全文获取类型
收费全文 | 1945篇 |
免费 | 34篇 |
国内免费 | 4篇 |
专业分类
林业 | 49篇 |
农学 | 5篇 |
127篇 | |
综合类 | 690篇 |
农作物 | 50篇 |
水产渔业 | 32篇 |
畜牧兽医 | 968篇 |
园艺 | 5篇 |
植物保护 | 57篇 |
出版年
2012年 | 109篇 |
2011年 | 162篇 |
2010年 | 30篇 |
2009年 | 17篇 |
2008年 | 106篇 |
2007年 | 139篇 |
2006年 | 138篇 |
2005年 | 136篇 |
2004年 | 118篇 |
2003年 | 133篇 |
2002年 | 121篇 |
2001年 | 75篇 |
2000年 | 129篇 |
1999年 | 59篇 |
1998年 | 9篇 |
1997年 | 14篇 |
1996年 | 19篇 |
1995年 | 22篇 |
1994年 | 21篇 |
1993年 | 18篇 |
1992年 | 19篇 |
1991年 | 31篇 |
1990年 | 22篇 |
1989年 | 27篇 |
1988年 | 7篇 |
1987年 | 16篇 |
1986年 | 15篇 |
1985年 | 10篇 |
1984年 | 8篇 |
1983年 | 18篇 |
1982年 | 13篇 |
1981年 | 11篇 |
1980年 | 11篇 |
1979年 | 12篇 |
1978年 | 9篇 |
1976年 | 10篇 |
1975年 | 5篇 |
1972年 | 8篇 |
1970年 | 8篇 |
1969年 | 6篇 |
1967年 | 6篇 |
1963年 | 8篇 |
1958年 | 6篇 |
1957年 | 11篇 |
1956年 | 11篇 |
1955年 | 6篇 |
1954年 | 6篇 |
1953年 | 13篇 |
1948年 | 6篇 |
1947年 | 4篇 |
排序方式: 共有1983条查询结果,搜索用时 31 毫秒
91.
In the fall of 1999, West Nile virus (WNV) was isolated for the first time in the Western Hemisphere during an outbreak of neurologic disease in humans, horses, and wild and zoo birds in the northeastern United States. Chickens are a potential reservoir for WNV, and little is known about the pathogenicity of WNV in domestic chickens. Seven-week-old chickens derived from a specific-pathogen-free flock were inoculated subcutaneously with 1.8 x 10(3) 50% tissue culture infectious dose of a crow isolate of WNV in order to observe clinical signs and evaluate the viremic phase, gross and microscopic lesions, contact transmission, and immunologic response. There were no observable clinical signs in the WNV-inoculated chickens during the 21-day observation period. However, histopathologic examination of tissues revealed myocardial necrosis, nephritis, and pneumonitis at 5 and 10 days postinoculation (DPI); moderate to severe nonsuppurative encephalitis also was observed in brain tissue from one of four inoculated birds examined at 21 DPI. WNV was recovered from blood plasma for up to 8 DPI. Virus titers as high as 10(5)/ml in plasma were observed at 4 DPI. Fecal shedding of virus was detected in cloacal swabs on 4 and 5 DPI only. The WNV also was isolated from myocardium, spleen, kidney, lung, and intestine collected from chickens euthanatized at 3, 5, and 10 DPI. No virus was isolated from inoculated chickens after 10 DPI. Antibodies specific to WNV were detected in inoculated chickens as early as 5 DPI by the plaque reduction neutralization test and 7 DPI by the indirect fluorescent antibody test. Chickens placed in contact with inoculated chickens at 1 DPI lacked WNV-specific antibodies, and no WNV was isolated from their blood plasma or cloacal swabs throughout the 21 days of the experiment. 相似文献
92.
Sprenger SJ Halvorson DA Nagaraja KV Spasojevic R Dutton RS Shaw DP 《Avian diseases》2000,44(3):725-729
Ornithobacterium rhinotracheale is a gram-negative, rod-shaped, pleomorphic bacterium that has been isolated from flocks of turkeys and broilers from around the world. Infections cause respiratory disease, mortality, and growth suppression, or clinical signs of infection may be absent. In layers, there have been few reports of disease caused by O. rhinotracheale. This is the first report of O. rhinotracheale infection in United States layer flocks. 相似文献
93.
Data derived from 340 dairy herds, mainly in southern England, between April 1998 and March 1999, showed that the average total culling rate was 22.1 per cent, with 5.6 per cent for infertility, 3.6 per cent for mastitis, 1.7 per cent for lameness, 2.0 per cent for poor milk yield, 3.7 per cent for age and 5.5 per cent for miscellaneous reasons which included death. The average annual rate of assisted calvings was 8.7 per cent, of injury 0.9 per cent, digestive disease 1.3 per cent ketosis 0.4 per cent, hypomagnesaemia 0.7 per cent, hypocalcaemia 5.3 per cent, mastitis 36.6 per cent, and lameness 23.7 per cent. There was a significant association (P<0.001) between higher rates of mastitis in cows housed in straw yards as opposed to cubicles and also between higher rates of lameness in cows housed in cubicles as opposed to yards (P<0.015). However, there were farms with low rates of mastitis in cows kept in straw yards and low rates of lameness in cows kept in cubicles. Larger herds tended to have more problems with lameness and higher bulk milk somatic cell counts (BMSCC). There was a positive association between BMSCC and mastitis rate. 相似文献
94.
95.
Miller CB Wilson DA Keegan KG Kreeger JM Adelstein EH Ganjam VK 《Veterinary surgery : VS》2000,29(1):1-7
OBJECTIVE: To determine if there is a difference in in vitro growth of fibroblasts isolated from the trunk and distal aspect of the limb of horses and ponies. To determine the effects of a corticosteroid and monokine on in vitro growth of fibroblasts isolated from the trunk and distal aspect of the limb of horses and ponies. STUDY DESIGN: Growth of fibroblasts from tissues harvested from the trunk and limb were compared from horse and pony samples grown in control media and control media with triamcinolone or monokine added. ANIMALS OR SAMPLE POPULATION: Dermal and subcutaneous tissue from 22 horses and 17 ponies of various ages and breeds. METHODS: Fibroblast growth was assessed by tritiated thymidine uptake using standard cell culture techniques. The effect of a monokine or triamcinolone plus control media were compared with control media for fibroblast growth. RESULTS: Fibroblast growth from tissues isolated from the horse limb was significantly less than growth from the horse trunk and the limb and trunk of ponies. Monokine was more effective than triamcinolone in suppressing fibroblast growth from tissues isolated from the trunk and limb in both horses and ponies. CONCLUSIONS: There are growth differences in fibroblasts isolated from the limb of horses compared with those isolated from the trunk and from the limb and trunk of ponies. CLINICAL RELEVANCE: The difference in fibroblast growth from tissues isolated from the trunk and limb of horses and ponies may provide evidence for the difference reported in the healing characteristics of limb wounds in horses and ponies. Influencing fibroblast growth may provide a key to controlling the development of exuberant granulation tissue in horses and ponies. 相似文献
96.
Stockham SL Kjemtrup AM Conrad PA Schmidt DA Scott MA Robinson TW Tyler JW Johnson GC Carson CA Cuddihee P 《Veterinary pathology》2000,37(1):11-21
A 6-year-old Simmental cow infected with Theileria buffeli had a clinical disease characterized by theilerial parasitemia, macrocytic normochromic anemia with acanthocytosis and spherocytosis, lymphoid hyperplasia (lymphocytosis, edematous lymphadenomegaly), dysproteinemia, evidence of liver disease, and a low serum antibody titer against T. buffeli. The cow was in a herd in which all cattle originated in Missouri; 22/75 (29%) of cattle had a theilerial parasitemia and 26/75 (35%) had titers to T. buffeli of > or =1:160. Classification of the Missouri bovine organism as T. buffeli was based on DNA sequencing and comparison to sequences for T. buffeli and Theileria sp. type A obtained from GenBank. Intraerythrocytic veils and piroplasms were seen during transmission electron microscopy. The organism was successfully transmitted to two splenectomized calves, which developed mild anemias while parasitemic. Blood from the second calf was used as the source of T. buffeli antigen for an indirect immunofluorescence antibody test. Theilerial isolates from a Missouri white-tailed deer were also sequenced and resembled Theileria sp. types F and G and were not consistent with the bovine organism. 相似文献
97.
ABSTRACT Coronatine is a non-host-specific chlorosis-inducing phytotoxin produced by the tomato and crucifer pathogen Pseudomonas syringae pv. tomato DC3000. How the chromosomal gene cluster controlling toxin synthesis in this strain is regulated in planta is unknown. Ice nucleation-active cor:inaZ marker-exchange derivatives of strain DC3000 were used to determine coronatine gene expression in various host and nonhost plants and in a minimal medium supplemented with selected tomato plant constituents. Ice nucleation activity, which was first detected 4 h after inoculation, was highest in cabbage, tomato, and soybean and lowest in melon and cucumber. No correlation existed between bacterial population size and expression level on the various plants. Crude tomato leaf extract and intercellular fluid were strong inducers of toxin synthesis. Based on high-performance liquid chromatography analyses and bioassays, we concluded that the active components of both preparations were malic and citric acids, with minor contributions coming from shikimic and quinic acid. Although several compounds including glucose and inositol activated the toxin genes when tested at high concentrations (3 to 5 mM), shikimic and quinic acids were the only ones with activity at concentrations below 0.1 mM. Neither acid could be used as a sole carbon source by strain DC3000. The signal activity of shikimic acid was enhanced 10-fold by the addition of glucose. None of the plant phenolics that we screened affected coronatine gene expression. 相似文献
98.
99.
King DA 《Tree physiology》1991,9(3):369-381
Relationships between tree height and crown dimensions and trunk diameter were determined for shade-tolerant species of old-growth forests of western Oregon. The study included both understory and overstory species, deciduous and evergreen angiosperms and evergreen conifers. A comparison of adult understory species with sapling overstory species of similar height showed greater crown width and trunk diameter in the former, whether the comparison is made among conifers or deciduous trees. Conifer saplings had wider crowns than deciduous saplings, but the crown widths of the two groups converged with increase in tree height. Conifer saplings had thicker trunks than deciduous saplings of similar crown width, possibly because of selection for resistance to stem bending under snow loads. The results suggest that understory species have morphologies that increase light interception and persistence in the understory, whereas overstory species allocate their biomass for efficient height growth, thereby attaining the high-light environment of the canopy. The greater crown widths and the additional strength requirements imposed by snow loads on conifer saplings result in less height growth per biomass increment in conifer saplings than in deciduous saplings. However, the convergence in crown width of the two groups at heights greater than 20 m, and the proportionately smaller effect of snow loads on large trees, may result in older conifers equalling or surpassing deciduous trees in biomass allocation to height growth. 相似文献
100.
Variation of sapflow velocity in Eucalyptus globulus with position in sapwood and use of a correction coefficient 总被引:1,自引:0,他引:1
Sapflow sensors were used to investigate variation in sapflow velocity at different positions in the sapwood of three-year-old Eucalyptus globulus ssp. globulus Labill. trees. Sapflow velocity was measured at 5-mm intervals across the sapwood by moving two probe sets simultaneously on two opposite radii. Another probe set placed in a fixed position at right angles to the first two sets acted as a control. A sapflow velocity ratio was defined as the velocity given by each moving sensor divided by that given by the static sensor. Correlation between observations of sapflow velocity at different positions exceeded 95%, and the ratio of velocity between any pair of sensors was constant. We observed radial variation in sapflow velocity across the sapwood with the lowest velocities at the center of the tree. Variation due to sensor position was high implying the need for large numbers of sensors for accurate estimates of sap flux. To overcome this need, we used a correction coefficient, namely a simple weighted average of the sapflow ratios with depth in the sapwood, for each fixed sensor. We recommend the use of three probe sets to estimate the correction coefficient. Subsequently, two probe sets can be placed at two fixed positions for routine measurements of sap flux. 相似文献