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   2049篇
  免费   136篇
  国内免费   1篇
林业   135篇
农学   76篇
基础科学   14篇
  278篇
综合类   352篇
农作物   83篇
水产渔业   237篇
畜牧兽医   820篇
园艺   62篇
植物保护   129篇
  2023年   17篇
  2022年   11篇
  2021年   39篇
  2020年   56篇
  2019年   60篇
  2018年   63篇
  2017年   57篇
  2016年   48篇
  2015年   45篇
  2014年   62篇
  2013年   85篇
  2012年   111篇
  2011年   163篇
  2010年   79篇
  2009年   64篇
  2008年   118篇
  2007年   117篇
  2006年   105篇
  2005年   102篇
  2004年   117篇
  2003年   115篇
  2002年   96篇
  2001年   30篇
  2000年   15篇
  1999年   15篇
  1998年   14篇
  1997年   18篇
  1996年   9篇
  1995年   25篇
  1994年   14篇
  1993年   8篇
  1992年   20篇
  1991年   27篇
  1990年   11篇
  1989年   20篇
  1988年   10篇
  1987年   26篇
  1986年   15篇
  1985年   13篇
  1984年   13篇
  1983年   15篇
  1982年   14篇
  1981年   9篇
  1980年   8篇
  1979年   8篇
  1978年   11篇
  1977年   9篇
  1973年   10篇
  1972年   8篇
  1970年   8篇
排序方式: 共有2186条查询结果,搜索用时 15 毫秒
91.
A geographic information system (GIS) is being developed to study the spread of pseudorabies virus (PRV) among swine herds in the state of Minnesota. This GIS features an interface with a database management system that stores and manages pertinent data. These data include herd size, type of production system, degree of confinement, topographical features surrounding the farm, density of swine herds and distance to the closest quarantined herd. A pilot study was conducted in one Minnesota county with 280 swine herds, of which the PRV status was known in 115. Cox regression analysis was used to determine factors associated with herd PRV status. Relative risks (RR) and 95% confidence intervals (CI) were calculated. No association was detected between the PRV status of the herd and distance to the nearest county road, highway or quarantined herd. However, the following factors were significant: located within 1 km of a river or lake (RR, 0.524; CI, 0.328–0.828); farrow to finish (RR, 2.120; CI, 1.194–3.791); complete confinement (RR, 3.423; CI, 1.639–7.139); density of swine herds within a 5 km radius (RR, 1.036 per herd; CI, 0.996–1.064).  相似文献   
92.
Abstract— Plasma concentrations of insulin-like growth factor-I (IGF-I) and insulin-like growth factor binding proteins (IGFBPs) were compared between market sized sunshine and palmetto bass. Differences between male and female fish were examined and both hybrids were sampled before and after a low-water confinement stress. There was no difference in body weight between the hybrid types, however, females were significantly heavier than males in both hybrids. Plasma IGF-I concentrations were significantly higher in pre-stressed palmetto bass when compared with sunshine bass, and male palmetto bass had significantly higher IGF-I concentrations than females. Two hours after a 15-min low-water stress, a significant decrease in palmetto bass IGF-I occurred, but there was no change in sunshine bass IGF-I concentrations. Levels of a 33-kDa IGFBP were similar in pre-stressed palmetto bass when compared with sunshine bass; however, no differences due to gender were evident for either hybrid. Two hours after a 15-min low-water exposure, there was no change in the levels of a 33-kDa IGFBP in palmetto or sunshine bass. A more complete understanding of IGF-I, IGFBPs, and IGF-I receptors may be necessary to explain the role of IGF-I in regulating fish growth.  相似文献   
93.
Abstract.— Similarities among multi-locus DNA fingerprints of five channel catfish Ictalurus punctatus strains and the ability to identify the strain of a fish based on its fingerprint pattern were investigated. Five restriction enzymes and 13 multi-locus DNA probes were screened to identify enzyme-probe combination useful for DNA fingerprinting channel catfish. Restriction enzymes Hinf I and Dpn II, in combination with probes (CAC)n, (CGC)n, (CTC)n, (ATCC)n, and (GATA)n, produced useful fingerprints (20–30 resolvable bands for each enzyme-probe combination). Thirty individuals (3 pools of 10 individuals each) from each of five channel catfish strains (albino, Mississippi normal, USDA-102, USDA-102 select, and USDA-103) were fingerprinted with all useful enzyme-probe combinations. Band sharing among samples was higher within strains than among strains and band sharing among strains was higher for strains whose breeding history indicated a high degree of relatedness. Individual fingerprints of 18 fish from each of the USDA-102 select and USDA-103 strains revealed no strain-specific bands, but several diagnostic bands (present at high frequencies in either USDA-102 select or USDA-103 strains and at a low frequencies in other strains) were identified. Band sharing at diagnostic bands was used to correctly identify fish as USDA-102 select or USDA-103 strains with 82% accuracy from fingerprints of 17 USDA-102 select strain fish, 18 USDA-103 strain fish, and 38 fish collected from three commercial farms.  相似文献   
94.
Monoclonal antibodies (mAbs) specific for bovine CD4 and CD5 antigens have been found to identify polymorphic determinants on these molecules. In the case of CD5, mAb IL-A67 recognises one allotypic form of the antigen while four other CD5-specific mAbs in the workshop (CC17, CC29, BLT-1 and 8C11) recognise a second allotype. The CD4-specific mAbs submitted to the workshop reacted with the cells of all animals tested. However, a further two mAbs (CC26 and IL-A18) specific for CD4 were found to react with cells only from about 85% of animals tested. Sequential immuno-precipitation experiments together with family studies showed that the allotypes of CD4 and CD5 are both inherited in a simple Mendelian manner and are co-dominantly expressed. One of the CD5 allotypes was not detected in Bos taurus animals while the gene frequency of the second allotype was only about 10% in the B. indicus animals tested. The gene frequency of the CD4 allotype detected by CC26 and IL-A18 was similar in the two sub-species.  相似文献   
95.
Following reports of an increase in the number of Corynebacterium pseudotuberculosis infections in horses in the western counties of Colorado in 1997, an epidemiological assessment of the problem was conducted using a survey of premises with affected horses. No gender, age or breed predilections were found. Most diagnoses of disease were made in the summer or fall of 1997. Abscesses were reported to occur in various locations with a mean healing time of 77 days. A majority of premises owners reported potential contact of horses with sheep or goats. The days of lost use and treatment costs per case had a mean of 13 days and $139, respectively.  相似文献   
96.
97.
A whole-farm mathematical programming model has been built for dryland crop-livestock farms in Western Australia. The multidisciplinary approach used for model building is presented and the resulting model structure is described. It is a mixed integer programming model which represents, in some detail, the biological, technological and financial relationships of the farming system and stresses the interdependencies of enterprises. The model is used to investigate the extent to which positive interactions between different enterprises influence the optimal farm plan. The management issue considered is the division of land between crop and pasture production. It is concluded that, for the farming system considered, interactions do have an influence on profit and the optimal farm plan.  相似文献   
98.
99.
OBJECTIVE: To optimize methods for the use of computed tomography (CT) to assess pathologic changes in the lungs of calves and to determine the effect of treatment on lung consolidation. ANIMALS: 10 male Holstein calves. PROCEDURES: Calves were anesthetized to facilitate CT imaging of the thorax. After initial images were obtained, pneumonia was induced in the calves by inoculation through a bronchoscope. Two calves were used in a preliminary study to refine the inoculation dose and optimize CT images. Four calves were administered florfenicol and 4 calves were untreated control animals. Serial images were obtained 24, 48, and 72 hours after inoculation. After final images were obtained, calves were euthanized, and lung consolidation was estimated by use of lung surface area scoring and water displacement. These estimates were compared with estimated lung consolidation obtained by use of CT. RESULTS: Calves had rapid disease progression. Percentage of lung consolidation was not significantly different between treatment groups for any of the estimation methods. Results of an ANOVA of the 3 assessment methods indicated significant differences among methods. Estimates of the percentage of lung consolidation obtained by use of surface area scoring and CT correlated well, whereas water displacement estimates correlated poorly with other methods of consolidation estimation. CONCLUSIONS AND CLINICAL RELEVANCE: Because of the correlation with other methods for estimation of lung consolidation, CT has the potential to be used to monitor disease progression in calves with experimentally induced respiratory tract disease.  相似文献   
100.
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