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   1482篇
  免费   107篇
林业   31篇
农学   17篇
基础科学   7篇
  144篇
综合类   392篇
农作物   67篇
水产渔业   44篇
畜牧兽医   775篇
园艺   11篇
植物保护   101篇
  2020年   21篇
  2019年   18篇
  2018年   17篇
  2017年   18篇
  2016年   19篇
  2015年   15篇
  2014年   25篇
  2013年   94篇
  2012年   56篇
  2011年   59篇
  2010年   28篇
  2009年   19篇
  2008年   45篇
  2007年   44篇
  2006年   46篇
  2005年   51篇
  2004年   50篇
  2003年   42篇
  2002年   41篇
  2001年   25篇
  2000年   31篇
  1999年   39篇
  1998年   21篇
  1997年   17篇
  1996年   12篇
  1995年   20篇
  1993年   15篇
  1992年   25篇
  1991年   33篇
  1990年   28篇
  1989年   28篇
  1988年   30篇
  1987年   24篇
  1986年   17篇
  1985年   29篇
  1984年   30篇
  1982年   16篇
  1979年   14篇
  1978年   22篇
  1977年   17篇
  1975年   19篇
  1974年   19篇
  1973年   12篇
  1972年   13篇
  1971年   16篇
  1970年   14篇
  1969年   11篇
  1968年   15篇
  1966年   11篇
  1937年   11篇
排序方式: 共有1589条查询结果,搜索用时 62 毫秒
71.
The frequency of claw regrowth, bony remnants, and complications in cats that underwent forelimb onychectomy using laser, scalpel, or guillotine techniques were evaluated. Eighty-seven client-owned cats were recruited from 27 veterinary clinics in the Canadian Atlantic provinces. At least 1 year after onychectomy the cats underwent a physical examination, gait analysis, and radiographic evaluation by 1 of 2 authors. There was no significant difference in the frequency of claw regrowth among the 3 methods (P = 0.283). Significantly more cats had bony remnants following guillotine onychectomy (P < 0.001). Bony remnants were significantly associated with claw regrowth (P = 0.001). Cats that underwent laser onychectomy had significantly less post-operative complications (P = 0.023). The long-term outcome was not significantly different among the 3 methods. We conclude that leaving remnants of the third phalanx in situ is associated with an increased frequency of claw regrowth. Laser onychectomy may be preferred to reduce the risk of post-operative complications, bony remnants, and claw regrowth.  相似文献   
72.
The aim of the present study was to determine the relationship of progesterone (P4), bovine pregnancy-associated glycoprotein-1 (bPAG-1) and nitric oxide (NO) levels with late embryonic (LEM; day 28 to day 42) and early fetal mortalities (EFM; > day 42 to day 56) in dairy cows. Transrectal ultrasonography (6–8 MHz) was performed in 100 Holstein-Friesian cows at days 28, 42 and 56 after artificial insemination (AI; day 0) to diagnose pregnancy and to monitor the fate of the embryo. After ultrasound scanning of each cow, a milk sample was collected for assessment of P4 by an ELISA test and a blood sample was collected for assessment of bPAG-1, by using a double-antibody radioimmunoassay, and serum NO metabolites (nitrate + nitrite). Based on ultrasonographic examinations and bPAG-1-RIA, 41 of 100 inseminated cows were confirmed pregnant at day 28 after AI. Nine cows suffered of LEM, and 6 cows suffered of EFM and the overall pregnancy loss rate was 36.6% (15/41) between days 28 and 56 of pregnancy. By logistic regression analysis, there were no significant relationships between the level of P4 and bPAG-1 at day 28 after AI and the occurrence of LEM and EFM. Also, there were no significant relationships between the levels of P4 and bPAG-1 at day 42 and the occurrence of EFM. On the other hand, a significant relationship (P<0.05) was found between NO level at day 28 and the occurrence of LEM. In conclusion, measurement of the serum NO concentration at day 28 of pregnancy might help to predict the outcome of pregnancy by day 42 in dairy cows but further studies are needed to confirm this.  相似文献   
73.
The effects of adding the nonlethal bird repellent methyl anthranilate (MA), at levels of 100 and 1000 mg kg?1, to fish feed on the bioaccumulation and growth of juvenile (10 g) hybrid striped bass (Morone chrysops×M. saxatilis) and juvenile (1 g) African cichlid fish Aulonocara jacobfreibergi were investigated under laboratory conditions. The bird repellent did not have any effect on the fish growth or survival over a period of 6 weeks. MA residues at low levels of 11.2±2.6 μg g?1 were found in lipophilic tissues (liver) of MA‐fed fish. Control fish, which had no MA added to their diet, had a much lower level of 0.6±0.3 μg g?1 MA in their liver. Fish muscle was found to contain negligible MA residues, while the outer body surface mucus did not contain any MA. Following a 6‐week depuration period, during which the previously MA‐fed hybrid striped bass were fed a feed to which no MA was added, the levels of MA residues detected were reduced by one order of magnitude.  相似文献   
74.
ObjectiveTo evaluate the effects of intravenous (IV) or intramuscular (IM) hyoscine premedication on physiologic variables following IV administration of medetomidine in horses.Study designRandomized, crossover experimental study.AnimalsEight healthy crossbred horses weighing 330 ± 39 kg and aged 7 ± 4 years.MethodsBaseline measurements of heart rate (HR), cardiac index (CI), respiratory rate, systemic vascular resistance (SVR), percentage of patients with second degree atrioventricular (2oAV) block, mean arterial pressure (MAP), pH, and arterial partial pressures of carbon dioxide (PaCO2) and oxygen (PaO2) were obtained 5 minutes before administration of IV hyoscine (0.14 mg kg?1; group HIV), IM hyoscine (0.3 mg kg?1; group HIM), or an equal volume of physiologic saline IV (group C). Five minutes later, medetomidine (7.5 μg kg?1) was administered IV and measurements were recorded at various time points for 130 minutes.ResultsMedetomidine induced bradycardia, 2oAV blocks and increased SVR immediately after administration, without significant changes in CI or MAP in C. Hyoscine administration induced tachycardia and hypertension, and decreased the percentage of 2oAV blocks induced by medetomidine. Peak HR and MAP were higher in HIV than HIM at 88 ± 18 beats minute?1 and 241 ± 37 mmHg versus 65 ± 16 beats minute?1 and 192 ± 38 mmHg, respectively. CI was increased significantly in HIV (p ≤ 0.05). Respiratory rate decreased significantly in all groups during the recording period. pH, PaCO2 and PaO2 were not significantly changed by administration of medetomidine with or without hyoscine.Conclusion and clinical relevanceHyoscine administered IV or IM before medetomidine in horses resulted in tachycardia and hypertension under the conditions of this study. The significance of these changes, and responses to other dose rates, requires further investigation.  相似文献   
75.
This article describes and contrasts the public health response to two human rabies cases: one organ recipient diagnosed within days of symptom onset and the transplant donor who was diagnosed 18 months post‐symptom onset. In response to an organ‐transplant‐related rabies case diagnosed in 2013, organ donor and recipient investigations were conducted by multiple public health agencies. Persons with potential exposure to infectious patient materials were assessed for rabies virus exposure. An exposure investigation was conducted to determine the source of the organ donor's infection. Over 100 persons from more than 20 agencies spent over 2700 h conducting contact investigations in healthcare, military and community settings. The 564 persons assessed include 417 healthcare workers [5.8% recommended for post‐exposure prophylaxis (PEP)], 96 community contacts (15.6% recommended for PEP), 30 autopsy personnel (50% recommended for PEP), and 21 other persons (4.8% recommended for PEP). Donor contacts represented 188 assessed with 20.2% recommended for PEP, compared with 5.6% of 306 recipient contacts recommended for PEP. Human rabies cases result in substantial use of public health and medical resources, especially when diagnosis is delayed. Although rare, clinicians should consider rabies in cases of encephalitis of unexplained aetiology, particularly for cases that may result in organ donation.  相似文献   
76.
A syndrome affecting cultured chinook salmon, Oncorhynchus tshawytscha (Walbaum), characterized by distended abdomens, gastric dilation, air sacculitis (GDAS), increased feed conversion rates and increased mortality has been recognized in New Zealand. Affected fish were most obvious in sea cages but were also present in fresh water. Mortality rates associated with this condition were highest in late summer and approached 6% per month. A dilated and flaccid stomach, without visible rugal folds containing copious oil, watery fluid or undigested feed was typical. Gastric mucosal ulceration or inflammation were not present. The air sacculitis consisted of a thickened, dilated bladder with a mixed mucosal inflammatory infiltrate and a luminal exudate associated with large numbers of morphologically diverse bacteria. Gastric dilation or air sacculitis occurred alone or together in the same fish. In a group of 20 subclinically affected fish with or without gastric dilation, there were no significant differences in weight, length, serum osmolality, sodium, total protein or packed cell volume. Twenty-three severely affected fish had significantly ( P  < 0.05) higher serum osmolality but similar sodium and total protein to that of clinically normal fish.  相似文献   
77.
The hatching envelope elevated around the spawned eggs of penaeoidean eggs presents a formidable barrier for manipulation of the embryo. By spawning eggs into seawater containing 3-amino-1,2,4-triazole and subsequently passing the eggs through Nitex screening approximately 45 min after spawning, the hatching envelope can be effectively removed with little or no damage to the embryos. The time and extent to which the envelope is removed can be varied. Embryos treated in this manner continue to develop and the method provides a new technique with which investigators can gain access to the surface of the egg or embryo.  相似文献   
78.
Backcross breeding programs have been used to transfer disease resistance and other traits from one forest tree species to another in order to meet restoration objectives. Evaluating the field performance of such material is critical for determining the success of breeding programs. In eastern North America, The American Chestnut Foundation has a backcross breeding program that uses Chinese chestnut (Castanea mollissima Blume) to introduce resistance of the fungal pathogen chestnut blight [Cryphonectria parasitica (Murr.) Barr.] to the native American chestnut [Castanea dentata (Marsh.) Borkh.]. We compared physiological and morphological characteristics among seedlings of American chestnut, Chinese chestnut, and BC1F3, BC2F3, and BC3F3 hybrid chestnuts during their fourth growing season after field-planting. American chestnut and the BC3F3 breeding generation displayed photosynthetic light-response curves that were similar to each other but different from Chinese chestnut. Rates of photosynthesis were higher for American chestnut and the BC3F3 breeding generation when compared to Chinese chestnut for light levels ≥800 μmol m?2 s?1 photosynthetic photon flux density and for maximum photosynthetic capacity. Leaf morphology variables were not different between American chestnut and any of the breeding generations, but leaf area (on a per leaf basis) of Chinese chestnut was lower than that of any other chestnut type. Our results suggest that backcross breeding can be used to transfer desirable traits for restoration of native species threatened by non-native pathogens.  相似文献   
79.
The Gram-positive bacterium Aerococcus viridans var. homari is a well-documented causative agent of the lethal systemic disease gaffkemia in both the American lobster, Homarus americanus, and the European lobster, Homarus gammarus. Previous phenotypic characterization has been unsuccessful at differentiating avirulent from virulent strains without performing lethal animal infection trials. Recent genetic characterization of A. viridans strains through 16S rRNA sequencing and random amplification of polymorphic DNA fingerprinting has revealed the presence of two subtypes. However, subtype 1 contains both virulent and avirulent strains which are genetically identical. The purpose of this study was to determine the proteomic mediators of virulence in A. viridans. Quantitative proteomic mapping of these two strains has revealed 29 differentially expressed protein spots, seven of which are only expressed in the virulent strain and could act as virulence factors. One protein, chaperonin 60 (Cpn60), is uniquely expressed in the virulent strain and has been shown to act as a virulence factor in many other bacteria. The proteomic mapping strategy employed in this study is the first to show phenotypic differences between virulent and avirulent strains. Cpn60 expression represents a potentially useful tool for identifying the virulent strains of A. viridans in epidemiological studies.  相似文献   
80.
A small population of resident T-lymphocytes is present in the normal epidermis of skin from humans, mice, sheep, and cattle. The objective of this study was to determine the prevalence of lymphocytes, CD3+ cells (T-lymphocytes) and CD79a+ cells (B-lymphocytes and plasma cells), in the epidermis and adnexal epithelia of alpacas. Skin-biopsy specimens from the normal skin of the dorsolateral thorax of 31 alpacas were examined histologically and immunohistochemically for the presence of CD3+ cells and CD79a+ cells in the epidermis and adnexal epithelia. CD3+ T-lymphocytes, but not CD79a+ cells, were present in the epidermis and adnexal epithelia. Therefore, in the absence of other signs of inflammation, the presence of lymphocytes in these structures in skin-biopsy specimens should be considered normal.  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