全文获取类型
收费全文 | 89757篇 |
免费 | 5538篇 |
国内免费 | 46篇 |
专业分类
林业 | 3735篇 |
农学 | 3466篇 |
基础科学 | 632篇 |
11247篇 | |
综合类 | 13528篇 |
农作物 | 3359篇 |
水产渔业 | 4939篇 |
畜牧兽医 | 47645篇 |
园艺 | 1167篇 |
植物保护 | 5623篇 |
出版年
2020年 | 732篇 |
2019年 | 953篇 |
2018年 | 1583篇 |
2017年 | 1845篇 |
2016年 | 1661篇 |
2015年 | 1405篇 |
2014年 | 1719篇 |
2013年 | 3717篇 |
2012年 | 2930篇 |
2011年 | 3587篇 |
2010年 | 2340篇 |
2009年 | 2329篇 |
2008年 | 3474篇 |
2007年 | 3210篇 |
2006年 | 3136篇 |
2005年 | 2670篇 |
2004年 | 2576篇 |
2003年 | 2592篇 |
2002年 | 2375篇 |
2001年 | 3390篇 |
2000年 | 3305篇 |
1999年 | 2554篇 |
1998年 | 984篇 |
1997年 | 1051篇 |
1996年 | 883篇 |
1995年 | 1024篇 |
1994年 | 898篇 |
1993年 | 897篇 |
1992年 | 1846篇 |
1991年 | 1920篇 |
1990年 | 1856篇 |
1989年 | 1859篇 |
1988年 | 1607篇 |
1987年 | 1595篇 |
1986年 | 1627篇 |
1985年 | 1527篇 |
1984年 | 1225篇 |
1983年 | 1032篇 |
1982年 | 713篇 |
1979年 | 1060篇 |
1978年 | 816篇 |
1975年 | 816篇 |
1974年 | 897篇 |
1973年 | 864篇 |
1972年 | 805篇 |
1971年 | 795篇 |
1970年 | 800篇 |
1969年 | 780篇 |
1968年 | 708篇 |
1967年 | 734篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
B Sarangi S K Chatterjee K Dutta S K Das 《Journal of the Association of Official Analytical Chemists》1985,68(3):547-548
A safe and simple thin layer chromatographic method has been developed for the estimation of niacin and niacinamide in pharmaceutical preparations containing other vitamins, enzymes, herbs, antiamoebic drugs, etc. The method involves removal of excipients from the preparation by ethanol precipitation and isolation of niacin or niacinamide from other ingredients by TLC on silica gel with water as developing solvent, followed by extraction in 0.1N HCl, and spectrophotometric estimation of the vitamin at 262 nm. The percent recoveries for niacin and niacinamide were 100.1 +/- 1.9 and 100.2 +/- 1.5, respectively. 相似文献
992.
S T Ferrell B R Weeks B Abbitt R W Radcliffe 《Journal of zoo and wildlife medicine》2000,31(2):215-220
Three juvenile sable antelope (Hippotragus niger) were diagnosed with bilateral aural hyalohyphomycosis based on histopathology. All three animals were suspected to be immunodeficient based on low IgG levels determined using the zinc sulfate turbidity test. The serum and hepatic copper levels of one animal were below the bovine reference range. Clinical signs in the three animals included bilateral ventral deviation of the pinnae with multifocal subcutaneous aural tumefaction and poor body condition. Numerous septate, nonpigmented fungal hyphae were found within the auricular cartilage, dermis, and subcutaneous granulomas. No significant fungal agents were isolated by culture, and no signs of systemic fungal dissemination were identified except for a concurrent fungal rhinitis in one animal. 相似文献
993.
Summary Nobilization in sugarcane is viewed from a genetic angle considering the significance of 2n gametes. The inbreeding coefficients resulting from 2n gametes, for first and second nobilizations are worked out. The implication of 2n gametes to sugarcane breeding are discussed. 相似文献
994.
Effects of different arginine/lysine level on growth performance,body composition and digestive enzyme activity of Macrobrachium rosenbergii 下载免费PDF全文
X. J. Dong J. Wu Y. Shen J. Y. Chen S. Y. Miao X. J. Zhang L. S. Sun 《Aquaculture Nutrition》2018,24(3):1101-1111
Given the universal presence of antagonism between arginine and lysine, determining the optimal dietary arginine/lysine level is of great importance. In this study, eight diets were formulated to contain different arginine/lysine level, followed by a 9‐week feeding trial with shrimps of 2.10 ± 0.01 cm in length and 0.38 ± 0.02 g in weight to determine the effect of dietary arginine/lysine level on growth performance, body composition and enzyme activity. The results showed that the Diet 6 (arginine/lysine level of 29.4/21.0 g/kg) group had significantly higher weight gain rate, and specific growth rate than the control group and most of the other groups (p < .05) did. Similarly, the retention efficiency of arginine and lysine in the Diet 6 group was significantly higher than the control group and most of the other groups (p < .05). Moreover, most digestive enzymes had higher activity levels in the Diet 6 group, and evident antagonism between dietary arginine and lysine was detected. In summary, dietary arginine/lysine level of 29.4/21.0 g/kg produced the best growth performance, suggesting that a better growth performance in Macrobrachium rosenbergii culture can be obtained by adjusting the dietary arginine/lysine level. 相似文献
995.
Apple replant disease (ARD) is a disease complex that reduces survival, growth and yield of replanted trees, and is often encountered in establishing new orchards on old sites. Methyl bromide (MB) has been the fumigant used most widely to control ARD, but alternatives to MB and cultural methods of control are needed. In this experiment, we evaluated the response of soil microbial communities and tree growth and yield to three pre-plant soil treatments (compost amendment, soil treatment with a broad-spectrum fumigant, and untreated controls), and use of five clonal rootstock genotypes (M.7, M.26, CG.6210, G.30 and G.16), in an apple replant site in Ithaca, New York. Polymerase chain reaction (PCR)—denaturing gradient gel electrophoresis (DGGE) analysis was used to assess changes in the community composition of bacteria and fungi in the bulk soil 8, 10, 18 and 22 months after trees were replanted. PCR-DGGE was also used to compare the community composition of bacteria, fungi and pseudomonads in untreated rhizosphere soil of the five rootstock genotypes 31 months after planting. Tree caliper and extension growth were measured annually in November from 2002 to 2004. Apple yield data were recorded in 2004, the first fruiting year after planting. Trees on CG.6210 rootstocks had the most growth and highest yield, while trees on M.26 rootstocks had the least growth and lowest yield. Tree growth and yield were not affected by pre-plant soil treatment except for lateral extension growth, which was longer in trees growing in compost-treated soil in 2003 as compared to those in the fumigation treatment. Bulk soil bacterial PCR-DGGE fingerprints differed strongly among the different soil treatments 1 year after their application, with the fingerprints derived from each pre-plant soil treatment clustering separately in a hierarchical cluster analysis. However, the differences in bacterial communities between the soil treatments diminished during the second year after planting. Soil fungal communities converged more rapidly than bacterial communities, with no discernable pattern related to pre-plant soil treatments 10 months after replanting. Changes in bulk soil bacterial and fungal communities in response to soil treatments had no obvious correlation with tree performance. On the other hand, rootstock genotypes modified their rhizosphere environments which differed significantly in their bacterial, pseudomonad, fungal and oomycete communities. Cluster analysis of PCR-DGGE fingerprints of fungal and pseudomonad rhizosphere community DNA revealed two distinct clusters. For both analyses, soil sampled from the rhizosphere of the two higher yielding rootstock genotypes clustered together, while the lower yielding rootstock genotypes also clustered together. These results suggest that the fungal and pseudomonad communities that have developed in the rhizosphere of the different rootstock genotypes may be one factor influencing tree growth and yield at this apple replant site. 相似文献
996.
Hitchcock AS 《Science (New York, N.Y.)》1905,21(543):828-832
997.
998.
This study examined the expression of three isoforms of nitric oxide synthase (NOS) in the testes of pigs. Immunohistochemical studies demonstrated the presence of nNOS, eNOS and iNOS in interstitial cells, primary spermatocytes and spermatids. Positive immunoreactions for eNOS and iNOS were detected in peritubular myoid cells. Some vascular endothelial cells were positive for nNOS and eNOS. The expression of nitrotyrosine was detected in interstitial cells. In addition, the histochemical study revealed that all the interstitial cells were stained positively for NADPH-diaphorase, although some spermatids and vascular endothelial cells displayed moderate enzymatic activity. These findings suggest that three isoforms of NOS are expressed in the testis of pig and that they play important roles in the biology of interstitial cells that produce testosterone, as well as in spermatogenesis in the seminiferous tubules. 相似文献
999.
Neto JO Azevedo SS Gennari SM Funada MR Pena HF Araújo AR Batista CS Silva ML Gomes AA Piatti RM Alves CJ 《Veterinary parasitology》2008,156(3-4):329-332
The prevalence and risk factors for anti-Toxoplasma gondii antibodies were investigated in goats of the Seridó Oriental microregion, Rio Grande do Norte state, Northeast region of Brazil. Three hundred and sixty-six blood samples from goats collected by jugular venopuncture were used. For the serologic diagnosis of Toxoplasma gondii infection, the indirect fluorescent-antibody test (IFAT) with cut-off value 1:64 was carried out. The prevalence of anti-T. gondii antibodies was 30.6% [95% CI=25.9-35.6%] with titers ranging from 1:64 to 1:16,384. The multivariate logistic regression analysis showed that the risk factors associated to anti-T. gondii antibodies were presence of cats in the herd, extensive/semi-intensive management systems and lack of mineral supplementation. 相似文献
1000.