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   4202篇
  免费   276篇
  国内免费   2篇
林业   377篇
农学   181篇
基础科学   30篇
  1094篇
综合类   291篇
农作物   206篇
水产渔业   351篇
畜牧兽医   1522篇
园艺   60篇
植物保护   368篇
  2024年   5篇
  2023年   58篇
  2022年   50篇
  2021年   109篇
  2020年   122篇
  2019年   58篇
  2018年   171篇
  2017年   182篇
  2016年   178篇
  2015年   141篇
  2014年   174篇
  2013年   280篇
  2012年   369篇
  2011年   336篇
  2010年   161篇
  2009年   114篇
  2008年   277篇
  2007年   271篇
  2006年   235篇
  2005年   240篇
  2004年   185篇
  2003年   199篇
  2002年   154篇
  2001年   86篇
  2000年   75篇
  1999年   39篇
  1998年   21篇
  1997年   17篇
  1996年   10篇
  1995年   9篇
  1994年   11篇
  1993年   16篇
  1992年   8篇
  1991年   4篇
  1990年   7篇
  1989年   8篇
  1988年   7篇
  1987年   3篇
  1985年   5篇
  1984年   4篇
  1983年   5篇
  1981年   6篇
  1980年   5篇
  1979年   3篇
  1976年   4篇
  1974年   5篇
  1969年   4篇
  1968年   5篇
  1953年   3篇
  1934年   3篇
排序方式: 共有4480条查询结果,搜索用时 31 毫秒
961.
OBJECTIVE: To assess the effectiveness of a behaviour change approach, with or without financial support, in improving vitamin A (VA) intake and serum retinol concentration through mango and liver consumption by children. DESIGN: A parallel design (no control area) was used to assess changes in VA intake and serum retinol over a 15-week period. SETTING AND SUBJECTS: A pilot study was implemented in the Department of Kokologho, a rural area in central west Burkina Faso. One hundred and fifty children aged 2-3 years were randomly selected and assigned to two treatment groups: PA$$ (promotional activities and financial support) and PA (promotional activities). RESULTS: The intervention significantly increased (P < 0.001) total VA intake by 56% in PA$$ and by 50% in PA. VA intake from liver increased significantly (P < 0.001) from 12.7 +/- 23.5 to 155.3 +/- 56.3 microg retinol activity equivalents (RAE) in PA$$ and from 21.6 +/- 29.7 to 135.3 +/- 44.9 microg RAE in PA. Changes in VA intake from liver were significantly higher (P = 0.004) in PA$$ compared with PA. Mean serum retinol concentration increased significantly by 26% (P < 0.001) in PA$$ and 30% (P < 0.001) in PA. Changes in serum retinol concentration (0.13 micromol l(-1) in PA$$ vs. 0.17 micromol l(-1) and in PA) did not differ significantly (P = 0.455) between groups over the intervention. CONCLUSION: Promotional activities on mango and liver intake effectively increased VA intake and serum retinol concentrations. Although an additional beneficial effect of financial support on liver intake was observed, this did not translate into a further increase in serum retinol concentration.  相似文献   
962.
AIM: The aim of this study was to determine the practices of primary health care (PHC) nurses in targeting nutritionally at-risk infants and children for intervention at a PHC facility in a peri-urban area of the Western Cape Province of South Africa. METHODOLOGY: Nutritional risk status of infants and children <6 years of age was based on criteria specified in standardised nutrition case management guidelines developed for PHC facilities in the province. Children were identified as being nutritionally at-risk if their weight was below the 3rd centile, their birth weight was less than 2500 g, and their growth curve showed flattening or dropping off for at least two consecutive monthly visits. The study assessed the practices of nurses in identifying children who were nutritionally at-risk and the entry of these children into the food supplementation programme (formerly the Protein-Energy Malnutrition Scheme) of the health facility. Structured interviews were conducted with nurses to determine their knowledge of the case management guidelines; interviews were also conducted with caregivers to determine their sociodemographic status. RESULTS: One hundred and thirty-four children were enrolled in the study. The mean age of their caregivers was 29.5 (standard deviation 7.5) years and only 47 (38%) were married. Of the caregivers, 77% were unemployed, 46% had poor household food security and 40% were financially dependent on non-family members. Significantly more children were nutritionally at-risk if the caregiver was unemployed (54%) compared with employed (32%) (P=0.04) and when there was household food insecurity (63%) compared with household food security (37%) (P<0.004). Significantly more children were found not to be nutritionally at-risk if the caregiver was financially self-supporting or supported by their partners (61%) compared with those who were financially dependent on non-family members (35%) (P=0.003). The weight results of the nurses and the researcher differed significantly (P<0.001), which was largely due to the different scales used and weighing methods. The researcher's weight measurements were consistently higher than the nurses' (P<0.00). The researcher identified 67 (50%) infants and children as being nutritionally at-risk compared with 14 (10%) by the nurses. The nurses' poor detection and targeting of nutritionally at-risk children were largely a result of failure to plot weights on the weight-for-age chart (55%) and poor utilisation of the Road to Health Chart. CONCLUSIONS: Problems identified in the practices of PHC nurses must be addressed in targeting children at nutritional risk so that appropriate intervention and support can be provided. More attention must be given to socio-economic criteria in identifying children who are nutritionally at-risk to ensure their access to adequate social security networks.  相似文献   
963.
This study was designed to investigate the occurrence of Salmonella spp and Campylobacter spp in faeces samples from 222 healthy Murciano-Granadina dairy goats reared on 12 farms in Spain and in samples of bulk tank milk from 11 of those herds. Neither Salmonella spp nor Campylobacter spp were isolated from any of the samples. Our results suggest that, under the management practices applied to this breed in Spain, Murciano-Granadina goats are not likely to be a significant reservoir for these food-borne pathogens.  相似文献   
964.
965.
Ramipril, an angiotensin-converting enzyme (ACE) inhibitor for use in dogs, is converted in vivo to its active form, ramiprilat, which is eliminated in the bile and urine in the dog. The objective of this study was to assess the effect of renal impairment on the pharmacokinetics (PKs) and pharmacodynamics (PDs) of ramipril and ramiprilat. Ten adult Beagle dogs were used. PK/PD studies were performed before and after the induction of subclinical renal impairment. Ramiprilat was given at 0.25 mg/kg by a single IV bolus. After a 2-week washout period, ramipril was administered PO at 0.25 mg/kg once daily for 8 days. Ramipril and ramiprilat PKs were studied by using a physiologically based model. The relationship between free plasma ramiprilat concentration and ACE activity was described by using the fractional Hill model. Glomerular filtration rate was decreased by 58%. No biologically relevant changes in usual plasma variables were observed between the 1st and the 8th day of oral treatment with ramipril under either condition. After an IV bolus of ramiprilat, the only changes in renal-impaired dogs were a 14 and 49% decrease in clearance of the free fraction of ramiprilat (P < .01) and free plasma concentration required to produce 50% of the maximal effect (P < .05), respectively. After repeated PO administration of ramipril, there were no alterations in any of the PK and PD parameters in healthy or renal-impaired dogs. No adjustment of the recommended PO dosage of ramipril is needed in dogs with moderate renal impairment.  相似文献   
966.
This study was performed to investigate the diagnostic relevance of cerebrospinal fluid (CSF) high resolution electrophoresis. The laboratory technique was applied to 100 paired samples of canine CSF and serum, with paired samples tested during the same analytical run, as recommended in human medicine. Ninety four of the dogs had a neurological disease and 6 healthy dogs served as a control group. A strong linear correlation between CSF total protein concentration and the albumin quota (AQ) was found in the control group and in the inflammatory (infectious or noninfectious), neoplastic, and miscellaneous groups: AQ = 0.015 CSF total protein--0.102, r = 0.990. This correlation suggests that an increased CSF total protein concentration can be an indicator of blood brain barrier dysfunction. The highest median AQ value was found in the aseptic suppurative meningitis group, but no statistical differences were found between this and the other groups. The AQ, calculated with this technique, did not provide any additional information. Moreover, although unexpected, the electrophoretic profiles were not characteristic of any particular disease. In conclusion, this study did not confirm high resolution electrophoresis of paired CSF and serum samples to be a valuable ancillary diagnostic tool for canine neurological diseases.  相似文献   
967.
The intradermal tuberculin (IDTB) test and the interferon-gamma (IFN-gamma) assay are used worldwide for detection of bovine tuberculosis in cattle, but little is known about the effect of co-infecting agents on the performance of these diagnostic tests. This report describes a field trial conducted in a cattle herd with dual infection (bovine tuberculosis and paratuberculosis) during 3.5 years. It has been based on a strategic approach encompassing serial parallel testing (comparative IDTB test, the IFN-gamma assay and serology of paratuberculosis) that was repeated 8 times over the period, and segregation of animals into two herds. The IDTB test detected 65.2% and the IFN-gamma test detected 69.6% of the Mycobacterium bovis culture-positive cattle. However, the IDTB test performed better during the first part of the trial, while the IFN-gamma test was the only method that detected infected animals during the following three samplings. The number of false positive reactors with the IDTB and/or the IFN-gamma tests was remarkably high compared to other reports, and could be caused by cross-reactivity with M. avium subsp. paratuberculosis. Also, the M. bovis isolates from cattle and wildlife from the same property were characterised using molecular techniques to disclose an epidemiological link. The IDTB test may not be appropriate to eradicate bovine tuberculosis in herds with dual mycobacterial infections. This report highlights the need to use several diagnostic techniques for the accurate detection of M. bovis infected animals in these herds.  相似文献   
968.
In shaded environments, minimizing dark respiration during growth could be an important aspect of maintaining a positive whole-plant net carbon balance. Changes with plant size in both biomass distribution to different tissue types and mass-specific respiration rates (R(d)) of those tissues would have an impact on whole-plant respiration. In this paper, we evaluated size-related variation in R(d), biomass distribution, and nitrogen (N) and total nonstructural carbohydrate (TNC) concentrations of leaves, stems and roots of three cold-temperate tree species (Abies balsamea (L.) Mill, Acer rubrum L. and Pinus strobus L.) in a forest understory. We sampled individuals varying in age (6 to 24 years old) and in size (from 2 to 500 g dry mass), and growing across a range of irradiances (from 1 to 13% of full sun) in northern Minnesota, USA. Within each species, we found small changes in R(d), N and TNC when comparing plants growing across this range of light availability. Consistent with our hypotheses, as plants grew larger, whole-plant N and TNC concentrations in all species declined as a result of a combination of changes in tissue N and shifts in biomass distribution patterns. However, contrary to our hypotheses, whole-plant and tissue R(d) increased with plant size in the three species.  相似文献   
969.
A new alkaloid, named gasabiimine (1), together with four known bisbenzyltetrahydroisoquinoline alkaloids (phaeanthine, 1,2-dehydroapateline, N-methylapateline, O-methylcocsoline) has been isolated from the roots of Triclisia sacleuxii. The structure of the new alkaloid was elucidated by spectroscopic data analysis.  相似文献   
970.
Water relations, leaf gas exchange, chlorophyll a fluorescence, light canopy transmittance, leaf photosynthetic pigments and metabolites and fruit quality indices of cherry cultivars 'Burlat', 'Summit' and 'Van' growing on five rootstocks with differing size-controlling potentials that decrease in the order: Prunus avium L. > CAB 11E > Maxma 14 > Gisela 5 > Edabriz, were studied during 2002 and 2003. Rootstock genotype affected all physiological parameters. Cherry cultivars grafted on invigorating rootstocks had higher values of midday stem water potential (Psi(MD)), net CO(2) assimilation rate (A), stomatal conductance (g(s)), intercellular CO(2) concentration (C(i)) and maximum photochemical efficiency of photosystem II (PSII) (F(v)/F(m)) than cultivars grafted on dwarfing rootstocks. The Psi(MD) was positively correlated with A, g(s) and C(i). Moreover, A was positively correlated with g(s), and the slopes of the linear regression increased from invigorating to dwarfing rootstocks, indicating a stronger regulation of photosynthesis by stomatal aperture in trees on dwarfing Edabriz and Gisela 5. The effect of rootstock genotype was also statistically significant for leaf photosynthetic pigments, whereas metabolite concentrations and fruit physicochemical characteristics were more dependent on cultivar genotype. Among cultivars, 'Burlat' leaves had the lowest concentrations of photosynthetic pigments, but were richest in total soluble sugars, starch and total phenols. Compared with the other cultivars, 'Summit' had heavier fruits, independent of the rootstock. 'Burlat' cherries were less firm and had lower concentrations of soluble sugars and a lower titratable acidity than 'Van' cherries. Nevertheless, 'Van' cherries had lower lightness, chroma and hue angle, representing redder and darker cherries, compared with 'Summit' fruits. In general, Psi(MD) was positively correlated with fruit mass and A was negatively correlated with lightness and chroma. These results demonstrate that: (1) water relations and photosynthesis of sweet cherry tree are mainly influenced by the rootstock genotype; (2) different physicochemical characteristics observed in cherries of the three cultivars suggest that regulation of fruit quality was mainly dependent on the cultivar genotype, although the different size-controlling rootstocks also had a significant effect.  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