全文获取类型
收费全文 | 272131篇 |
免费 | 15971篇 |
国内免费 | 3347篇 |
专业分类
林业 | 26385篇 |
农学 | 17095篇 |
基础科学 | 4807篇 |
46235篇 | |
综合类 | 30745篇 |
农作物 | 18859篇 |
水产渔业 | 17520篇 |
畜牧兽医 | 97481篇 |
园艺 | 7380篇 |
植物保护 | 24942篇 |
出版年
2022年 | 2300篇 |
2021年 | 3731篇 |
2020年 | 4012篇 |
2019年 | 4827篇 |
2018年 | 4682篇 |
2017年 | 5499篇 |
2016年 | 5674篇 |
2015年 | 5465篇 |
2014年 | 6884篇 |
2013年 | 17740篇 |
2012年 | 7755篇 |
2011年 | 9586篇 |
2010年 | 8977篇 |
2009年 | 9515篇 |
2008年 | 8628篇 |
2007年 | 7640篇 |
2006年 | 8025篇 |
2005年 | 7137篇 |
2004年 | 6476篇 |
2003年 | 6336篇 |
2002年 | 5810篇 |
2001年 | 6213篇 |
2000年 | 5768篇 |
1999年 | 5502篇 |
1998年 | 4436篇 |
1997年 | 4527篇 |
1996年 | 4165篇 |
1995年 | 4723篇 |
1994年 | 4126篇 |
1993年 | 3732篇 |
1992年 | 4274篇 |
1991年 | 4478篇 |
1990年 | 4143篇 |
1989年 | 4109篇 |
1988年 | 3633篇 |
1987年 | 3613篇 |
1986年 | 3554篇 |
1985年 | 3850篇 |
1984年 | 3641篇 |
1983年 | 3385篇 |
1982年 | 2846篇 |
1981年 | 2730篇 |
1980年 | 2713篇 |
1979年 | 3177篇 |
1978年 | 2827篇 |
1977年 | 2686篇 |
1976年 | 2503篇 |
1975年 | 2346篇 |
1974年 | 2511篇 |
1973年 | 2512篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
Techniques for in vitro production of embryos have been developed world-wide in different species, with promising results in human and ruminants. Thousands of human IVF-babies have been born during the last 20 years and thousands of in vitro -produced calves have been born since the late 1980s. With current methods for bovine in vitro fertilization, about 30–40% of in vitro -fertilized bovine oocytes develop further to the blastocyst stage and can be used for transfer. A proper evaluation of blastocyst quality remains however, an important challenge for every researcher involved in embryology and for every clinician who wants to select the best embryos for transfer. This review attempts to summarize the different methods available for estimation of blastocyst quality with a special emphasis upon differential staining. 相似文献
142.
S. J. Delaney A. S. Hill R. C. Backus G. L. Czarnecki-Maulden & Q. R. Rogers 《Journal of animal physiology and animal nutrition》2001,85(3-4):88-100
The objective of the present study was to examine the interaction between graded levels of leucine and dietary crude protein. Dose–response curves were generated using four 3 × 3 Latin squares (two dogs/square). Each square represented one of two concentrations of crude protein (140 or 280 g/kg diet) and one of two combinations of three concentrations of leucine (5.0, 7.0 and 9.0 g/kg diet or 9.0, 11 and 13 g/kg diet). An additional experiment was performed by feeding crude protein at 210 g/kg diet with either 7.0 or 11 g leucine/kg diet. Weight gain, food intake, nitrogen retention, plasma albumin and plasma amino acids were measured. The requirement was determined to be the minimum leucine concentration required to maximize weight gain and nitrogen retention. For 8–14-week-old male Beagle dogs, 140 g crude protein/kg diet in a diet containing 18 kJ metabolizable energy/g does not appear to support maximal growth. The leucine requirement was not affected by doubling the dietary crude protein level from 140 to 280 g/kg diet. From these results, the leucine requirement of 8–14-week-old Beagle dogs appears to be 11 g leucine/kg diet independent of the level of dietary crude protein, whereas dogs over 14 weeks require only 7 g leucine/kg diet for maximal nitrogen retention. 相似文献
143.
随着瘦肉生长率的提高 ,猪对氨基酸的需要量也增加了。能影响生产水平的一切因素都能影响猪对氨基酸的需要量 ,比如遗传、健康、温度 ,等等。在确定氨基酸需要量的时候对这些因素都应加以考虑。 相似文献
144.
E.-M. BERNOTH 《Journal of fish diseases》1991,14(3):419-422
Abstract. A method is described which improves both the specificity and paracticability of immune identification of Aeromonas salmonicida. The modified assay employs antisera raised against outer membrane proteins (OMP) of A. salmonicida cells and is carried out as a dot blot test on nitrocellulose membranes. Performance of the test with 55 non- A. salmonicida bacterial isolates from fish and water revealed weak cross reactivity in five cases. However, these cross reactive only occur at very high antigen concentrations and can be overcome by adequate dilution. 相似文献
145.
A stripspawning methodology was evaluated for tilapia (oreochromid) species. This technique achieved an average hatching success of 68.6 ± 3.6% (N= 31). Female fecundity and spawning frequency were dependant on both genetical and husbandry factors. Egg yields for Oreochromis niloticus, O. mossambicus, and O. niloticus±O. mossambicus hybrids averaged 4.54, 10.86 and 10.36 eggs/g female/spawn, respectively. Female broodstock that were adapted to an intensive spawning regime exhibited a significant increase in fecundity. Additionally, egg survival was not affected by hydration for up to 15 minutes prior to fertilization. Results suggest that the strip spawning of tilapia species may be an efficient method of providing viable gametes for hatchery purposes. 相似文献
146.
Y. SANTOS I. BANDIN S. NUÑEZ K. GRAVNINGEN A. E. TORANZO 《Journal of fish diseases》1991,14(3):407-411
Abstact. The potency of a whole-cell bacterin (WCB) and a toxoid enriched whole-cell vaccine (WCEB) administered intraperitoncally into rainbow trout, Oncorhynchus mykiss (Richardson), were compared. The most effective vaccine was further evaluated by bathing turbot, Scophthalmus maximus (L.). These vaccines were composed of three strains of V. anguillarun , of the serotypes 01 and 02. Both vaccines conferred the highest protection against strains of serotype 01 within 4 weeks. With the toxoid enriched vaccine giving the best results (77 RPS). When trout were revaccinated after 7 weeks with this vaccine, good protection was achieved against strains of serotypes 01 and 02. Interestingly, when the WCEB was administered by bath to turbot, acceptable levels of protection against strains of both serotypes were obtained after 4 weeks of immunization. 相似文献
147.
148.
149.
Polysaccharide-induced protection of carp, Cyprinus carpio L., against bacterial infection 总被引:5,自引:0,他引:5
Abstract. The efficacy of 10 polysaccharides (curdlan, inulin, krestin, laminaran, lentinan, levan, schizophylian, selerogiucan, yeast glucan and zymosan) to enhance protection of carp, Cyprinus carpio L., against bacterial infection was investigated. Carp were intraperitoneally injected with the polysaceharides (2–l0 mgkg-1 ) on days 1 and 4, and challenged with Edwardsiella tarda on day 7. Among the polysaccharides tested, lentinan, schizophyllan and scleroglucan, which are l,6-branchcd-β-l,3-glucans, significantly increased the survival rate. They also induced a protective effect against Aeromonas hydrophila at a dose of 5 mg kg-1 . The ability of the polysaccharides to activate the alternative complement pathway (ACP) was examined by incubating the polysaccharides with carp serum and measuring the residual ACP activity. At a final concentration of 0.l mgml-1 , l,6-branched-β-1,3-glucans greatly reduced (76–77%) the ACP activity. Therefore, it is suggested that the protective effect of the l,6-branched-β-1,3-glucans may be associated with the activation of ACP. 相似文献
150.
RICHARD G. HARVEY 《Veterinary dermatology》1993,4(4):175-179
Abstract— The physiology of essential fatty acid metabolism in the cat is reviewed. Emphasis is placed on those aspects of the n:6 and n:3 fatty acids, their metabolites and interactions, which relate primarily to the skin. The functional roles, if known, of the fatty acids are discussed. Recent clinical research into the use of essential fatty acid supplements in the management of feline dermatoses is presented. Current indications for the therapeutic supplementation with essential fatty acids are summarised. 相似文献