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   2258篇
  免费   112篇
林业   127篇
农学   59篇
基础科学   14篇
  296篇
综合类   377篇
农作物   54篇
水产渔业   170篇
畜牧兽医   1130篇
园艺   43篇
植物保护   100篇
  2021年   32篇
  2020年   34篇
  2019年   25篇
  2018年   38篇
  2017年   39篇
  2016年   44篇
  2015年   33篇
  2014年   38篇
  2013年   70篇
  2012年   106篇
  2011年   125篇
  2010年   69篇
  2009年   76篇
  2008年   125篇
  2007年   140篇
  2006年   130篇
  2005年   144篇
  2004年   105篇
  2003年   107篇
  2002年   122篇
  2001年   43篇
  2000年   60篇
  1999年   42篇
  1998年   21篇
  1997年   23篇
  1996年   18篇
  1995年   21篇
  1994年   28篇
  1993年   17篇
  1992年   25篇
  1991年   17篇
  1990年   24篇
  1989年   33篇
  1988年   25篇
  1987年   19篇
  1986年   30篇
  1985年   19篇
  1984年   20篇
  1983年   13篇
  1982年   16篇
  1980年   9篇
  1979年   12篇
  1978年   11篇
  1976年   10篇
  1974年   19篇
  1973年   13篇
  1972年   14篇
  1970年   13篇
  1969年   15篇
  1967年   9篇
排序方式: 共有2370条查询结果,搜索用时 15 毫秒
1.
2.
3.
Context

Human appropriation of net primary productivity (HANPP) is employed as a measure of human pressures on biodiversity, though largely at global and national scales rather than landscape to regional scales where many conservation decisions take place. Though gaining in familiarity, HANPP is not widely utilized by conservation professionals.

Objectives

This study, encompassing the US side of the Great Lakes basin, examines how regional distributions of HANPP relate to landscape-based biodiversity proxy metrics used by conservation professionals. Our objectives were (1) to quantify the HANPP of managed lands at the county scale; and (2) to assess spatial patterns of HANPP in comparison to landscape diversity and local habitat connectedness to determine if the metric can provide useful information to conservation professionals.

Methods

We aggregated forest and cropland NPP data between 2005 and 2015 and coupled it with previously published potential vegetation maps to quantify the HANPP of each county in the study region. We mapped the outputs at 500 m resolution to analyze spatial relationships between HANPP and landscape metrics of biodiversity potential.

Results

Area-weighted HANPP across our study region averaged 45% of NPP, down to 4.9% in forest-dominated counties. Greater HANPP correlated with reduced landscape diversity (p?<?0.001, r2?=?0.28) and reduced local habitat connectedness (p?<?0.001, r2?=?0.36).

Conclusion

HANPP could be used as an additional tool for conservation professionals during regional-scale land use planning or conservation decision-making, particularly in mixed-use landscapes that both support important biodiversity and have high levels of primary production harvest.

  相似文献   
4.
Earthworm activity is beneficial in most natural and agricultural systems, but excessive earthworm casting is a problem on sports fields worldwide. Expulsion of soil‐rich earthworm fecal matter, or casts, as mounds of soil on the turf canopy can muddy the surface, reduce photosynthesis, and lead to thinning, weed invasion and surface softening. Casts affect ball roll on sports fields, cause fouling of maintenance equipment, and dull mower blades. Build‐up of cast material on reel mower units can affect height and quality of cut. Casting activity is dependent on environmental conditions such as soil texture, temperature, moisture, pH, and organic matter. Response to environmental conditions varies by species. Management options are limited, because no pesticides are specifically labeled for earthworm control at this time, and cultural control methods such as soil modification, turfgrass clipping removal, and sand topdressing have limited and inconsistent efficacy. Products containing plant‐derived saponins and irritants show promise for earthworm management. Pest management practices to mitigate excessive earthworm casting will likely need to be species‐specific, but limited knowledge of earthworm identification by end‐users further inhibits the efficacy of control measures. © 2019 Society of Chemical Industry  相似文献   
5.
6.
A retrospective study of the demographics of the Canadian dairy industry was conducted using data derived from Statistics Canada’s Census of Agriculture from 1991 to 2011. This longitudinal study shows that Canada’s dairy industry has undergone considerable consolidation. From 1991 to 2011, the number of dairy producers and dairy farms decreased by 48.9% and 61.9%, respectively. Furthermore, this trend in consolidation is likely to continue; nearly half (45.8%) of all dairy producers in Canada were ≥ 50 y of age in 2011. Not only will most of these producers be retired by 2021, but younger producers will also exit the industry for other reasons. These findings mirror what is occurring in Canada’s cow-calf industry, underscoring that supply-managed sectors experience demographic consolidation similar to non-supply managed sectors. These substantial changes will have ramifications for the veterinary profession. There will be fewer but larger farms, and the services and knowledge demanded of veterinarians will change accordingly, which has implications for food animal veterinary education.  相似文献   
7.
Gross and histologic postmortem studies were performed on the hind feet of feedlot cattle that had, or were free from, lesions of toe tip necrosis. The hind feet of feedlot cattle were collected by 3 veterinary feedlot practices in southern Alberta, Canada. Three studies of these feet were conducted: i) prediction of disease based on the presence or absence of apical white line separation, ii) gross assessment of the distribution and severity of lesions within affected claws, and iii) microscopic evaluation of the distal phalanx and surrounding soft tissues of affected claws. Prediction of toe tip necrosis based on the presence of apical white line separation was statistically significant (P < 0.0001). This, combined with a pattern of lesions indicative of an ascending infection of the distal phalanx and the absence of other lesions, suggests that the pathogenesis involves bacterial infection originating from the most distal aspect of the toe, at the apical white line.  相似文献   
8.

Background

Hereditary ataxias with similar phenotypes were reported in the Smooth-Haired Fox Terrier, the Jack Russell Terrier and the Parson Russell Terrier. However, segregation analyses showed differing inheritance modes in these breeds. Recently, molecular genetic studies on the Russell group of terriers found independent mutations in KCNJ10 and CAPN1, each associated with a specific clinical subtype of inherited ataxia. The aim of this study was to clarify whether or not Smooth-Haired Fox Terriers with hereditary ataxia and dogs of other related breeds harbor either of the same mutations. A sub goal was to update the results of KCNJ10 genotyping in Russell group terriers.

Findings

Three Smooth-Haired Fox Terriers with hereditary ataxia and two Toy Fox Terriers with a similar phenotype were all homozygous for the KCNJ10 mutation. The same mutation was also found in a heterozygous state in clinically unaffected Tenterfield Terriers (n = 5) and, in agreement with previous studies, in Jack Russell Terriers, Parson Russell Terriers, and Russell Terriers.

Conclusions

A KCNJ10 mutation, previously associated with an autosomal recessive spinocerebellar ataxia in Jack Russell Terriers, Parson Russell Terriers, and Russell Terriers segregates in at least three more breeds descended from British hunting terriers. Ataxic members of two of these breeds, the Smooth-Haired Fox Terrier and the Toy Fox Terrier, were homozygous for the mutation, strengthening the likelihood that this genetic defect is indeed the causative mutation for the disease known as “hereditary ataxia” in Fox Terriers and “spinocerebellar ataxia with myokymia, seizures or both” in the Russell group of terriers.  相似文献   
9.
Canine osteosarcoma is an aggressive cancer, comprising 85% of canine bone neoplasms. Current treatment practices of surgery and chemotherapy increase 1-year survival by only 45%. The curcumin analogue RL71, has demonstrated potent in vitro and in vivo efficacy in several models of human breast cancer through increased apoptosis and cell cycle arrest. Thus, the present study aimed to investigate efficacy of curcumin analogues in two canine osteosarcoma cell lines. Osteosarcoma cell viability was assessed using the sulforhodamine B assay and mechanisms of action were determined by analysing the levels of cell cycle and apoptotic regulatory proteins via Western blotting. Further evidence was obtained using flow cytometry to detect cell cycle distribution and the number of apoptotic cells. RL71 was the most potent curcumin analogue with EC50 values of 0.64 ± 0.04 and 0.38 ± 0.009 μM (n = 3) in D-17 (commercial) and Gracie canine osteosarcoma cells, respectively. RL71 significantly increased the ratio of cleaved-caspase 3 to pro-caspase 3 and the level of apoptotic cells at the 2× and 5× EC50 concentration (p < 0.001, n = 3). Furthermore, at the same concentration, RL71 significantly increased the number of cells in the G2/M phase. In conclusion, RL71 has potent cytotoxic activity in canine osteosarcoma cells triggering G2/M arrest and apoptosis at concentrations achievable in vivo. Future research should further investigate molecular mechanisms for these changes in other canine osteosarcoma cell lines prior to in vivo investigation.  相似文献   
10.
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