全文获取类型
收费全文 | 278389篇 |
免费 | 15960篇 |
国内免费 | 686篇 |
专业分类
林业 | 24046篇 |
农学 | 14086篇 |
基础科学 | 2985篇 |
44942篇 | |
综合类 | 30056篇 |
农作物 | 17539篇 |
水产渔业 | 17458篇 |
畜牧兽医 | 112817篇 |
园艺 | 6397篇 |
植物保护 | 24709篇 |
出版年
2021年 | 2510篇 |
2020年 | 2918篇 |
2019年 | 3732篇 |
2018年 | 4301篇 |
2017年 | 4732篇 |
2016年 | 4939篇 |
2015年 | 4377篇 |
2014年 | 5743篇 |
2013年 | 16305篇 |
2012年 | 7276篇 |
2011年 | 9530篇 |
2010年 | 7839篇 |
2009年 | 8380篇 |
2008年 | 8894篇 |
2007年 | 7968篇 |
2006年 | 8308篇 |
2005年 | 7629篇 |
2004年 | 7401篇 |
2003年 | 7186篇 |
2002年 | 6398篇 |
2001年 | 7022篇 |
2000年 | 6807篇 |
1999年 | 6114篇 |
1998年 | 4223篇 |
1997年 | 4280篇 |
1996年 | 3996篇 |
1995年 | 4655篇 |
1994年 | 4011篇 |
1993年 | 3705篇 |
1992年 | 4816篇 |
1991年 | 4995篇 |
1990年 | 4696篇 |
1989年 | 4682篇 |
1988年 | 4138篇 |
1987年 | 4172篇 |
1986年 | 4175篇 |
1985年 | 4416篇 |
1984年 | 3991篇 |
1983年 | 3708篇 |
1982年 | 3018篇 |
1981年 | 2874篇 |
1980年 | 2871篇 |
1979年 | 3555篇 |
1978年 | 3114篇 |
1977年 | 2850篇 |
1976年 | 2731篇 |
1975年 | 2648篇 |
1974年 | 2834篇 |
1973年 | 2813篇 |
1972年 | 2460篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
941.
942.
Although agronomically interesting, selection of creeping-rootedness inlucerne, that is, the ability to form adventitious shoots on horizontal,spreading roots, remains difficult because of the complex and still unclear geneticcontrol of this character, coupled with the possible occurrence of external factorsthat affect unpredictably its expression. Two experiments were undertaken onprogenies of creeping-rooted plants, to verify previous findings and inferences,and test novel hypotheses, concerning the effect of various plant factors on theexpression of creeping-rootedness. The ultimate aim was to provide information onthe most appropriate germplasm, procedures, or indirect selection criteria, that wouldimprove the efficiency of breeding for this character. The factors here examinedincluded genotype, age, kind of progeny, vigour, spreading ability, and undergroundmorphology. Some genotypes appeared to be good donors of the character, whereasothers did not. The proportion of creeping-rooted plants increased in allcases with the plant age. Selfing consistently depressed the expression ofthe character compared with clonal propagation or crossing. It is suggestedthat the effect of age and selfing on the penetrance of creeping-rootedness ismediated through their effect on plant vigour. Both aerial and root vigour showedindeed an effect per se on the presence of the character. Beside thevigour, possible indirect selection criteria were identified in the frequenciesof swollen zones and latent stem primordia on horizontal roots. Proportion ofcreeping-rooted plants in progenies after one year of growth appeared as a simple anduseful criterion to predict future trends. Consistent with previous conclusions, theresults provided evidence that a combination of family and individualselection may enhance the character expression. 相似文献
943.
944.
This study was the first to examine the seroprevalence of Ornithobacterium rhinotracheale (ORT) within a commercial egg layer population. Serum samples collected from egg production companies were examined by serum plate agglutination test (SPAT) and outer membrane protein-based enzyme-linked immunosorbent assay (ELISA). Results show that 90% of layer flocks were positive by SPAT and 100% by ELISA. Of the pullet flocks examined, 43% and 52% were positive by SPAT and ELISA, respectively. Our study indicates that the prevalence of ORT antibody is high in the commercial layer population, suggesting that this respiratory pathogen can easily spread through multiple-age layer farms from older flocks to newly housed pullet flocks. 相似文献
945.
Melody L. Danley P. Brett Kenney Patricia M. Mazik Rodney Kiser Joseph A. Hankins 《Journal of the World Aquaculture Society》2005,36(3):249-261
Rainbow trout Oncorhynchus mykiss (261.6 × 24.7 g initial weight, mean × SEM) at 13.1 × 0.2 C were exposed for 94 d to one of three CO2 treatments: control (22.1 × 2.8 mg/L), medium (34.5 × 3.8 mg/L), or high (48.7 × 4.4 mg/L). Trout were checked daily for survival, and fish were sampled at 0, 28, 56, and 84 d for physiological responses, growth, and fillet quality assessments. Trout were also challenged to a 15-min crowding stress at 93 d to assess their ability to initiate a stress response during hypercapnia. Chronically exposed trout showed nearly 100% survival through 84 d exposure (1 of 1,500 fish died). Growth and physiological results showed that increasing elevated CO2 , concentrations result in corresponding decreased growth rates and CO2 specific physiological parameters: The medium and high CO2 treatments had significantly slower growth and subsequently smaller fish by 84 d. Exposed trout also showed significantly ( P < 0.05) decreased plasma chloride for medium and high CO2 treatments compared to the control from 28 through 84 d. Decreased growth and smaller fish in the medium and high CO2 treatments resulted in correspondingly smaller fresh and smoked fillet weights. Chronic CO2 exposure did not result in notable changes in ultimate muscle pH. Exposure to 15-min crowding stress at 93 d resulted in significant changes in hematocrit, plasma cortisoI, glucose, and chloride for all treatment groups. CO2 -specific changes were detected in hematocrit, plasma cortisoI, and plasma chloride responses following the 15-min crowding stress. 相似文献
946.
P. Laffaille A. Acou J. Guillouët B. Mounaix A. Legault 《Ecology of Freshwater Fish》2006,15(4):583-588
Abstract – Changes in the numbers and size-class structure of European silver eels, Anguilla anguilla , in the River Frémur (France) were examined over a 9-year period after installation of downstream eel passes. The number of silver eels migrating downstream peaked in 1999, then decreased strongly and steadily after 2000, reaching relatively low levels. At the same time, a gradual shift in the silver eel sex ratio from a dominance of males (size from 270 to 442 mm, age from 3 to 6 years) to females (size from 366 to 1112 mm, age from 4 to 9 years) was recorded. Possible explanations for the escapement patterns observed are environmental sex determination and the installation of eel passes on the main hydraulic engineering structures in 1992 and 1996. 相似文献
947.
D. M. Zhao S. Tateyama N. Miyoshi K. Uchida R. Yamaguchi T. Yamagami T. Hayashi 《Research in veterinary science》1995,59(3)
Cloning of the canine yes oncogene was attempted from a c
library derived from a healthy canine spleen using a human c-yes-1 probe. The nucleotide and amino acid sequences revealed that the canine yes gene contained an open reading frame consisting of 539 amino acids. Its product had a molecular mass of 60,368 Daltons and showed 95·9 per cent and 90·4 per cent homology with human and chick p61c-yes, respectively. Moreover, the product had a myristylation signal, src homology region (SH) 3, SH2, and tyrosine kinase domains showing 98·8 per cent and 96·0 per cent homology with those of human beings and chickens, respectively. These findings indicate that the products of the canine yes gene may have non-receptor-type tyrosine kinase activity on the cell membrane, as is the case in human and chick p61c-yes 相似文献
948.
Marie Engel David W. Hird Tim E. Carpenter Martin Wierup 《Preventive veterinary medicine》1995,24(4):245-252
A cohort of 53 swine seronegative to Aujeszky's disease virus (ADV) was monitored in a 1 year study of a chronically infected commercial Swedish weaner pig producing herd. Serum samples were acquired from all 134 adult swine and analyzed by enzyme-linked immmunosorbent assay (ELISA). Animals testing negative, along with introduced replacement gilts, were followed serologically every second month. Movements of animals were recorded for 319 days and exposure to seropositive animals was calculated for each seronegative pig in the cohort. The accumulated daily pig contact between the 53 ADV-non-infected swine and 43 infected swine was 35 660 days and the median number of days in contact for the non-infected swine with infected was 222. Despite the frequent contact with seropositive pigs, no seronegative animals seroconverted during the first 11 months of observation. Forty-six of 53 pigs seroconverted after a clinical outbreak during the twelfth month of observation. 相似文献
949.
SUMMARY: Confinement of horses with their heads elevated for periods up to 24 hours was used to evaluate the extent and the effects of bacterial contamination of the equine lower respiratory tract. Significant (P < 0.05) increases in bacterial numbers (up to 109 colony forming units/mL in transtracheal aspirate derived samples) occurred within 6 or 12 hours in most horses. Pasteurella/Actinobacillus spp and Streptococcus spp were most commonly isolated. Lowering of the head for 30 minutes every 6 hours to facilitate postural drainage did not prevent multiplication of organisms to levels equivalent to those achieved by horses where the head was elevated for 24 hours. When horses were released from confinement and heads were no longer maintained in an elevated position, clearance of accumulated secretions and bacteria occurred within 8 to 12 hours. Thus, confinement with the head elevated resulted in significant bacterial contamination and multiplication within the lower respiratory tract during a period often encountered in routine management procedures, such as transportation. The clearance of accumulated secretions occurred over a prolonged period after release from such confinement. 相似文献
950.
Yunita T. Winarto 《Agriculture and Human Values》1995,12(4):47-57
The 1989 National Integrated Pest Management Program in Indonesia is a case of a breakthrough in national policy to enhance the ecological balance by conserving natural enemies and diminishing the indiscriminate use of pesticides in the protection of food crops. The Program provided training to agricultural officials and farmers to shift their perspectives in pesticide use through “knowledge transmission” rather than the transferal of “technological packages.” This paper examines how farmers, with the novel understanding they had, responded to the persisting top-down policy of the “complete credit packages” from the government rice intensification agencies and the recurrent outbreak of pests in 1990–1992. IPM knowledge and training provided initial understanding of, and the stimulus to discover, unknown phenomena on the basis of which farmers improved their knowledge and learning capacity. Constraints largely came from the inflexibility of the subsidized scheme of inputs and the inadequate explanation and extension services provided to farmers 相似文献