全文获取类型
收费全文 | 3606篇 |
免费 | 254篇 |
国内免费 | 6篇 |
专业分类
林业 | 170篇 |
农学 | 58篇 |
基础科学 | 23篇 |
422篇 | |
综合类 | 908篇 |
农作物 | 85篇 |
水产渔业 | 368篇 |
畜牧兽医 | 1545篇 |
园艺 | 100篇 |
植物保护 | 187篇 |
出版年
2023年 | 24篇 |
2022年 | 22篇 |
2021年 | 61篇 |
2020年 | 74篇 |
2019年 | 95篇 |
2018年 | 86篇 |
2017年 | 105篇 |
2016年 | 83篇 |
2015年 | 75篇 |
2014年 | 88篇 |
2013年 | 148篇 |
2012年 | 169篇 |
2011年 | 230篇 |
2010年 | 143篇 |
2009年 | 103篇 |
2008年 | 182篇 |
2007年 | 211篇 |
2006年 | 169篇 |
2005年 | 181篇 |
2004年 | 166篇 |
2003年 | 160篇 |
2002年 | 125篇 |
2001年 | 57篇 |
2000年 | 58篇 |
1999年 | 39篇 |
1998年 | 17篇 |
1996年 | 23篇 |
1995年 | 29篇 |
1994年 | 21篇 |
1992年 | 38篇 |
1991年 | 36篇 |
1990年 | 27篇 |
1989年 | 34篇 |
1988年 | 20篇 |
1987年 | 46篇 |
1986年 | 66篇 |
1985年 | 61篇 |
1984年 | 44篇 |
1983年 | 44篇 |
1982年 | 36篇 |
1981年 | 36篇 |
1980年 | 36篇 |
1979年 | 36篇 |
1978年 | 36篇 |
1977年 | 28篇 |
1976年 | 28篇 |
1975年 | 28篇 |
1974年 | 32篇 |
1973年 | 19篇 |
1971年 | 17篇 |
排序方式: 共有3866条查询结果,搜索用时 15 毫秒
71.
72.
Salinity and geomorphology drive long‐term changes to local and regional fish assemblage attributes in the lower Pecos River,Texas 下载免费PDF全文
River systems throughout arid regions worldwide have been heavily impacted by human activities, resulting in long‐term ecological consequences. The lower Pecos River in the Trans‐Pecos region of Texas is no exception, having undergone anthropogenic changes that include decreased flow, elevated salinity, species loss and species invasion. We compared historical and contemporary fish assemblage attributes from the Pecos River at local (site‐specific) and regional (Trans‐Pecos region) scales across a 24‐year time period. Fish assemblage data were collected in October 1987 and 2011, by seining at 15 sites spanning 430 km of the river in Texas. Additionally, we examined contemporary environmental conditions to determine species–environment relationships. We found that fish assemblages were significantly different between time periods, likely due to increased salinisation in the upper half of the study region. Decreased species richness, species replacement and increases in euryhaline species were documented in the upstream sites. Freshwater springs lower the salinity and maintain flows in the downstream reach, allowing for maintenance of the native fish fauna. Careful management of regional aquifers, irrigation practices and petroleum waste water will be necessary for protecting biodiversity and environmental flows in the lower Pecos River. 相似文献
73.
Anaerobic Treatment of Brackishwater Aquaculture Sludge: An Alternative to Waste Stabilization Ponds
Natella Mirzoyan Ryan Christopher McDonald Amit Gross 《Journal of the World Aquaculture Society》2012,43(2):238-248
Environmental pressure, land utilization, and economic feasibility have resulted in the development of recirculating aquaculture systems (RAS). For many RAS, sludge is collected and washed from the system to waste stabilization ponds (WSPs). However, disposal of brackishwater aquaculture sludge into WSP is often prohibited because the high salinity can interfere with treatment. Moreover, there are problems associated with WSPs because of elevated salt content, such as the common practice of reusing treated water and land application of stabilized sludge. We tested and compared the treatment of brackishwater aquaculture sludge in an upflow anaerobic sludge blanket (UASB) reactor as an alternative to a WSP. In UASB, wastewater flows upward through a blanket of granular sludge and is treated by anaerobic micro‐organisms. Reduction in organic matter and 5‐d biochemical oxygen demand by 97 and 91%, respectively, was achieved in a UASB as compared to corresponding reductions of 22 and 41% in a WSP. During the UASB digestion process, methane is produced and recovered. Overall, a reduction in potential environmentally harmful factors such as salinization, land requirements, greenhouse gas emissions, as well as transportation costs are achieved, making the UASB reactor an attractive possible alternative for saline aquaculture sludge management. 相似文献
74.
Christopher Inglesfield 《Pest management science》1989,27(4):387-428
The discovery and development of the pyrethroid insecticides represents a major advance in the techniques of crop protection and disease vector control. These compounds combine outstanding efficacy against a broad spectrum of noxious insects with low toxicity to birds and mammals. This paper considers the effects of pyrethroids on other components of the terrestrial non-target fauna, with particular reference to effects on ‘beneficial’ organisms, including natural pest-control agents, pollinators, and organisms responsible for the maintenance of soil structure and fertility. This paper uses laboratory and field data to identify which groups of organisms may be potentially at risk from the use of pyrethroids, and draws on extensive field data to consider the significance of such hazards under conditions of normal use. Emphasis will be placed throughout on the evaluation of these compounds under practical conditions, including both crop and non-crop usages. Limitations to, and opportunities for, the use of pyrethroids are discussed, and the potential for further development is considered. 相似文献
75.
Steven W. Lindsay M. Ismai Hossainl Stephen Bennett Christopher F. Curtis 《Pest management science》1991,32(4):397-411
The insecticidal properties of twelve pyrethroid preparations impregnated into polyester netting were compared, before and after washing, with a view to assessing their suitability for treating bednets (mosquito nets) in the field. Bioassays were carried out by exposing the mosquitoes, Anopheles gambiae Giles sensu stricto andAedes aegypti L. to netting with two concentrations of insecticide, on washed and unwashed samples, for 30 s and 3 min at different times after impregnation. Mortality was recorded after 24 h. Permethrin applied at 97°C at pH 3.4 resulted in increased uptake by polyester fibres. The performance of most insecticides on unwashed netting had not declined appreciably 12 weeks after impregnation. However, all the insecticides were depleted by washing. The treatments with best insecticidal activity after washing were cypermethrin, lambda-cyhalothrin applied at ambient temperature and normal pH and permethrin applied in a hot acid solution. 相似文献
76.
Stuart M. Taylor Christopher T. Elliott William J. Blanchflower 《Pest management science》1987,21(1):39-43
Ten dairy cows were separated into two groups of five. One group was ear tagged with a PVC ear tag containing 8-5 g kg?1 cypermethrin in each ear. A PVC strip impregnated with the same quantity of cypermethrin was attached to the tail of each cow in the other group. The concentration of cypermethrin in hair samples from the neck, flanks and the anterior, sides and the posterior of the udder was measured for a period of 10 weeks after application. Concentrations were found to be high in those body areas nearest to the points of application, moderate on the flanks but low on the front and sides of the udder. 相似文献
77.
Zinc and copper status in ewes supplemented with sulfate- and amino acid-complexed forms of zinc and copper 总被引:3,自引:0,他引:3
Hatfield PG Swenson CK Kott RW Ansotegui RP Roth NJ Robinson BL 《Journal of animal science》2001,79(1):261-266
Thirty 6-yr-old Targhee ewes were randomly allotted to one of five supplemental treatments to evaluate supplementation effects on liver and fecal Zn and Cu concentrations and serum alkaline phosphatase activity: 1) control, 2) Zn complex, 3) Zn and Cu (ZnCu) complex, 4) Zn sulfate, and 5) ZnCu sulfates. Supplements were administered daily in gelatin capsules for 56 d. Liver biopsies and serum samples were collected every 14 d starting on d 0. Supplemental Zn and Cu levels were formulated to provide 90 mg/kg Zn and 10 mg/kg Cu, respectively, on a daily dry matter intake basis. Form (complex vs sulfate) x type (Zn vs ZnCu) interactions were not detected (P > 0.35). Therefore, contrast statements were used to make the following treatment comparisons: 1) control vs supplement, 2) Zn vs ZnCu, and 3) complex vs sulfate. Ewe BW at the end of the study (P = 0.09) and ewe BW change from beginning to end of the study (P = 0.07) were greater for supplemented than control ewes. Body weight and BW change did not differ between sulfate and complex (P > 0.39) or Zn- and ZnCu- (P > 0.40) supplemented ewes. Liver Cu concentrations did not differ (P = 0.41) between control and supplemented ewes. Liver Cu concentrations were higher (P < 0.10) for ewes supplemented with ZnCu than Zn and complex than sulfate forms of supplement. Liver Zn concentration tended (P = 0.13) to be higher in ZnCu than Zn-supplemented ewes. Liver and fecal Zn concentration were higher (P < 0.06) in ewes fed complex than sulfate supplements. However, serum alkaline phosphatase activity tended (P = 0.12) to be greater in ewes fed sulfate than complex supplements. Supplementing mature ewes with complexed minerals resulted in higher concentrations of Zn and Cu in the liver. In addition, supplemental Cu tended to increase concentrations of Zn in the livers of ewes; however, high levels of supplemental Zn did not negatively impact liver Cu concentrations. 相似文献
78.
79.
Alford P Geller S Richrdson B Slater M Honnas C Foreman J Robinson J Messer M Roberts M Goble D Hood D Chaffin M 《Preventive veterinary medicine》2001,49(3-4):209-222
Risk factors for equine laminitis were examined in a prospective case-control study of the 258 cases seen at six collaborating veterinary teaching hospitals over a 32-month period. Case-control pairs were matched on institution, clinician, and season of diagnosis. The 90% of case-control pairs (78 acute, 155 chronic) that had complete data for age, gender, and breed were used in separate conditional logistic-regression models for acute and chronic laminitis. There was an increase in risk for horses with acute laminitis from 5 to 7 years of age (OR 4.7, 95% CI 1.3–16) and from 13 to 31 years of age (OR 3.9, 95% CI 1.3–12) (both compared to <5 years); risk was increased for chronic laminitis from 10 to 14 years (OR 3, 95% CI 1.4–6.8) and from 15 to 38 years (OR 2.9, 95% CI 1.4–6.1) (both compared to <6 years). Mares — but not stallions — were more likely than geldings to develop acute laminitis (OR 2.6, 95% CI 1.1–6.2) and chronic laminitis (OR 2.0, 95% CI 1.1–3.6). In the small acute-laminitis data set, the breed variable was collapsed into three categories: Thoroughbred (THB, reference), the Quarter Horse (QH), and other (non-QH-THB). The non-QH-THB group was at increased risk of acute laminitis (OR 3.8, 95% CI 1.2–11.8). For the seven breed-group categories used in the chronic-laminitis model, however, all non-THB breed groups appeared significantly at risk as compared to the THB, with odds ratios ranging from 3.3 (95% CI 1.3–8.30) for the QH to 9.1 (95% CI 2.1–39.3) for ponies. 相似文献
80.
Jane Oakey Judy Forbes-Faulkner Christopher Gilpin Bronwyn Murdoch Steven Johnson 《Journal of veterinary diagnostic investigation》2005,17(4):378-382
In 2003, a steer carcass was condemned at a Central Queensland abattoir because of metastatic tumors. In addition, a granulomatous lesion was found in the mediastinal lymph node. Histological examination showed this to be a pyogranuloma, typically associated with Rhodococcus or the Nocardia/Streptomyces group. However, in this case, the only etiological agent was an acid-fast bacillus, which would normally be associated with a more fibrous lesion. A number of nucleic acid-based techniques were used, and the isolate was identified as Mycobacterium asiaticum. This organism is a rarely encountered opportunistic pathogen of humans, associated with subtropical climates. This is the first report of this organism causing infection in cattle. The similarities between this case and cases of human disease are discussed. 相似文献