首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   16478篇
  免费   1150篇
  国内免费   3篇
林业   1428篇
农学   635篇
基础科学   101篇
  3827篇
综合类   546篇
农作物   683篇
水产渔业   1522篇
畜牧兽医   7082篇
园艺   259篇
植物保护   1548篇
  2023年   123篇
  2022年   178篇
  2021年   359篇
  2020年   344篇
  2019年   249篇
  2018年   697篇
  2017年   742篇
  2016年   732篇
  2015年   568篇
  2014年   655篇
  2013年   985篇
  2012年   1163篇
  2011年   1049篇
  2010年   608篇
  2009年   532篇
  2008年   875篇
  2007年   853篇
  2006年   700篇
  2005年   677篇
  2004年   595篇
  2003年   556篇
  2002年   465篇
  2001年   381篇
  2000年   372篇
  1999年   318篇
  1998年   75篇
  1997年   63篇
  1996年   67篇
  1992年   161篇
  1991年   133篇
  1990年   131篇
  1989年   148篇
  1988年   141篇
  1987年   138篇
  1986年   129篇
  1985年   119篇
  1984年   98篇
  1983年   92篇
  1982年   49篇
  1981年   61篇
  1980年   54篇
  1979年   117篇
  1978年   105篇
  1977年   97篇
  1976年   55篇
  1975年   51篇
  1974年   113篇
  1973年   99篇
  1972年   110篇
  1971年   103篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
We determined the thyroxine (T4) concentration in 63 heifers on 0th (day of insemination), 6th and 21st day after first and in repeat-breeder cows after second and third insemination in relation with the breed, age and weight of animals. The examination was carried out with a feed ration balanced according to the Czechoslovak State Standard CSN 46 7070, with all-the-year-round housing, with keeping the uniform time of insemination (8.00--9.00 a. m.) as well as the time of blood sampling (10.00--12.00 a. m.). The animals were inseminated from August to November. The average T4 concentrations, with a successful insemination, fluctuated on 0th day after first, second and third insemination at levels of 56.15 +/- 13.6; 84.6 +/- 10.66 and 89.75 +/- 13.62 nmol X l-1 of serum. As the results show, the lowest T4 concentrations (P less than 0.001; P less than 0.001) on 0th day were recorded in animals becoming pregnant in first insemination. The comparison with non-pregnant animals did not show any statistically significant difference in T4 concentrations. On 0th day of the second insemination, the T4 concentrations in pregnant animals were higher (P less than 0.01) as compared with non-pregnant. On 6th and 21st day after successful first insemination the T4 content increased significantly (P less than 0.001). After second and third successful insemination we recorded in T4 concentrations a moderate decrease on 6th day (P less than 0.01 for third insemination), with insignificant increase on 21st day.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
982.
Impregno-architectonics of neurons in the nuclei of the medial part of hypothalamus was studied in two-month and adult female and male sheep using the methods after Ramón-Moliner and Golgi-Cox. Types of neurons in the hypothalamic nuclei are given, morphology of synaptic conjugations and orientation of nerve processes are described. Ventromedial nucleus has the most interesting structure as a part of the medial hypothalamus: it is ring-shaped in cross-section with paucicellular center, contains various types of neurons and all kinds of synapses which can occur in hypothalamus. Only small numbers of neurons in the pars caudalis of tuberomammillary nucleus, similarly like in nucl. supraopticus and nucl. paraventricularis, are impregnated so that they can be supposed to belong to true neurosecretory nuclei.  相似文献   
983.
The occurrence of pathologically changed muscular fibers in some wild species of birds and in economically important domesticated species of birds is described. The hydrops of muscular fibers, necrosis and atrophy with connective tissue infiltration in muscular bundles were detected in breast and thigh muscles in wild birds, e. g. in raven, pigeon and pheasant. The same pathological processes were also found in domesticated species, e. g. in guinea fowl, less often in geese and duck. Their incidence in turkeys and laying types of fowl was more frequent, they were observed most often in muscles of broiler hens. Fission of muscular fibers, very thin, but also hypertrophic fibers and resorption of necrotic fibers were detected in hens besides the above changes. The described histological picture is confronted with the picture of hereditary myodystrophy in chickens.  相似文献   
984.
985.
A group of 401 patients suspected for toxoplasmosis was examined by the indirect immunofluorescent antibody test (IFAT). All patients positive in IFAT (176) were examined by the immunosorbent agglutination assay (IgM ISA). In the IgM ISA 154 of them were negative, 10 temporarily and 12 high-positive. Some of high-positive patients were examined repeatedly; decrease of high levels of specific IgM antibodies occurred 2-9 months after the first examination. For the IgM ISA antigen prepared from peritoneal exudate of experimentally infected mice was used. The antigen was stable at 4 degrees C or in liquid nitrogen at least 1 year. The IgM ISA combined with IFAT and IgM IFAT was proved satisfactory for the diagnosis of acquired acute toxoplasmosis and can be recommended for laboratories with lower capacity.  相似文献   
986.
The duration and type of sleep and activity were monitored in a group of 9 Duroc piglets weaned at 7 weeks of age and placed into a cage of 1.2 x 1.2 m. Average air temperature in the cage ranged between 20 and 23 degrees C and relative humidity was around 70%. The piglets were fed and watered ad libitum. The measurements were performed one week after the pigs had been transferred to the cages, in the period between 8 a.m. and 1 p.m. Approximately half of the 5-hour period of observation was occupied by sleep. There were, however, marked individual differences (ranges: 66 and 24%). Non-REM sleep occupied 79% while REM sleep 21% of the whole sleep time. REM episodes lasted, on the average, 3.8 + 0.58 min. The other half of the period studied was occupied by wakefulness which was devoted to movement, lying and, also, massaging and sucking each other. In some animals this activity was high and was the cause of unrest in the whole group. Differences in respiratory rate between non-REM and REM sleep recorded in heavier animals were the result of their heat load.  相似文献   
987.
Three strains of the Pasteurella aerogenes complex were isolated as sole pathogens from aborted fetuses of a sow aborted at the 12th week of gestation on a farm of 600 sows. Gross pathology showed no characteristic lesions. The isolates were biochemically identical and resembled P. pneumotropica on the basis of their strong indole and urease positivity but they produced gas, were ornithine decarboxylase negative and fermented mannitol but not trehalose. Only a few differences were apparent in biochemical characteristics between the isolated strains and P. aerogenes. They differed from the type strain of P. aerogenes in ornithine decarboxylase activity, indole production and lactose and mannitol fermentation; however, such strains do occur within this heterogeneous species. At the time of abortion the antibody titre of the aborted sow was 1 in 16 when examined with live bacterial suspension and 1 in 128 if boiled antigen was used. Similar strains could not be isolated from the vaginas of aborted sows or pregnant and newly farrowed sows in the same group. The bacteriological, serological and histological findings support the opinion of other workers on the occasional pathogenic nature of P. aerogenes.  相似文献   
988.
Tubulin was estimated to account for 0.3% of the total soluble protein in Trichinella spiralis cytosolic fractions. Tubulin from T. spiralis was partially purified by precipitation with either taxol or vinblastine sulphate. Immunoblotting with alpha- and beta-tubulin monoclonal antibodies revealed the presence of tubulin in T. spiralis partially purified preparations. Electrophoretic mobility of T. spiralis tubulin in sodium dodecyl sulphate-polyacrylamide gels was very similar to that shown by pig brain tubulin. Further studies with colchicine binding assays indicated that T. spiralis tubulin has binding features similar to that of tubulin from other nematodes: colchicine association constant = 8.1 x 10(-4) M and competitive inhibition of colchicine binding by podophyllotoxin, with an inhibition constant of 1.3 x 10(-6) M. Finally, inhibition of colchicine binding by several benzimidazoles (mebendazole, fenbendazole, oxibendazole and albendazole) was investigated. All the benzimidazoles inhibited colchicine binding in a competitive manner, with inhibition constant values ranging from 1.4 x 10(-7) M (mebendazole) to 3.9 x 10(-6) M (fenbendazole).  相似文献   
989.
The author analyses the interrelationship between the hygienic prerequisites of reproduction and performance in poultry farming, with regard to their impact on human health. The main factors influencing the hatching rate of poultry eggs are reviewed along with their epizootiological and economic aspects, and the possibilities of the prevention of egg-transmitted diseases are discussed.  相似文献   
990.
Analysis of growth curves of fowl. I. Chickens.   总被引:4,自引:0,他引:4  
1. The Richards function was used to describe the growth curves (n = 989) of 9 broiler lines. Chickens were fed ad libitum and body weight was recorded every second week from hatching to 26 weeks of age. 2. The accuracy of curve fit measured by the coefficient of determination (R2) was better for males than for females (0.9986-0.9995 vs 0.9972-0.9988, respectively). 3. The estimation of the asymptotic final weight (A) for different lines enabled the degree of maturity (ut = yt/A) to be determined at any fixed point of the curve. At the age of 7 weeks this had a value of 0.318-0.369 for cockerels and 0.325-0.377 for pullets and represented the slaughter maturity of individual lines. The ratio of inflection/asymptotic weight (y+/A = 0.370-0.388) indicated that in some cases chicken growth can be described approximately by the Gompertz function (y+/A = 0.368). 4. It was found that the age at the inflection point of curves (t+ 48.2-55.7 d for cockerels and t+ = 47.8-52.8 d for pullets) roughly corresponds to the slaughter age of the chickens. 5. The interline differences in the parameters of maturation rate for weight (y+/A, k, t+, u7) are low in comparison with the differences in body weight (A, y+, y7) and absolute growth rate (v, v+). 6. The intragroup phenotypic correlation among growth parameters and the importance of the mathematical models are discussed.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号