全文获取类型
收费全文 | 231655篇 |
免费 | 14204篇 |
国内免费 | 655篇 |
专业分类
林业 | 23138篇 |
农学 | 13904篇 |
基础科学 | 2935篇 |
41309篇 | |
综合类 | 19343篇 |
农作物 | 17011篇 |
水产渔业 | 16079篇 |
畜牧兽医 | 83567篇 |
园艺 | 6193篇 |
植物保护 | 23035篇 |
出版年
2021年 | 2425篇 |
2020年 | 2802篇 |
2019年 | 3612篇 |
2018年 | 3793篇 |
2017年 | 4159篇 |
2016年 | 4628篇 |
2015年 | 4219篇 |
2014年 | 5563篇 |
2013年 | 16110篇 |
2012年 | 5566篇 |
2011年 | 7238篇 |
2010年 | 6995篇 |
2009年 | 7678篇 |
2008年 | 6703篇 |
2007年 | 5763篇 |
2006年 | 6484篇 |
2005年 | 5745篇 |
2004年 | 5591篇 |
2003年 | 5366篇 |
2002年 | 4727篇 |
2001年 | 5031篇 |
2000年 | 4668篇 |
1999年 | 4621篇 |
1998年 | 4036篇 |
1997年 | 4082篇 |
1996年 | 3832篇 |
1995年 | 4398篇 |
1994年 | 3778篇 |
1993年 | 3496篇 |
1992年 | 3679篇 |
1991年 | 3804篇 |
1990年 | 3528篇 |
1989年 | 3493篇 |
1988年 | 3075篇 |
1987年 | 3204篇 |
1986年 | 3065篇 |
1985年 | 3442篇 |
1984年 | 3312篇 |
1983年 | 3138篇 |
1982年 | 2686篇 |
1981年 | 2648篇 |
1980年 | 2602篇 |
1979年 | 2788篇 |
1978年 | 2613篇 |
1977年 | 2431篇 |
1976年 | 2290篇 |
1975年 | 2100篇 |
1974年 | 2259篇 |
1973年 | 2153篇 |
1971年 | 1960篇 |
排序方式: 共有10000条查询结果,搜索用时 250 毫秒
971.
972.
973.
A moth egg assignable to the family Noctuidae (Lepidoptera) is described from 75-million-year-old sediments from Martha's Vineyard, Massachusetts. This sample, which extends the fossil record of this farnily and modern heteroneuran moths back to the Cretaceous, may provide insight into the coevolution of moths and flowering plants, as well as have implications for the evolution of bats. 相似文献
974.
Intensive microearthquake swarms with the appearance of volcanic tremor have been observed in the southwest part of Long Valley caldera, southeastern California. This activity, possibly associated with magma injection, began 6 weeks after several strong (magnitude 6+) earthquakes in an area south of the caldera and has continued sporadically to the present time. The earthquake sequence and magmatic activity are part of a broad increase in tectonic activity in a 15,000-square-kilometer region surrounding the "White Mountains seismic gap," an area with high potential for the next major earthquake in the western Great Basin. 相似文献
975.
Keane DP Horney FD Ogilvie TH 《The Canadian veterinary journal. La revue veterinaire canadienne》1983,24(2):57-59
A four day old Holstein calf was presented with a history of recurrent free gas bloat. An exploratory laparotomy was performed and no abnormalities were discovered in the gastrointestinal tract. An esophagotracheal fistula was suspected and confirmed by endoscopy. Surgical correction was performed by ligating the tubular connection between the trachea and esophagus and suturing closed the defects in these structures. 相似文献
976.
Examination of the origin of increased equine serum alkaline phosphatase concentrations 总被引:2,自引:0,他引:2
Trueman KF Lumsden JH McSherry BJ 《The Canadian veterinary journal. La revue veterinaire canadienne》1983,24(4):108-111
Serum alkaline phosphatase activity was found to be increased in 32.6% of equine samples analyzed at the Ontario Veterinary College over an 18 month period. An attempt was made using sensitivity to L-phenylalanine and heat to identify the origin of increased serum alkaline phosphatase isoenzymes present in 44 clinical cases. No difference in sensitivity to either procedure was observed for serum alkaline phosphatase from groups of foals and horses representing different clinical problems. Alkaline phosphatase of osseous tissue origin appeared to be the major source of activity for each group of animals reported. 相似文献
977.
978.
This paper concerns the taxonomic status of the F38-like group (MacOwan), a prime determinant of contagious caprine pleuropneumonia (CCPP). Extensive biochemical and serological investigations on strain F38 are reported. Some complex serological relationships with other mycoplasma species are revealed. The results, taken in conjunction with earlier published work on geno-typic characters, lead to the conclusion that final classification of these organisms should await further comparative studies of a number of field strains with a related group of strains classified as M. capri-colum.The characterization of F38 confirms its partial relationship to the “M. mycoides group” of ovine/caprine/bovine mycoplasmas, and has also revealed a very close phenotypic relationship to the bovine mycoplasma serogroup 7, a finding of potential diagnostic and epidemiological importance.Key words: mycoplasmas, classification, F38-like group 相似文献
979.
Endocrine effects of heat stress in boars 总被引:1,自引:0,他引:1
K. Larsson S. Einarsson K. Lundstrm J. Hakkarainen 《Acta veterinaria Scandinavica》1983,24(3):305-314
The purpose of this study was to describe the temporal changes in peripheral plasma levels of testosterone and Cortisol in boars during and after heat stress. A total of 8 boars were utilized, 4 of them were exposed to 35°C, for 100 h in a climatic room, and 4 served as controls and were kept at 20 °C for 100 h in the climatic room.Blood samples were obtained via permanent vein catheters 3 times daily from 5 days before heat stress until 20 days after termination of heat stress. Testestorone levels were determined by radioimmunoassay and Cortisol by a competitive protein binding technique.For both hormones the pre-exposure levels were similar in both groups of boars. The control boars had significantly higher testosterone levels, while being in the climatic room, than during any other period. The experimental boars had slightly increased testosterone levels during the first day of heat stress and thereafter continuously decreased levels. In the control boars the testosterone levels returned to pre-exposure levels immediately after removal from the climatic room, whereas in the experimental boars the testosterone levels were dramatically increased during the first 5 days after exposure. The differences in Cortisol levels, between the 2 groups of boars were restricted to the period spent in the climatic room. During this period the experimental boars had significantly higher Cortisol levels.Key words: boars, heat stress, testosterone, cortisol 相似文献
980.
Using 12 years of data from the common grazing areas at Store Vildmose on the incidence of foot rot in cattle, it was shown that the disease incidence varied significantly between the sections in which the animals were grazing. Differences also existed between breeds in their foot rot incidence. Extraordinarily wet summers were noted to influence foot rot in cattle, and a possible effect of soil pH was also observed.Key words: foot rot, cattle, incidence, epidemiology, breed differences, climatic factors, lameness 相似文献