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   167751篇
  免费   9055篇
  国内免费   89篇
林业   6589篇
农学   5126篇
基础科学   1077篇
  19140篇
综合类   33082篇
农作物   6897篇
水产渔业   7890篇
畜牧兽医   84715篇
园艺   2066篇
植物保护   10313篇
  2018年   2044篇
  2017年   2293篇
  2016年   2189篇
  2015年   1921篇
  2014年   2252篇
  2013年   6288篇
  2012年   4401篇
  2011年   5332篇
  2010年   3538篇
  2009年   3478篇
  2008年   5237篇
  2007年   4977篇
  2006年   4761篇
  2005年   4475篇
  2004年   4428篇
  2003年   4412篇
  2002年   4233篇
  2001年   5254篇
  2000年   5160篇
  1999年   4005篇
  1998年   1749篇
  1997年   1750篇
  1996年   1567篇
  1995年   1933篇
  1994年   1789篇
  1993年   1718篇
  1992年   3555篇
  1991年   3706篇
  1990年   3615篇
  1989年   3725篇
  1988年   3322篇
  1987年   3460篇
  1986年   3644篇
  1985年   3505篇
  1984年   2812篇
  1983年   2581篇
  1982年   1805篇
  1981年   1652篇
  1979年   2545篇
  1978年   2055篇
  1977年   1697篇
  1976年   1641篇
  1975年   1756篇
  1974年   2213篇
  1973年   2298篇
  1972年   2235篇
  1971年   2150篇
  1970年   1996篇
  1969年   1836篇
  1967年   1570篇
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
991.
A young, female Maltese dog was presented with intermittent vomiting of bile. Biochemical evidence of persistent mild hepatopathy had been present for 11 months. Exploratory celiotomy was performed. Absence of the gallbladder with malformation of the quadrate lobe of the liver was identified. There was histological evidence of bile duct proliferation and portal fibrosis.  相似文献   
992.
Histochemical and ultrastructural studies were performed on the lateral musculature from individual female whitemouth croaker, Micropogonias furnieri, at the anterior, medium and posterior regions. Based upon histochemical myosin-ATPase (m-ATPase) determination, diverse types of red, pink and white fibres were discerned. Red muscle had abundant mitochondria and stained intensely for aerobic enzymes, white muscle scarcely stained for the same enzymes and pink muscle responded in an intermediate manner. Both white and pink muscle had few mitochondria. The relative proportion of red muscle increased towards the caudal region; pink muscle diminished towards this region and white muscle modified its proportion only in the anterior region. m-ATPase activity showed differences in relation to the gonadal condition along the body, particularly in the white fibres at the anterior and medium regions.  相似文献   
993.
994.
Two experiments were conducted to determine (i) the effects of protein-rich trees (PRTs) and (ii) the effects of leucaena treated with NaOH solution and leucaena plus polyethylene glycol (PEG) on nutrient digestibility, nitrogen (N) balance, urinary purine derivatives excretion and blood metabolites in Thai swamp buffaloes. In Experiment 1, animals were fed with rice straw as a basal diet and one of the four PRT supplements: (i) oven-dried rain tree pods (RTPP, control); (ii) sun-dried leucaena leaves; (iii) sun-dried cassia leaves and (iv) sun-dried mulberry leaves. Fibre digestibility and N balance were lower (P < 0.05), but microbial N in the rumen was higher (P < 0.05) in animals supplemented with RTPP than in those fed with the other supplements. In Experiment 2, animals were fed with rice straw as a basal diet and leucaena were treated in one of three ways: (i) untreated (control), (ii) leucaena treated with NaOH solution and (iii) leucaena + PEG. Fibre digestibility and N balance in the rumen improved (P < 0.05) in animals supplemented with leucaena + PEG, but microbial N was not increased. The study demonstrated a way of using local PRTs for a suitable and worthwhile method to improve the quality of buffalo feeding systems in the tropics.  相似文献   
995.
Three juvenile sable antelope (Hippotragus niger) were diagnosed with bilateral aural hyalohyphomycosis based on histopathology. All three animals were suspected to be immunodeficient based on low IgG levels determined using the zinc sulfate turbidity test. The serum and hepatic copper levels of one animal were below the bovine reference range. Clinical signs in the three animals included bilateral ventral deviation of the pinnae with multifocal subcutaneous aural tumefaction and poor body condition. Numerous septate, nonpigmented fungal hyphae were found within the auricular cartilage, dermis, and subcutaneous granulomas. No significant fungal agents were isolated by culture, and no signs of systemic fungal dissemination were identified except for a concurrent fungal rhinitis in one animal.  相似文献   
996.
OBJECTIVE: To evaluate in vitro holding power and associated microstructural and thermal damage from placement of positive-profile transfixation pins in the diaphysis and metaphysis of the equine third metacarpal bone. SAMPLE POPULATION: Third metacarpal bones from 30 pairs of adult equine cadavers. PROCEDURE: Centrally threaded positive-profile transfixation pins were placed in the diaphysis of 1 metacarpal bone and the metaphysis of the opposite metacarpal bone of 15 pairs of bones. Tensile force at failure for axial extraction was measured with a materials testing system. An additional 15 pairs of metacarpal bones were tested similarly following cyclic loading. Microstructural damage was evaluated via scanning electron microscopy in another 6 pairs of metacarpal bones, 2 pairs in each of the following 3 groups: metacarpal bones with tapped holes and without transfixation pin placement, metacarpal bones following transfixation pin placement, and metacarpal bones following transfixation pin placement and cyclic loading. Temperature of the hardware was measured with a surface thermocouple in 12 additional metacarpal bones warmed to 38 C. RESULTS: The diaphysis provided significantly greater resistance to axial extraction than the metaphysis. There were no significant temperature differences between diaphyseal and metaphyseal placement. Microstructural damage was limited to occasional microfractures seen only in cortical bone of diaphyseal and metaphyseal locations. Microfractures originated during drilling and tapping but did not worsen following transfixation pin placement or cyclic loading. CONCLUSIONS AND CLINICAL RELEVANCE: Centrally threaded, positive-profile transfixation pins have greater resistance to axial extraction in the diaphysis than in the metaphysis of equine third metacarpal bone in vitro. This information may be used to create more stable external skeletal fixation in horses with fractures.  相似文献   
997.
This study was undertaken to determine the hydrophobicity of the luminal surface of the equine stomach and to elucidate the ultrastructure of the lining imparting that property. Gastric and duodenal mucosal samples from 5 horses were collected immediately after euthanasia and subjected to surface contact angle measurement using a goniometer. Gastric mucosal samples from 4 horses and a foal were examined by electron microscopy following a fixation procedure known to preserve phospholipids and oligolamellar structures. Contact angles for the equine gastric glandular mucosal surface (mean +/- s.e. 78.0 +/- 11.0 degrees) were greater than for the duodenum (33.4 +/- 8.7 degrees), (P = 0.003). The contact angles for gastric squamous tissue (50.4 +/- 4.5 degrees) tended to be greater than for duodenum (P = 0.15). Electron microscopy revealed the existence of surfactant as abundant osmiophilic phospholipid material within both squamous and glandular gastric mucosae. These results indicate the hydrophobic nature of the equine gastric mucosae. We propose that the water-repellent nature of the stomach contributes to the 'gastric mucosal barrier' and is imparted by surface-active phospholipid adsorbed to the surface. Phospholipids may also be utilised as a physical barrier to back-diffusion of acid by lining intracellular canaliculi and oxyntic ducts where other defence mechanisms are absent.  相似文献   
998.
999.
Two experiments, each with 39 high-lean-gain potential barrows, were conducted to evaluate the organ weights, body chemical composition, and tissue accretion rates of pigs fed corn-soybean meal diets (CONTROL) and low-protein diets supplemented with crystalline lysine, threonine, tryptophan, and methionine either on an ideal protein basis (IDEAL) or in a pattern similar to that of the control diet (AACON). Amino acids were added on a true ileally digestible basis. The initial and final BW were, respectively, 31.5 and 82.3 kg in Exp. 1 and 32.7 and 57.1 kg in Exp. 2, and pigs were fed for 55 and 27 d in Exp. 1 and 2, respectively. In Exp. 1, the CONTROL and IDEAL diets were offered on an ad libitum basis, or by feeding 90 or 80% of ad libitum intake. In Exp. 2, the CONTROL, IDEAL, and AACON diets were offered on an ad libitum basis, or by feeding 80% of the ad libitum intake. Three pigs were killed at the start of the experiments and three from each treatment were killed at the end of each experiment to determine body chemical composition. In both trials, the whole-body protein concentration (g/kg) and the accretion rates of protein (g/d) were greater (P < 0.05) for pigs fed the CONTROL than for pigs fed the IDEAL and AACON diets. In Exp. 1, pigs fed the CONTROL diet had a trend (P < 0.10) for greater water and lower lipid concentration and had greater (P < 0.05) water and ash accretion rates. Whole-body protein concentration was greatest (P < 0.05) in pigs fed at 80% of ad libitum, but protein, water, and ash accretion rates were greatest (P < 0.05) in pigs allowed ad libitum access to feed. In summary, pigs fed the IDEAL and the AACON diets had less protein in the body and lower protein accretion rates than pigs fed the CONTROL diet. It seems that reductions in protein deposition in pigs fed the IDEAL and AACON diets may have been due to a deficiency of one or more essential amino acids or possibly to increases in the NE for metabolic processes leading to increases in adipose tissue deposition.  相似文献   
1000.
This study evaluated the relative bioactivities of natural and synthetic stereoisomers of alpha-tocopherol in swine. Deuterium-labeled vitamin E (150 mg each of d3-RRR- [natural] and d6-all-rac- [synthetic] alpha-tocopheryl acetates) was administered orally to adult female pigs (n = 3) with the morning feed. Blood samples were obtained at 0, 3, 6, 9, 12, 36, 48, and 72 h after the dose. The time of maximum plasma d3-alpha-tocopherol concentration (0.486 microg/mL) occurred at 12 h, and d6-alpha-tocopherol peaked earlier (at 9 h) and at a lower (P < 0.05) concentration (0.288 microg/mL). The d3-/d6-alpha-tocopherol ratio increased from 1.35 (SD = 0.73) at 3 h after dosing to 2.0 (SD = 0.14) at 72 h (P = 0.03). The plasma disappearance rates of d3- and d6-alpha-tocopherols (post-maximum concentrations) were similar and were estimated to be 0.013 microg/mL per hour. In summary, swine discriminated between RRR- and all-rac-alpha-tocopherols, which resulted in an approximately twofold higher plasma alpha-tocopherol concentration arising from the RRR-form. This 2:1 ratio of RRR- to all-rac- is higher than the currently accepted USP definition of RRR-:all-rac- of 1.36:1.00.  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