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   2918篇
  免费   192篇
  国内免费   4篇
林业   198篇
农学   70篇
基础科学   22篇
  490篇
综合类   701篇
农作物   64篇
水产渔业   236篇
畜牧兽医   1027篇
园艺   90篇
植物保护   216篇
  2024年   6篇
  2023年   20篇
  2022年   28篇
  2021年   71篇
  2020年   77篇
  2019年   91篇
  2018年   80篇
  2017年   77篇
  2016年   89篇
  2015年   69篇
  2014年   79篇
  2013年   108篇
  2012年   192篇
  2011年   225篇
  2010年   146篇
  2009年   97篇
  2008年   224篇
  2007年   213篇
  2006年   198篇
  2005年   190篇
  2004年   194篇
  2003年   184篇
  2002年   177篇
  2001年   32篇
  2000年   23篇
  1999年   28篇
  1998年   16篇
  1997年   7篇
  1996年   18篇
  1995年   14篇
  1994年   12篇
  1993年   9篇
  1992年   5篇
  1991年   11篇
  1990年   6篇
  1989年   10篇
  1988年   6篇
  1987年   10篇
  1986年   7篇
  1985年   8篇
  1984年   4篇
  1982年   5篇
  1981年   3篇
  1980年   6篇
  1976年   5篇
  1974年   8篇
  1973年   3篇
  1970年   2篇
  1961年   2篇
  1957年   2篇
排序方式: 共有3114条查询结果,搜索用时 46 毫秒
991.
992.
The molecular motion of water was studied in glucono-δ-lactone-acidified skim milk powder (SMP) solutions with various pH values and dry matter contents. NMR relaxometry measurements revealed that lowering the pH in SMP solutions affected 17O and 1H T2 relaxation rates almost identically. Consequently, the present study indicates that the proteins present in the samples do not affect the 1H relaxation behavior markedly, even at relatively high SMP concentrations (15-25%). Comparison of rheological measurements and NMR measurements suggested that the collapse of κ-casein during acidification could contribute to the initial decrease in 17O and 1H relaxation rate in the pH range between 6.6 and 5.5 for 15% SMP and in the pH range between 6.6 and 5.9 for 25% SMP. However, below pH 5.5 the viscosity and 17O and 1H NMR relaxation rates did not correlate, revealing that the aggregation of casein micelles, which increases viscosity below pH 5.5, does not involve major repartitioning of water.  相似文献   
993.
Spatial prioritisations are increasingly being undertaken to allocate the investment of limited conservation resources. These techniques are highly effective at identifying where important areas for conservation are located, but of limited use for deciding how implementation of conservation action should be undertaken. Conservation planning should be a collaborative social process, and necessitates that spatial prioritisations be complemented with an implementation strategy. We undertook a conservation planning initiative in the Subtropical Thicket biome in South Africa, complementing a spatial prioritisation with an implementation strategy. Believing strongly in the importance of developing a ‘safe-fail’ culture in conservation, we detail our activities and document our successes and failures. We synthesize lessons from our experience of collaboratively developing an implementation strategy with stakeholders, so as to ensure it effectively complements a spatial prioritisation.  相似文献   
994.
Spatial prioritization techniques are increasingly applied in the design of protected area networks, which are regarded as the cornerstone of nature conservation efforts. These techniques are becoming ever more sophisticated, but are still founded primarily upon biological data. A common assumption made in most spatial prioritizations is that land throughout a planning region is available for acquisition. We interviewed land managers in the Eastern Cape province, South Africa, and mapped their willingness-to-sell their land using a psychometric analytical technique. We examined the, (i) degree to which vegetation type targets are achieved across a planning region, (ii) areal and cost efficiency, and (iii) spatial configuration, of candidate protected areas identified as important for achieving conservation targets. We found that only 10 out of 48 land managers were willing-to-sell their land. Only seven, five and one of the 19 vegetation types, respectively, could achieve their conservation targets of 10%, 30% and 50% when unwilling land managers were removed from the analysis. Assuming unwilling land managers could be convinced to sell if offered a premium price, the cost of acquiring all lands was between 6.20% and 30.67% more expensive than 2006 land prices. Accounting for implementation opportunities and constraints, such as land manager willingness-to-sell, not simply identifying biological priorities, is of fundamental importance for ensuring spatial prioritizations deliver maps with the potential to usefully guide expansion of protected area networks which can be feasibly implemented.  相似文献   
995.
Escherichia coli O157:H7 remains a threat to humans via cattle-derived fecal contamination of food and water. Preharvest intervention strategies represent a means of reducing the pathogen burden before harvest. In this study, the efficacy of a commercially produced type III secreted protein (TTSP) vaccine was evaluated with the use of a commingled experimental calf infection model (30 placebo-treated animals and 30 vaccinates). The calves were vaccinated on days 0, 21, and 42 and infected with 10(9) colony-forming units (CFU) of E. coli O157 by oral-gastric intubation on day 56. Fecal shedding was monitored daily for 14 d. Serologic assessment revealed a robust immune response to vaccination; the serum titers of antibodies against EspA, Tir, and total TTSPs were significantly higher in the vaccinates than in the placebo-treated animals on days 21, 42, 56, and 70. Significantly less (P = 0.011) of the challenge organism was shed by the vaccinates than by the placebo-treated animals on days 3 to 10. Peak shedding occurred in both groups on days 3 to 6; during this period the vaccinates showed a mean log reduction of 1.4 (P = 0.002) and a mitigated fraction of 51%. The number of animals shedding was significantly lower among the vaccinates compared with the placebo group on days 3 to 6 (P ≤ 0.05), with a mean prevented fraction of 21%. No differences in the duration of shedding were observed. Owing to the low challenge shedding in both groups on days 11 to 14 (mean CFU/g < 10; median = 0), no significant differences were observed. These data indicate that TTSP vaccination had protective effects through significant reductions in the number of animals shedding and the number of challenge organisms shed per animal and provides evidence that TTSP vaccination is an effective preharvest intervention strategy against E. coli O157.  相似文献   
996.
Ammonia concentrations in water can affect the severity of Flavobacterium columnare infections in fish. Two trials lasting 7 d each were conducted to determine the effect of a single immersion flush treatment of total ammonia nitrogen (TAN; 15 mg/L) on the survival of channel catfish Ictalurus punctatus infected with E columnare; the chemical was added while the water flowed continuously through the tanks. Both trials consisted of four treatments: (1) no ammonia exposure and no bacterial challenge (control), (2) ammonia exposure only, (3) bacterial challenge only, and (4) both ammonia exposure and bacterial challenge. Two hours after exposure to ammonia, the highest un-ionized ammonia level was 0.43 mg/L. The percent un-ionized ammonia is based on TAN, temperature, and pH. Caudal fins from three fish in each treatment were sampled at 24 h posttreatment to be analyzed by quantitative real-time polymerase chain reaction (qPCR). No significant difference in survival (mean +/- SE) was noted between the channel catfish in treatment 1 (95.2 +/- 1.2%) and those in treatment 2 (95.6 +/- 1.0%); however, survival in both treatments 1 and 2 differed significantly from that in treatments 3 (8.5 + 4.5%) and 4 (41.8 +/- 12.7%). Treatment 4 catfish had significantly higher survival than treatment 3 catfish. Quantitative PCR data showed that treatment 4 fish had significantly less F. columnare (7.6 x 10(5)) than did treatment 3 fish (1.2 x 10(7)), and treatment 2 fish (8.5 x 10(3)) had significantly less bacteria than did treatment 1 fish (6.9 x 10(4)), indicating that ammonia limited the F. columnare infection. The highest mean concentration of the bacteria (3.9 x 10(7)) was found on moribund fish. The ammonia concentrations tested did not negatively influence fish survival but interfered with the infection process. An in vitro assay was also conducted to evaluate the direct effects of ammonia on F columnare.  相似文献   
997.
Head and lateral line erosion syndrome (HLLES) is a common but very poorly understood disease of marine aquarium fish. One suspected etiology is the use of granulated activated carbon (GAC) to filter the water. Seventy-two ocean surgeons Acanthurus bahianus were distributed among three carbon-negative control systems and three GAC-treated systems such that each tank contained approximately the same total body mass. Each replicate system was made up of two 250-L circular tanks with a common filtration system (6 fish per tank, 12 fish per replicate system). The GAC-treated tanks were exposed to full-stream, extruded coconut shell activated carbon, which produced a mean total organic carbon content of 0.4 mg/L. The results of this study indicate that extruded coconut shell activated carbon filtering at full-stream rates can cause HLLES-type lesions in ocean surgeons. The HLLES developed exponentially over 15 d, beginning in the chin region. This was followed by pitting in the cheek region, which expanded until erosions coalesced. Once the carbon was discontinued, the processes reversed in a mean time of 49 d. As the lesions healed, they reverted from the coalesced to the pitted stage and then darkened before returning to normal.  相似文献   
998.
A 2-yr-old male intact alpaca (Vicugna pacos) was admitted for a 4-day history of anorexia and colic. Five months prior, the alpaca had undergone surgical removal of a duodenal trichophytobezoar and had recovered uneventfully. The alpaca died under anesthesia, and diaphragmatic herniation of the third gastric compartment (C3) was diagnosed at necropsy. A defect was identified in the left dorsal hemidiaphragm accompanied by herniation of 80% of C3 and the aboral portion of the second gastric compartment into the pericardial sac. The smooth margins and dorsal location of the diaphragmatic defect suggested a congenital origin. Diaphragmatic herniation is uncommon in camelids, and only one other case has been reported. Due to the dorsal location of the diaphragmatic defect in this animal, positioning during the previous surgery may have initiated a partial entrapment of gastric compartments, leading to a more complete incarceration between when the animal was discharged and presented again.  相似文献   
999.
Studies from both tropical and temperate systems show that scatter‐hoarding rodents selectively disperse larger seeds farther from their source than smaller seeds, potentially increasing seedling establishment in larger‐seeded plants. Size‐biased dispersal is evident in many oaks (Quercus) and is true both across and within species. Here, we predict that intraspecifc variation in seed size also influences acorn dispersal by the Blue Jay (Cyanocitta cristata Linnaeus), but in an opposite manner. Blue Jays are gape‐limited and selectively disperse smaller acorn species (e.g. pin oaks [Quercus palustris Münchh]), but often carry several acorns in their crop during a single dispersal event. We predict that jays foraging on smaller acorns will load more seeds per trip and disperse seeds to greater distances than when single acorns are carried in the bill. To test this, we presented free‐ranging Blue Jays with pin oak acorns of different sizes over a 2‐year period. In each of 16 experimental trials, we monitored the birds at a feeding station with remote cameras and determined the number of acorns removed and the distance acorns were dispersed when cached. Jays were significantly more likely to engage in multiple seed loading with smaller seeds in both years of the study. During the second year, these smaller acorns were dispersed farther than larger acorns, and during the first year, larger acorns were dispersed farther, revealing an inconsistent response to seed size during our study. We suggest that in some circumstances, multiple seed loading by Blue Jays may favor dispersal in some plant species.  相似文献   
1000.
The purpose of this study was to quantify the effect of flaxseed (Linum usitatissimum) supplementation on the skin test response of atopic horses. Six horses that displayed a positive skin test for allergy to extract from Culicoides sp. participated in the 42-day, placebo-controlled, double-blind, cross-over trial. Results showed that supplementation with flaxseed for 42 days in our experimental horses reduced the mean skin test response to Culicoides sp. This observation was concurrent with a significant decrease in the long-chain saturated fatty acids; behenic acid (22:0) and lignoceric acid (24:0), in the hair of horses receiving flaxseed. There was also a significant decrease in aspartate aminotransferase, and increase in serum glucose in the treatment animals at specific sampling points. It was concluded that; in this small pilot study, flaxseed was able to reduce the lesional area of the skin test response of atopic horses, alter the fatty acid profile of the hair, reduce inflammation, and did not elicit any negative side-effects in the experimental horses.  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