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   227522篇
  免费   13812篇
  国内免费   917篇
林业   23158篇
农学   14019篇
基础科学   3008篇
  41658篇
综合类   15129篇
农作物   17236篇
水产渔业   16098篇
畜牧兽医   82602篇
园艺   6224篇
植物保护   23119篇
  2021年   2518篇
  2020年   2917篇
  2019年   3709篇
  2018年   3976篇
  2017年   4329篇
  2016年   4759篇
  2015年   4297篇
  2014年   5674篇
  2013年   16217篇
  2012年   5704篇
  2011年   7445篇
  2010年   7185篇
  2009年   7810篇
  2008年   6913篇
  2007年   5773篇
  2006年   6461篇
  2005年   5685篇
  2004年   5569篇
  2003年   5296篇
  2002年   4619篇
  2001年   5017篇
  2000年   4662篇
  1999年   4558篇
  1998年   4033篇
  1997年   4085篇
  1996年   3819篇
  1995年   4352篇
  1994年   3724篇
  1993年   3414篇
  1992年   3574篇
  1991年   3780篇
  1990年   3446篇
  1989年   3417篇
  1988年   3008篇
  1987年   3020篇
  1986年   2925篇
  1985年   3241篇
  1984年   3127篇
  1983年   3016篇
  1982年   2610篇
  1981年   2601篇
  1980年   2584篇
  1979年   2785篇
  1978年   2552篇
  1977年   2403篇
  1976年   2271篇
  1975年   2062篇
  1974年   2141篇
  1973年   2058篇
  1972年   1793篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
161.
162.
Twenty-five domestic rice (Oryza sativa L.) cultivars were evaluated in a simulated rice/crawfish double cropping system in southwest Louisiana during 1983 and 1984. Wide variation was observed among cultivars in parameters believed to be important for maximizing productivity in such a system. Mean maturity ranged from 99 (Labelle) to 118 days (Starbonnet), while height ranged from 76 (Bellemont) to 130 cm (Della, Nato). Significant differences in grain yield response were observed, with yields ranging from 4,101 (Bluebelle) to 6,665 kg/hectare (Newbonnet). Straw dry matter production varied greatly, from 3,831 (Bellemont) to 9,170 kg/hectare (Della), and was positively correlated with plant height (r = 0.64; P = 0.0001). Ratoon biomass production was negatively correlated with both grain yield (r = -0.39; P = 0.0001) and maturity group (r = -0.69; P = 0.0001). Plant senescence at grain maturity also varied greatly and appears to be a factor in post-harvest ratoon capability. Early or very early maturing cultivars with both proven grain production and good ratooning capability appear best suited. Cultivar selection should be based upon individual farm situations and currently employed agronomic practices.  相似文献   
163.
164.
Zusammenfassung Das bayerische Arten- und Biotopschutzprogramm (ABSP) wird seit 1984 auf der Basis der naturr?umlichen Gegebenheiten erarbeitet und jeweils landkreisbezogen dargestellt. über die H?lfte der 71 bayerischen Landkreise ist vollst?ndig bearbeitet. In der ersten Arbeitsphase wurden nur die innerhalb der Naturschutzverwaltung zug?nglichen Datenquellen ausgewertet. Neuerdings werden in Absprache mit der Obersten Forstbeh?rde verst?rkt forstliche Informationen eingearbeitet. Bereits herausgegebene ?ltere B?nde werden aktualisiert. Das Programm hat keine unmittelbare Rechtswirksamkeit. Die Forstbeh?rden und Waldbesitzer sind aufgerufen, die im Programm dargestellten Ziele des Arten- und Biotopschutzes zu unterstützen. Diese umfassen u. a. die verst?rkte fachliche Zusammenarbeit, die Erhaltung und F?rderung von Sonderstandorten, Alt- und Totholz, die Reduktion der Wilddichte und die Vermehrung der Waldfl?che.
Bavaria's program for the protection of species and biotopes, and its effects on forestry
Summary Bavaria's program for the protection of species and biotopes (ABSP) is being worked out since 1984, by counties, on the basis of given natural environmental conditions. Work has been completed for more than half of the 71 Bavarian counties. During the first phase of the work, only data sources of the nature conservation authorities were evaluated. More recently, also forestry-related information is increasingly incorporated in agreement with the State's top forestry authorities. Already issued volumes are updated. The program has no direct legal status. Forestry authorities and woodland owners are called upon to support the program's objectives of species and biotope protection. Such are, amongst others: increased professional cooperation; preservation and promotion of special sites, old timber and dead snags; reduction of game animal population densities; and increase of woodland areas.
  相似文献   
165.
Abstract –  The interaction between brown trout ( Salmo trutta ; fork length (FL) range 255–390 mm) and inanga ( Galaxias maculatus ; FL range 55–115 mm) was tested during summer through autumn in an artificial stream consisting of a single run-riffle-pool sequence with a natural food supply. Each experimental trial lasted for 15 days, and consisted of two brown trout and 50 inanga collected fresh from a nearby stream, with each species given prior residence in four replicate tests, totalling eight trials in all. In addition, two control trials (each 10 days), with 50 inanga in each, were run. Brown trout almost exclusively occupied the pool, whereas inanga occupied all habitat types, although in different proportions, when tested with and without brown trout. The proportion of inanga in the pool was appreciably lower in the experimental trials with brown trout than in the control trials with no brown trout; prior residence had no significant effect on inanga habitat use. Mortality of inanga attributable to predation by brown trout ranged from 0 to 40% with a mean of 14.5 ± 4.7%. The results suggest that habitat use and survival of inanga populations in small streams can be adversely affected by brown trout.  相似文献   
166.
Abstract. Experiments were conducted for one year on two different soil types. On a clay soil straw was either (a) burnt, (b) baled leaving the stubble, or (c) chopped and spread. The soil was tine cultivated to depths of 5, 10 or 15 cm or ploughed to 20 or 30 cm before winter wheat was sown conventionally. In addition, a direct-drilled crop was sown after each straw treatment. On a silt loam soil the direct-drilled, tine cultivated to 15 cm and ploughed to 30 cm treatments following burning or chopping and spreading straw were repeated.
Tine cultivation incorporated less straw than ploughing, decreased plant establishment and early growth but did not decrease yield. Direct-drilling through chopped straw decreased yield on the silt loam but not on the clay soil. Short straw (< 5 cm) was easier to incorporate than longer straw. Ploughing was the most efficient method of straw incorporation because it inverts soil. Early effects on crop growth and nutrient uptake following straw incorporation were transient and associated with large amounts of straw in the seeded layer of soil.  相似文献   
167.
168.
The present study shows that milk is an appropriate source for detection of seroreactors to bovine virus diarrhoea virus (BVDV). There was close agreement between antibody titres in serum and in skim milk, as determined by an indirect enzyme-linked immunosorbent assay. The antibody titres were usually lower in skim milk than in serum, but all seropositive cows (n = 84) were also skim milk-positive and all but one seronegative cow (n = 55) proved negative in skim milk. During lactation, the level of antibodies to BVDV in milk showed an inverse relationship to the amount of milk produced. However, there was a sufficient level of antibodies in milk throughout lactation to permit an adequate determination of BVDV antibody status in dairy cows. There was a mutual good agreement between milk antibody titre in the four mammary quarters, irrespective of milk cell count. Milk can be used to detect seroreactors to BVDV. Milk is preferable to blood in large-scale epidemiological studies, since the sampling procedure is much simpler.  相似文献   
169.
M. Fladung 《Plant Breeding》1993,111(3):242-245
The iaaL gene of Pseudomonas syringae subsp. savastanoi encodes an indoleacetic acid-lysine synthetase which conjugates free indoleacetic acid (IAA) with lysine. lAA-lys is biologically less active than free IAA. The iaaL coding region was expressed under the control of the cauliflower mosaic virus 35S promoter and transgenic potato plants were produced (Spena et al. 1991). 35S iaaL potato plants are characterized by increased internodal length and epinastic bending of older leaves. In three greenhouse experiments with plants grown in pots of different size and in two growth chamber experiments tuber number increased in iaaL transgenic plants compared to untransformed and vector-transformed controls of the same genotype. The increase in tuber numbers observed under controlled conditions was reflected in tuber yield which increased in the pot grown transgenics.  相似文献   
170.
Efficacy of ivermectin treatment (0.2 mg/kg) against 28-day experimental infections of Parascaris equorum was determined in 18 pony foals6–17.5 weeks old. There were 6 foals in each group: nontreated control, ivermectin injectable or oral paste. In comparison with larvae found in the nontreated controls, ivermectin injectable or paste was 96.0% and 99.9% efficacious. There was a distinct difference in drug effect against the larger (ca 26mm.) vs the smaller (13–19mm) larvae by the 2 formulations of ivermectin. There were no adverse signs related to treatment of the young foals.  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