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   2128篇
  免费   79篇
林业   326篇
农学   68篇
基础科学   3篇
  244篇
综合类   423篇
农作物   52篇
水产渔业   120篇
畜牧兽医   857篇
园艺   41篇
植物保护   73篇
  2021年   31篇
  2020年   31篇
  2019年   47篇
  2018年   39篇
  2017年   37篇
  2016年   45篇
  2015年   30篇
  2014年   49篇
  2013年   96篇
  2012年   89篇
  2011年   123篇
  2010年   63篇
  2009年   64篇
  2008年   111篇
  2007年   108篇
  2006年   138篇
  2005年   100篇
  2004年   96篇
  2003年   90篇
  2002年   86篇
  2001年   31篇
  2000年   39篇
  1999年   32篇
  1997年   11篇
  1996年   10篇
  1995年   18篇
  1993年   12篇
  1992年   19篇
  1991年   19篇
  1990年   19篇
  1989年   23篇
  1988年   12篇
  1987年   16篇
  1986年   16篇
  1985年   17篇
  1984年   19篇
  1983年   19篇
  1980年   11篇
  1979年   23篇
  1978年   13篇
  1975年   10篇
  1974年   10篇
  1973年   14篇
  1972年   23篇
  1971年   13篇
  1970年   15篇
  1969年   13篇
  1968年   12篇
  1967年   14篇
  1965年   12篇
排序方式: 共有2207条查询结果,搜索用时 27 毫秒
991.
OBJECTIVE: To analyse the association between socio-economic indicators and diet among 2-year-old children, by assessing the independent contribution of parental education and equivalent income to food intake. DESIGN: The analysis was based on data from a prospective birth cohort study. Information on diet was obtained using a semi-quantitative food-frequency questionnaire. Low and high intake of food was defined according to the lowest and the highest quintile of food consumption frequency, respectively. SETTING: Four German cities (Munich, Leipzig, Wesel, Bad Honnef), 1999-2001.Subjects Subjects were 2637 children at the age of 2 years, whose parents completed questionnaires gathering information on lifestyle factors, including parental socio-economic status, household consumption frequencies and children's diet. RESULTS: Both low parental education and low equivalent income were associated with a low intake of fresh fruit, cooked vegetables and olive oil, and a high intake of canned vegetables or fruit, margarine, mayonnaise and processed salad dressing in children. Children with a low intake of milk and cream, and a high intake of hardened vegetable fat, more likely had parents with lower education. Low butter intake was associated with low equivalent income only. CONCLUSIONS: These findings may be helpful for future intervention programmes with more targeted policies aiming at an improvement of children's diets.  相似文献   
992.
To evaluate the manner in which a cow's claws make contact with the ground at the walk, the gait, and in particular the claw-ground contact pattern, were studied in 12 healthy, lactating dairy cows, using high-speed cinematography (500frames/s) while the animals were walking on a treadmill. The results showed that the limbs were advanced around the contralateral limbs in a sigmoid curve. The feet contacted the ground with the foot axis and the tips of the claws rotated slightly outwards. In all cows the lateral claws contacted the ground before the medial claws in the hindlimbs, and in 10/12 cows in the forelimbs. The heel of the lateral claws was the region of initial contact with the ground in the hindlimbs of all cows and in the forelimbs in 9/12 cows. Lateral 'heel first' contact in the fore and hindlimbs appeared to be the normal gait pattern in these animals. Compared with a previous study of heifers, lactating cows had a larger step width in the hindlimbs and a smaller step width in the forelimbs. These ground contact patterns offer an explanation for the predisposition to claw disorders of the lateral claw of the hindlimb. The results of this study reinforce the suggestion that soft floor surfaces should be provided for cattle to prevent mechanical injury to the claws.  相似文献   
993.
Objective  To identify anesthesia-related variables which may independently predict time to standing in horses anesthetized with ketamine/diazepam/isoflurane.
Study design  Retrospective case series.
Animals  Three hundred and eighty-one horses.
Methods  Case records were searched for the years 2000–2003 and 381 horses older than 12 months which weighed at least 200 kg were identified. Data were extracted from the records, and only horses that were anesthetized with xylazine, ketamine, diazepam and isoflurane were included in the analysis. Multiple linear regression was used to relate time to standing with demographic, intraoperative and anesthetic variables.
Results  Most (326; 86%) horses recovered unassisted and 55 (14%) were assisted in recovery. The model for unassisted recovery had an R 2 of 0.228 with colic ( p  < 0.0001), anesthesia duration ( p  < 0.02), temperature nadir ( p  < 0.02) and duration of hypotension ( p  < 0.0001) being significant predictors of time to standing. The final model for predicting assisted recovery time had an R 2 of 0.314 with emergency status ( p  < 0.0001), warm-blood breed ( p  < 0.04) and intraoperative administration of ketamine ( p  < 0.004) being the significant predictor.
Conclusions and clinical relevance  Variables which could be impacted by the anesthetist which would result in a faster time to standing include duration of anesthesia, hypothermia and intraoperative hypotension. However, the contribution of anesthesia factors explained <23% of the variability in recovery time, suggesting that other, more important factors contribute to anesthesia recovery time in horses.  相似文献   
994.
ABSTRACT Verticillium wilt caused by the vascular fungal pathogen Verticillium longisporum is one of the most important pathogens of oilseed rape (Brassica napus sp. oleifera) in northern Europe. Because production of this major oilseed crop is expanding rapidly and no approved fungicides are available for V. longisporum, long-term control of the disease can only be achieved with cultivars carrying effective quantitative resistance. However, very little resistance to V. longisporum is available within the gene pool of oilseed rape, meaning that interspecific gene transfer from related species is the only possibility for broadening levels of resistance in current varieties. The amphidiploid species B. napus can be resynthesized by crossing the two progenitor species Brassica oleracea and Brassica rapa, hence resistant accessions of these two diploid species can be used as resistance donors. In this study a total of 43 potential B. rapa and B. oleracea resistance donors were tested with regard to their reaction to a mixture of two aggressive V. longisporum isolates, and resistances from diverse lines were combined by embryo rescue-assisted interspecific hybridization in resynthesized rapeseed lines. Progenies from crosses of the two B. rapa gene bank accessions 13444 and 56515 to the B. oleracea gene bank accessions BRA1008, CGN14044, 8207, BRA1398, and 7518 showed a broad spectrum of resistance in pathogenicity tests. Of 45 tested resynthesized lines, 41 lines exhibited a significantly higher level of resistance than the moderately V. longisporum-tolerant oilseed rape cultivar Express. These lines represent a promising basis for the combination of different resistance resources in new varieties.  相似文献   
995.
Results from crop variety trials may vary between geographical regions because of differences in climate and soil types. Results are usually presented at regional level. To evaluate the importance of the regions used in the Swedish variety trial programs, we examined which regions produced similar levels of yield and similar ratios in yield between cultivars; the amount by which variance could be reduced by division into regions or clusters of regions; and the amount of trials per region and year, replicates per trial, and trials per year required in order to fulfill specifications on the precision of results. Yield data from spring barley and winter wheat trials performed during 1997–2006 were studied using cluster analysis and variance component estimation. The objectives were (1) to discuss the effects of regions on precision when the number of trials has decreased; (2) to demonstrate the method; and (3) to report the results obtained. In spring barley, clusters of regions produced different levels of yield, but similar yield ratios between cultivars. In winter wheat, clusters of regions giving different yield ratios were identified. When the option of a single analysis was compared with that of region-wise analysis, the reduction in variance with the former, due to the larger number of trials, outweighed the reduction in variance with the latter due to decreased random interaction between trials and cultivars.  相似文献   
996.
Downy mildew of lettuce, caused by Bremia lactucae, is difficult to control in soilless systems by using conventional methods of disease management because few chemicals are registered, while resistant cultivars face the problem of resistance break down; therefore other methods for disease control need to be investigated. The effect of silicon salt as well as increased electrical conductivities against downy mildew was evaluated in four experiments carried out in hydroponically systems, using the cultivar of lettuce “Cobham Green”, known for its susceptibility to the pathogen. Silicon, as potassium silicate, was added at 100 mg l−1 of nutrient solution at three levels of electrical conductivity: 1.5–1.6 mS cm−1 (EC1), 3.0–3.5 mScm−1 (EC2, 0.70 g l−1 NaCl) and 4.0–4.5 mS cm−1 (EC3, 0.95 g l−1 NaCl) respectively. Lettuce plants, grown for 14–20 (trials 1 and 2) and 36–45 (trials 3 and 4) days in the different nutrient solutions tested, were inoculated with B. lactucae conidia with a maximum of two inoculations before final disease assessment carried out 14–21 days after the inoculation able to give symptoms. EC and potassium silicate significantly influenced downy mildew incidence and severity, while their interaction was not a significant factor. The addition to the standard nutrient solution (EC1) of potassium silicate resulted in a significant reduction of downy mildew severity in trials 1 and 2 where plants were artificially inoculated 15 and 20 days after transplanting. This efficacy was slight on plants grown for 36 and 45 days before inoculation in a soil drenched with EC1 amended with potassium silicate. EC2 gave a significantly similar downy mildew reduction than EC2 added with potassium silicate in trial 3. Plants grown for 36 and 45 days at the highest electrical conductivity (EC3) showed a significant reduction in severity of downy mildew compared with that observed at EC2 level. The best results, in terms of disease control, were given by the addition of potassium silicate to the EC3 solution. This combination also led to a significantly increased plant biomass. The possibility and benefits of applying potassium silicate and increased EC amendments in practice is discussed.  相似文献   
997.
Resistance of nematodes to anthelmintics such as avermectins has emerged as a major global health and agricultural problem, but genes conferring natural resistance to avermectins are unknown. We show that a naturally occurring four-amino-acid deletion in the ligand-binding domain of GLC-1, the alpha-subunit of a glutamate-gated chloride channel, confers resistance to avermectins in the model nematode Caenorhabditis elegans. We also find that the same variant confers resistance to the avermectin-producing bacterium Streptomyces avermitilis. Population-genetic analyses identified two highly divergent haplotypes at the glc-1 locus that have been maintained at intermediate frequencies by long-term balancing selection. These results implicate variation in glutamate-gated chloride channels in avermectin resistance and provide a mechanism by which such resistance can be maintained.  相似文献   
998.
999.
1000.
    
Ohne Zusammenfassung  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