全文获取类型
收费全文 | 13693篇 |
免费 | 1520篇 |
国内免费 | 2671篇 |
出版年
2024年 | 37篇 |
2023年 | 348篇 |
2022年 | 281篇 |
2021年 | 507篇 |
2020年 | 598篇 |
2019年 | 702篇 |
2018年 | 598篇 |
2017年 | 669篇 |
2016年 | 681篇 |
2015年 | 684篇 |
2014年 | 673篇 |
2013年 | 868篇 |
2012年 | 575篇 |
2011年 | 642篇 |
2010年 | 536篇 |
2009年 | 695篇 |
2008年 | 739篇 |
2007年 | 771篇 |
2006年 | 669篇 |
2005年 | 693篇 |
2004年 | 523篇 |
2003年 | 498篇 |
2002年 | 476篇 |
2001年 | 449篇 |
2000年 | 368篇 |
1999年 | 375篇 |
1998年 | 329篇 |
1997年 | 264篇 |
1996年 | 280篇 |
1995年 | 261篇 |
1994年 | 215篇 |
1993年 | 228篇 |
1992年 | 212篇 |
1991年 | 181篇 |
1990年 | 194篇 |
1989年 | 185篇 |
1988年 | 175篇 |
1987年 | 116篇 |
1986年 | 103篇 |
1985年 | 99篇 |
1984年 | 96篇 |
1983年 | 43篇 |
1982年 | 61篇 |
1981年 | 40篇 |
1980年 | 32篇 |
1979年 | 27篇 |
1978年 | 27篇 |
1977年 | 12篇 |
1976年 | 12篇 |
1974年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 112 毫秒
21.
22.
C. M. Calkins J. D. Scasta T. Smith M. M. Stayton S. L. Lake 《Medical and veterinary entomology》2019,33(4):546-551
The horn fly Haematobia irritans (Diptera: Muscidae) is a blood obligate ectoparasite of bovids that causes annual losses to the U.S. beef cattle industry of over US$1.75 billion. Climate warming, the anthropogenic dispersion of bovids and the cross‐breeding of beef cattle with other bovid species may facilitate novel horn fly–host interactions. In particular, hybridizing yaks [Bos grunniens (Artiodactyla: Bovidae)] with beef cows (Bos taurus) for heterosis and carcass improvements may increase the exposure of yak × beef hybrids to horn flies. The present paper reports on the collection of digital images of commingled beef heifers (n = 12) and F1 yak × beef hybrid bovids (heifers, n = 7; steers, n = 5) near Laramie, Wyoming (~ 2200 m a.s.l.) in 2018. The total numbers of horn flies on beef heifers and F1 yak × beef heifers [mean ± standard error (SE): 88 ± 13 and 70 ± 17, respectively] did not differ significantly; however, F1 yak × beef steers had greater total horn fly abundance (mean ± SE: 159 ± 39) than female bovids. The present report of this experiment is the first such report in the literature and suggests that F1 yak × beef bovids are as susceptible as cattle to horn fly parasitism. Therefore, similar monitoring and treatment practices should be adopted by veterinarians, entomologists and producers. 相似文献
23.
24.
25.
Novel strategies for assessing and managing the risks posed by invasive alien species to global crop production and biodiversity 总被引:3,自引:0,他引:3
RICHARD BAKER RAY CANNON PAUL BARTLETT IAN BARKER 《The Annals of applied biology》2005,146(2):177-191
International actions to combat the threat posed by invasive alien species (IAS) to crops and biodiversity have intensified in recent years. The formulation of 15 guiding principles on IAS by the Convention on Biological Diversity (CBD) stimulated the International Plant Protection Convention (IPPC) to review its role in protecting biodiversity. IPPC standards now demonstrate clearly that the risks posed by any organism that is directly or indirectly injurious to cultivated or uncultivated plants can be assessed and managed under the IPPC. Since the IPPC, unlike the CBD, constitutes an international legal instrument recognised by the World Trade Organization, greater protection from the introduction of IAS is now available. However, phytosanitary measures can only be enacted if they can be justified by risk analysis and we outline some novel strategies to improve the assessment and management of the risks posed by IAS, highlighting some of the key challenges which remain. 相似文献
26.
M.J. Tisdale 《Chemico-biological interactions》1981,34(1):75-83
The tumour growth inhibitor L-2-amino-4-methoxy-trans-3-butenoic acid (Ro07-7957) inhibits serine hydroxymethyltransferase in cytosolic extracts of Walker carcinoma non-competitively with respect to L-serine with an apparent inhibition constant similar to the Km-value for L-serine. The kinetics of inactivation suggest that it reacts as an irreversible substrate analogue. Incubation of Walker cells with Ro07-7957 causes an increase in serine hydroxymethyltransferase activity which is most pronounced at concentration ≤LD50. This increase in enzyme activity does not occur in the presence of cycloheximide. These results suggest that inhibition of serine hydroxymethyltransferase in intact cells is accompanied by an increase in enzyme biosynthesis and that the growth inhibitory property of Ro07-7957 does not involve interference with the conversion of serine to glycine. 相似文献
27.
Summary The rhizosphere microflora of arecanut palm under continuous application of organic manures and inorganic fertilizers was
studied. The nutrients applied are 100 g N, 40 g P2O5 and 140 g K2O/palm/year in the form of organics and inorganics. The application of organic manure increased the microbial population.
The increase in microbial population was observed between the rhizosphere samples collected at 0–30cm and 30–60 cm depths.
The surface cultivation of soil increased the microbial population.Trichoderma sp. andAspergillus sp. dominated in therhizosphere of arecanut palm.
Contribution No. 208. Central Plantation Crops Research Institute, Vittal-574243, Karnataka, India. 相似文献
28.
29.
30.
Summary The influence of total nitrification to nitrate or partial nitrification to nitrite on the soil organic nitrogen status was examined. NH
4
+
–15N was added to the soil in the absence and the presence of NaClO3, respectively nitrapyrin. The first chemical inhibits only nitrate formation, the second inhibits total nitrification. The accumulation of nitrite nitrogen in the soil at levels up to 5 mg kg–1 increased the loss of nitrogen. Yet, it did not increase the binding of mineral nitrogen into soil organic matter, relative to the control soil. The data suggest that the biochemistry of the nitrite formation process, rather than the levels of nitrite ions formed, are of primary importance in the role of nitrification mediated nitrosation of soil organic matter. 相似文献