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   83895篇
  免费   4706篇
  国内免费   56篇
林业   2927篇
农学   2283篇
基础科学   566篇
  9243篇
综合类   14901篇
农作物   3093篇
水产渔业   3913篇
畜牧兽医   45745篇
园艺   908篇
植物保护   5078篇
  2018年   1044篇
  2017年   1180篇
  2016年   1099篇
  2015年   941篇
  2014年   1185篇
  2013年   3068篇
  2012年   2138篇
  2011年   2719篇
  2010年   1661篇
  2009年   1764篇
  2008年   2604篇
  2007年   2420篇
  2006年   2413篇
  2005年   2137篇
  2004年   2095篇
  2003年   2084篇
  2002年   1971篇
  2001年   2700篇
  2000年   2804篇
  1999年   2134篇
  1998年   815篇
  1997年   825篇
  1996年   812篇
  1995年   1017篇
  1994年   888篇
  1993年   842篇
  1992年   1930篇
  1991年   2081篇
  1990年   1924篇
  1989年   1936篇
  1988年   1826篇
  1987年   1929篇
  1986年   1975篇
  1985年   1872篇
  1984年   1483篇
  1983年   1298篇
  1982年   881篇
  1979年   1366篇
  1978年   1123篇
  1977年   973篇
  1976年   937篇
  1975年   984篇
  1974年   1272篇
  1973年   1298篇
  1972年   1234篇
  1971年   1147篇
  1970年   1088篇
  1969年   965篇
  1968年   832篇
  1967年   869篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
In a replicated 4 × 4 Latin square experiment conducted in summer, eight mature Thoroughbred horses received two diets, control and fat-supplemented, and were exercised at two fitness levels designed to increase daily digestible energy (DE) requirements to approximately 150% (fitness level I) and 200% (fitness level II) of maintenance. In a second experiment during the winter, horses received the two diets in a switchback design and exercised at fitness level II. After 3 wk adaptation to treatments, feed and fecal samples were collected and horses galloped a standardized exercise test (SET) designed to increase the heart rate above 185 bpm for 1200 m. Vital signs were monitored and blood samples were obtained. Feed intake increased as fitness level increased (P < .05). Horses on the fat-supplemented diet required less feed (P < .05) to meet the energy requirements at a given fitness level. Daily DE intake was higher (P < .05) for the horses exercised at fitness level II to meet the increased metabolic demands. Heart rate, respiration rate and rectal temperature all increased (P < .05) with exercise. No treatment effects were found for heart rate (P >. 10) or rectal temperature (P > .30). Horses exercised in the winter had lower (P < .05) respiration rates, indicative of less problems dissipating excess body heat. Plasma aldosterone concentrations increased (P < .05) with exercise, corresponding to an increase in plasma K concentrations. Horses exercised in the summer exhibited higher (P < .05) plasma aldosterone concentrations than horses worked in the winter. Plasma CI and Na concentrations did not change (P > .30) with any treatment or exercise, indicating that the horses were not sufficiently stressed to induce any significant dehydration.  相似文献   
992.
Use of ultrasound in the reproductive management of dairy cattle   总被引:1,自引:0,他引:1  
The significant decrease in fertility observed in dairy cattle during the last few decades and increasing requirements by the farmers have made a regular control of reproduction indispensable to urgently identify and solve potential problems affecting reproductive efficiency. Traditionally, the main diagnostic methods used for reproductive control in cattle included rectal palpation, inspection of vaginal discharge and vaginoscopy. Since the 1990 s, the use of ultrasound (US) has become a common diagnostic method as a result of the new advances made in the development of US scans: smaller size, high level of autonomy, high image quality and accessible prices. Ultrasound improves accuracy in the diagnoses of stages of the oestrous cycle, ovarian and uterine pathologies, and pregnancy diagnosis. In addition, it facilitates the diagnosis of alterations during pregnancy (embryo mortality, foetal malformations, etc.) and helps determining foetal sex from day 55 of pregnancy.  相似文献   
993.
994.
Parvovirus-like enteropathy in Missouri turkeys   总被引:1,自引:0,他引:1  
A viral enteric disease of young turkeys characterized by stunting of affected birds, diarrhea, and increased mortality is described. Eosinophilic intranuclear inclusion bodies were found in the absorptive epithelial cells of the ileum. Electron microscopy of formalin-fixed tissue revealed that the intestinal inclusions contained numerous loosely packed 15-to 20-nm hexagonal particles. The size, shape, and intranuclear location have been used to tentatively identify these particles as parvoviruses.  相似文献   
995.
996.
Brahman (n = 9) and 1/2 Simmental x 1/4 Brahman x 1/4 Hereford (n = 11) calves were utilized to determine the influence of exposure to cold on the physiology of the neonate. All calves were removed from their dams within 20 min of birth and prior to suckling. Calves were assigned randomly within breed to either a warm (W; 31 degrees C) or cold (C; 4 degrees C) environmental treatment group. Jugular blood samples were collected via indwelling catheters at 20-min intervals for 180 min. At 100 to 120 min of sampling, all calves were given 1.2 liters of colostrum from their dams via stomach tube. At 120 min, C calves were placed in the W environment. Calf vigor score (CVS) and rectal temperature were determined at each time blood was collected. Serum or plasma was analyzed for glucose (GLU), lactate (LAC), blood urea nitrogen (BUN), hemoglobin (HEM), triglyceride (TRG), triiodothyronine (T3), thyroxine (T4), insulin (INS), cortisol (CORT) and nonesterified fatty acid (NEFA) concentration. Rectal temperature was lower (P less than .01) in C Brahman than in W Brahman and C or W crossbred calves. Crossbred calves had higher (P less than .01) CVS than Brahman calves. Calves in W had lower (P less than .01) GLU than C calves. Brahman calves had higher GLU, LAC, BUN, TRG, T3, T4 and CORT (P less than .05) than crossbred calves. The C Brahman calves had the highest (P less than .05) TRG, CORT, T3 and T4 of all groups. Concentration of NEFA were higher (P less than .01) in C than in W calves.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
997.
Three horses with sudden onset severe lameness were admitted for further diagnostic investigation. All horses had variable changes on radiographs in the distal tarsal region. Because of the sudden onset and severe degree of lameness, a magnetic resonance imaging (MRI) examination was performed. All horses showed areas of increased signal intensity in short tau inversion recovery (STIR) images involving the central and/or third tarsal bones. These lesions involved both the subchondral bone and bone marrow and are currently defined as bone marrow lesions (BML). Two horses were treated with shockwave therapy, one received intra‐articular medication. Two of the horses returned to previous athletic level and one is still in rehabilitation.  相似文献   
998.
With the advent of web-based recording and analysis systems, individual cow composite somatic cell count (SCC) data are being increasingly used for decision support in mastitis control at both the individual cow and herd level. SCC data from first and second lactation dairy cows (n=1912) from 12 farms were analysed using multinomial logistic regression to investigate possible associations between high SCC patterns in the first lactation and the subsequent lactation. Animals with three non-consecutive counts >200,000 cells/mL in their first lactation were significantly more likely to have three non-consecutive counts >200,000 cells/mL (OR 3.11; 95% CI 1.72-5.62) or three consecutive counts >200,000 cells/mL (OR 2.00; 95% CI 1.09-3.68) in their second lactation. Similarly animals with three consecutive counts >200,000 cells/mL in their first lactation were significantly more likely to have three non-consecutive counts >200,000 cells/mL (OR 1.90; 95% CI 1.13-3.19) or three consecutive counts >200,000 cells/mL (OR 4.14; 95% CI 2.81-6.08) in their second lactation. These findings suggest that patterns established in the first lactation may have an impact on udder health in the subsequent lactation. However, simulation modelling of positive predictive values for the first lactation cell count patterns as predictors of second lactation patterns demonstrated that, at prevalences likely to be encountered on UK dairy farms, the associations were not a sufficient basis for major management decisions such as culling.  相似文献   
999.
Nine groups of rats (n = 5 per group) received an intramuscular (IM) injection of one of the following drugs or drug combinations: saline, atropine (0.05 mg/kg), glycopyrrolate (0.5 mg/kg), ketamine:xylazine (85:15 mg/kg), ketamine:detomidine (60:10 mg/kg), atropine:ketamine:xylazine (0.05: 85:15 mg/kg), glycopyrrolate: ketamine:xylazine (0.5:85:15 mg/kg), atropine:ketamine:detomidine (0.05: 60:10 mg/kg) or glycopyrrolate: ketamine:detomidine (0.5:60:10). Similarly six groups of rabbits (n = 5) received an IM injection of either saline, atropine (0.2 mg/kg), atropine (2 mg/kg), glycopyrrolate (0.1 mg/kg), ketamine:xylazine (35:10 mg/kg) or glycopyrrolate:ketamine:xylazine (0.1:35:10 mg/kg). In rats, atropine sulfate (0.05 mg/kg) and glycopyrrolate (0.5 mg/kg) produced an increase in heart rate for 30 and 240 min, respectively. In rabbits atropine sulfate at either 0.2 or 2.0 mg/kg did not induce a significant increase in heart rate, but glycopyrrolate (0.1 mg/kg) elevated the heart rate above saline treated animals for over 50 min. Both atropine and glycopyrrolate provided protection against a decrease in heart rate in rats anesthetized with ketamine: xylazine (85:15 mg/kg) or ketamine: detomidine (60:10 mg/kg); however, glycopyrrolate was significantly more effective in maintaining the heart rate within the normal range. Glycopprrolate also prevented a decrease in heart rate in rabbits anesthetized with ketamine:xylazine (35:5 mg/kg). Neither glycopyrrolate nor atropine influenced respiration rate, core body temperature or systolic blood pressure when used alone or when combined with the injectable anesthetic. Glycopyrrolate is an effective anticholinergic agent in rabbits and rodents and more useful as a preanesthetic agent than atropine sulfate in these animals.  相似文献   
1000.
Mastitis is a common reproductive disorder in bitches, reaching a prevalence of 0.71%. Mastitis has a wide range of forms, from asymptomatic to severe gangrenous mastitis that can lead to septic shock and death of the bitch and nurslings. However, most of the time it is overlooked, undiagnosed or mistreated. The present systematic review was performed to revise and summarize the existing knowledge related to this disorder, including diagnosis, treatment and prevention.  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