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   112194篇
  免费   6213篇
  国内免费   59篇
林业   4590篇
农学   3780篇
基础科学   665篇
  13005篇
综合类   18665篇
农作物   3932篇
水产渔业   5748篇
畜牧兽医   59496篇
园艺   1344篇
植物保护   7241篇
  2018年   1553篇
  2017年   1729篇
  2016年   1581篇
  2015年   1316篇
  2014年   1618篇
  2013年   4165篇
  2012年   2999篇
  2011年   3601篇
  2010年   2475篇
  2009年   2442篇
  2008年   3610篇
  2007年   3439篇
  2006年   3222篇
  2005年   3040篇
  2004年   2957篇
  2003年   3072篇
  2002年   2732篇
  2001年   3734篇
  2000年   3767篇
  1999年   2904篇
  1998年   1109篇
  1997年   1120篇
  1996年   1028篇
  1995年   1345篇
  1994年   1146篇
  1993年   1098篇
  1992年   2402篇
  1991年   2599篇
  1990年   2572篇
  1989年   2491篇
  1988年   2313篇
  1987年   2352篇
  1986年   2348篇
  1985年   2204篇
  1984年   1908篇
  1983年   1631篇
  1982年   1090篇
  1979年   1695篇
  1978年   1308篇
  1977年   1204篇
  1976年   1125篇
  1975年   1218篇
  1974年   1462篇
  1973年   1486篇
  1972年   1454篇
  1971年   1328篇
  1970年   1283篇
  1969年   1282篇
  1968年   1041篇
  1967年   1151篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Gangwar  R. K.  Makádi  M.  Demeter  I.  Táncsics  A.  Cserháti  M.  Várbíró  G.  Singh  J.  Csorba  Á.  Fuchs  M.  Michéli  E.  Szegi  T. 《Eurasian Soil Science》2021,54(7):1007-1018
Eurasian Soil Science - This study was conducted with the aim to assess the effect of land use on chemical properties (organic carbon; pH; electrical conductivity; available P, K, Ca, Mg, Na),...  相似文献   
2.
3.
Two experiments were conducted to evaluate the effects of Fe injection timing after birth on suckling and subsequent nursery and growing-finishing pig performance. The injectable Fe source used in both experiments was GleptoForte (Ceva Animal Health, LLC., Lenexa, KS). GleptoForte contains gleptoferron which is a Fe macromolecule complex. In Exp. 1, a total of 324 newborn pigs (DNA 241 × 600, initially 1.6 ± 0.04 kg body weight [BW]) within 27 litters were used. Two days after birth, all piglets were weighed, and six barrows and six gilts per litter were allotted to 1 of 6 treatments consisting of no Fe injection or 200 mg of injectable Fe provided in a single injection on d 2, 4, 6, 8, or 10 of age. Pigs were weaned (~21 d of age) and allotted to nursery pens with all pigs in each pen having received the same Fe treatment. In Exp. 2, a total of 1,892 newborn pigs (PIC 359 × C40; initially 1.5 ± 0.02 kg BW) within 172 litters were used. One day after birth, piglets were weighed, and 11 pigs within each litter were allotted to 1 of 6 treatments consisting of no Fe injection or 200 mg of injectable Fe provided on d 1, 3, 5, or 7 of age, or 200 mg on d 1 plus 200 mg on d 12 of age. Pigs were weaned (19 d of age) and placed in a commercial wean-to-finish facility in a total of 15 pens with equal representation of treatments in each pen. In both experiments, not providing an Fe injection after birth decreased (P < 0.05) preweaning average daily gain (ADG), weaning weight, and hemoglobin and hematocrit values compared with all other treatments. In Exp. 1, increasing the age that piglets received an Fe injection until 4 or 6 d after birth provided marginal evidence for an improvement (quadratic; P = 0.070) in preweaning ADG. For the nursery period, increasing the age that piglets received an Fe injection improved (quadratic; P = 0.013) d 80 BW, but there was no evidence of a difference (P > 0.10) in d 173 BW at the end of the grow-finish period. In Exp. 2, increasing the age that piglets received a 200 mg Fe injection showed no evidence of difference (P > 0.10) for subsequent nursery and growing-finishing ADG. In both experiments, hemoglobin and hematocrit values were decreased (linear; P < 0.05) at weaning with increasing age when pigs received an Fe injection. These experiments suggest that providing a 200 mg Fe injection within 7 d after farrowing is sufficient for optimizing preweaning and subsequent growth performance.  相似文献   
4.
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   
5.
Agroforestry Systems - While many agroforestry research projects contribute to improving food security, livelihoods and management of natural resources, few have had a significant role in achieving...  相似文献   
6.
  1. Intermediate metrics of translocation success are useful for long-lived, slow to mature species where survival and reproduction happen over decades.
  2. With fewer than 150 individuals in the wild, the Critically Endangered Philippine crocodile (Crocodylus mindorensis) is one of the most threatened species on Earth. This study presents the first analysis of diet and body condition of wild Philippine crocodiles and headstarted (i.e. captive-reared) individuals released into the wild over the last decade, and uses these results to show how diet and body condition can be pertinent intermediate metrics of translocation success.
  3. Analyses of stomach contents revealed 17 different aquatic and terrestrial invertebrate and vertebrate prey species. Interestingly, 70% of Philippine crocodiles showed snails to be the predominent prey type, followed by fish (36.7%), birds (33.3%) and reptiles (33.3%). More than 50% of crocodiles consumed the invasive golden apple snail, a leading agricultural pest. Regardless of crocodile history (wild vs. headstarted) or size class (juvenile vs. adult), no evidence was found for dietary differences in percentage occurrence, percentage composition or prey diversity.
  4. Body condition was significantly higher in wild compared with headstarted individuals when analysed together in a pooled group, although neither group differed significantly from the standardized expectation, and headstarted individuals were not significantly different when body condition was derived independently for the two groups.
  5. This study provides a working example of how assessing the convergence of diet and body condition between translocated and wild individuals can provide complementary monitoring parameters to demonstrate post-release establishment of translocated crocodylians. The congruent dietary composition and comparable body condition observed in this study suggest that headstarted crocodiles adapt well following release.
  6. Crocodylus mindorensis survives in an agricultural landscape and is likely to play an ecologically important role by exploiting invasive species, reinforcing the importance of this species to local communities.
  相似文献   
7.
Floral resources, such as carbohydrate-rich nectar or pollen, can bolster fitness and raise reproductive output of adult lepidopterans. Here, we used laboratory experiments to assess how those plant-derived foods impact adult fecundity, reproductive physiology and flight performance of an invasive strain of the fall armyworm, Spodoptera frugiperda(FAW; Lepidoptera: Noctuidae) in China. More specifically, supplementary feeding on bee pollen and honey enhanced FAW flight duration, testis size, ovarian development, longevity and adult fecundity. FAW adults attained the longest pre-oviposition(10.8 days) and oviposition period(6.8 days) and longevity(19.2 days) on 5% acacia honey. Upon access to 2.5% acacia honey and 2.5‰ pine pollen, S. frugiperda attained the highest mating rate(79.7%), fecundity(644.9 eggs/female) and egg hatching rate(82.3%). Feeding on honey further delayed decay of male testes, while ovarian development was enhanced when female moths were allowed access to 2.5% honey and 2.5‰ pine pollen. Upon feeding on 5% honey solution, S. frugiperda engaged in flight over the longest duration(9.5 h), distance(29.9 km) and speed(3.1 km h~(–1)). Honey had a comparatively greater effect on the above parameters than pollen. Our findings help decipher FAW invasion patterns and population dynamics, facilitate the development of nutritional attractants, and contribute to integrated pest management of this newly-invasive pest in eastern Asia.  相似文献   
8.
  1. The amplitude, duration, frequency, and predictability of runoff and inundation of aquatic habitats are key hydrological characteristics linked to aquatic ecosystem functioning and biodiversity, but they are seldom integrated into analyses of Amazon floodplain ecology. Remote sensing approaches, measurements and modelling of floodplain hydrology provide a basis for this integration.
  2. Effective legislation to protect floodplains and other wetlands depends on operational definitions that require application of hydrological data.
  3. Extent and changes of flooded areas are linked to fish diversity and to presence and growth of flooded forests and floating plants.
  4. Dam construction reduces river system connectivity and modifies the flood pulse, with major negative implications for floodplain ecosystems adapted to and dependent on a natural flood regime.
  5. Trends and variability in climate plus deforestation are altering the Amazon's hydrological cycle, causing changes in discharge and flooded area with concomitant ecological impacts.
  相似文献   
9.
Eurasian Soil Science - The differences between eutrophic peat soils (Hypereutric Sapric Histosols) and peaty gleyzems (Eutric Histic Gleysols) 20 years after a forest-peat fire have been studied...  相似文献   
10.
Bouteldja  M.  Malek  I.  Posta  K.  Kampfl  G.  Fóti  S.  Pintér  K.  Nagy  Z.  Balogh  J. 《Eurasian Soil Science》2021,54(8):1183-1195
Eurasian Soil Science - There is a need for long-term data on N2O emission in Central Europe to provide a thorough understanding of the variability of the emission processes as affected by...  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