine ehrlichiosis, even in a geographic area with relatively high disease prevalence.  相似文献   
93.
Intoxication by pods of Prosopis juliflora (mesquite beans) causes an impairment of cranial nerve function in cattle and goats. In goats, vacuolation of neurons in the trigeminal motor nuclei has been reported. To study the lesions in cattle caused by consumption of P. juliflora pods and dry ground pods, eight 6- to 12-month-old male cattle were divided into 4 groups: group 1 was fed a ration containing 50% of pods; groups 2 and 3 received a ration containing 50 and 75% of dry ground pods, respectively; group 4 was the control. After 200 days, all cattle were killed and sampled for histologic evaluation. Samples of the trigeminal motor nucleus were examined by electron microscopy. All cattle from groups 1, 2, and 3 showed clinical signs resulting from impaired function of cranial nerves V, IX, X, and XII, starting 45-75 days after consumption of the plant. The main histologic lesions were vacuolation and loss of neurons in trigeminal motor nuclei and other motor cranial nerve nuclei with Wallerian-like degeneration in the cranial nerves. Mild denervation atrophy was observed in the masseter and other masticatory muscles. On electron microscopy, neurons of the trigeminal nuclei had markedly swollen mitochondria, with the mitochondrial cristae displaced peripherally, disoriented and disintegrating. Intoxication by P. juliflora seems to have a novel pathogenesis, characterized by a selective, primary, chronic, and progressive injury to mitochondria of neurons of the trigeminal and other cranial nerve nuclei. Cranial nerve degeneration and denervation atrophy of the muscles occurs as a consequence of the neuronal lesion.  相似文献   
94.
The influence of seasonal changes on epidemiological and entomological indices of malaria transmission in North Central Nigeria was elucidated in a series of studies carried out between January 2004 and December 2009. The climate in the study area was divided into three seasonal periods namely, rainy (May-October), dry (December-March) and transitional (April and November), during which larval and adult anopheline mosquito collections were carried out and assessed for densities, sporozoite infection and parity rates and potentials for malaria transmission. The results indicated that the climate in the study area was clearly seasonal, with close similarities in the patterns of distribution of the climatic factors in the study sites. Mosquito densities, both at the adult and larval stages (i.e., 29.35 +/- 5.10 adult mosquitoes/man/night and 10.36 +/- 3.34 larvae/dip, respectively), were significantly (p<0.05) highest during the rainy season. However, while the former varied significantly in the three seasonal periods, the latter was not significantly different during the dry and transitional seasons. Malaria transmission risks, in terms of sporozoite rates and entomological inoculation rates, was significantly (p<0.05) least in the dry season (i.e., 2.89 +/- 1.25% and 0.37 +/- 0.21 infective bite/man/night, respectively) but the two variables were not significantly (p>0.05) different during the transitional and rainy seasons. Adult mosquito daily survival rate and adult longevity were least in the dry season (26.52 +/- 11.80% and 6.80 days, respectively) and significantly (p<0.05) highest during the rainy season (72.28 +/- 4.00% and 16.95 +/- 4.20 days, respectively). Parous rates of the mosquitoes and duration of sporogony had distinct distribution pattern from the other variables investigated. While, significantly highest parous rates were recorded in the transitional season (86.00 +/- 4.30%), duration of sporogony was not significantly (p>0.05) different during the three seasons. The epidemiology of urban malaria in North Central Nigeria was discussed from the view points of the these results and concluded that the findings should promote the development of informed temporally-targeted vector control programs for the area.  相似文献   
95.
沈咏舟  潘鑫  郑新勇 《安徽农业科学》2006,34(19):4941-4941
从猪身上分离到NDV AS株,其HA和HI效价均为1∶5 124,ELD50/0.1 ml为10-6.0。  相似文献   
96.
97.
我们根据委托方的要求,做了保力胺添加剂饲养肉鸡的试验.根据介绍;保力胺(Polian)是由多种天然活性酶组成的。它主要含有三种酶,即凤梨酶、胰凝乳蛋白酶和多糖酶.它具有促进吸收、增强机体活力、缓解炎症、对抗应激、促进药物效力等作用。  相似文献   
98.
OBJECTIVE: To evaluate the effects of a comprehensive physical activity (PA) promotion programme in elementary schools on children's total PA levels, leisure-time PA, physical fitness and psychosocial correlates of PA. DESIGN: A pre-test-post-test design over two school years. SETTING AND SUBJECTS: Sixteen elementary schools (764 children, mean age: 11.2 +/- 0.7 years) were randomly assigned to the intervention condition (n = 8) and the control condition (n = 8). The intervention included a health-related physical education programme, an extracurricular PA promotion programme and classroom-based PA education lessons. In the total sample, leisure-time PA, psychosocial correlates of PA and physical fitness were measured using a PA questionnaire and the Eurofit test battery. In a sub-sample, total PA levels were measured using an accelerometer. RESULTS: According to accelerometer data, children's moderate PA and moderate-to-vigorous PA (MVPA) levels decreased less in the intervention schools than in the control schools (P < 0.01). The average time spent on MVPA decreased by 9 min per day in the intervention schools compared with 33 min per day in the control schools. Children in the intervention schools reported significantly more moderate PA in leisure time than the controls (P < 0.05). No overall improvement of physical fitness and no effects on the psychosocial correlates of PA were found. CONCLUSIONS: The comprehensive PA promotion programme was successful in preventing a decline in children's total activity levels. Furthermore, the intervention increased children's PA engagement in leisure time. Therefore, implementation needs to be encouraged.  相似文献   
99.
OBJECTIVES: To investigate the association between fruit and vegetable consumption and self-reported physical and mental functional health measured by an anglicised short-form 36-item questionnaire (UK SF-36). DESIGN: Population-based cross-sectional study. SETTING: General community in Norfolk, UK. SUBJECTS: A total of 16,792 men and women aged 40-79 years recruited from general practice population registers as part of the European Prospective Investigation into Cancer (EPIC)-Norfolk study, who completed food-frequency questionnaires in 1993-1997 and Health and Life Experiences Questionnaires 18 months later, were enrolled in the study. RESULTS: Mean SF-36 physical component summary scores increased significantly with increasing total fruit and vegetable consumption in both men and women (P < 0.0001 for trend). Men and women in the top quartile of consumption compared with the bottom quartile had a significantly higher likelihood of reporting good physical health (defined as a score > or = 55); odds ratio (OR) 1.30, 95% confidence interval (CI) 1.11-1.53 for men and OR 1.28, 95% CI 1.11-1.48 for women, after controlling for age, body mass index, smoking, education, social class, prevalent illness and total energy intake. Exclusion of current smokers and people with prevalent illness did not alter the associations. CONCLUSION: Higher fruit and vegetable consumption is associated with better self-reported physical functional health within a general population. Increasing daily intake by two portions of fruit and vegetables was associated with an 11% higher likelihood of good functional health. Since the current average consumption of fruit and vegetables in the UK is about three portions, the recommended 'five a day' strategy may have additional benefit for functional as well as other health outcomes in the population.  相似文献   
100.
The olive fruit fly Bactrocera oleae (Gmelin) (Diptera: Tephritidae) is the most important pest of olives in countries around the Mediterranean basin. Its control has been based mostly on bait sprays with organophosphate insecticides (usually dimethoate or fenthion) for about 40 years. In the present study, the resistance status of olive fruit fly populations to dimethoate was examined in Greece and Cyprus over 2 years. Thirty-one populations from various regions of Greece, nine from Cyprus and one laboratory susceptible strain, which served as a control, were assayed by topical application of dimethoate. Considerable variation in the resistance levels to dimethoate was recorded in the populations of B. oleae, with resistance ratios ranging from 6.3 to 64.4 (ED(50) values 12.5-128.7 ng dimethoate per insect). The highest resistance ratios were found in populations from Crete, and the lowest in those from Cyprus. This variation could be attributed to different selection pressures from insecticidal applications among populations from the various regions. Migration of resistant genotypes, either autonomous or via commerce, may also be involved.  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