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   105788篇
  免费   5448篇
  国内免费   53篇
林业   4322篇
农学   3040篇
基础科学   733篇
  11729篇
综合类   20757篇
农作物   4300篇
水产渔业   4638篇
畜牧兽医   54449篇
园艺   1195篇
植物保护   6126篇
  2018年   1207篇
  2017年   1336篇
  2016年   1291篇
  2015年   1092篇
  2014年   1264篇
  2013年   3937篇
  2012年   2586篇
  2011年   3200篇
  2010年   2081篇
  2009年   2061篇
  2008年   3214篇
  2007年   2946篇
  2006年   2811篇
  2005年   2642篇
  2004年   2750篇
  2003年   2690篇
  2002年   2627篇
  2001年   3278篇
  2000年   3293篇
  1999年   2452篇
  1998年   1072篇
  1997年   1083篇
  1995年   1252篇
  1994年   1165篇
  1993年   1121篇
  1992年   2367篇
  1991年   2533篇
  1990年   2447篇
  1989年   2497篇
  1988年   2274篇
  1987年   2351篇
  1986年   2485篇
  1985年   2398篇
  1984年   1924篇
  1983年   1776篇
  1982年   1166篇
  1981年   1124篇
  1980年   1026篇
  1979年   1765篇
  1978年   1425篇
  1977年   1152篇
  1976年   1131篇
  1975年   1167篇
  1974年   1528篇
  1973年   1612篇
  1972年   1588篇
  1971年   1517篇
  1970年   1408篇
  1969年   1264篇
  1967年   1096篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A case report of a male 6-day-old male layer chick featuring incomplete polymelia of the hind limbs and hindgut malformations is presented. The chick was submitted to computed tomography (CT) examination and subsequent anatomical dissection. Apart from the two supernumerary hind limbs, the anatomical dissection revealed additional hindgut alterations: three uniform-sized caeca flanked the ileum, and the rectum branched into paired cloacae. The supernumerary hind limbs were localized caudal to the normal hind limbs in an inverted position and were attached to pelvic girdle elements and to a curtate pygostyle. They featured a prominent unpaired femur besides paired tibiotarsi, tarsometatarsi and species-specific phalanges of the toes. Additionally, two separate bones attached to the caudoventral aspect of the regular hip bones were developed. The supernumerary limbs were in part mobile and received nerve and vascular supply. Digital 3D-reconstruction based on the CT datasets revealed the osseous components of the malformed body parts. The possible morphogenesis including an in-depth literature review and the clinical implications of the reported malformations are discussed.  相似文献   
992.
Seventy-four 9-week old female C57BL/6J mice housed in a conventional facility were manipulated to induce experimental autoimmune encephalomyelitis, among which 26 developed clinical signs including lethargy, absence of defecation, and abdominal distension. By gross necropsy examination, there was distension of the cecum and colon with fecal impaction. By histologic examination, there was severe ulcerative and proliferative typhlocolitis. Fecal ELISA confirmed the presence of toxins A and B of Clostridium difficile. Alteration in immune status of the immunocompetent mice, due to stress caused by experimental manipulation or autoimmune disease, may have led to intestinal dysbiosis, followed by opportunistic infections resulting in C. difficile-associated disease. This report brings to light the occurrence of the disease in immunocompetent laboratory mice during experimental manipulations associated with alteration in immune status, and it discusses potential hazards associated with conventional housing within a hospital-associated research institute.  相似文献   
993.
994.
This study determined whether early experiences by sheep with monotonous or diverse diets influence intake of unfamiliar flavors and feeds later in life. Thirty 2-mo-old lambs were randomly assigned to 3 treatment diets (n = 10): diverse (DIV), diverse with plant toxins (DIV+T), and monotonous (MON). Lambs in DIV received in 9 successive periods of exposure 4-way choice combinations of 2 foods high in energy and 2 foods high in protein from an array of 6 foods: 3 high in energy [beet pulp, oat grain, and a mix of milo:grape pomace (60:40)] and 3 high in digestible protein (DP) (soybean meal, alfalfa, corn gluten meal). Lambs in DIV+T received the same exposure as DIV, but 2 plant toxins, oxalic acid (1.5%) and quebracho tannins (10%), were randomly added to 2 of the feeds in each of the choice combinations. Lambs in MON received a monotonous balanced diet, made with a mixture of all 6 feeds detailed before. All treatments received their feed in 4 separate buckets. During exposure, treatments did not differ in total daily DMI (P = 0.31), but daily intake of ME was less (P < 0.02) and daily intake of DP was greater (P < 0.03) for lambs in DIV and DIV+T than for lambs in MON. Treatments did not differ in ADG or G:F (P > 0.05). After exposure, lambs were offered a familiar feed (wheat bran) containing novel flavors (maple, garlic, or bitter) and 2-way choices of novel feeds (fescue hay vs. corn distillers grains, rice vs. calf manna, and green peas vs. rolled oats). Intake of maple-flavored wheat bran tended (P = 0.08) to be greater for lambs in DIV than for lambs in DIV+T and MON. Intake of bitter-flavored and garlic-flavored wheat bran were greater (P = 0.03 and P = 0.04, respectively) for lambs in DIV and DIV+T than for lambs in MON. During 2-way choice trials, lambs in DIV, but not in DIV+T, showed greater intakes of fescue hay (P = 0.05) and rice (P = 0.04) than lambs in MON. Intake of green peas was greater (P = 0.03) for lambs in DIV and DIV+T than for lambs in MON. At the end of testing, lambs in DIV but not in DIV+T showed greater ADG than lambs in MON (P = 0.05). Thus, early exposure to diverse foods enhanced acceptance of novel flavors relative to early exposure to a monotonous ration. Early experience with diverse feeds plus plant toxins led to a less diverse diet than early experience with diverse feeds. Early exposure to diverse feeds may be beneficial in production systems that require rapid acceptance and high intake of unfamiliar feeds.  相似文献   
995.
996.
997.
Objectives To determine the prevalence of canine vector‐borne diseases (CVBD: Babesia spp., Anaplasma spp., Ehrlichia spp., haemotropic mycoplasmas and Hepatozoon) in Australian dogs; namely, dogs from pounds in south‐east Queensland and an indigenous Aboriginal community in the north‐east of the Northern Territory. Design and procedure Blood samples were collected from 100 pound dogs and 130 Aboriginal community dogs and screened for the CVBD pathogens using polymerase chain reaction (PCR). All positive PCR products were sequenced for species confirmation. Results In total, 3 pound dogs and 64 Aboriginal community dogs were infected with at least one CVBD pathogen. Overall, B. vogeli was detected in 13 dogs, A. platys in 49, M. haemocanis in 23, Candidatus Mycoplasma haematoparvum in 3 and C. M. haemobos in 1 dog. Co‐infections were detected in 22 Aboriginal community dogs. Conclusions This study found B. vogeli, A. platys and haemotropic mycoplasma infections to be common in dogs in subtropical and tropical areas of Australia. This study also reports for the first time the prevalence and genetic characterisation of haemotropic mycoplasmas in dogs in Australia.  相似文献   
998.
Craniofacial malformation in 64 sheep was phenotypically described as mandibular distoclusion. Digital radiographs were examined in order to determine the degree of morphological changes in certain bones of the skull. Therefore, laterolateral standardised digital radiographs were used to determine anatomic reference points. Subsequently, five reference lines were defined and 16 linear and seven angular measurements were determined to describe malformations in the bones of the skull. Statistical analysis revealed a significant shortening of the rostral part of the corpus mandibulae and of the ramus mandibulae. However, the molar part of the mandible remained unchanged. These morphological changes caused premolar and molar malocclusion. No further craniofacial abnormalities, such as an elongation of the maxilla or of the incisive bone, were identified. In conclusion, the phenotypically observed mandibular distoclusion is caused by a shortening of specific parts of the mandible. This form of ovine craniofacial malformation is therefore best described as brachygnathia inferior.  相似文献   
999.
QUANTITATIVE CONTRAST ULTRASOUND ANALYSIS OF RENAL PERFUSION IN NORMAL DOGS   总被引:1,自引:1,他引:1  
Eight normal dogs with no evidence of renal disease, weighing between 8 and 25 kg were imaged using contrast harmonic ultrasound after injection of a microbubble contrast medium. All dogs received three separate bolus injections of 0.05 ml of commercial contrast medium (Definity). Time/mean pixel value (MPV) curves were generated for selected regions in the cortex and medulla of the left kidney in each dog. Upslope, downslope, baseline, peak intensity, and time to peak were calculated for each zone. For a bolus injection, within the renal cortex (averaging all subjects) the upslope was 7.4 +/- 1.5 MPV/s, downslope was -0.4 +/- .2 MPV/s, baseline was 66.8 +/- 9.3 MPV, peak was 103.6 +/- 8.2 MPV, time to peak (from injection) was 12.8 +/- 5.3 s and from time of contrast medium reaching the kidney was 5.1 +/- 2.0 s. Within the renal medulla (averaging all subjects), upslope was 2.8 +/- 1.7 MPV/s, downslope was -0.3 +/- .2 MPV/s, baseline was 39.3 +/- 6.0 MPV, peak was 65.2 +/- 14.3 MPV, time to peak from injection was 20.9 +/- 6.4 s and from time of contrast reaching the kidney was 11.6 +/- 4.1 s. These baseline data may prove useful in the evaluation of dogs with diffuse disease or vascular compromise.  相似文献   
1000.
Canine separation anxiety is a common behavioral problem presented to veterinarians. Associated behaviors are distressing to both dog and owner, have the potential to disrupt the human-companion animal bond, and may lead to euthanasia. The results of this study demonstrate the clinical efficacy and safety of Reconcile (fluoxetine, 1 to 2 mg/kg/day [0.45 to 0.91 mg/lb/day]), in conjunction with behavior management, for the treatment of canine separation anxiety. The beef flavored chewable formulation was palatable to treated dogs and easy to administer. This study provides to veterinarians and owners valuable information about an effective separation anxiety treatment plan that combines use of Reconcile with behavior modification.  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