全文获取类型
收费全文 | 8540篇 |
免费 | 429篇 |
国内免费 | 58篇 |
专业分类
林业 | 1271篇 |
农学 | 232篇 |
基础科学 | 67篇 |
1438篇 | |
综合类 | 758篇 |
农作物 | 272篇 |
水产渔业 | 390篇 |
畜牧兽医 | 3848篇 |
园艺 | 225篇 |
植物保护 | 526篇 |
出版年
2023年 | 60篇 |
2022年 | 81篇 |
2021年 | 123篇 |
2020年 | 163篇 |
2019年 | 118篇 |
2018年 | 228篇 |
2017年 | 189篇 |
2016年 | 226篇 |
2015年 | 190篇 |
2014年 | 173篇 |
2013年 | 433篇 |
2012年 | 407篇 |
2011年 | 400篇 |
2010年 | 264篇 |
2009年 | 186篇 |
2008年 | 344篇 |
2007年 | 390篇 |
2006年 | 345篇 |
2005年 | 287篇 |
2004年 | 287篇 |
2003年 | 256篇 |
2002年 | 230篇 |
2001年 | 160篇 |
2000年 | 153篇 |
1999年 | 125篇 |
1998年 | 52篇 |
1992年 | 96篇 |
1991年 | 118篇 |
1990年 | 133篇 |
1989年 | 148篇 |
1988年 | 117篇 |
1987年 | 103篇 |
1986年 | 103篇 |
1985年 | 101篇 |
1984年 | 95篇 |
1983年 | 75篇 |
1979年 | 104篇 |
1978年 | 76篇 |
1977年 | 65篇 |
1976年 | 51篇 |
1975年 | 70篇 |
1974年 | 86篇 |
1973年 | 93篇 |
1972年 | 74篇 |
1971年 | 82篇 |
1970年 | 80篇 |
1969年 | 85篇 |
1968年 | 68篇 |
1967年 | 65篇 |
1965年 | 58篇 |
排序方式: 共有9027条查询结果,搜索用时 31 毫秒
11.
R W Morgan J Gelb C S Schreurs D Lütticken J K Rosenberger P J Sondermeijer 《Avian diseases》1992,36(4):858-870
Recombinant strains of herpesvirus of turkeys (HVT) were constructed that contain either the fusion protein gene or the hemagglutinin-neuraminidase gene of Newcastle disease virus (NDV) inserted into a nonessential gene of HVT. Expression of the NDV antigens was regulated from a strong promoter element derived from the Rous sarcoma virus long terminal repeat. Recombinant HVT strains were stable and fully infectious in cell culture and in chickens. Chickens receiving a single intra-abdominal inoculation at 1 day of age with recombinant HVT expressing the NDV fusion protein had an immunological response and were protected (> 90%) against lethal intramuscular challenge at 28 days of age with the neurotropic velogenic NDV strain Texas GB. Recombinant HVT expressing the NDV hemagglutinin-neuraminidase provided partial protection (47%) against the same challenge. Chickens vaccinated with recombinant HVT vaccines had low levels of protection against NDV replication in the trachea when challenged ocularly. Recombinant HVT vaccines and the parent HVT strain provided similar levels of protection to chickens challenged with the very virulent RB1B strain of Marek's disease virus, indicating that insertion of foreign sequences into the HVT genome did not compromise the ability of HVT to protect against Marek's disease. 相似文献
12.
Nephrogenic diabetes insipidus is characterized by polyuria, hyposthenuria and compensatory polydipsia. With conventional clinical and laboratory examinations the diagnosis can be established and the differential diagnostic diseases excluded. In order to verify the diagnosis, differentiate the condition from central and nephrogenic diabetes insipidus and rule out psychogenic polydipsia, additional laboratory studies are required: of these, the concentration test, modified Carter-Robbins test and AVP stimulation test are described. 相似文献
13.
14.
A tumor of the third eyelid gland was diagnosed in a 13 year old female Chihuahua. Histopathology of the excised tissue revealed an apocrine adnexoma with squamous metaplasia. The literature regarding prolapse and tumors of the third eyelid gland was reviewed. No case with similar morphology to this tumor was described to our knowledge. 相似文献
15.
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
16.
A line of Munich Miniature Swine (MMS) Troll showing a high incidence of spontaneous benign and malignant cutaneous melanocytic lesions has been developed since 1986. The inheritance of cutaneous melanocytic lesions was studied by establishing the F1 -, F2 - and reciprocal B1 -generations with one melanoma MMS-Troll boar and four unaffected German Landrace sows as founders. A total of 176 animals were available, 27 in the F1 -, 111 in the F2 -, 19 in the B1-DL -, and 14 in the B1-Troll -generation. Benign melanocytic lesions were observed in 42% of F1 -, 18% of F2 -, 11% of B1-DL - and 50% of B1-Troll -animals. Malignant melanomas developed in 3.6% of F2 - and 7.1% of B1-Troll -animals, although no animal with white coat colour was affected. A mixed major gene model with arbitrary gene action explained the segregation of benign lesions sufficiently well. For melanomas a mixed major gene model required additional dominant acting suppressor loci to obtain a sufficient fit to the data. An influence of SLA haplotypes on the penetrance of melanocytic lesions was not evident. The association analysis of the white phenotypes strongly indicated that the dominant allele I at the I-locus suppresses malignant melanocytic lesions. A possible explanation is the lack of melanocytes in the skin of dominant white pigs caused by a mutation of the KIT-gene, which leads to a failure of melanoblast migration and development. 相似文献
17.
J. Müller 《Journal of Agronomy and Crop Science》1993,171(4):217-235
Dry Matter Production, CO2 Exchange, Carbohydrate and Nitrogen Content of Winter Wheat at Elevated CO2 Concentration and Drought Stress
Methods of mathematical modelling and simulation are being used to an increasing degree in estimating the effects of rising atmospheric CO2 concentration and changing climatic conditions on agricultural ecosystems. In this context, detailed knowledge is required about the possible effects on crop growth and physiological processes. To this aim, the influence of an elevated CO2 concentration and of drought stress on dry matter production, CO2 exchange, and on carbohydrate and nitrogen content was studied in two winter wheat varieties from shooting to milk ripeness. Elevated CO2 concentration leads to a compensation of drought stress and at optimal water supply to an increase of vegetative dry matter and of yield to the fourfold value. This effects were caused by enhanced growth of secondary tillers which were reduced in plants cultivated at atmospheric CO2 concentration. Analogous effects in the development of ear organs were influenced additionally by competitive interactions between the developing organs. The content and the mass of ethanol soluble carbohydrates in leaves and stems were increased after the CO2 treatment and exhausted more completely during the grain filling period after drought stress. Plants cultivated from shooting to milk ripeness at elevated CO2 concentration showed a reduced response of net photosynthesis rate to increasing CO2 concentration by comparison with untreated plants. The rate of dark respiration was increased in this plants. 相似文献
Methods of mathematical modelling and simulation are being used to an increasing degree in estimating the effects of rising atmospheric CO
18.
H Müller 《Tier?rztliche Praxis》1991,19(6):591-595
Bluetongue virus (BTV), an arthropod-borne virus, is transmitted primarily by biting midges of the genus Culicoides. Some insect species, which might serve as a potential vector, are prevalent in Central Europe. In sheep, bluetongue is acute and mortality is high, whereas in cattle, goats and most wild ruminants the infection is usually clinically inapparent. Viremia is of short duration in sheep, but cattle experience a prolonged viremia and provide a reservoir for the dissemination of BTV. At least 24 different BTV serotypes have been identified. Antigenic variations occur in the polypeptides of the outer viral capsid and the segmented nature of the viral genome provides the potential for evolution of the virus by a mechanism of reassortment. This renders the use of polyvalent vaccines inefficient and emphasizes the significance of import/export restrictions on ruminants from BTV endemic areas. 相似文献
19.
The insecticide formulation BI 58 EC was tested for teratogenicity in chicken embryos, with particular reference to degradation of the active ingredient (dimethoate) after the treatment of embryonated eggs. The pesticide was diluted in water to a concentration level of 0.8%, and the emulsion was injected into the air space in a volume of 0.1 ml/egg, or hen's eggs were treated by the immersion technique. Residues of dimethoate were measured in the samples on days, 13, 15 and 19 of the incubation of chicken embryos, and morphological examinations were performed simultaneously. Analytical chemistry data indicated a slower degradation of dimethoate in embryos after the immersion of eggs, and cyllosis was remarkable in this group among the sporadic developmental anomalies. The liver tissues of both treated groups exhibited severe fatty infiltration. 相似文献
20.
1. This paper addresses the possibility of using a monthly model for the genetic evaluation of laying hens, based on the definition of a test day model with fixed regression as used in dairy cattle, in which monthly records were treated as repeated measurements of the same trait. 2. Production records of 6450 hens, daughters of 180 sires and 1335 dams were analysed using an animal model with restricted maximum likelihood (REML). The traits considered were individual monthly egg production and cumulative egg production in 11 months. Four different models were fitted to various combinations of monthly and cumulative records. The covariates were derived from the regression of Ali and Schaeffer (1987). 3. Spearman rank correlations were computed to compare breeding values from different models. Two types of correlations were computed: between individual breeding values and between sire breeding values based on subsets of full-sib records. 4. The results indicated that a monthly model with nested covariates produced higher heritability and permanent environmental variance than the models with non-nested or without covariates. The estimates of heritability obtained from monthly model were lower than the estimates from the cumulative model. The monthly model resulted in higher correlations of sire breeding values between two subsets of full-sib records than those from cumulative models. 5. In conclusion, the monthly model with nested covariates appears to be better than the model with non-nested covariates or without covariate. Although the heritability estimates obtained from the monthly model were lower, the monthly model with nested covariates could be better than the cumulative model for genetic evaluation of laying hens in the 1st cycle of laying period when using either full or part records. The use of information from odd months of production could be of interest for the evaluation of full records. 相似文献