全文获取类型
收费全文 | 419854篇 |
免费 | 21032篇 |
国内免费 | 729篇 |
专业分类
林业 | 26880篇 |
农学 | 14797篇 |
基础科学 | 3094篇 |
54737篇 | |
综合类 | 76410篇 |
农作物 | 18456篇 |
水产渔业 | 21017篇 |
畜牧兽医 | 190077篇 |
园艺 | 7021篇 |
植物保护 | 29126篇 |
出版年
2018年 | 5354篇 |
2017年 | 5851篇 |
2016年 | 5862篇 |
2015年 | 5006篇 |
2014年 | 6382篇 |
2013年 | 17323篇 |
2012年 | 10703篇 |
2011年 | 13879篇 |
2010年 | 9615篇 |
2009年 | 9633篇 |
2008年 | 12969篇 |
2007年 | 12396篇 |
2006年 | 12180篇 |
2005年 | 11384篇 |
2004年 | 11211篇 |
2003年 | 10977篇 |
2002年 | 10183篇 |
2001年 | 11804篇 |
2000年 | 11393篇 |
1999年 | 9389篇 |
1998年 | 4797篇 |
1997年 | 4815篇 |
1996年 | 4543篇 |
1995年 | 5285篇 |
1994年 | 4859篇 |
1993年 | 4529篇 |
1992年 | 7863篇 |
1991年 | 8286篇 |
1990年 | 8315篇 |
1989年 | 8326篇 |
1988年 | 7566篇 |
1987年 | 7670篇 |
1986年 | 7755篇 |
1985年 | 7626篇 |
1984年 | 6484篇 |
1983年 | 5957篇 |
1982年 | 4391篇 |
1981年 | 4097篇 |
1979年 | 6038篇 |
1978年 | 5111篇 |
1977年 | 4500篇 |
1976年 | 4299篇 |
1975年 | 4474篇 |
1974年 | 5345篇 |
1973年 | 5417篇 |
1972年 | 5136篇 |
1971年 | 4910篇 |
1970年 | 4761篇 |
1969年 | 4540篇 |
1967年 | 4017篇 |
排序方式: 共有10000条查询结果,搜索用时 521 毫秒
911.
A Campylobacter jejuni isolate obtained from a turkey liver, designated C101, and a C. jejuni isolate obtained from the feces of a chicken, designated C111, were used to inoculate their respective hosts. Isolate C101 depressed weight gain by 20% when inoculated into newly hatched poults or 4-day-old poults. It also caused death, hepatic necrosis, and generalized hemorrhages in turkey embryos. The chicken-derived isolate, C111, did not reduce weight gain in newly hatched chicks, but it did induce mortality in chicken embryos. The supernatant of the cultures of both C. jejuni isolates also caused mortality in embryos. 相似文献
912.
Two mungbean varieties, Guj-1 and PIMS-1 differing in their productivity potential, were examined to investigate their photosynthetic system at various stages of growth in relation to yield with two different dates of sowing. Vertical leaves were found to be beneficial. On the other hand, excessive leaf area during the later stages of growth may be detrimental to yield. Lower LAD at seedling stage and overall high NAR values may be reflected in higher grain yield. NAR declined as season progressed and this, in turn, may be related with increase in LAI. The grain yield was considerably more at the first date of sowing as compared to second date of sowing, the effect was being more pronounced in var. PIMS-1 . Various environmental factors recorded at two different dates of sowing and at various stages of growth indicated that these factors may have influenced the yield potential at two different dates of sowing. 相似文献
913.
V. Slipevi I. Vedrina-Dragojevi L. Balint J. Momirovi-uljat 《Journal of Agronomy and Crop Science》1992,168(2):73-84
Dynamics of the cumulation of phosphorus, potassium, calcium and magnesium in developing seed of four soybean genotypes under the defined climatic conditions in the course of three years was observed. Samples were being taken at weekly intervals from the point at which the grain reached a size suitable to be analyzed up to the full maturity of the seed. By analyzing the results it was found out that the intense cumulation of the minerals tested took place during the stages III and IV of the seed development, that is, in the course of intense cumulation of dry matter and the most expressive metabolic activity in the seed. Investigation of the impact of climatic conditions provides evidence that cumulation of the tested macroelements has been favourably influenced by increased precipitation along with moderate air temperatures within the seed developing period. The highest content of the minerals found with the cultivar Maksimirka independently of the year investigated reveals that mineral cumulation in soybean seed is affected by the cultivar and genetic particularity. 相似文献
914.
Chloroform-methanol (2:1 v/v), absolute methanol, and 90% acetone were evaluated for their effectiveness as extractants of chlorophyll a from samples of phytoplankton communities collected from catfish ponds. Chloroform-methanol consistently extracted more chlorophyll a than either 90% acetone or methanol. Precision for the methanol extraction was also unacceptably low, with an average coefficient of variation of 17%. Average coefficients of variation for the chloroform-methanol and 90% acetone extraction procedures were 6 and 5%, respectively. Filtered samples should be steeped in chloroform-methanol for at least 4 h to obtain maximum chlorophyll extraction, and the addition of MgCO3 to the extractant as a buffer is not necessary. 相似文献
915.
H J Rapp 《Tier?rztliche Praxis》1988,16(2):167-173
During the combination of oral and intravenous application of saline solution for treatment of the COPD of horses the level of hydraemia basing on the total protein concentration in the serum, the urine production and the specific weight of urine was determined. Additionally the development of serum concentration and of renal excretion rates of potassium, calcium and magnesium were ascertained. The level of hydraemia resulting from the combined method is almost identical with the solely intravenous performed hyperinfusion therapy. Due to the excessive application of fluid an extremely high level of urine production is reached which causes a reduced specific weight as well as an increased renal excretion of potassium, calcium and magnesium. The result is a real loss of electrolytes which is - apart from hydraemia - the reason for the lower level of the corresponding serum concentrations. This seems to be important, especially for potassium and magnesium, because the organism is unable to compensate the loss of these electrolytes in the same way as the loss of calcium. In connection with the renal loss of electrolytes during the high level of urine production glucosuria is observed. 相似文献
916.
Response time of broiler chickens to cimaterol: meat tenderness, muscle composition fiber size, and carcass characteristics. 总被引:4,自引:0,他引:4
The response time to cimaterol (CIM), a beta-adrenergic agonist, by broiler chickens for carcass characteristics, muscle composition, muscle fiber size, catheptic enzyme activity, and tenderness was determined. Two trials were conducted in which chickens were fed a control diet (CON) containing 0 ppm of CIM or a diet containing 1 ppm of CIM. Trial 1 consisted of 55, 31-d-old broiler chickens individually fed for up to 48 h. At 0, 6, 12, 18, 24, and 48 h, five CON and five CIM-fed chickens were killed. Trial 2 consisted of 160, 33-d-old broiler chickens group-fed for up to 14 d. At 2, 4, 6, 8, 10, 12, and 14 d, 10 CON and 10 CIM-fed chickens were killed. The breast muscle (BM) and leg muscle (LM) weight, cathepsin B and L activities, DNA, RNA, and protein concentration, and BM shear force value (SFV) were measured in both trials. Thigh muscle (TM) SFV were measured in Trial 2 only. Fiber size of BM was measured (five birds per treatment) at d 2, 6, 10, and 14. In Trial 1, BM weight and SFV were lower in CIM-fed birds at 6 h (P less than .05). In Trial 2 BM SFV were higher at d 8 (P = .06) and d 10 (P less than .05) in CIM-fed chickens. The SFV of CIM-fed chickens were higher at d 4, 8, 10, 12, and 14 (P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
917.
In a nine-year-old female cat, the diagnostic procedure, therapy and differential diagnoses of a rare, primary, malignant, epibulbar melanoma are presented. Post-operatively no recidivation or metastases occurred within 18 months. 相似文献
918.
W. E. HAWKINS J. W. FOURNIE R. M. OVERSTREET W. W. WALKER 《Journal of fish diseases》1988,11(3):259-266
Abstract. Three cases of skeletal muscle neoplasms occurring in two small fish species used in carcinogen tests are reported. The cases illustrated a wide range of histologic patterns, and consisted of a well-differentiated juvenile type, a well-differentiated pleomorphic type and a poorly differentiated pleomorphic type. A rhabdomyosarcoma with juvenile type features developed in the Japanese medaka, Oryzias latipes , exposed to 0·5 ml/1 of the solvent dimethylformamide. A pleomorphic type rhabdomyosarcoma occurred in one medaka and in one guppy, Poecilia reticulata , exposed to the carcinogen methylazoxymethanol acetate. These cases indicate the potential for skeletal muscle cells in medaka and guppy to become neoplastic but, because the tumours occurred at a low frequency, there may not necessarily be a chemical aetiology. 相似文献
919.
With the help of a new synthetic material follicles and corpora lutea from the ovaries of 80 cows in different stages of the sexual cycle were examined for modifications of their blood vessel architecture. In the theca externa the main artery which runs towards the follicle, separates into arterioles of which capillaries derive in the theca interna, building a basket-like network around the zona granulosa. After ovulation the follicle wall forms characteristic fields. Capillaries grow into the zona granulosa and thus the folds form the lobular structure of the corpus luteum. Capillaries which rise from the ruptured part of the follicle, spread centripetally and together with lutein cells they build up the not lobulated cupola of the corpus luteum. The regression of the corpus luteum starts around the 16th day of the cycle, beginning at the capillary network of the cupola. A contraction of the vessels and a quick decrease of the volume follow. 相似文献
920.
G L Bennett 《Journal of animal science》1992,70(1):51-56
Lean tissue growth rate is usually estimated from indirect measurements including growth rate. A procedure to determine prediction equations for lean tissue growth rate is proposed. The procedure restricts the regression of fat growth rate on predicted lean growth rate to be equal to the regression of fat growth rate on actual lean growth rate. The restriction can be phenotypic or genetic if suitable parameter estimates are available. When applied phenotypically, selection on predicted lean tissue growth rate will result in selection differentials for both fat and lean tissue growth rates that are proportional to those obtained by direct selection for lean tissue growth rate. This restriction is desirable because expected correlated changes in fat are used to justify selection for lean tissue growth. Conventional prediction procedures have ignored correlated changes and obscured the original intent of using lean tissue growth rate as a biological selection criterion. When using conventional procedures to predict a biological selection criterion from indirect measurements, changes in important correlated traits may depend more on the choice of indirect measurements than on the choice of selection criterion. 相似文献