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   132507篇
  免费   7269篇
  国内免费   93篇
林业   5382篇
农学   4271篇
基础科学   831篇
  17042篇
综合类   21487篇
农作物   5397篇
水产渔业   6691篇
畜牧兽医   67984篇
园艺   1800篇
植物保护   8984篇
  2020年   1177篇
  2019年   1375篇
  2018年   2031篇
  2017年   2330篇
  2016年   2095篇
  2015年   1742篇
  2014年   2224篇
  2013年   5290篇
  2012年   4115篇
  2011年   5094篇
  2010年   3245篇
  2009年   3390篇
  2008年   4971篇
  2007年   4667篇
  2006年   4353篇
  2005年   4088篇
  2004年   3850篇
  2003年   3973篇
  2002年   3542篇
  2001年   4151篇
  2000年   4252篇
  1999年   3372篇
  1998年   1475篇
  1997年   1388篇
  1996年   1288篇
  1995年   1531篇
  1994年   1358篇
  1993年   1232篇
  1992年   2587篇
  1991年   2702篇
  1990年   2591篇
  1989年   2612篇
  1988年   2369篇
  1987年   2350篇
  1986年   2428篇
  1985年   2342篇
  1984年   1884篇
  1983年   1678篇
  1982年   1154篇
  1979年   1748篇
  1978年   1397篇
  1977年   1152篇
  1976年   1213篇
  1975年   1293篇
  1974年   1500篇
  1973年   1514篇
  1972年   1405篇
  1971年   1304篇
  1970年   1251篇
  1969年   1284篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
In more than 10 Spanish dairy cows, a bovine herpesvirus 4 (BHV4) associated postpartum metritis was confirmed by virus isolation, BHV4-glycoprotein B (gB) PCR and/or serology. In this study, 12 cows with, and, at the time of sampling, 3 cows without clinical signs of acute postpartum metritis from one large dairy herd in Spain were examined for bacterial and viral infections. Blood, placenta/caruncles and uterine contents were collected between day 1 and day 20 post-calving, and examined for the presence of bacteria and for viruses by virus isolation, BHV4 DNA by BHV4-gB PCR and/or BHV4 antibody titres. Bovine herpesvirus 4 was detected in 83% of the cases with clinical signs of acute postpartum metritis by virus isolation and/or BHV4-gB PCR. An increase of BHV4 antibodies was detected in all examined postpartum metritis cows and in the 3 cows without clinical metritis. Two of these 3 cows developed severe metritis a few dayss after collecting the first blood sample. A concurrent infections of BHV4 and bacteria, mainly Arcanobacterium pyogenes and Streptococcus sp., were detected in 73% of the examined uterine contents collected from postpartum metritis affected cows. This case-report study showed a clear association between BHV4 infections and acute postpartum metritis in dairy cows. In addition, the BHV4-associated postpartum metritis appeared to be an emerging syndrome in this Spanish herd.  相似文献   
992.
In humans and animals, ossifying fibroma is a benign neoplasm that most frequently affects the mandible, often resulting in cosmetic deformities and malocclusion. It is considered rare in animals and most frequently affects young horses. A surgical biopsy of a solitary mass located beneath the gingiva in the right maxillary region, which had overgrown teeth and expanded the adjacent hard palate from a 6-year-old miniature Rex rabbit was submitted for light microscopic examination. The submitted incisional biopsy specimen was pale pink, firm, and nodular. Histopathologically, the neoplasm was composed of fibroblastic cells separated by abundant collagen. The neoplastic cells were interwoven with osteoblasts surrounding islands of mineralized, bony matrix containing few, widely spaced, often empty, lacunae. Minimal inflammation was present. Based on the histopathologic features, the tumor was diagnosed as an ossifying fibroma. To our knowledge, this is the first report of an ossifying fibroma in a rabbit.  相似文献   
993.
Mouse epidermal development: effects of retinoic acid exposure in utero   总被引:2,自引:0,他引:2  
Epidermal morphogenesis was studied in vivo following prenatal exposure to retinoic acid (RA). In pregnant mice, a single oral dose of RA on day 11.5 of gestation failed to induce histological changes in fetal epidermal development except in epidermal thickness. Epidermal thickness increased from 16.5 days post-coitum (dpc) onwards, and temporal and spatial epidermal modifications in keratins K5 and K14 related to proliferative activity of keratinocytes were observed. An RA effect on cell proliferation was supported by a statistically significant increase in the number of epidermal S-phase cells, containing BrdU-incorporated DNA in RA-exposed mice compared with nonexposed animals. The prolonged in utero action of RA on epidermal proliferative activity in fetuses and newborns suggests a long-term RA effect that may play a role on the development and evolution of diseases in adult skin.  相似文献   
994.
Thirty mink dams nursing litters of six kits were assigned to one of three dietary treatments [high protein (HP), medium protein (MP) and low protein (LP)], fed ad libitum for 4 week from parturition, to investigate the effects of protein supply on milk yield and milk composition in order to estimate the amino acid requirement of the lactating mink. Twelve dams were held in an intensive care unit and subjected to balance experiments and the kits were injected with deuterium oxide to determine water kinetics and milk yield. Eighteen dams were kept under normal farm conditions but with feed intake of dams and live weight gain of kits being determined and milk samples collected. The ME intake was higher (p < 0.05) in dams fed the LP and MP diets than in dams fed the HP diet, whereas the amino acid intake (g/day) was lowest (p < 0.05) in dams fed the LP diet. In the third and fourth weeks of lactation milk yield was higher (p < 0.05) in dams fed the LP and MP diets than in dams fed the HP diet. Chemical composition of milk was not affected (p > 0.05) by dietary treatment. However, protein content tended (p = 0.06) to be lower in dams fed the LP diet. Amino acid content (g/16 g N) of milk was higher (p < 0.05) in dams fed the LP and MP diets than in dams fed the HP diet. This resulted in the highest (p < 0.05) amino acid intake and highest (p < 0.001) live weights of kits nursed by dams fed the LP and MP diets, which may be explained by a combined effect of higher ME intake and reduced energetic costs for glucose production through less amino acids being used in gluconeogenesis. In conclusion, the improved performance of dams fed the LP diet suggested that their requirement of essential amino acids and non-specific N were covered, and the requirement of digestible amino acids of lactating mink (kg(0.75)) was, thereby, estimated by use of a factorial approach including the amino acid excretion in milk of LP dams.  相似文献   
995.
Several hormones regulate Na+, K+-ATPase content in the muscle cell membrane, which is essential for maintaining muscle cell excitability. Chronic glucocorticoid excess is associated with muscle weakness and reduced endurance. We hypothesized that chronic glucocorticoid excess affects Na+, K+-ATPase content in canine skeletal muscle, and contributes to reduced endurance and muscle weakness associated with pituitary-dependent hyperadrenocorticism (PDH) in dogs. Therefore, Na+, K+-ATPase content in skeletal muscle was evaluated before and after hypophysectomy and hormone replacement (cortisone and l-thyroxin) in dogs with PDH (n = 13), and in healthy controls (n = 6). In addition, baseline and exercise-induced changes in plasma electrolyte concentrations and acid–base balance were evaluated before and after hypophysectomy in dogs with PDH. Na+, K+-ATPase content of gluteal muscle in dogs with PDH was significantly lower than in control dogs (201 ± 13 pmol/g versus 260 ± 8 pmol/g wet weight; P < 0.01). Similar differences were found in palatine muscle. After hypophysectomy and on hormone replacement, Na+, K+-ATPase was increased (234 ± 7 pmol/g wet weight). Both plasma pH and base excess in dogs with PDH (7.44 ± 0.01; 1.7 ± 0.6 mmol/l, respectively) were significantly higher (P < 0.05) than after hypophysectomy and hormone replacement (7.41 ± 0.01; −0.2 ± 0.4 mmol/l, respectively). Exercise induced respiratory alkalosis, but did not result in hyperkalemia in dogs with PDH. In conclusion, chronic glucocorticoid excess in dogs with PDH is associated with decreased Na+, K+-ATPase content in skeletal muscle. This may contribute to reduce endurance in canine PDH, although dogs with PDH did not exhibit exercise-induced hyperkalemia. Na+, K+-ATPase content normalized to values statistically not different from healthy controls after hypophysectomy and hormone replacement.  相似文献   
996.
In semi-arid Mediterranean areas, small grain aftermath stubble represents an important summer source of food for grazing flocks of small ruminants. Wheat stubble is a mediocre source of forage and flocks are grazed in summer under harsh conditions of temperature and air dustiness. However, stubble grazing procedures are changing, water and shading are more frequently available between grazing sessions (“improved management”), and the biological soundness of this ancestral practice needs to be re-visited. The present study was aimed at evaluating the cost in energy of “improved” wheat stubble grazing, compared with feeding a similar diet indoors. The intake of stubble was first quantitatively and qualitatively evaluated in Awassi sheep. Ewes consumed daily 980 ± 100 g day− 1 of wheat stubble. Ewes were then housed and fed diets consisting of wheat hay, straw and grain formulated to be iso-energetic and iso-nitrogenous to diets consumed from wheat stubble. The average intake of ME was similar during the confinement and the pasture periods (6.4 ± 0.5 and 7.6 ± 0.8 MJ day− 1of ME, respectively). During 2 days of each period, animals were fitted with external electrodes and data loggers of heart rate and skin temperature. Energy expenditure (EE) was calculated from oxygen consumption estimated as the product of heart beats rate measured for the two days by the amount of oxygen delivered to body tissues at each heart beat (O2 pulse). The O2 pulse was determined by simultaneously measurement of oxygen consumption and HR twice daily on two occasions, while grazing stubble and indoors. Energy expenditure and energy balance were not different in sheep while grazing wheat stubble (11.1 and − 3.5 MJ day− 1) or fed indoors (11.1 and − 4.8 MJ day− 1). Our data show that stubble did not cover nitrogen and energy requirements for maintenance, and that the cost of summer stubble grazing carried out under conditions described here is less than thought before.  相似文献   
997.
The prevalence of capsular and somatic serotypes were studied among 123 Pasteurella multocida strains isolated from chickens (n = 94), ducks (22), quails (4), turkeys (2) and geese (1) from different geographical regions of India. All strains exhibited similar cultural and morphological characteristics. Ninety-two of the isolates belonged to serotype A:1, the most prevalent serotype, with serotypes A:3, A:1,3, D:3 and F:3 having two isolates each. Only one isolate was positive for serotypes A:4 and D:1. Twenty isolates were untyped. A multiplex capsular PCR assay generated amplicons of sizes 460, 1044, 657 and 854 bp in 106 isolates identified as capsular serotype-A, 15 in serotype D and two in serotype F. Capsular types B and E were not detected in any of the avian isolates studied. The present findings suggest that a multiplex capsular PCR assay may be suitable for the rapid initial identification serotypes P. multocida during epidemiological studies of fowl cholera.  相似文献   
998.
研究表明,猪繁殖和呼吸综合症病毒是造成近10年来美国所谓非典型猪繁殖和呼吸综合症流行的原因。美国的养猪业尽管取得了重大进展,但仍然容易受到各种疾病的侵害,因而需要开发出能快速鉴别新感染的手段。  现在我们称之为猪繁殖和呼吸综合症的复杂的临床症候群,首次在1989年报告于北美。自那时以来,大量调查研究得出结论认为猪繁殖和呼吸综合症病毒是这一灾难性疾病的病原因子。九十年代早期展现在美国猪兽医面前的猪繁殖和呼吸综合症临床症状是:妊娠后期广泛性繁殖失败、妊娠母猪或公猪临床症状轻微或没有其它临床症状,以及幼猪出现呼吸…  相似文献   
999.
Feeding increased levels of dietary vitamin E can inhibit lipid oxidation. The aim of this study was to investigate the effect of levels of dietary alpha-tocopherol acetate (VE) and feeding duration on meat quality and lipid oxidation. Eighty-one pigs were allocated to 1 of 3 diets containing 40, 200, or 400 IU of VE/kg of feed, and each diet group was divided into 3 feeding periods (3, 6, or 9 wk). Carcass characteristics and meat quality were evaluated. Oxidative stability of fresh and cooked pork patties and pork chops was determined after chilled or frozen storage. Increasing dietary concentrations of VE did not affect any growth performance parameter. Drip loss, however, decreased (P < 0.05) with increased dietary VE levels. Moreover, an increased duration of VE feeding improved (P < 0.05) pH and drip loss. Less lipid oxidation (P < 0.05) was detected in fresh ground pork from pigs fed greater concentrations of VE after 4 d of storage. A greater (P < 0.05) resistance to oxidation in cooked ground pork was observed in pigs fed 200 or 400 IU of VE/kg at 2 and 6 d of storage. Fresh and cooked pork patty oxidation decreased (P < 0.05) linearly as feeding duration increased from 3 to 9 wk. After 6 mo of freezer storage, lipid oxidation of pork chops from pigs fed 200 or 400 IU of VE/kg was lower (P < 0.05) than for pigs fed 40 IU of VE/kg. Likewise, lipid oxidation of pork chops of pigs fed VE for an extended period of time (6 wk) was lower (P < 0.05) after 9 mo of storage. Fatty acid profiles of neutral lipid fraction of the LM became more unsaturated (P < 0.05) with added VE to the feed. These results indicate an increased intake of dietary VE concentration, and prolonged feeding of VE can improve drip loss and reduce lipid oxidation in ground pork and pork chops. This study suggests that supplementation with 200 IU of VE/kg of feed for 6 wk before market is beneficial in improving lipid stability and pork quality.  相似文献   
1000.
The objective of this study was to determine the effects of boar presence on aggression, shoulder scratches, and salivary cortisol in group-housed sows during the period after mixing, which is when dominance hierarchies are formed. A total of 225 York-shire sows were used. Five groups of 15 sows were each exposed to 1 of 3 levels of boar presence (n = 15): physical (boar in pen with sows; 2.15 m2 per sow), fenceline (boar housed adjacent to sows; 2.3 m2 per sow), or control (no boar in the room; 2.3 m2 per sow). The experiment was divided into 2 phases. In phase 1, behavioral measurements were taken for 48 h after mixing and boar introduction. In phase 2, behavioral measurements were taken beginning on d 6 after mixing and included 24 h before and 48 h after boar removal. In phases 1 and 2, shoulder scratches were scored 24 h before and 24 h after mixing and boar removal, respectively. Saliva samples were collected each morning (0600 to 0700) and afternoon (1600 to 1700) during both phases. Frequencies of intersow aggressive contact (bite, body knock, and head knock) and threats as well as frequencies and durations of fighting bouts were determined from video recordings during daily feeding (49.6 +/- 1.4 min) and nonfeeding (21 +/- 0.2 h) periods. During phase 1, boar presence did not affect the frequency of threats during either the feeding or nonfeeding periods. Control groups had fewer aggressive contacts at feeding during the 25- to 48-h period compared with the fenceline groups (P = 0.01). Total duration of fighting and average fighting bout duration were unaffected by treatment. However, the number of fights occurring during feeding 25 to 48 h postmixing was lower in the physical groups than in the fenceline groups (P = 0.023); measures for the control groups were intermediate. Physical sows also received fewer shoulder scratches postmixing than did control sows (P = 0.048). After mixing, physical sows showed greater morning (P = 0.08) and afternoon (6 h postmixing, P = 0.01; 32 h postmixing, P = 0.08) salivary cortisol concentrations compared with fenceline sows, although they were only numerically greater than those for control sows. During phase 2, removing the boar did not increase fighting among sows or affect shoulder scratches or salivary cortisol concentrations. The presence of a boar was minimally effective at reducing fighting and scratches during the postmixing period, and sows showed a greater stress response in the presence of a boar.  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