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   87305篇
  免费   5188篇
  国内免费   42篇
林业   2978篇
农学   2713篇
基础科学   526篇
  9930篇
综合类   16188篇
农作物   3655篇
水产渔业   4302篇
畜牧兽医   45547篇
园艺   1082篇
植物保护   5614篇
  2019年   807篇
  2018年   1193篇
  2017年   1376篇
  2016年   1291篇
  2015年   1145篇
  2014年   1361篇
  2013年   3453篇
  2012年   2558篇
  2011年   3042篇
  2010年   1981篇
  2009年   1947篇
  2008年   2935篇
  2007年   2836篇
  2006年   2689篇
  2005年   2536篇
  2004年   2384篇
  2003年   2414篇
  2002年   2276篇
  2001年   2905篇
  2000年   2782篇
  1999年   2268篇
  1998年   975篇
  1997年   893篇
  1996年   790篇
  1995年   957篇
  1994年   863篇
  1993年   857篇
  1992年   1812篇
  1991年   1811篇
  1990年   1751篇
  1989年   1816篇
  1988年   1570篇
  1987年   1680篇
  1986年   1739篇
  1985年   1665篇
  1984年   1312篇
  1983年   1167篇
  1982年   842篇
  1981年   723篇
  1979年   1107篇
  1978年   901篇
  1977年   781篇
  1976年   734篇
  1975年   810篇
  1974年   999篇
  1973年   1041篇
  1972年   994篇
  1971年   946篇
  1970年   887篇
  1969年   811篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
The objective of this study was to determine the effect of a monensin controlled-release capsule administered intraruminally at drying-off on body condition score (BCS) at calving, milk yield, fertility and concentration of energy-related blood metabolites in Holstein cows dried-off with low BCS (< or = 3.0, scale 1 to 5 with a 0.25 point of increment). Between July and August, 2001, 220 cows from parity 2 or more and dried-off 50-70 days before expected parturition, with a BCS < or = 3.0 were randomly assigned to either a treatment group (n=110; oral capsule of monensin releasing 335 mg/day for 95 days) or a control group (no capsule, n=110). At assignment, on day 21 before expected parturition, at calving, and at 7, 14, and 21 days in milk a blood sample was obtained from a random sub sample of 10 cows per group. Effects of monensin on serum NEFA, BHBA and glucose were measured. Milk yield, milk fat and protein content (%) at DHIA test days during the entire lactation, 305 ME milk production and reproductive responses were compared. Monensin significantly improved BCS at calving, increased milk yield at test days 4 and 8, decreased the percentage of milk protein, did not change the percentage of milk fat, and decreased NEFA and BHBA during the post-partum period.  相似文献   
972.
973.
974.
975.
The acute and chronic toxicities of three benzoylphenyl ureas, diflubenzuron, hexaflumuron and teflubenzuron were assessed under laboratory conditions against two-day-old second (II) instar Desert Locust, Schistocerca gregaria (ForskÅl) nymphs. Following exposure by ingestion of a single precise dose applied to short pieces of spring barley, nymphs were monitored for two moults until the fourth (IV) instar. Analysis of acute response data gave three significantly different LD50 statistics (P < 0·05), 68·0, 26·6 and 0·71 μg per nymph respectively for diflubenzuron, hexaflumuron and teflubenzuron. The probit regression slopes also differed significantly, indicating distinct tolerance distributions for the three compounds, the narrowest response being to diflubenzuron and the widest range of response being to teflubenzuron. The timing of death was found to vary between the compounds; most nymphs died during the first moult following treatment with either hexaflumuron or teflubenzuron. However, the majority of nymphs that died after exposure to diflubenzuron did so after completing the first moult after treatment, but before the second. The mean development times of nymphs during the II and especially third (III) instars were significantly longer (P < 0·05) than those of the controls following exposure to diflubenzuron and hexaflumuron. Teflubenzuron had no significant effect (P < 0·05) on the duration of the II instar. The potential of the three compounds to control S. gregaria populations in the field is discussed with particular reference to the timing and nature of acute and chronic responses.  相似文献   
976.
1. The environment of the island of Anglesey, North Wales, UK, is described and related to the ecology of the lake systems present. 2. Estimates are given of the number and types of lakes present on the island. Sixteen biological Sites of Special Scientific Interest (SSSIs) have standing water as a recognized habitat feature and their reasons for receiving a conservation designation are discussed. 3. Brief reviews are given of the lacustrine flora, invertebrates, fish and aquatic bird populations of Anglesey and their importance in assessing the conservation value of a site. 4. These reviews are followed by detailed accounts of the current ecology of three of the island's lake systems—Llyn Coron, Llyn Dinam and Llyn Penrhyn—which occur within biological Sites of Special Scientific Interest. Original data are presented on their physical and chemical characteristics, diatom assemblages, macrophyte communities, littoral and open water zooplankton communities and macroinvertebrate assemblages. 5. Artificial enrichment, the introduction of alien species, the legacy of metal mining operations and recreational pressures are identified as the main threats to lake conservation on Anglesey. 6. The Anglesey Wetland Strategy, an informal operational level liaison procedure, is described. Using this mechanism, broad targets and indicative strategies are highlighted by the partner organizations active in wetland conservation on the island, and opportunities for collaboration are developed.  相似文献   
977.
S. C. Gupta 《Plant Breeding》1994,112(2):135-140
Screening of a population of Ocimum basilicum var. glabratum Benth., raised from seeds collected from France, West Africa and Kerala (India) was carried out for chemical composition of its essential oils. In all three collections, plants were found to be comprised of 3 chemotypes which were morphologically indistinguishable, but each chemotype was found to be capable of intercrossing freely with the other two. Selfing was carried out during 4 consecutive generations, after which three pure lines were established. Study of the selfed progeny of each collection exhibited an interesting segregation in S2 and S3 generations. Along with segregation in F2 and F3 generations of various hybrids the results can be explained by assuming a gene ‘M’ existing in 3 (or more) allelic forms: The allele M0 for biosynthesis of estragole (methyl chavicol) is dominant to allelcs M1 , for eugenol and M2 for camphor. Also, allele M1 is dominant to allele M2 . The assumption was further confirmed by testcrosses of F1 hybrids of each cross with their recessive parent.  相似文献   
978.
Marine biofouling is an epibiotic biological process that affects almost any kind of submerged surface, causing globally significant economic problems mainly for the shipping industry and aquaculture companies, and its prevention so far has been associated with adverse environmental effects for non-target organisms. Previously, we have identified bromosphaerol (1), a brominated diterpene isolated from the red alga Sphaerococcus coronopifolius, as a promising agent with significant antifouling activity, exerting strong anti-settlement activity against larvae of Amphibalanus (Balanus) amphitrite and very low toxicity. The significant antifouling activity and low toxicity of bromosphaerol (1) motivated us to explore its chemistry, aiming to optimize its antifouling potential through the preparation of a number of analogs. Following different synthetic routes, we successfully synthesized 15 structural analogs (2–16) of bromosphaerol (1), decorated with different functional groups. The anti-settlement activity (EC50) and the degree of toxicity (LC50) of the bromosphaerol derivatives were evaluated using cyprids and nauplii of the cirriped crustacean A. amphitrite as a model organism. Derivatives 2, 4, and 6–16 showed diverse levels of antifouling activity. Among them, compounds 9 and 13 can be considered as well-performing antifoulants, exerting their activity through a non-toxic mechanism.  相似文献   
979.
Dilated cardiomyopathy is the usual cause of congestive heart failure in the dobermann pinscher and was the presumptive diagnosis in an eight-year-old female dobermann pinscher presented for dyspnoea and syncope. This was supported by thoracic radiography and electrocardiographic evidence of ventricular tachycardia. An echocardiogram revealed a left atrial tumour as the principal cause of cardiac failure. Pathological examination confirmed that the tumour was a large aortic body carcinoma in an unusual location. Multifocal myocardial infarctions were also found. This unusual case demonstrates the importance of a complete cardiological examination as part of a diagnostic work up and confirms the necessity of echocardiography as one of the examination methods.  相似文献   
980.
Efforts to control viral diseases of grapevine include the production of certified material and development of virus-resistant transgenic grapevines. However, effective antiviral agents, once the viruses have infected the plants, are still lacking. This study shows that a crude garlic extract has significant antiviral activity against grapevine viruses. Replication of grapevine leafroll-associated virus 2 (GLRaV-2) was obviously inhibited in grapevine cv. Cabernet Sauvignon calli treated with diluted (1:100) garlic extract. The relative RNA levels of GLRaV-2 and grapevine fleck virus (GFkV) in cv. Summer Black grapevine in in vitro-grown plantlets 10 days after treatment with diluted (1:100) garlic extract were about 22% and 20%, respectively, of that in controls. The viral RNA accumulation of GLRaV-2, GFkV, grapevine virus A (GVA), grapevine fanleaf virus (GFLV) and grapevine rupestris stem pitting-associated virus (GRSPaV) in field-grown grapevine cv. Centennial Seedless plants sprayed with diluted (1:100) garlic extract were about 31–40%, 26–38%, 18–31%, 17–42% and 15–18%, respectively, of that in controls. Moreover, the garlic extract treatment led to a significant decrease in viral RNA accumulation of GLRaV-3, GLRaV-2, GVA, GFkV, GFLV, GRSPaV and grapevine Pinot Gris virus in pot-grown grapevine cv. Shine Muscat plants, and viral disease symptoms in these plants were obviously attenuated. In addition, this extract significantly induced expression of pathogenesis-related protein genes and stimulated activity of antioxidant enzymes in grapevines. Taken together, these results indicate that the crude garlic extract acts as a significant inhibitor against a broad range of grapevine viruses.  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