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   135633篇
  免费   7785篇
  国内免费   70篇
林业   5576篇
农学   4644篇
基础科学   862篇
  17483篇
综合类   18955篇
农作物   5087篇
水产渔业   7089篇
畜牧兽医   72969篇
园艺   1798篇
植物保护   9025篇
  2020年   1171篇
  2019年   1410篇
  2018年   2229篇
  2017年   2489篇
  2016年   2295篇
  2015年   1999篇
  2014年   2451篇
  2013年   5731篇
  2012年   4350篇
  2011年   5163篇
  2010年   3502篇
  2009年   3547篇
  2008年   5152篇
  2007年   4970篇
  2006年   4641篇
  2005年   4262篇
  2004年   4108篇
  2003年   4174篇
  2002年   3808篇
  2001年   4773篇
  2000年   4641篇
  1999年   3747篇
  1998年   1471篇
  1997年   1452篇
  1996年   1341篇
  1995年   1599篇
  1994年   1327篇
  1993年   1359篇
  1992年   2762篇
  1991年   2901篇
  1990年   2866篇
  1989年   2897篇
  1988年   2610篇
  1987年   2592篇
  1986年   2539篇
  1985年   2400篇
  1984年   1956篇
  1983年   1715篇
  1982年   1154篇
  1979年   1649篇
  1978年   1284篇
  1977年   1135篇
  1976年   1100篇
  1975年   1164篇
  1974年   1357篇
  1973年   1448篇
  1972年   1378篇
  1971年   1195篇
  1970年   1247篇
  1969年   1146篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
OBJECTIVES: To evaluate the efficacy of juvenile pubic symphysiodesis (JPS) in a clinical setting for the early treatment of canine hip dysplasia (CHD), and to identify its indications and contraindications. METHODS: The final degree of CHD using the FCI (Fédération Cynologique Internationale) CHD classification in 5 Grades (A, B, C, D, E) was assessed at skeletal maturity in two homogeneous groups of dogs assessed at the age of 14 to 22 weeks and selected according to their susceptibility to CHD; one group was treated with JPS and one group was conservatively managed. Two hundred seventeen puppies completed the study; 81 were treated with JPS (group 1) and 76 were conservatively managed (group 2). A third group of 60 puppies with normal hips was followed as a negative control group. RESULTS: In group 1, 43.2% of the puppies had regression or a lack of progression of the disease in the final evaluation (Grade A & B), 25.9% had mild CHD (Grade C) and 30.9% had moderate and severe CHD (Grade D & E). In group 2, 23.6% of the puppies did not show any development of the disease (Grade A & B), 21.1% had mild CHD (Grade C) and 55.3% developed moderate to severe CHD (Grade D & E). Further investigation was done by comparing the severity of early signs of susceptibility to CHD with the final FCI Grades at adulthood in both groups. CLINICAL SIGNIFICANCE: The JPS procedure increased the odds of arresting or limiting the progression of CHD in mild to moderate grades of CHD, while it was less effective or ineffective in more severe forms.  相似文献   
992.
An extensible internal device (EID) was developed to preserve growth plate during the treatment of fracture complications or segmental bone loss from tumour resection in children. Since this type of extensible, transphyseal, internal fixation device has only been used in a few paediatric cases; the aim of this study was to evaluate an in vivo canine study, a surgical application of this device, and its interference with longitudinal growth of the non-fractured distal femur. Ten clinically healthy two- to three-month-old poodles weighing 1.5-2.3 kg were used. Following a medial approach to the right distal femur, one extremity of the EID, similar to a T-plate, was fixed in the femoral condyle with two cortical screws placed below the growth plate. The other extremity, consisting of an adaptable brim with two screw holes and a plate guide, was fixed in the third distal of the femoral diaphysis with two cortical screws. The EID was removed 180 days after application. All of the dogs demonstrated full weight-bearing after surgery. The values of thigh and stifle circumferences, and stifle joint motion range did not show any difference between operated and control hindlimbs. The plate slid in the device according to longitudinal bone growth, in all but one dog. In this dog, a 10.5% shortening of the femoral shaft was observed due to a lack of EID sliding. The other dogs had the same longitudinal lengths in both femurs. The EID permits longitudinal bone growth without blocking the distal femur growth plate if appropriately placed.  相似文献   
993.
994.
Abstract Congenital portosystemic shunts generally arise as single vascular anomalies that cause the portal blood to bypass the liver and enter the systemic venous circulation directly. The liver is primarily affected, as it is deprived of perfusion by portal hepatotrophic factors such as insulin, glucagon, and amino acids. There is progressive hepatic atrophy, and as a consequence, dysfunction. Hepatic encephalopathy can result from increased levels of ammonia and gamma-aminobutyric acid within the systemic circulation. Variably toxic amines, captans and short chain fatty acids may act as false neurotransmitters. Hypoglycaemia will exacerbate the effects of these substances. Increased concentrations of ammonia and uric acid in the urine predispose to the precipitation of ammonium biurate crystals and the formation of calculi. Haematological changes include anaemia, microcytosis, hypoproteinaemia, leucocytosis, and coagulation abnormalities. Gastrointestinal effects are common. They may be displayed as anorexia, vomiting, ptyalism, pica, diarrhoea, or polyphagia. Most dogs are less than 1 year of age at initial presentation. Diagnosis from a laboratory viewpoint will involve a consideration of the history, clinical findings, haematology, serum biochemistry and urinalysis. If the findings are suggestive of a congenital portosystemic shunt, the demonstration of elevated fasting or, more consistently, post-prandial serum bile acid concentrations, and subsequent histological examination of a liver biopsy will provide a definitive diagnosis.  相似文献   
995.
1. The amino acid requirements of laying type pullets during the growing period can be estimated by measuring the growth of different components of the body and making use of nutritional constants that define the amount of each amino acid that is required for the production of the tissues being formed.

2. In this experiment, carcase analyses of each of three breeds of pullets were conducted at weekly intervals throughout the growth of the pullets, to 18 weeks of age. Measurements were made of body weight, gut‐fill and feather weight, and chemical analyses consisted of water, protein, lipid and ash measurements of both the body and the feathers. Each age group comprised 10 birds of each breed.

3. Gompertz functions accurately estimated the growth of both body protein and feather protein, to 18 weeks of age, from which the rate of growth of these two components of the body could be estimated. The mature weight of pullets was overestimated by the Gompertz growth curve, which may indicate that a pullet ceases to increase in body protein content once sexual maturity has been reached.

4. Using allometric relationships between the chemical components of the body and of feathers, all the components of growth could be estimated from the growth of body protein and feather protein. These components were then added together to determine the growth rate of the body as a whole.

5. The daily amino acid requirements for 4 functions were calculated, namely, those for the maintenance of body protein and feather protein, and for the gain in body protein and feather protein. These requirements were then summed to determine the requirement of pullets on each day of the growing period.

6. Using the ‘effective energy’ system, the amount of energy required by these pullets was calculated for each day of the growing period, from which the desired daily food intake of the pullets could be predicted. By dividing the amino acid requirement by this daily food intake it was possible to determine the concentration of amino acids that would be needed in the diet in order to meet the requirements of a pullet.

7. The results indicate that the ratio between the requirement for lysine and for methionine and cysteine changes dramatically during the growing period, negating the concept of a fixed ratio between all the amino acids during growth.

8. The above process is the first step in determining the optimal feeding programme for a population of pullets of a given genotype. The constraining effects, of the diet being offered and of the environment in which the pullets are housed, on the food intake and growth rate of each pullet have to be estimated, and such a theory can then be expanded to include all the individuals in the population. Only by the use: of simulation models can all these constraining effects be considered simultaneously.  相似文献   

996.
A dog undergoing diagnostic myelography inadvertently received 0.5 ml diatrizoate meglumine (66%) and (10%) diatrizoate sodium via intrathecal injection. This resulted in post-operative seizures requiring extended sedation of the patient. The clinical course and management of this patient are presented. Complications associated with the inappropriate administration of a hyperosmolar contrast agent into the subarachnoid space and the results of similar accidents in humans are also discussed. The dog in the report recovered with no evidence of long trem neurological dam  相似文献   
997.
A five-year-old male, West Highland terrier was treated for a gastrointestinal foreign body via a gastrotomy incision and enterotomies. The dog was asymptomatic for several months but was eventually re-presented because of vomiting. Endoscopic and surgical evaluation confirmed a large mass within the pyloric canal that probably resulted from a reaction to the polypropylene suture material used to close the original gastrotomy incision. The mass was excised via a Y-U pyloroplasty incision and the dog has been free of vomiting for over a year.  相似文献   
998.
Genetic factors affecting female reproductive performance in lines of mice with a known history of selection were estimated from a 5 X 5 diallel cross. Lines were selected as follows: large litter size at birth (L+); large 6-wk body weight (W+); an index for large litter size and small 6-wk body weight (L+W-); the complementary index (L-W+) and randomly (K). Partitioning of direct and correlated responses for litter size, 6-wk body weight and related traits into average direct genetic (li) and average maternal genetic (mi) effects indicated that the magnitude of differences in li exceeded those in mi. Lines having positive responses in li were W+ greater than L+ greater than L-W+ for dam body weight, L+ greater than L+W- greater than W+ for litter size and L+ greater than (W+, L+W-) for litter birth weight, whereas L-W+ responded negatively for litter size. A positive association was found between mi for litter size and dam body weight, W+ and L-W+ being high and L+ and L+W- low for both traits. Female infertility and time from male exposure to parturition had relatively small correlated responses. Line rankings in general combining ability (gi) and net line effects were similar for the respective traits. Depending upon the line and trait involved, the relative contribution of average direct genetic and line direct heterotic (hi) effects to general combining ability [gi = (1/2) li + hi] varied. Line heterosis refers to average heterosis in crosses involving that line. Direct heterosis ( hij ) for each trait differed considerably among crosses. The three crosses showing the highest hij for litter size at birth, W+ X L-W+ (1.78), L+ X W+ (1.28) and L-W+ X L+W- (1.22), possibly had loci contributing directional dominance to litter size with frequencies of parental lines deviating in opposite directions relative to mean gene frequency. The correlation between absolute difference in parental line means and hij for litter size was not significant, suggesting that the magnitudes of absolute differences in parental means were not reliable predictors of divergence in gene frequency. Crossbred performance increased linearly with midparent values for litter size at birth (b = .88 +/- .09, R2 = .92) and dam parturition body weight (b = 1.13 +/- .04, R2 = .99), the latter trait showing an increase (P less than .01) in heterosis as midparent values increased.  相似文献   
999.
Fifty-one treponemas were isolated from pigs. Twenty-three isolates with typical morphology and growth characteristic were beta hemolytic, enteropathogenic, produced indole and with exception of three strains did not ferment fructose. These strains were classified as typical T. hyodysenteriae and were usually isolated from pigs with symptoms of mucohemorrhagic diarrhoea. The seventeen other isolates were weakly beta hemolytic after 48 h incubation, enteropathogenic, 12 out of 17 produced indole, 10 out 17 fermented fructose. These strains were usually isolated from pigs with symptoms of gray-green diarrhoea and classified as T. hyodysenteriae 2 biotype or intermediate type. They may be compared with Treponema sp. isolated by Taylor et al. Eleven non enteropathogenic strains showed typical characteristic for T. innocens. Gas chromatography analysis of the fatty acids production from glucose, showed that all isolated treponemas produced acetate and butyrate. Typical T. hyodysenteriae produced additionally propionate. Strains of T. hyodysenteriae biotype 2 produced propionate or isobutyrate as well.  相似文献   
1000.
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