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   202955篇
  免费   10784篇
  国内免费   136篇
林业   8604篇
农学   6305篇
基础科学   1363篇
  24964篇
综合类   36524篇
农作物   8181篇
水产渔业   9694篇
畜牧兽医   102617篇
园艺   2649篇
植物保护   12974篇
  2019年   1838篇
  2018年   2695篇
  2017年   3070篇
  2016年   2829篇
  2015年   2364篇
  2014年   2971篇
  2013年   7941篇
  2012年   5585篇
  2011年   6939篇
  2010年   4581篇
  2009年   4636篇
  2008年   6919篇
  2007年   6446篇
  2006年   6036篇
  2005年   5705篇
  2004年   5579篇
  2003年   5637篇
  2002年   5244篇
  2001年   6132篇
  2000年   6201篇
  1999年   4822篇
  1998年   2149篇
  1997年   2094篇
  1996年   1952篇
  1995年   2361篇
  1994年   2152篇
  1993年   2009篇
  1992年   4110篇
  1991年   4387篇
  1990年   4239篇
  1989年   4241篇
  1988年   3916篇
  1987年   3918篇
  1986年   4091篇
  1985年   3981篇
  1984年   3224篇
  1983年   2937篇
  1982年   1950篇
  1979年   3036篇
  1978年   2454篇
  1977年   2001篇
  1976年   2039篇
  1975年   2147篇
  1974年   2611篇
  1973年   2728篇
  1972年   2602篇
  1971年   2471篇
  1970年   2314篇
  1969年   2275篇
  1967年   1951篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
161.
The uterine and umbilical vessels of 12 pregnant ponies were catheterised to study foetal metabolism. The effects of this procedure on maternal and foetal cardiovascular, respiratory, metabolic and adrenocortical activity were monitored during and after surgery. Premedication with acepromazine-butorphanol-detomidine was followed by induction of anaesthesia with detomidine and ketamine and maintenance, using mechanical ventilation, with halothane in oxygen and nitrous oxide. Mean maternal arterial blood pressure was greater than 70 mmHg during anaesthesia and arterial oxygen tension remained over 100 mmHg. The foetuses were adequately oxygenated but were hypercapnic and lactic acidaemic. Most maternal and foetal blood gases and metabolites had returned to normal by 24 h, although foetal plasma lactate fell more slowly. The maternal adrenocortical discharge was less severe than reported previously and plasma cortisol had fallen to basal levels by 48 h after surgery. Foetal plasma cortisol remained low and did not change during or after surgery. Arterio-venous metabolite and gas tension differences across the uterine and umbilical circulations were slightly greater at operation than in the recovery period, suggesting that uteroplacental perfusion may have been impaired during surgery. Post-operative recovery of the mare and foetus was satisfactory and subsequent problems associated with the foetal catheters were not related to the anaesthesia or surgery.  相似文献   
162.
Experiments were conducted to determine comparative populations of Salmonella typhimurium in the most commonly infected body organs of long-term carrier swine. Naturally farrowed Salmonella-free pigs (n = 58) were orally exposed to S typhimurium when they were 47 days old. Necropsy of 3 to 5 randomly selected pigs was conducted at 3, 7, 10, 14, and 17 days and at 3, 4, 5, 6, 8, 12, 16, 20, 24, and 28 weeks after exposure. Mean populations (log10/g) of S typhimurium in palatine tonsils, ileum, cecum (wall and contents), ascending colon (wall and contents), and mandibular and ileocolic lymph nodes were estimated at each necropsy, using a most-probable-number method of bacteriologic examination. Populations of organisms in cecum and colon were similar to each other throughout the duration of the study. Mean populations (log10/g) associated with cecal and colonic walls decreased from 6.1 and 6.6, respectively, during the first postexposure (PE) week to less than or equal to 1.67 from PE weeks 4 to 28. Populations (log10/g) associated with cecal and colonic contents decreased from 5.6 and 5.5, respectively, at PE day 3 to 2.5 and 2.7, respectively, at PE week 4, and remained less than or equal to 2.8 until week 28. Populations (log10/g) associated with intestinal walls and contents were closely correlated during the study. Population (log10/g) in the ileum was greater than or equal to 5.3 from PE days 3 to 17, then varied between 5.4 and -0.4 up to PE week 28.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
163.
Xylazine and tiletamine-zolazepam anesthesia in horses   总被引:4,自引:0,他引:4  
The cardiopulmonary and anesthetic effects of xylazine in combination with a 1:1 mixture of tiletamine and zolazepam were determined in 6 horses. Each horse was given xylazine IV or IM, as well as tiletamine-zolazepam IV on 4 randomized occasions. Anesthetics were administered at the rate of 1.1 mg of xylazine/kg of body weight, IV, 1.1 mg of tiletamine-zolazepam/kg, IV (treatment 1); 1.1 mg of xylazine/kg, IV, 1.65 mg of tiletamine-zolazepam/kg, IV (treatment 2); 1.1 mg of xylazine/kg, IV, 2.2 mg of tiletamine-zolazepam/kg, IV (treatment 3); and 2.2 mg of xylazine/kg, IM, 1.65 mg of tiletamine-zolazepam/kg, IV (treatment 4). Tiletamine-zolazepam doses were the sum of tiletamine plus zolazepam. Xylazine, when given IV, was given 5 minutes before tiletamine-zolazepam. Xylazine, when given IM, was given 10 minutes before tiletamine-zolazepam. Tiletamine-zolazepam induced recumbency in all horses. Duration of recumbency in group 1 was 31.9 +/- 7.2 (mean +/- 1 SD) minutes. Increasing the dosage of tiletamine-zolazepam (treatments 2 and 3) significantly (P less than 0.05) increased the duration of recumbency. Xylazine caused significant (P less than 0.05) decreases in heart rate and cardiac output and significant (P less than 0.05) increases in central venous pressure and mean pulmonary artery pressure 5 minutes after administration. Respiratory rate was decreased. Arterial blood pressures increased significantly (P less than 0.05) after xylazine was administered IV in treatments 1 and 3, but the increases were not significant in treatment 2. Xylazine administered IM caused significant (P less than 0.05) increases in central venous pressure and significant (P less than 0.05) decreases in cardiac output.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
164.
Chinese catfish ( Clarias fuscus ) were successfully spawned in Hawaii using human chorionic gonadotropin (HCG) at dosage rates of two and four international units (IU) per gram body weight. Fish not injected with HCG did not produce viable eggs. Successful spawns with HCG occurred between May and October. Hatch rates of up to 80% were obtained during June, July, and August for those fish given either a 2 or 4 IU per gram body weight injection of HCG. Fish spawned in either May or October yielded significantly higher hatch rates when injected with 4 rather than 2 IU per gram body weight. Fish held at elevated temperatures (28 to 30 C) prior to the normal spawning season developed significantly larger oocyte diameters, 60 days earlier than fish held under ambient temperature conditions (21.5 to 24 C). Photoperiod manipulation at ambient temperature conditions was associated with earlier oocyte maturation, but photoperiod effects were much less important than temperature.  相似文献   
165.
Flowering events in relation to smut susceptibility in pearl millet   总被引:1,自引:0,他引:1  
In field experiments at ICRISAT, selected lines of pearl millet (Pennisetum glaucum) resistant and susceptible to smut (Tolyposporium penicillariae) were evaluated for the timing of flowering events. In smut-resistant lines, the time from the boot-leaf stage (inoculation) to stigma emergence varied in the range 46–120 h, from boot to anther emergence 105–190 h, and from stigma emergence to anther emergence (protogyny period) 59–131 h; in smut-susceptible lines, the corresponding periods were 62–140 h, 146–200 h and 44–120 h, respectively. There were no significant correlations between timing of events and smut severity. Three cytoplasmically male-sterile lines showed longer protogyny periods and higher smut severity than their corresponding maintainer lines. Four lines having short protogyny periods (22–52 h) and resistance to ergot (Claviceps fusiformis) also showed high resistance to smut. Resistance to smut in most ergot-susceptible lines was independent of the timing of flow ering events, but in ergot-resistant lines it could be closely related to flowering events.  相似文献   
166.
Systemic and pulmonary antibody responses of calves to Pasteurella haemolytica were evaluated by measuring immunoglobulin production in blood for 9 days and in pulmonary lavage fluid for 7 days after intrapulmonary inoculation. Clinical signs, pulmonary lesions, pulmonary and systemic inflammatory response, and amount of antigen in lavage fluid were used to evaluate the response of calves to challenge with P haemolytica. The pulmonary response consisted of production of IgG, IgE, and IgM antibodies to P haemolytica antigens and a 17- to 68-fold increase of cells in lavage fluid 8 hours after inoculation, with a gradual decrease toward normal. Antibodies of the IgM isotype to P haemolytica were demonstrated as early as 8 hours through 7 days after inoculation in 3 of 3 calves. Of the anti-P haemolytica isotypes, IgM was found in the highest concentration. In all of the inoculated calves, IgE was found 1 to 2 days after inoculation, and IgG was found in 2 of 3 inoculated calves from day 1 through 7 after inoculation. Detection of IgG correlated with smaller pulmonary lesions. Immunoglobulin A was not detected in lavage fluid. Serum was evaluated for IgG and IgM antibody response to P haemolytica. Specific IgM was detectable 5 days after inoculation, and IgG was detectable 7 days after inoculation. Pasteurella haemolytica antigens were not detected in serum or plasma. A transient increase in neutrophil count was found 8 hours after inoculation, with return to baseline values by 24 hours after inoculation.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
167.
Twelve nonlactating dairy cows, free of signs of liver disease and with normal serum activities of liver-derived enzymes and normal liver biopsy tissue, were examined over a 72-hour period for serum total bile acid concentrations. The cattle were fed hay twice daily, and blood samples were obtained every hour for 24 hours, every other hour for 24 hours, then every hour for 24 hours. After 3 weeks, the study was repeated on 6 of the cattle, thus providing data for eighteen 72-hour periods. Serum bile acid concentration varied greatly over the 72 hours, with the range being from one third to 3 times the median. There were variations by as much as 60 mumol/L from 1 hour to the next. After another 3 weeks, 8 of the cattle were deprived of hay for 48 hours and then fed hay morning and afternoon of the third (last) day of the study. There was no significant reduction in bile acid concentration after withholding the hay, but the variability was reduced (P = 0.02) during the last 20 hours of the hay-deprivation period. In 3 ancillary studies, serum bile acid concentrations were examined over a 48-hour period in 2 cows in early lactation, 3 cows in midlactation, and two 6-month-old heifers. The cows were fed hay and grain twice daily, and the heifers were fed only hay twice daily. In comparison with values for the 12 nonlactating cows fed hay twice daily, mean serum bile acid concentration in the recently freshened cows was significantly (P < 0.002) higher (62.9 vs 22.0 mumol/L).(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
168.
Antibody titers for infectious bursal disease virus (IBDV), infectious bronchitis virus, Newcastle disease virus, and reovirus from chicks with chicken anemia agent (CAA) antibodies were compared with antibody titers from their CAA-antibody-negative counterparts. These comparisons were made in 396 chickens that were 1 day, 2 weeks, 8-9 weeks, 10 weeks, 17 weeks, or 29-32 weeks old. Only one serum sample was collected from any given chick or chicken. There were no significant differences between the antibody titers at any age for any antigen, with one exception: at 29-32 weeks, the IBDV titers were higher (t = 2.62, df = 142, P less than 0.01) in chickens with CAA antibody. Although not at all likely, we believe that the observation of high IBDV antibody titers in CAA-antibody-positive chicks could have been a spurious one.  相似文献   
169.
A technique is described for biopsy of the bovine udder, employing sedation and local anaesthesia. Tissue samples of approximately 5 g were obtained by electrocautery from two quarters of the udder of a cow laterally recumbent. Care was taken to ensure complete haemostasis which was achieved by electrocoagulation and ligation. Postoperative recovery was rapid, and loss of yield was no greater in biopsied glands than in control glands of the same cow. Yield from all quarters returned to preoperative levels within 48 h.  相似文献   
170.
Monoclonal antibodies to bovine viral diarrhea virus (BVDV) were examined for binding with a large number of North American BVDV isolates and eight strains of the serologically related pestivirus, hog cholera virus (HCV). No single BVDV monoclonal antibody reacted with all BVDV isolates. The most cross-reactive monoclonal antibody was an anti-p80/p125 antibody which showed a positive reaction with 173 of 180 (96%) North American isolates. From a fewer number of isolates tested, one anti-gp53 monoclonal antibody also showed a high cross-reactivity (94%). All BVDV isolates showed a positive reaction with at least one of the seven monoclonal antibodies in the panel. Thus, the results indicated that a pool of these monoclonal antibodies may be used in place of polyclonal antisera for the detection of BVDV contamination of cell lines or for virus isolation. For HCV, all three anti-p80/p125 monoclonal antibodies reacted positively with all eight virus strains. In contrast, none of the anti-gp53 monoclonal antibodies were reactive to HCV strains. Thus, the anti-gp53 monoclonal antibodies may be useful for distinguishing between usually innocuous BVDV infections and the highly significant HCV infections in swine for foreign animal disease surveillance.  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