全文获取类型
收费全文 | 663篇 |
免费 | 47篇 |
专业分类
林业 | 24篇 |
农学 | 16篇 |
基础科学 | 8篇 |
103篇 | |
综合类 | 70篇 |
农作物 | 26篇 |
水产渔业 | 26篇 |
畜牧兽医 | 386篇 |
园艺 | 17篇 |
植物保护 | 34篇 |
出版年
2021年 | 11篇 |
2020年 | 8篇 |
2019年 | 10篇 |
2018年 | 8篇 |
2017年 | 10篇 |
2016年 | 12篇 |
2015年 | 10篇 |
2014年 | 14篇 |
2013年 | 40篇 |
2012年 | 27篇 |
2011年 | 35篇 |
2010年 | 30篇 |
2009年 | 25篇 |
2008年 | 41篇 |
2007年 | 39篇 |
2006年 | 44篇 |
2005年 | 30篇 |
2004年 | 31篇 |
2003年 | 33篇 |
2002年 | 39篇 |
2001年 | 13篇 |
2000年 | 4篇 |
1999年 | 9篇 |
1998年 | 16篇 |
1997年 | 8篇 |
1996年 | 19篇 |
1995年 | 7篇 |
1994年 | 10篇 |
1993年 | 10篇 |
1992年 | 11篇 |
1991年 | 14篇 |
1990年 | 7篇 |
1989年 | 11篇 |
1988年 | 8篇 |
1987年 | 5篇 |
1986年 | 5篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 4篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 3篇 |
1977年 | 3篇 |
1976年 | 4篇 |
1975年 | 3篇 |
1974年 | 5篇 |
1969年 | 3篇 |
1968年 | 2篇 |
1966年 | 2篇 |
1965年 | 2篇 |
排序方式: 共有710条查询结果,搜索用时 15 毫秒
31.
Larry G. Hansen Elizabeth M. Cozzi Robert L. Metcalf Terry K. Hansen 《Pesticide biochemistry and physiology》1985,24(1):136-148
The oxons of leptophos, its desbromo analog, and its ethoxy analog all inhibit hen brain neurotoxic esterase (NTE) to a similar degree in vitro, but have large differences in minimum effective oral doses for organophosphorus-induced delayed neurotoxicity. The potencies of leptophos and ethoxyleptophos are increased 10-fold when administered to hens intravenously (iv), but the potency of desbromoleptophos is nearly the same whether administered iv or po. Leptophos distributes nearly five times more rapidly from the central compartment than does desbromoleptophos, and its attenuated oral potency may be due to slower net absorption and/or dilution by compartments other than nervous tissue. Intravenously administered phenylphosphonothionates are excreted rapidly into the gastrointestinal tract and a large proportion of the dose is eliminated in the first 48 hr. Indirect evidence indicates that ethoxyleptophos is more rapidly degraded and ethoxyleptophos-inhibited NTE recovers more rapidly than does NTE inhibited by leptophos or desbromomleptophos. It is proposed that differential aging of ethoxyleptophos chiral isomers as well as pharmacokinetic factors may contribute to the apparently anomalous behavior of these three analogs. 相似文献
32.
Christel C. Kern Brian J. Palik Terry F. Strong 《Forest Ecology and Management》2006,230(1-3):162-170
We evaluated ground-layer plant diversity and community composition in northern hardwood forests among uncut controls and stands managed with even-age or uneven-age silvicultural systems. Even-age treatments included diameter-limit cuttings (20-cm diameter at 30-cm stem height) in 1952 and shelterwood removals in 1964. Uneven-age treatments included three intensities of selection harvest (light, 20.6 m2/ha residual basal area after harvest; medium, 17.2 m2/ha residual basal area; and heavy, 13.8 m2/ha residual basal area) that were applied in 1952, 1962, 1972, and 1982. All treatments were winter logged over snow pack. In 1991, plant diversity and community composition were examined. Species richness for spring ephemerals ranged from 1 to 6 species/150-m2, spring ephemeral diversity (Shannon's Index of Diversity) averaged 0.57 ± 0.04 and evenness averaged 0.45 ± 0.03. Summer flowering species richness ranged from 1 to 18 species/1-m2, with an average diversity of 0.71 ± 0.07 and evenness of 0.42 ± 0.03. We found no significant differences among treatments for any of these variables, although power to detect a difference (at p = 0.05) was low in all cases (0.15–0.55) due to high variance and low replication. Community composition was not significantly different among the treatments, for either spring (p = 0.09) or summer (p = 0.79) flora. Few exotic species were found in any treatment. Lack of exotic invasions and minimal differences in plant diversity or composition among treatments may be due, in part, to the negligible amount of soil disturbance that resulted from winter logging. While other (unmeasured) ecosystem components may differ among these silvicultural treatments, our results suggest that ground-layer plant communities in northern hardwood ecosystems are either resistant to change or have recovered within the 40 years since disturbance in the even-age treatments and within 10 years since disturbance in the uneven-age treatments. 相似文献
33.
34.
Field studies were conducted from 2011 to 2013 near Stuttgart, Arkansas. The impacts of nitrogen rate (0, 45, 90, 135, and 180 kg of N/ha) and harvest moisture content (HMC) (23, 19, and 15%, wet basis) on physicochemical properties and milling yields were determined. Trends were similar for the cultivars evaluated: Cheniere, CL XL745, and Wells. Milled rice yields were only minimally impacted by either N rate or HMC level. Increasing N rate reduced kernel length and thickness of brown rice, chalkiness of brown rice and head rice, and viscosities of head rice flour, and it increased brown rice and head rice crude protein content and head rice yield (HRY). In terms of milling yields and head rice functionality, these data suggest that N rates as low as 90 kg of N/ha could be utilized, should production recommendations be changed. Significant interactions between N rate and HMC level were infrequent and were associated with the 0 kg of N/ha rate, unrealistic for rice production. Decreasing HMC from 23 to 19% reduced kernel length and thickness and increased crude protein content and chalkiness; further decreasing HMC to 15% also increased kernel fissuring and decreased HRY. 相似文献
35.
36.
Eddy covariance (EC) systems are being used to assess the accuracy of remote sensing methods in mapping surface sensible and
latent heat fluxes and evapotranspiration (ET) from local to regional scales, and in crop coefficient development. Therefore,
the objective was to evaluate the accuracy of EC systems in measuring sensible heat (H) and latent heat (LE) fluxes. For this
purpose, two EC systems were installed near large monolithic weighing lysimeters, on irrigated cotton fields in the Texas
High Plains, during the months of June and July 2008. Sensible and latent heat fluxes were underestimated with an average
error of about 30%. Most of the errors were from nocturnal measurements. Energy balance (EB) closure was 73.2–78.0% for daytime
fluxes. Thus, daylight fluxes were adjusted for lack of EB closure using the Bowen ratio/preservation of energy principle,
which improved the resulting EC heat flux agreement with lysimetric values. Further adjustments to EC-based ET included nighttime
ET (composite) incorporation, and the use of ‘heat flux source area’ (footprint) functions to compensate ET when the footprint
expanded beyond the crop field boundary. As a result, ET values remarkably matched lysimetric ET values, with a ‘mean bias
error ± root mean square error’ of −0.03 ± 0.5 mm day−1 (or −0.6 ± 10.2%). 相似文献
37.
38.
39.
40.
Washburn KE Streeter RN Lehenbauer TW Snider TA Rezabek GB Ritchey JW Meinkoth JH Allison RW Rizzi TE Boileau MJ 《Journal of the American Veterinary Medical Association》2007,230(2):228-232
OBJECTIVE: To determine whether antemortem core needle biopsy and fine-needle aspiration of enlarged peripheral lymph nodes could be used to distinguish between inflammation and lymphosarcoma in cattle. DESIGN: Prospective study. ANIMALS: 25 cattle with enlarged peripheral lymph nodes. PROCEDURES: Antemortem biopsies of the selected lymph nodes were performed with an 18-gauge, 12-cm core needle biopsy instrument. Fine-needle aspirates were performed with a 20-gauge, 4-cm needle. Specimens were analyzed by pathologists who were unaware of clinical findings and final necropsy findings, and specimens were categorized as reactive, neoplastic, or nondiagnostic for comparison with necropsy results. RESULTS: Sensitivity and specificity of core needle biopsy ranged from 38% to 67% and from 80% to 25%, respectively. Sensitivity of fine-needle aspiration ranged from 41% to 53%, and specificity was 100%. Predictive values for positive test results ranged from 77% to 89% for core needle biopsy and were 100% for fine-needle aspiration. Predictive values for negative test results were low for both core needle biopsy and fine-needle aspiration. CONCLUSIONS AND CLINICAL RELEVANCE: Results indicated that core needle biopsy and fine-needle aspiration can aid in the antemortem diagnosis of bovine enzootic lymphosarcoma. Results of fine-needle aspiration of enlarged peripheral lymph nodes were more specific and more predictive for a positive test result than were results of core needle biopsy. 相似文献