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   274篇
  免费   15篇
林业   4篇
农学   4篇
基础科学   1篇
  24篇
综合类   66篇
农作物   10篇
水产渔业   34篇
畜牧兽医   135篇
园艺   3篇
植物保护   8篇
  2022年   3篇
  2021年   4篇
  2020年   7篇
  2019年   7篇
  2017年   4篇
  2016年   3篇
  2015年   7篇
  2014年   5篇
  2013年   9篇
  2012年   16篇
  2011年   14篇
  2010年   9篇
  2009年   11篇
  2008年   22篇
  2007年   21篇
  2006年   10篇
  2005年   18篇
  2004年   15篇
  2003年   15篇
  2002年   13篇
  2001年   6篇
  2000年   3篇
  1999年   5篇
  1996年   2篇
  1995年   4篇
  1993年   2篇
  1992年   3篇
  1991年   3篇
  1989年   2篇
  1987年   2篇
  1984年   5篇
  1983年   4篇
  1982年   1篇
  1981年   1篇
  1980年   2篇
  1979年   1篇
  1978年   4篇
  1977年   1篇
  1975年   4篇
  1973年   1篇
  1972年   1篇
  1970年   3篇
  1968年   1篇
  1966年   3篇
  1962年   1篇
  1961年   1篇
  1960年   1篇
  1937年   1篇
  1892年   1篇
  1886年   2篇
排序方式: 共有289条查询结果,搜索用时 443 毫秒
51.
Background: Bronchoalveolar lavage (BAL) fluid is evaluated for the diagnosis and study of lung disease and airway inflammation. Cytologic profiles for BAL fluid have not been reported for badgers and may be useful in understanding the pathogenesis of pulmonary diseases such as Mycobacterium bovis. Objective: The aim of this study was to evaluate cytologic and microbial findings in BAL fluid from captive European badgers (Meles meles) and identify correlates with the results of concurrently collected blood and fecal samples. Methods: BAL fluid (by a nonbronchoscopic method) and jugular venous blood samples (for routine CBC) were obtained from 23 captive tuberculosis‐free anesthetized badgers on 2 occasions 4 weeks apart. Fecal samples were collected for routine parasitology. Morphologic evaluation and 100‐cell differentials were done on cytocentrifuged BAL specimens. Pellets from centrifuged BAL were aerobically cultured for bacteria. Results: With the 2 BAL samples from each of the 23 badgers combined, the median (range) cell percentages were 73.0% (5–95%) neutrophils, 7.5% (2–16%) macrophages, 8.0% (0–27%) lymphocytes, and 9.5% (0–92%) eosinophils. Macrophages frequently contained silica‐like crystals. Other findings included ciliated epithelial cells, goblet cells, mucus, and Aelurostrongylus sp. larvae. A light growth of Streptococcus, Pasteurella, or Escherichia coli was cultured in 6 badgers. Trypanosoma pestanai were identified in blood from 10 badgers and fecal parasites (mainly coccidia) were found in 20 badgers. No correlation was found between BAL and CBC results and the presence of parasites. Conclusions: The predominance of neutrophils in BAL fluid from badgers differs from the predominance of macrophages found in BAL from other species. This difference may reflect the burrowing lifestyle or the unique immune response of badgers.  相似文献   
52.
53.
The objective of this study was to determine the effect of supplementing increasing levels of dried tagasaste (Chamaecytisus palmensis) leaf on the nutrition and performance of sheep fed a basal diet of barley straw. The study had four treatments, which involved supplementation of dried tagasaste leaf at 100, 200, 300, and 400 g/day. Twenty-four yearling Menz sheep (weight 17 ± 0.83 kg) were assigned to one of the four treatments in a randomized complete block design. The experiment comprised a feeding trial lasting for 90 days, a digestibility trial, and carcass evaluation using all animals. Samples of the feed consumed, refused, and feces were analyzed for nutrients. Intake and digestibility of dry matter (DM) and nutrients increased linearly (P < 0.001) as tagasaste supplementation increased from 100 to 400 g/day. Similarly, average daily body weight gain increased linearly (P < 0.001) from 20 to 73 g/day, feed conversion efficiency from 0.04 to 0.10, and dressing percentage from 40 to 48%, as the supplementation increased from 100 to 400 g/day. Proportion of tagasaste leaf in the diet increased from 20 to 50%, while the ratio of neutral detergent fiber (NDF) to crude protein (CP) decreased from 7.9 to 5.3, as the level of supplementation increased. It was thus concluded that supplementation of dried tagasaste leaf up to 50% of the diet DM, resulting in a NDF:CP ratio of 5.3, produces no deleterious effects on the performance of sheep, and inclusion to this level can be applied for superior growth performance and carcass yield in sheep fed crop residue-based diets.  相似文献   
54.
Production losses from abortions, stillbirths, and early calf mortality were described for the 2002 calf crop in 203 beef herds in western Canada. A total of 1689 calves were examined. A summary diagnosis was reported for 64% of aborted calves, 78% of stillborn calves, 88% of neonatal calves, and 94% of the calves > 3 d of age. Diagnoses for aborted calves included: thyroid gland lesions, pneumonia, developmental anomalies, placentitis, and myocardial necrosis or myopathy. For stillborn calves, diagnostic findings included: dystocia, thyroid gland lesions, myocardial necrosis or myopathy, developmental anomalies, and skeletal myopathy or necrosis. The most common diagnoses for neonatal calves (≤ 3 d) were: pneumonia, skeletal myopathy or necrosis, myocardial necrosis or myopathy, accident or trauma, and septicemia. For older calves (3 d to 3 mo), the most common diagnoses included: starvation, abomasal ulcer or perforation, enteritis or colitis, pneumonia, and intestinal volvulus, obstruction, or perforation.  相似文献   
55.
Pulsating aurora, a spectacular emission that appears as blinking of the upper atmosphere in the polar regions, is known to be excited by modulated, downward-streaming electrons. Despite its distinctive feature, identifying the driver of the electron precipitation has been a long-standing problem. Using coordinated satellite and ground-based all-sky imager observations from the THEMIS mission, we provide direct evidence that a naturally occurring electromagnetic wave, lower-band chorus, can drive pulsating aurora. Because the waves at a given equatorial location in space correlate with a single pulsating auroral patch in the upper atmosphere, our findings can also be used to constrain magnetic field models with much higher accuracy than has previously been possible.  相似文献   
56.
Streptococcus uberis, strain 0140J, contains a single copy sortase A (srtA), encoding a transamidase capable of covalently anchoring specific proteins to peptidoglycan. Unlike the wild-type, an isogenic mutant carrying an inactivating ISS1 insertion within srtA was only able to infect the bovine mammary gland in a transient fashion. For the first 24 h post challenge, the srtA mutant colonised at a similar rate and number to the wild type strain, but unlike the wild type did not subsequently colonise in higher numbers. Similar levels of host cell infiltration were detected in response to infection with both strains, but only in those mammary quarters infected with the wild type strain were clinical signs of disease evident. Mutants that failed to express individual sortase substrate proteins (sub0135, sub0145, sub0207, sub0241, sub0826, sub0888, sub1095, sub1154, sub1370, and sub1730) were isolated and their virulence determined in the same challenge model. This revealed that mutants lacking sub0145, sub1095 and sub1154 were attenuated in cattle. These data demonstrate that a number of sortase anchored proteins each play a distinct, non-redundant and important role in pathogenesis of S. uberis infection within the lactating bovine mammary gland.  相似文献   
57.
The exotic invasive insect, hemlock woolly adelgid (Adelges tsugae Annand), is causing mortality in eastern hemlocks (Tsuga canadensis [L.] Carr.) throughout the eastern U.S. Because hemlocks produce dense shade, and are being replaced by hardwood species that produce less shade, their loss may increase understory light levels. In the southern Appalachians, increases in light could increase stream temperatures, threatening species such as brook trout (Salvelinus fontinalis). We studied changes in light and stream temperature with eastern hemlock decline at a headwater southern Appalachian brook trout stream. Our results indicate that stream light levels have increased significantly with adelgid infestation. Leaf-on light levels are currently significantly higher (P < 0.02) in plots containing high basal areas of hemlock (mean global site factor (GSF)(SE) = 0.267(0.01)) compared with plots containing no hemlock (mean GSF(SE) = 0.261(0.01)), suggesting that increases in light have occurred with hemlock decline. The Normalized Difference Vegetation Index (NDVI), a remotely sensed metric of vegetation density, decreased with hemlock decline from 2001 to 2008. In 2001, NDVI showed no relationship (R2 = 0.003; F = 0.14; P = 0.71) with hemlock basal area, but by 2008, there was a significant negative relationship (R2 = 0.352; F = 19.55; P < 0.001) between NDVI and hemlock basal area. A gap experiment showed that light levels may increase by up to 64.7% more (mean increase in GSF = 27.5%) as hemlocks fall, creating gaps in the canopy. However, stream temperatures did not increase with hemlock decline during the study period, and we found that ground water inputs have a stronger influence on water temperature than light levels at this site. Linear regression showed a significant negative relationship between water temperature and proximity to ground water sources (R2 = 0.451; F = 13.14; P = 0.002), but no relationship between water temperature and light levels (R2 < 0.02; P > 0.05). In addition, by comparing light levels between plots containing hemlock and those containing only hardwoods, we found that if hemlocks are replaced by hardwoods, light levels under an all-hardwood canopy (mean GSF(SE) = 0.240(0.005)) are unlikely to be higher than they are under the current forest (mean GSF(SE) = 0.254(0.007)). These results suggest that loss of hemlock along southern Appalachian headwater streams could have short-term impacts on light levels, but that long-term changes in light levels, increases in water temperature, and adverse effects on brook trout may be unlikely.  相似文献   
58.
Ecological patterns and processes operate at a variety of spatial scales. Those which are regional in nature may not be effectively captured through the combination of conservation plans derived at the local level, where land use planning frequently takes place. Conversely, regional conservation plans may not identify resources important for conservation of intraregional ecological variation. We compare modeled conservation networks derived at regional and local scales from the same area in order to analyze the impact of scale effects on conservation planning. Using the MARXAN reserve selection algorithm and least cost corridor analysis we identified a potential regional conservation network for the Central Valley ecoregion of California, USA, from which we extracted those portions found within five individual counties. We then conducted the same analysis for each of the five counties. An overlay of the results from the two scales shows a general pattern of large differences in the identified networks. Especially noteworthy are the trade-offs and omissions evident at both scales of analysis and the disparateness of the identified corridors that connect core reserves. The results suggest that planning efforts limited to one scale will neglect biodiversity patterns and ecological processes that are important at other scales. An intersection of results from the two scales can potentially be used to prioritize areas for conservation found to be important at several spatial scales.  相似文献   
59.
60.
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