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   8022篇
  免费   396篇
  国内免费   59篇
林业   1225篇
农学   209篇
基础科学   60篇
  1289篇
综合类   750篇
农作物   235篇
水产渔业   354篇
畜牧兽医   3667篇
园艺   217篇
植物保护   471篇
  2023年   52篇
  2022年   73篇
  2021年   95篇
  2020年   138篇
  2019年   74篇
  2018年   209篇
  2017年   173篇
  2016年   209篇
  2015年   190篇
  2014年   166篇
  2013年   416篇
  2012年   379篇
  2011年   374篇
  2010年   241篇
  2009年   158篇
  2008年   323篇
  2007年   360篇
  2006年   331篇
  2005年   273篇
  2004年   259篇
  2003年   240篇
  2002年   222篇
  2001年   153篇
  2000年   158篇
  1999年   114篇
  1992年   100篇
  1991年   112篇
  1990年   131篇
  1989年   139篇
  1988年   117篇
  1987年   98篇
  1986年   105篇
  1985年   95篇
  1984年   96篇
  1983年   74篇
  1979年   102篇
  1978年   70篇
  1977年   61篇
  1976年   49篇
  1975年   66篇
  1974年   82篇
  1973年   93篇
  1972年   68篇
  1971年   82篇
  1970年   76篇
  1969年   84篇
  1968年   65篇
  1967年   65篇
  1966年   49篇
  1965年   56篇
排序方式: 共有8477条查询结果,搜索用时 15 毫秒
91.
Susceptibility of crops to drought may change under atmospheric CO2 enrichment. We tested the effects of CO2 enrichment and drought on the older malting barley cultivar Golden Promise (GP) and the recent variety Bambina (BA). Hypothesizing that CO2 enrichment mitigates the adverse effects of drought and that GP shows a stronger response to CO2 enrichment than BA, plants of both cultivars were grown in climate chambers. Optimal and reduced watering levels and two CO2 concentrations (380 and 550 ppm) were used to investigate photosynthetic parameters, growth and yield. In contrast to expectations, CO2 increased total plant biomass by 34 % in the modern cultivar while the growth stimulation was not significant in GP. As a reaction to drought, BA showed reduced biomass under elevated CO2, which was not seen in GP. Grain yield and harvest index (HI) were negatively influenced by drought and increased by CO2 enrichment. BA formed higher grain yield and had higher water‐use efficiency of grain yield and HI compared to GP. CO2 fertilization compensated for the negative effect of drought on grain yield and HI, especially in GP. Stomatal conductance proved to be the gas exchange parameter most sensitive to drought. Photosynthetic rate of BA showed more pronounced reaction to drought compared to GP. Overall, BA turned out to respond more intense to changes in water supply and CO2 enrichment than the older GP.  相似文献   
92.
Prunus avium is primarily cultivated for its fruit, sweet cherries. However, it is also used to produce high‐quality timber. In a P. avium seed orchard, gametophytic self‐incompatibility is a restriction for free pollen flow and should be considered when establishing basic forest materials. In this study, S‐locus diversity and cross‐incompatibility of wild cherry individuals in clonal banks established for breeding for timber production were investigated. Wild cherry trees (140) with outstanding forest growth habit, collected in northern Spain, grafted and planted in two clonal banks, were genotyped at the S‐locus. The self‐incompatibility S‐locus genes, S‐RNase and SFB, were analysed by PCR. Twenty‐two S‐haplotypes, resulting in 72 different S‐genotypes, were identified. The genotypes were grouped into 33 incompatibility groups and 39 unique genotypes. This initial S‐locus analysis revealed large genetic diversity of wild cherry trees from the Spanish northern deciduous forest, and provides useful information for seed orchard design. Wild P. avium displays significantly more genetic diversity than what is detected in local cultivars, revealing a narrowing of genetic diversity during local domestication.  相似文献   
93.
There is a growing recognition for the need to develop sensitive indicators of soil quality that reflect the effects of land management on soil and assist land users in promoting sustainability of agro-ecosystems. Three soil enzymes (dehydrogenase, phosphatase and invertase) microbial biomass as biological variables and soil organic matter content (SOM) were investigated relative to fertilization and soil fertility (estimated by crop yield) at a long-term fertilization trial (Keszthely, Hungary). 0-34.7-69.4-104.1t farmyard manure (FYM) ha m 1 5 yr m 1 and the corresponding amount of mineral fertilizers (NPK) were applied in two different crop rotation systems. There were four straw and/or stalk incorporating treatments in the second crop rotation 'B'. Enzyme activities, microbial biomass and the amount of SOM were generally higher in the fertilized soils than in the unfertilized soils. The type of amendments (mineral, FYM or mixed) had significant effects only on the amount of SOM. The correlations among the biological variables and the crop yield were generally low (r < 0.250. The differences in field management resulted only in the invertase activity.  相似文献   
94.
The use of live animals for educational purposes is an old practice that is still employed in teaching and research institutions. However, there are several objections to this practice, whether for ethical or humanitarian reasons. Surgical techniques teaching using anatomical pieces and/or preserved cadavers promotes greater learning efficiency, provides exercise repetition and increases the confidence and satisfaction of the students when compared to the use of live animals. The current work aimed to analyse the feasibility of using fresh swine urinary bladder and small intestines (jejunum), obtained from slaughterhouses, fixed in 99.8% ethyl alcohol (EA) and preserved in sodium chloride hypersaturated solution (SCHS) at 30%, for 7, 14 and 21 days, as an alternative method for surgical skills training (SST). Swine viscera, fixed in EA and preserved in SCHS, presented a realistic appearance, absence of odour and maintained the viable morphological characteristics during the performance of the operative techniques. Preservation solutions had low cost, were easy to acquire and did not offers risks to human health. Therefore, urinary bladders and small intestines fixed in 99.8% EA for 30 days and maintained in 30% SCHS at different periods were demonstrated as a good viable option as a preservation method for surgical skills training.  相似文献   
95.
In the present study, the location, histology and number of corpuscles of Stannius (Sc), which are endocrine glands associated with the kidneys of teleost fish, were investigated for the first time in Lake Van fish (Alburnus tarichi), an anadromous and endemic inhabiting Turkey's Lake Van Basin. The Sc, which were ovoid or spheroid and white or cream in colour, were found to vary in number between three and five among the examined fish. The glands were located in the caudal part of the kidney, and either partially or completely embedded, and found to be present on both the ventral and dorsal surface of either side of the caudal part of the kidney. The Sc were surrounded by a connective tissue capsule that penetrated and divided the gland into incomplete lobules. Two types of cells were determined in the parenchyma of the gland. Type-I cells were predominant throughout the parenchyma and larger than the second (type-II). In the type-I cells, the cytoplasm was observed as weakly or moderately eosinophilic with haematoxylin and eosin staining and weakly or moderately acidophilic with Mallory's triple staining. In the type-I cells, the cytoplasm exhibited weak to moderate periodic acid-Schiff staining and slight or uniform staining with aldehyde fuchsin. The type-II cells were round, had a darkly stained spherical nucleus and were dispersed among the type-I cells. They displayed no cytoplasmic staining with the abovementioned stains.  相似文献   
96.
The hepatic lipidosis (HL) in fattening turkeys is a disease has been known for a long time, but the cause and pathogenesis is still not clarified. A recent study reported unexplained high levels of iron in liver tissue of fattening turkeys suffering from HL. In this study, the iron status, possible infectious or inflammatory influences in form of an acute phase reaction and the analysis of fatty acid pattern in liver tissue of turkeys affected by HL were examined. Three cases of HL on three different fattening turkey farms were investigated during the outbreak of the disease. Clinically affected and non-affected animals were subjected to a pathological examination, where the diagnosis HL or non-affected was made. In total, 70 birds were examined (40 with HL, 30 without HL) and blood and liver samples were taken. Additionally, samples from 15 slaughtered birds were taken as a further control group. In liver tissue, the iron content and the content of long-chain fatty acids were determined; in blood samples, ferritin and transferrin were measured. The iron content in liver tissue was more than three times higher for animals with HL than among non-affected animals and the control group. The transferrin levels were lowest for animals with HL, highest in the control group and in between for non-affected animals. The fatty acid pattern in liver tissue of affected animals indicated a shift from polyunsaturated fatty acids to saturated fatty acids and monounsaturated fatty acids compared to the control group and the non-affected animals. Overall, the non-affected animals of a flock affected by HL were similar to the healthy animals of the abattoir. The low acute phase protein levels for animals with HL together with high iron contents could indicate a previous malnutrition/starvation period and/or severe liver damage for those animals suffering from HL.  相似文献   
97.
2021年1月12—19日,山东黄河三角洲国家级自然保护区大汶流管理站出现35只死亡的野生疣鼻天鹅。经现场调查,市级和省级实验室检测,诊断为疑似H5N8亚型高致病性禽流感,经国家禽流感参考实验室进行病毒分离鉴定,最终确定为H5N8亚型高致病性禽流感疫情。疫情发生后,通过现场调查、座谈及实验室检测等方式,对此次疫情开展了紧急流行病学调查,追溯疫情的可能来源,分析疫情的扩散风险,继而提出针对性的应急处置措施。调查发现,此次疫情由迁徙候鸟带毒引入引起的可能性较大;野生鸟类未及时开展免疫,导致体内抗体水平低下是疫情暴发的内源因素。由于迅速采取了合理的应急处理措施,此次疫情得到迅速控制,避免了疫情扩散。本次疫情警示,必须切实做好禽流感的强制免疫和监测工作,高度重视自然保护区及其附近场所珍稀禽类的免疫,以降低疫情发生风险。  相似文献   
98.
自2018年非洲猪瘟(African swine fever,ASF)传入中国以来疫情得到了有效控制。但2021年以后,我国的ASF流行情况出现了新的变化,临床中出现了“自然变异株”。这些毒株可能导致感染后临床症状不典型,容易与其他疫病混淆等问题。这与国外的流行演变规律一致,即当ASF在一个国家或地区流行较长时间后,其临床表现将由急性发病转变为缓慢发病或出现新的临床表现。经过多年的观察积累,国外已经对该病毒的流行规律特别是自然弱毒株的演变进程有了大量记载,但国内目前尚无这方面的描述。为此,就ASF自然弱毒株的演变进程进行综述,从而提出加强血清学诊断产品研发、加快产业化报批进程、适时开展血清学监测追溯等建议,以期为我国ASF科学防控提供参考。  相似文献   
99.
ObjectiveTo determine the intraoperative and early postoperative opioid requirement after ultrasound-guided sciatic and/or femoral nerve block or epidural anaesthesia in dogs undergoing tibial plateau levelling osteotomy (TPLO).Study designProspective, masked, pilot, randomized, clinical trial.AnimalsA total of 40 client-owned dogs undergoing TPLO.MethodsEach dog was randomly assigned to group SF (combined sciatic and femoral nerve block), group S (sciatic nerve block), group F (femoral nerve block) or group E (epidural anaesthesia). A total of 0.3 mL kg–1 of ropivacaine 0.5% was administered to each nerve or in the epidural space. Intraoperatively, fentanyl (2 μg kg–1) was administered intravenously when heart rate, mean arterial pressure or respiratory rate increased by >30% compared with baseline values. Postoperatively, a visual analogue scale (VAS) and a modified German version of the French pain scale (4AVet) were used to assess pain every 30 minutes for 150 minutes and again once the morning after surgery. Methadone (0.1 mg kg–1) was administered intravenously if the VAS was ≥ 4 cm [maximal value 10 cm; median (interquartile range)] or the composite pain score was ≥5 [maximal value 15; median (interquartile range)]. Significance was defined as p ≤ 0.05.ResultsGroups SF and E required less total intraoperative and early postoperative opioid doses compared with groups S and F (p = 0.031). No dogs in group SF had a block failure or required postoperative methadone. A reduced methadone requirement was found in group SF compared with all the other groups up to 150 minutes after recovery (p = 0.041).Conclusions and clinical relevanceCombined sciatic and femoral nerve block and epidural anaesthesia lead to less cumulative consumption of perioperative opioids than single nerve blockade. Sciatic or femoral nerve block alone might be insufficient to control nociception and early postoperative pain in dogs undergoing TPLO.  相似文献   
100.
The aim of the present study was to clarify the effect of extracerebral dopamine (DA) on salsolinol (SAL)‐induced prolactin (PRL) secretion in goats. An intravenous injection of SAL or thyrotropin‐releasing hormone (TRH) was given to female goats before and after treatment with an extracerebral DA receptor antagonist, domperidone (DOM), and the PRL‐releasing response to SAL was compared with that to TRH. DOM alone increased plasma PRL concentrations and the PRL‐releasing response to DOM alone was greater than that to either SAL alone or TRH alone. The PRL‐releasing response to DOM plus SAL was similar to that to DOM alone, and no additive effect of DOM and SAL on the secretion of PRL was observed. In contrast, the PRL‐releasing response to DOM plus TRH was greater than that to either TRH alone or DOM alone and DOM synergistically increased TRH‐induced PRL secretion. The present results demonstrate that the mechanism involved in PRL secretion by SAL differs from that by TRH, and suggest that the extracerebral DA might be associated in part with the modulation of SAL‐induced PRL secretion in goats.  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