全文获取类型
收费全文 | 87892篇 |
免费 | 5332篇 |
国内免费 | 42篇 |
专业分类
林业 | 3007篇 |
农学 | 2725篇 |
基础科学 | 529篇 |
10014篇 | |
综合类 | 16255篇 |
农作物 | 3668篇 |
水产渔业 | 4326篇 |
畜牧兽医 | 45996篇 |
园艺 | 1109篇 |
植物保护 | 5637篇 |
出版年
2019年 | 844篇 |
2018年 | 1214篇 |
2017年 | 1402篇 |
2016年 | 1313篇 |
2015年 | 1163篇 |
2014年 | 1390篇 |
2013年 | 3480篇 |
2012年 | 2614篇 |
2011年 | 3112篇 |
2010年 | 2017篇 |
2009年 | 1970篇 |
2008年 | 2993篇 |
2007年 | 2885篇 |
2006年 | 2726篇 |
2005年 | 2561篇 |
2004年 | 2419篇 |
2003年 | 2446篇 |
2002年 | 2310篇 |
2001年 | 2911篇 |
2000年 | 2784篇 |
1999年 | 2275篇 |
1998年 | 981篇 |
1997年 | 897篇 |
1996年 | 794篇 |
1995年 | 961篇 |
1994年 | 871篇 |
1993年 | 859篇 |
1992年 | 1816篇 |
1991年 | 1815篇 |
1990年 | 1753篇 |
1989年 | 1817篇 |
1988年 | 1571篇 |
1987年 | 1681篇 |
1986年 | 1739篇 |
1985年 | 1666篇 |
1984年 | 1313篇 |
1983年 | 1167篇 |
1982年 | 842篇 |
1981年 | 723篇 |
1979年 | 1107篇 |
1978年 | 900篇 |
1977年 | 781篇 |
1976年 | 734篇 |
1975年 | 810篇 |
1974年 | 1000篇 |
1973年 | 1041篇 |
1972年 | 994篇 |
1971年 | 946篇 |
1970年 | 887篇 |
1969年 | 811篇 |
排序方式: 共有10000条查询结果,搜索用时 125 毫秒
71.
72.
F C Battaglia 《Journal of animal science》1992,70(10):3258-3263
Fetal amino acid nutrition and metabolism have been studied primarily in pregnant sheep. The umbilical uptake of amino acids changes during gestation, but at both mid- and late gestation the total supply exceeds that required for growth. Weight-specific protein synthetic rate decreases with increasing gestational age, and these changes are proportional to the changes in metabolic rate. The use of multiple tracer methodology coupled with measurement of net tracer fluxes into and out of fetal and placental tissues can be used to delineate amino acid metabolism in considerable detail. Such studies demonstrate that even essential amino acids can be oxidized extensively by the fetus. The oxidation rate of leucine exceeds its rate of accretion in tissue proteins. Glycine metabolism is unique in several ways; there is a large umbilical uptake of glycine without a measurable uterine uptake. In late gestation there is no significant umbilical uptake of serine, although there is a significant uterine uptake, suggesting net uteroplacental utilization. Glycine is oxidized within the fetal liver and used for serum production. The interorgan exchange of amino acids between the fetal liver and placenta is clearly of major importance for serine and glycine metabolism and is likely to be of major importance for most nonessential amino acids. 相似文献
73.
E Bezirtzoglou M B Romond C Romond 《Comparative immunology, microbiology and infectious diseases》1992,15(1):71-74
Studies were undertaken to determine the regulation of the bacterial intestinal implantation in 19 newborns delivered by caesarean section. Correlation was made with the infant feeding mode. The effect of human milk seemed to be the result of B. bifidum proliferation, in contrast to artificial alimentation that seemed to favour C. perfringens implantation. The question was raised by us as to whether this opposition was only related to alimentation. In fact, B. bifidum itself also had an effect as demonstrated by the lower mean counts of C. perfringens in bottle-fed infants carrying the bifido-bacterial flora (P = 0.05). None of the other faecal bacteria investigated in this study led to the same decrease. 相似文献
74.
Soil pH declined from 5.9 to 5.0 in 8 years beneath plantations of Eucalyptus saligna (Sm.) in Hawaii. In stands of Albizia falcataria, (L.) Fosberg, the soil pH change was more dramatic, declining from 5.9 to 4.6. We measured several components of soil acidity beneath four mixtures of the two tree species to gain insight on the processes responsible for the decline in soil pH. These components were studied using an empirical method of comparing acid quantity, degree of neutralization (depletion of base cations), and acid strength. The decline in soil pH differed between species as a result of differences in the degree of neutralization of the soil exchange complex; the larger decrease in soil pH under Albizia was produced by greater acidification of the exchange complex. Empirical titration curves suggested that differences in acid strength moderated the divergence in soil pH beneath the species. Had the acids accumulating in the soil under Albizia been as strong as those in the Eucalyptus soil, the difference in soil pH would have been greater. Though the two species had contrasting effects on soil pH, the differences in degree of neutralization, responsible for the pH decline, were small compared with differences in the amount of cations stored in tree biomass. Continued supply of nutrient cations (from weathering or fertilization) will ultimately control both the extent of soil pH decline and the level of productivity sustained by the forest. 相似文献
75.
A technique is described for biopsy of the bovine udder, employing sedation and local anaesthesia. Tissue samples of approximately 5 g were obtained by electrocautery from two quarters of the udder of a cow laterally recumbent. Care was taken to ensure complete haemostasis which was achieved by electrocoagulation and ligation. Postoperative recovery was rapid, and loss of yield was no greater in biopsied glands than in control glands of the same cow. Yield from all quarters returned to preoperative levels within 48 h. 相似文献
76.
Precise localization of the needle tip during CT-guided percutaneous biopsy is considered to be a key element of a successful procedure. To ensure accuracy, the true needle tip must be differentiated from a false or simulated tip which appears when the CT slice encompasses only the shaft of an angled needle. By obtaining images of an aspiration biopsy needle inserted vertically into a phantom and then incrementally tilting the gantry, the authors were able to compare the characteristic features of the true tip to the simulated tip. The true tip was abrupt and distinct and had an adjacent flame-like low density artifact. The simulated tip was indistinct and tapered, yet still produced the adjacent artifact. We concluded that the shape and distinctness of the end portion of the needle itself, rather than the attendant artifact, were the most reliable criteria for accurate needle tip identification. 相似文献
77.
78.
79.
J W Tyler J S Cullor M C Thurmond D W Hird K M Parker 《American journal of veterinary research》1989,50(1):126-130
Serologic recognition of common lipopolysaccharide core antigens has been related to enhanced resistance to gram-negative bacterial disease in several species. Class-specific titers (IgG, IgM) were determined by direct ELISA, using intact Escherichia coli (J5) as a plate antigen. Serum samples were obtained from 224 neonatal swine between the ages of 36 and 60 hours. The mean (+/- SEM) log10 IgG titer against gram-negative core antigens was 1:1,713 +/- 0.4718 and the mean log10 IgM titer was 1:202 +/- 0.5644. The IgG titer was directly related with litter size, birth weight, and serum total IgG concentration; IgM titer was directly related with dam parity and serum total IgG concentration. 相似文献
80.
Abstract. A two year field experiment was carried out in a semiarid Mediterranean area in order to evaluate, the effect on soil erosion of adding different urban organic wastes: a stabilized municipal waste (compost), an unstabilized municipal waste, and an aerobic sewage sludge. All the treatments significantly reduced soil erosion, compared to the control soil. The soil amended with compost was the most effective treatment, reducing soil loss by 94% and runoff by 54%. 相似文献