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   333篇
  免费   21篇
林业   13篇
农学   14篇
基础科学   7篇
  71篇
综合类   39篇
农作物   11篇
水产渔业   36篇
畜牧兽医   122篇
园艺   6篇
植物保护   35篇
  2023年   4篇
  2022年   3篇
  2021年   6篇
  2020年   2篇
  2019年   9篇
  2018年   7篇
  2017年   4篇
  2016年   6篇
  2015年   5篇
  2014年   10篇
  2013年   18篇
  2012年   17篇
  2011年   26篇
  2010年   11篇
  2009年   14篇
  2008年   22篇
  2007年   21篇
  2006年   21篇
  2005年   31篇
  2004年   20篇
  2003年   20篇
  2002年   18篇
  2001年   8篇
  2000年   5篇
  1999年   1篇
  1998年   5篇
  1997年   8篇
  1996年   2篇
  1995年   1篇
  1994年   1篇
  1993年   3篇
  1992年   2篇
  1990年   1篇
  1988年   3篇
  1986年   1篇
  1985年   3篇
  1984年   3篇
  1983年   5篇
  1982年   1篇
  1981年   1篇
  1979年   2篇
  1973年   1篇
  1963年   1篇
  1957年   1篇
排序方式: 共有354条查询结果,搜索用时 15 毫秒
81.
OBJECTIVE: To determine the prevalence of lameness as a function of season (summer vs winter), housing type (free stalls vs tie stalls), and stall surface (sand vs any other surface) among lactating dairy cows in Wisconsin. DESIGN: Epidemiologic survey. ANIMALS: 3,621 lactating dairy cows in 30 herds. PROCEDURE: Herds were visited once during the summer and once during the winter, and a locomotion score ranging from 1 (no gait abnormality) to 4 (severe lameness) was assigned to all lactating cows. Cows with a score of 3 or 4 were considered to be clinically lame. RESULTS: Mean +/- SD herd lameness prevalence was 21.1 +/- 10.5% during the summer and 23.9 +/- 10.7% during the winter; these values were significantly different. During the winter, mean prevalence of lameness in free-stall herds with non-sand stall surfaces (33.7%) was significantly higher than prevalences in free-stall herds with sand stall surfaces (21.2%), tie-stall herds with non-sand stall surfaces (21.7%), and tie-stall herds with sand stall surfaces (12.1%). CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that the prevalence of lameness among dairy cattle in Wisconsin is higher than previously thought and that lameness prevalence is associated with season, housing type, and stall surface.  相似文献   
82.
83.
This research compared ring block lidocaine anesthesia (L) and compression (C) for velvet antler removal in elk. Thirty-two wapiti were given 1 mg/kg body weight of zuclopenthixol acetate. The next day, they were restrained in a hydraulic chute and given either a compression device or a lidocaine ring block on the antler pedicle. Behavioral and physiological responses to treatment application and antler removal were recorded, and blood was collected for cortisol analysis. During application of L and C, increases in mean heart rate and systolic arterial blood pressure were greater in the C treatment group (P < 0.05, and P = 0.05, respectively). When antler was removed, more behavioral responses occurred in the C treatment group (P = 0.02) and its median behavior score was higher (P = 0.03). Mean heart rates increased for both treatment groups when antlers were removed (P < 0.01). It was concluded that application of C may be painful, and that C was not as effective as L for analgesia for velvet antler removal.  相似文献   
84.
The rates of uptake and loss of ethylenethiourea (ETU) in lettuces were studied following the application of ethylenethiourea or selected ethylenebisdithiocarbamate fungicides to the soil or to the leaves of potgrown plants. Analyses were carried out using high performance liquid chromatographic analysis of ETU. The amounts of ETU residues on leaves decreased rapidly and were negligible within a week, unless ethylenebisdithiocarbamate fungicides had been applied to the soil, as the latter were degraded simultaneously to give additional ETU which was then taken up by the plants.  相似文献   
85.
86.
Very high cypermethrin and fenvalerate resistance frequencies were recorded in Helicoverpa armigera (Hübner) populations in central India during the 1993–94, 1994–95 and 1995–96 cropping seasons. Synergism assays and biochemical analyses of detoxification enzyme levels indicated that mono-oxygenases and esterases were important metabolic mechanisms mediating pyrethroid resistance. Piperonyl butoxide- (PBO) and profenofos-suppressible pyrethroid resistance were correlated with enhanced levels of cytochrome P450 and general esterases respectively. Enzyme assay data indicated that high cytochrome P450 levels generally coincided with low esterase activity and vice versa. Similarly, synergist bioassays showed that PBO-insensitive resistance was frequently associated with profenofos-sensitive resistance and vice versa. Oxidase- and esterase-mediated mechanisms evidently alternated in a reciprocal manner, with perceptible shifts in relative importance occurring during mid-October in all three seasons and in late January in 1995. Apart from metabolic mechanisms, a synergist-insensitive resistance mechanism (believed to be nerve insensitivity), accounted for an average of 51, 30 and 28% of cypermethrin resistance during the 1993–94, 1994–95 and 1995–96 seasons respectively. © 1997 SCI.  相似文献   
87.
  • 1. A proposed trial reintroduction of the Eurasian beaver (Castor fiber L.) to Scotland has recently been approved (May 2008). A previous proposal was turned down by the licensing authority, partly over the perceived risks to woodland within a Special Area of Conservation.
  • 2. This paper presents data on two years of willow (Salix spp.) and aspen (Populus tremula L.) regrowth following tree felling by captive beavers within two large semi‐natural enclosures in eastern Scotland. Both willow and aspen are highly preferred by beavers.
  • 3. Regrowth of willow stems was quantified from cut stumps, felled logs and partially severed (‘incompletely felled’) trees, and compared with growth of stems in the canopies of untouched willows.
  • 4. Maximum annual regrowth was observed in incompletely felled trees, followed by cut stumps and then unfelled control trees. Mean regrowth rates were ca 18 m regrowth per metre of measured tree section per year in incompletely felled trees. This was approximately 12 times more annual regrowth than that observed in unfelled trees, despite apparently selective browsing of the regrowth of incompletely felled trees by roe deer.
  • 5. Although incompletely felled willows represent a relatively small component of felling activity (9% of all felled willow), they contribute disproportionately to the biomass of beaver‐engineered riparian woodlands, creating a more multi‐layered habitat structure.
  • 6. Willow regrowth in inundated areas was avoided by deer, and harvesting of regrowth by beavers was rare.
  • 7. Significant regrowth from 11 stumps of aspen in 2 years post‐felling occurred by suckering, with 85 suckers of mean height 2.1 m being recorded, all of which were subsequently avoided by browsing deer and beavers.
  • 8. These data suggest that rapid regeneration of willow and aspen will occur in riparian woodlands in the event of major felling activity by Eurasian beaver, even in the presence of low to moderate levels of roe deer browsing, and that the conservation status of both these trees or the wider habitats that they form would not be threatened by a well planned and managed reintroduction of beavers to Scotland.
Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   
88.
Seasonally managed wetlands in the Grasslands Basin on the west-side of California’s San Joaquin Valley provide food and shelter for migratory wildfowl during winter months and sport for waterfowl hunters during the annual duck season. Surface water supply to these wetlands contain salt which, when drained to the San Joaquin River (SJR) during the annual drawdown period, can negatively impact water quality and cause concern to downstream agricultural riparian water diverters. Recent environmental regulation, limiting discharges salinity to the SJR and primarily targeting agricultural non-point sources, now also targets return flows from seasonally managed wetlands. Real-time water quality management has been advocated as a means of continuously matching salt loads discharged from agricultural, wetland and municipal operations to the assimilative capacity of the SJR. Past attempts to build environmental monitoring and decision support systems (EDSS’s) to implement this concept have enjoyed limited success for reasons that are discussed in this paper. These reasons are discussed in the context of more general challenges facing the successful implementation of a comprehensive environmental monitoring, modelling and decision support system for the SJR Basin.  相似文献   
89.
A representative group of 1, 4-naphthoquinone derivatives, bearing alkoxy, phenoxy and acyloxy substitutents in the 2-position, has been synthesised and assayed against Botrytis cinerea, Cladosporium fulvum and Venturia inaequalis. The alkoxy compounds exhibited moderate activity, which was enhanced by the presence of a 3-chloro and, to a lesser extent, a 3-bromo substituent. 2-Chloro-3-phenoxy derivatives were highly active, but diphenoxy compounds were inactive. Acyloxy derivatives possessed only weak activity unless a halogen atom was present in the 3-position; benzoyloxy compounds, however, were generally poor fungicides.  相似文献   
90.
Politics     
New Zealand politics during the 1990s was dominated by debate over a new electoral system, MMP (mixed member proportional), recommended in 1986 and endorsed by voters in 1992 and 1993. The four elections held during the 1990s were influenced, and at times overshadowed, by moves to adopt a new voting system based on proportional representation principles. The loss of confidence in New Zealand's two major parties, National and Labour, was accompanied by a rise in support for smaller parties. A related theme of 1990s politics revolves around the political career of Winston Peters, whose odyssey parallels the rise and fall in public esteem of the new electoral system itself. By the end of the 1990s, it was far from clear that the new system – and the political leadership which has come to power under its auspices – would be able to achieve for New Zealanders the goals of consensus and a sense of common enterprise valued by so many.  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