全文获取类型
收费全文 | 285773篇 |
免费 | 16441篇 |
国内免费 | 1396篇 |
专业分类
林业 | 24328篇 |
农学 | 14469篇 |
基础科学 | 3227篇 |
45629篇 | |
综合类 | 33688篇 |
农作物 | 17514篇 |
水产渔业 | 17483篇 |
畜牧兽医 | 115789篇 |
园艺 | 6653篇 |
植物保护 | 24830篇 |
出版年
2021年 | 2745篇 |
2020年 | 3080篇 |
2019年 | 3859篇 |
2018年 | 4336篇 |
2017年 | 4780篇 |
2016年 | 5062篇 |
2015年 | 4625篇 |
2014年 | 5932篇 |
2013年 | 16593篇 |
2012年 | 7560篇 |
2011年 | 9782篇 |
2010年 | 8111篇 |
2009年 | 8510篇 |
2008年 | 8944篇 |
2007年 | 8016篇 |
2006年 | 8402篇 |
2005年 | 7739篇 |
2004年 | 7320篇 |
2003年 | 7158篇 |
2002年 | 6475篇 |
2001年 | 7086篇 |
2000年 | 6906篇 |
1999年 | 6149篇 |
1998年 | 4284篇 |
1997年 | 4285篇 |
1996年 | 4035篇 |
1995年 | 4572篇 |
1994年 | 4052篇 |
1993年 | 3740篇 |
1992年 | 5039篇 |
1991年 | 5262篇 |
1990年 | 5071篇 |
1989年 | 4890篇 |
1988年 | 4415篇 |
1987年 | 4433篇 |
1986年 | 4425篇 |
1985年 | 4558篇 |
1984年 | 4188篇 |
1983年 | 3869篇 |
1982年 | 3098篇 |
1981年 | 2966篇 |
1980年 | 2951篇 |
1979年 | 3701篇 |
1978年 | 3257篇 |
1977年 | 2975篇 |
1976年 | 2887篇 |
1975年 | 2732篇 |
1974年 | 3040篇 |
1973年 | 3010篇 |
1972年 | 2730篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
A. V. TYLER 《Fisheries Oceanography》1992,1(1):97-107
In many cases when ecologists want to investigate a process, they often look for the best system on which to conduct the research, "best" meaning that the possibility of discovering mechanisms is optimized or made easier in some way. In fisheries we do it backwards. The species and system are given to us by economics, and we then fly in the face of the difficult circumstances to find mechanisms that are elusive anyway. These difficult circumstances constitute in some sense the first set of statistical problems. Using examples from the Northeast Pacific, I review the characteristics of cohort time series that make some species more tractable; propose a conscious process of conceptualization to assist in the formulation of clear, germane hypotheses; highlight the contrast between modeling in the sense of statistical fitting versus simulation models of processes; explore how the first round of models integrates with the second round of planning for new data collection at sea and in the lab; and, finally, propose how to judge success in terms of an operational approach. 相似文献
62.
福建黄兔繁殖性状相关研究 总被引:5,自引:0,他引:5
本文对310只母兔的繁殖性状进行了测定分析,结果表明;妊娠期与产仔数、产活仔数及活仔窝重呈负相关,相关系数分别为-0.426*、-0.350*和-0.515头4胎内母兔,胎次对产仔性能无影响,对母兔泌乳性能有一定影响。同样条件下胎次增加。泌乳性能提高,第3胎较第2胎有显著提高。 相似文献
63.
Methods of augmenting bovine mononuclear cell responsiveness during physiological transitions of the udder may enhance resistance of the mammary gland to intramammary infections. Interleukin-2 is required for proliferation of T-lymphocytes and may contribute to B-lymphocyte proliferation. Recombinant bovine interleukin-2 (rBoIL-2) was evaluated as a potential immunoenhancer of bovine mammary gland mononuclear cells. Bovine mononuclear cells were isolated from five primiparous Holstein cows at 14-18 and 28-32 days of involution and at 7-13 days prior to parturition. Bovine blood and mammary gland mononuclear cells were highly responsive to rBoIL-2. Response of mammary gland mononuclear cells to rBoIL-2 was comparable with response of blood mononuclear cells. These data suggest that rBoIL-2 may be an effective immunoenhancer of bovine mononuclear cells during the non-lactating and prepartum periods. 相似文献
64.
A disease causing the decline of papaya (Carica papaya) plants was noticed in the Jordan Valley in 1982. The disease caused severe yellowing of the upper leaves and dieback of the apex. The disease was not transmissible mechanically and continued efforts to associate viruses and viroids have so far been unsuccessful. Epidemiological observations suggested that the disease is airborne and probably caused by a mollicute. In order to test this hypothesis, plots were covered by an insect-proof net and with white nets providing 15, 30 and 50% shade. The 30%, 50% and insect-proof nettings provided complete protection, and the 15% netting reduced disease incidence to <2%, compared with>37% in the uncovered control plot. Spraying plants at weekly intervals with a commercial whitewash solution was also found to be effective in reducing disease incidence. The advantages of using a range of netting field treatments as a simple means for obtaining information on the epidemiological nature of a new disease are described. 相似文献
65.
P. Buracco R. Guglielmino O. Abate V. Bocchini E. Cornaglia D. B. DeNicola† M. Cilli P. Ponzio 《The Journal of small animal practice》1992,33(6):279-284
The clinical course of a feline leukaemia virus (FeLV)-negative and feline immunodeficiency virus (FIV)-positive cat affected with a large granular lymphocyte lymphoma is presented. Cyto-logical examination showed neoplastic cells in the pleural effusion and in two abdominal masses. Bone marrow and peripheral blood were moderately involved and chemotherapy was used to control the tumour. Cytochemistry, immunohis-tochemistry and ultrastructural studies were applied to define the cellular lineage; cytochemistry suggested a T-cell lineage. 相似文献
66.
B Mermer P Hillman R Harris T Krogmann Q Tonelli W Palin P Andersen 《Veterinary immunology and immunopathology》1992,35(1-2):133-141
We have developed an antibody detection enzyme-linked immunosorbent assay (ELISA) for the identification of animals infected by feline immunodeficiency virus (FIV). The ELISA solid-phase antigen consists of recombinant FIV gag proteins expressed in bacteria. The proteins are purified from bacterial lysates as insoluble inclusion bodies. In the case of bacterially expressed p24gag, it is shown that all of the linear, sequential epitopes presented by viral p24 during infection are retained. Purified preparations can be substituted for solid-phase whole virus in the IDEXX PetChektm immunoassay. The antibody ELISA duplicates the sensitivity and specificity of the whole virus based PetChek plate assay. 相似文献
67.
E. Conde A. M. Moreno I. Martin-Lacave A. Fernandez y H. Galera 《Anatomia, histologia, embryologia》1992,21(1):94-100
A systematic immunohistochemical study of the ultimobranchial tubule (UBT) has been carried out in 45 Wistar rats of different ages (0, 5, 10, 15, 20, 25, 30, 60 and 120 days). The existence of calcitonin immunoreactive cells in the UBT wall has been demonstrated in a 5-days old rat. In addition, immunohistochemical studies for thyroglobulin revealed positive staining in follicular cells connected to the UBT and, occasionally, in isolated cells lying within solid clusters from the UBT. These last results together with the continued and repeated existence of numerous mitosis and PAS (+) microfollicles, apparently rising from the UBT, support the hypothesis that the ultimobranchial body (UBB) may contribute partially to the formation of a part of the follicular component. 相似文献
68.
The effect of porcine somatotropin (pST) on the lipid profiles of adipose tissue and muscle was investigated. Sixteen crossbred barrows were injected daily with either 3 mg of pST or a placebo. After slaughter, total lipid and fatty acid composition of raw subcutaneous (SC) adipose and intermuscular (IM) adipose tissue and longissimus muscle were determined. The SC adipose tissue from pST-treated pigs had a 7.5% decrease in total lipid content; specific fatty acids 16:0, 18:0, and 18:1(n-9)c decreased most. The IM fat from pST-treated pigs had lower levels of 16:0 and 20:0. There was no effect of pST treatment on the lipid profile of the longissimus muscle. The data suggest that pST treatment produces small but significant changes in the saturated fatty acid content of adipose tissue in pigs. 相似文献
69.
Abstract. The use of an indirect enzyme-linked immunosorbent assay ( elisa ) for the detection of channel catfish antibody to Edwardsiella ictaluri is described. Changes in agglutination titre in fish immunized with Edwardsiella ictaluri heat killed whole bacterins or lipopolysaccharides were reflected by corresponding changes in elisa readings. Relatively high correlations were observed among elisa OD readings, computed elisa titres and corresponding agglutination titres. 相似文献
70.
Serum amyloid A protein (SAA) in horses: objective measurement of the acute phase response 总被引:1,自引:0,他引:1
M B Pepys M L Baltz G A Tennent J Kent J Ousey P D Rossdale 《Equine veterinary journal》1989,21(2):106-109
A sensitive and precise immunoassay for equine serum amyloid A protein (SAA) was established and used to determine, for the first time, the circulating concentration of this protein in health and disease. As in other species, equine SAA was present only at trace levels in healthy animals but behaved as an extremely sensitive and rapidly responding acute phase reactant following most forms of tissue injury, infection and inflammation, objectively reflecting the extent and activity of disease. Measurements of SAA should make a significant contribution to diagnosis and management of viral and bacterial infection in horses, and routine serial assays could provide an objective criterion for monitoring prospectively the general health of horses in training and racing. 相似文献