全文获取类型
收费全文 | 74391篇 |
免费 | 3952篇 |
国内免费 | 47篇 |
专业分类
林业 | 3433篇 |
农学 | 2249篇 |
基础科学 | 370篇 |
9202篇 | |
综合类 | 13595篇 |
农作物 | 2791篇 |
水产渔业 | 3599篇 |
畜牧兽医 | 37707篇 |
园艺 | 840篇 |
植物保护 | 4604篇 |
出版年
2019年 | 690篇 |
2018年 | 1117篇 |
2017年 | 1242篇 |
2016年 | 1123篇 |
2015年 | 926篇 |
2014年 | 1193篇 |
2013年 | 2682篇 |
2012年 | 2210篇 |
2011年 | 2516篇 |
2010年 | 1716篇 |
2009年 | 1780篇 |
2008年 | 2527篇 |
2007年 | 2474篇 |
2006年 | 2271篇 |
2005年 | 2134篇 |
2004年 | 1987篇 |
2003年 | 2023篇 |
2002年 | 1864篇 |
2001年 | 2271篇 |
2000年 | 2220篇 |
1999年 | 1814篇 |
1998年 | 716篇 |
1997年 | 735篇 |
1995年 | 760篇 |
1994年 | 685篇 |
1993年 | 708篇 |
1992年 | 1342篇 |
1991年 | 1432篇 |
1990年 | 1337篇 |
1989年 | 1303篇 |
1988年 | 1267篇 |
1987年 | 1295篇 |
1986年 | 1254篇 |
1985年 | 1278篇 |
1984年 | 1053篇 |
1983年 | 931篇 |
1979年 | 1022篇 |
1978年 | 807篇 |
1977年 | 744篇 |
1976年 | 751篇 |
1975年 | 805篇 |
1974年 | 1000篇 |
1973年 | 896篇 |
1972年 | 945篇 |
1971年 | 910篇 |
1970年 | 808篇 |
1969年 | 866篇 |
1968年 | 710篇 |
1967年 | 821篇 |
1966年 | 694篇 |
排序方式: 共有10000条查询结果,搜索用时 847 毫秒
101.
A 13-month-old Angus steer was examined with a 6-week history of lethargy, malaise and dribbling urine. Laboratory exam revealed crystalluria and poor renal function. Ultrasound revealed hydronephrosis and hydroureter. Euthanasia was chosen because of a poor prognosis for economic recovery. Necropsy demonstrated numerous calculi causing partial urethral obstruction approximately 25 cm from the end of the penis. Secondary renal changes were confirmed. Urolithiasis occurs commonly in ruminants. Secondary obstruction is usually complete with severe consequences. This is the first report of chronic partial obstructive urolithiasis resulting in endstage renal disease. 相似文献
102.
An account is given of the methodology for fractionation of cock spermatozoa into head and tail fractions by ultrasonication, followed by sucrose density gradient centrifugation. Quantitative estimates of DNA attested to 89.4% purity of the head fraction and low contamination of tails with heads. Recovery of protein and malic dehydrogenase (MDH) activity, following sperm fractionation, averaged 94.3% and 95.7%, respectively. Contamination of the head fraction with tails, as assessed by MDH assay, was only 4.65%, and the purity of the tail fraction was 91%. Intensive succinic dehydrogenase (SDH) activity was histochemically localised in the separated tail fraction and in the tail portion of intact spermatozoa. However, SDH activity was discernible neither in the head fraction nor in the head of intact spermatozoa. 相似文献
103.
Chickens were given daily injections of cyproterone acetate (CA) and the effects on plasma corticosterone, bodyweight, weights of the adrenals and lymphoid organs, numbers of circulating peripheral blood lymphocytes and their proliferation in the presence of lectins, concanavalin A (Con A) and phytohaemagglutinin (PHA), were investigated. Five daily doses of 10 or 30 mg CA kg-1 bodyweight each week over a three-week period caused a decrease in weight gain and a reduction in the relative weights of the bursa and thymus but not the spleen. There was a small decrease in the adrenals after treatment with 10 mg CA kg-1. When daily injections of CA were given over a seven-day period doses of 6 and 10 mg CA kg-1 bodyweight caused a significant (P less than 0.05) decrease in plasma corticosterone concentration after four days. However, after eight daily injections of CA a single injection of corticotrophin (10 iu ACTH kg-1) increased circulating corticosterone indicating CA had not completely blocked adrenal synthesis. CA had no effect on numbers of circulating peripheral blood lymphocytes or their ability to proliferate in the presence of Con A or PHA. The results indicate that CA is effective in lowering circulating corticosterone in the fowl but this did not affect the numbers or responsiveness of peripheral blood lymphocytes. 相似文献
104.
The relationship between the levels of CO and airborne Pb was evaluated against conditions of traffic flow, ambient air temperature and the ventilation rates in 3 underground car parks. During the study period significant variations were noted in the levels of the pollutants in each of the car parks which frequently exceeded the proposed Malaysian Air Quality Standard of 0.7 μg m?3Pb and 9 μg L?1 (8 hr average) CO for indoor environs. High correlation coefficient values were obtained between the levels of CO and traffic flow and Pb and traffic flow. High traffic flow also increased the mean air temperature inside the car parks. Reductions in the ventilation rates in the car parks caused an almost two-fold increase in the level of CO and vice versa. A simple mathematical model depicting the CO concentrations in relation to other parameters was formulated. 相似文献
105.
Quantitative determination of plant opal content in soils, using a combined method of heavy liquid separation and alkali dissolution 总被引:3,自引:1,他引:2
The aim of this study was to improve the quantitative determination of the plant opal content (i.e. phytoliths) in soils. The proposed method is based on: (i) the separation of plant opal from the silt and sand fractions of the soil, using heavy liquid flotation (aqueous solution of ZnBr2, density = 1.92 g cm?3); (ii) the subsequent determination of alkali-soluble silicon by atomic absorption spectrometry. Extraction and analytical procedures were tested on a broad sample of temperate and tropical soils with very different phytolith contents. Our investigations lead to the following conclusions: (i) a selective dissolution of opal in alkaline solutions (e.g. hot 0.5 m NaOH as proposed by Jones, 1969) is inaccurate so that a sink-float method must be used before any dissolution procedure; (ii) to dissolve opal completely, a 0.5 M NaOH dissolution treatment at 150°C can be easily and successfully carried out in steel PTFE-lined pressure vessels; (iii) the reproducibility of the determination is satisfactory for a step-by-step procedure (mean coefficient of variation = 13.4%). The comparison of this new method of quantitative assessment of soil opal with two other methods (gravimetric and phytolith-counting methods), shows very highly significant correlations (P<0.001). Therefore, this procedure is a useful tool in studies connected with pedological and environmental history. 相似文献
106.
107.
108.
While isotopic techniques have largely superseded traditional markers for the determination of the volume of fluid compartments in vivo, they are not always convenient, especially with diarrhoeic animals. A direct comparison was therefore made in week-old calves between Evans blue and radio-iodinated serum albumin as measures of plasma volume and thiocyanate or 24sodium as measures of extracellular fluid space. The correlation coefficients were excellent (1.00, 0.96; P < 0.001) and the calves had plasma and extracellular fluid volumes of 72 +/- 2 and 438 +/- 2 ml kg-1, respectively. The latter value is, though high, typical of young animals and comparable with other data in calves. 相似文献
109.
110.
Effect of dietary protein content on episodic growth hormone secretion and on heat production of male broiler chickens. 总被引:1,自引:0,他引:1
1. The effect of the crude protein content (200 and 150 g/kg) of isoenergetic diets on episodic growth hormone (GH) release and on heat production was investigated in male broiler chickens. 2. Decreasing the crude protein content of isoenergetic diets from 200 g/kg (HP diet) to 150 g/kg (LP diet) resulted in depressed body weight gain, impaired food conversion efficiency and increased abdominal fat deposition. 3. The pattern of growth hormone secretion was markedly affected by dietary treatment. Broiler chickens fed on the LP diet had higher overall mean, amplitude, baseline and peak frequency than the HP chickens. 4. The LP chickens produced more heat per unit of metabolic body weight than the HP chickens. 5. The hypothesis relating the pattern of GH secretion to protein conversion efficiency was corroborated. 相似文献