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   337篇
  免费   37篇
林业   8篇
农学   8篇
基础科学   2篇
  11篇
综合类   49篇
农作物   8篇
水产渔业   1篇
畜牧兽医   281篇
园艺   1篇
植物保护   5篇
  2023年   2篇
  2021年   4篇
  2020年   9篇
  2019年   8篇
  2018年   8篇
  2017年   9篇
  2016年   12篇
  2015年   14篇
  2014年   14篇
  2013年   11篇
  2012年   13篇
  2011年   16篇
  2010年   6篇
  2009年   11篇
  2008年   13篇
  2007年   17篇
  2006年   7篇
  2005年   15篇
  2004年   9篇
  2003年   14篇
  2002年   12篇
  2001年   11篇
  2000年   16篇
  1999年   10篇
  1998年   5篇
  1997年   3篇
  1996年   10篇
  1995年   5篇
  1994年   10篇
  1993年   4篇
  1992年   7篇
  1991年   6篇
  1990年   10篇
  1989年   8篇
  1988年   4篇
  1987年   6篇
  1984年   2篇
  1983年   2篇
  1978年   2篇
  1976年   2篇
  1974年   3篇
  1970年   2篇
  1928年   3篇
  1925年   2篇
  1901年   1篇
  1900年   1篇
  1899年   1篇
  1898年   1篇
  1897年   1篇
  1896年   1篇
排序方式: 共有374条查询结果,搜索用时 15 毫秒
71.

Introduction

To determine the relationship between aortoseptal angle (AoSA) and the short- and long-term systolic pressure gradient (PG) reduction following combined cutting and high-pressure balloon valvuloplasty (CB/HPBV) in dogs with severe subaortic stenosis.

Animals

Retrospective study of 22 client-owned dogs of various breeds with severe subaortic stenosis (mean left ventricular to aortic PG = 143 mmHg; range = 80–322 mmHg) that underwent CB/HPBV.

Materials and methods

Initial angiographic and left apical and right-sided parasternal long-axis view echocardiographic video loops were used for measuring the angle between the plane of the interventricular septum and the longitudinal axis of the ascending aorta. The PG reduction ratio immediately after CB/HPBV and 6 and 12 months later were compared with AoSA.

Results

Weak correlations were observed for all instances of PG reduction ratio and AoSA type. Significantly greater mean differences of PG reduction ratio were observed for angles >160° than for angles <160° at 24 h (>160° mean: 54.45, standard error [SE]: ±3.8; <160° mean: 39.88, SE: ±2.09), 6 months (>160° mean: 57.73, SE: ±10.9; <160° mean: 28.22, SE: ±3.42), and 12 months (>160° mean: 76.11, SE: ±17.5; <160° mean: 27.61, SE: ±6.44; p=0.003).

Conclusions

Dogs with AoSA >160° on right-sided parasternal long-axis view echocardiograms responded with a greater PG reduction following CB/HPBV than did dogs with AoSA <160°. This suggests that AoSA is associated with long-term outcomes of CB/HPBV, and measurement could help in the evaluation of dogs that are candidates for CB/HPBV.  相似文献   
72.
73.
Conventional clay capping for post-closure management of landfill commonly cracks and deteriorates over time. As a consequence, water ingress into waste increases as a function of time, potentially causing a range of environmental issues. An alternative approach is known as phytocapping, which utilizes select plant species to control cap stability and moisture percolation. In this study, growth of Arundo donax L. (giant reed), Brassica juncea (L.) Czern. (Indian mustard), and Helianthus annuus L. (sunflower) on a landfill site was studied with different biosolid amendment rates (0, 25, and 50?Mg?ha?1). Cultivation of the landfill cap and amendment with biosolids significantly improved the characteristics of the soil. Growth of each plant species increased due to biosolid addition. Giant reed produced the largest biomass in the 50?Mg?ha?1 biosolid amendment rate (38?Mg?ha?1 dry weight). The high pH and clay content of landfill cap soil, and the low metal concentrations of the biosolid resulted in low heavy metal (copper, zinc, cadmium, and lead) accumulation in leaves of most treatments. The improvement in growth and limited uptake of metal contaminants to plant shoots indicated that biosolid application to landfill clay caps improves the application of phytocapping of old landfill sites.  相似文献   
74.
Restoration of degraded tropical forest landscapes   总被引:5,自引:0,他引:5  
The current scale of deforestation in tropical regions and the large areas of degraded lands now present underscore the urgent need for interventions to restore biodiversity, ecological functioning, and the supply of goods and ecological services previously used by poor rural communities. Traditional timber plantations have supplied some goods but have made only minor contributions to fulfilling most of these other objectives. New approaches to reforestation are now emerging, with potential for both overcoming forest degradation and addressing rural poverty.  相似文献   
75.
76.
77.
Stem cellulose and lignin concentrations are major determining factors of alfalfa (Medicago sativa ssp. sativa L.) forage quality. Only limited information is available on the genetic variability and the influence of environmental effects on these two stem-quality traits. Our objectives were to: evaluate the variability for stem cellulose and lignin concentrations in modern alfalfa germplasms across several harvests; observe the environmental stability of these two quality traits in 32 alfalfa clones selected high or low for either stem cellulose or lignin concentration; and examine the relationships between these two stem-quality traits and leaf and stem crude protein (CP). Fifty alfalfa entries (cultivars and experimental populations) were established May 1993, and sampled for stem acid-detergent lignin (ADL), stem acid-detergent cellulose (ADC), and stem CP on Sep 1993, June and Aug 1994. Clones were vegetatively propagated from individual plants selected for extremes in stem ADL and ADC and transplanted into blocks at two locations in May 1994 and sampled for quality analysis in September 1994. For all samples, leaves were hand-separated from stems and stem ADL, ADC, CP and leaf CP concentration were determined using near infrared reflectance spectroscopy. Entry differences for stem ADL and ADC were detected only at the June 1994 harvest date in the cultivar study. Spearman's ranked correlations over years in the clonal study demonstrated greater environmental stability for stem ADC (r = 0.70, P ≥ 0.01) than for stem ADL (r = 0.54, P # 0.05). Environmental effects had an impact on both traits, but stem ADC showed greater potential for improving forage quality. Simple correlations showed that decreasing stem ADL or ADC would have minimal effect on leaf CP and may increase stem CP.  相似文献   
78.
Deterministic computer models were used to simulate the cow-calf segment of an integrated production system. Angus, Charolais, Hereford, Limousin, and Simmental breeds were included in three mating systems: pure-breeding (PB) or two- (2R) or three-breed (3R) rotational crossbreeding. Breed data were taken from the literature. Herds were evaluated over the production year. Sires represented breed averages and were available from sources outside their herds, and 100 replacement heifers were saved annually. Females in 3R had the highest average energy requirements (8,144 Mcal of ME.cow-1.yr-1) and production costs ($322.31.cow-1.yr-1), and PB females had the lowest average requirements (7,748 Mcal of ME.cow-1.yr-1) and costs ($313.2.cow-1.yr-1). Purebred systems were the least biologically and economically efficient (64.9 Mcal of ME/kg of steer equivalent, $2.35/kg of steer equivalent), respectively, and 3R systems were the most efficient (56.6 Mcal of ME/kg of steer equivalent, $1.95/kg of steer equivalent). On average, 3R systems were more efficient biologically and economically than 2R systems. However, some 2R systems were as efficient as some 3R systems. Crossbred combinations containing Angus and(or) Hereford ranked more biologically and economically efficient than other breed combinations. Conversely, British purebreds ranked more biologically efficient, whereas Continental purebreds ranked more economically efficient.  相似文献   
79.
80.
Plant litter and livestock excreta are two important ways of carbon and nutrient input to soil in grassland grazing systems. Grazing livestock often deposit dung on plant litter, which may affect litter decomposition through a changing microenvironment. We assessed effects of yak dung on litter mixing effects on litter decomposition in a Tibetan alpine grassland. Six common species were selected, including low-quality litter species Kobresia capillifolia, Elymus nutans, and Ligularia virgaurea and high-quality litter species Anemone rivularis, Saussurea nigrescens, and Thermopsis lanceolata. Litter bags containing each species alone and all two-species combinations were allowed to decompose with and without experimental dung addition in the field. Mass loss of the leaf litter was measured after 6 and 12 mo. High-quality litter species had significantly greater mass loss than low-quality litter species. Dung significantly accelerated litter mass loss after both 6 and 12 mo for low-quality litter species, but only after 12 mo for high-quality litter species. Litter mixtures containing both high- and low-quality species showed positive nonadditive effects (NAEs) on mass loss after 6 mo but additive effects after 12 mo. Dung increased the strength of NAEs after 6 mo and shifted litter mixing effects from positive to negative NAEs after 12 mo. Our results support previous findings that litter mixing could produce NAEs on litter decomposition and that these NAEs could change with incubation time. Most importantly, we show that dung can modify NAEs, demonstrating that litter mixing effects are dependent on the microenvironment. Our findings also demonstrate that yak dung can influence soil processes by varying both single-species litter decomposition rates and litter interactions within mixtures. Furthermore, the results suggest yak dung is closely related to material and nutrient cycling, so we believe dung should remain and not be substantially removed from this grazing ecosystem.  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