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   16399篇
  免费   1篇
林业   3621篇
农学   1293篇
基础科学   138篇
  2729篇
综合类   720篇
农作物   2095篇
水产渔业   1779篇
畜牧兽医   1052篇
园艺   1110篇
植物保护   1863篇
  2019年   1篇
  2018年   2744篇
  2017年   2702篇
  2016年   1178篇
  2015年   63篇
  2014年   14篇
  2013年   5篇
  2012年   788篇
  2011年   2122篇
  2010年   2101篇
  2009年   1252篇
  2008年   1312篇
  2007年   1575篇
  2006年   31篇
  2005年   97篇
  2004年   100篇
  2003年   150篇
  2002年   59篇
  2001年   4篇
  2000年   41篇
  1997年   1篇
  1995年   1篇
  1993年   12篇
  1992年   7篇
  1990年   1篇
  1989年   5篇
  1988年   11篇
  1987年   1篇
  1985年   1篇
  1977年   4篇
  1973年   1篇
  1972年   1篇
  1970年   1篇
  1969年   2篇
  1968年   4篇
  1967年   1篇
  1965年   1篇
  1945年   1篇
  1941年   1篇
  1937年   2篇
  1936年   1篇
  1932年   1篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Each species is characterized by a specific set of chromosomes, which is described as the chromosome portrait or karyotype. In general, such a karyotype is the same for all individuals in the population. An exception to that rule has recently been found in the orchid Erycina pusilla, which has been reported to have two cytotypes with chromosome numbers of 2n = 10 and 2n = 12. Here, we examined the karyotypes of the two cytotypes and found differences in arm ratios and heterochromatin patterns as well as in the presence of satellite chromosomes and in the number and location of rDNA and telomeric repeat sites. These differences are extensive and would have required multiple chromosome rearrangements to generate the differences between the two karyotypes. We also found that F1 hybrids between the parents with the two different chromosome numbers resulted in sterile offspring, in accordance with our previous findings. The combination of hybrid sterility and extensively rearranged chromosomes supports the hypothesis that these two reported cytotypes are, in fact, two different species.  相似文献   
72.
Soybean [Glycine max (L.) Merrill] yield and seed fatty acids, protein, and oil content are important traits for which an improved understanding of significant genomic regions would be useful. To accomplish this, a soybean population consisting of 203 F5 derived recombinant inbred lines (RILs) was developed and genotyped with 11,633 polymorphic single nucleotide polymorphisms (SNPs). Each RIL was grown in a single plot at Knoxville, TN in 2010; followed by replicated, multi-location field trials in 2013 and 2014. The data from 2010, 2013, and 2014 were analyzed together in order to detect quantitative trait loci (QTL) for these traits, and 30 total QTLs were detected. Five QTLs are candidates for confirmed status and one QTL is a candidate for positional confirmation. Many of the genes with mutations in close proximity to the fatty acid QTLs are involved in biological processes for fatty acids and/or lipids and could be considered possible candidate genes. Similarly, genes with mutations in genomic regions near yield, protein, and oil QTLs were plentiful and may contribute to the variation observed in these traits. Except for yield and stearic acid, each trait displayed pleiotropic effects with other traits in this study. Notable are the pleiotropic effects for oleic and linolenic acid on chromosomes 9, 13, and 19. Overall, the findings from this research contribute new information to the genetic understanding of soybean yield and seed fatty acids, protein and oil content. This understanding will be useful in making trait improvements.  相似文献   
73.
Salinity is one of the major abiotic stresses that severely effects rice production throughout the world. Previously, there have not been many studies which focused on studying diversity among rice germplasm based on specific physiological traits for salt tolerance. Our diversity study was based on physiological traits such as Na+ concentration, K+ concentration, K+/Na+ ratio, osmotic potential, and biomass which are major components determining salt tolerance. This study has systematically analyzed phenotypic data of 191 germplasms in two different salt concentrations apart from the control. The current study identified salt tolerant germplasms based on their response to a single physiological trait as well as a combination of different physiological traits. Some of the germplasm identified outperformed known salt tolerant cultivar Pokkali. This study identifies correlation among various physiological traits. The salt tolerant germplasms can be taken forward into developing better varieties by conventional breeding and exploring genes for salt tolerance.  相似文献   
74.
Determining effective measures to alleviate the impact of climate change on crops under various regional environments is one of the most urgent issues facing agriculture. In this study, geographic regions of South Korea for future-adjusted barley cultivation were outlined and the impact of climate change on barley production in the next 100 years was evaluated under two greenhouse gas concentration trajectory scenarios: the representative concentration pathway (RCP) 4.5 and RCP 8.5. To achieve our intended study goals, a geospatial crop simulation modeling (GCSM) scheme was formulated using CERES-barley model of Decision Support System for Agricultural Technology (DSSAT) crop model package version 4.6 to simulate grid-based geospatial crop yields. Two experiments were carried out at an open field to obtain model coefficients for the nation and at temperature gradient field chambers to evaluate the performance of the CERES-barley model under elevated temperature conditions. Suitable cultivation regions for three different types of barley (naked, hooded, and malting) under changing climate were projected to expand to the northern regions under both RCP 8.5 and RCP 4.5. However, they were projected to expand more rapidly under RCP 8.5 than those under RCP 4.5. Projected yields of four barley varieties were increased with a slow phase as year progressed under RCP 4.5 scenario. However, they were rapidly increased under RCP 8.5 scenario. It appears that geospatial variation in barley yield under changing climate can be effectively outlined. Therefore, GCSM system might be useful for determining impacts of climate change on geospatial variations of crops, potentially providing means to impede food insecurity.  相似文献   
75.
Gluten protein determines the processing quality of both durum wheat and bread wheat. The glutenin subunits compositions and associated quality traits of 20 Ethiopian durum wheat varieties were systematically analyzed using SDS-PAGE and Payne numbers. A total of 16 glutenin patterns were identified. At the Glu-A1 locus, all varieties scored the null allele. The predominant glutenin alleles at the Glu-B1 locus were Glu-B1b (7+8) and Glu-B1e (20). In Glu-3, the most abundant glutenin subunits were Glu-A3a and Glu-B3c. Based on the Payne scores, the varieties Yerer, Ginchi, Candate, and Foka were identified to have allelic composition suitable for pasta making. The cluster analysis using agglomerative hierarchical clustering (AHC) method classified the varieties into four similarity classes. Based on the findings of this experiment, suggestions were made for allelic composition improvement through introgression of superior alleles from known Glu-1 and Glu-3 sources.  相似文献   
76.
Cold stress is one of the major abiotic factors that influence the productivity and geographical distribution of many agriculturally important crops like Hevea brasiliensis. Cultivation of H. brasiliensis in India is being extended to northeastern regions, where low temperature during winter adversely affects its survival, growth, and productivity. Developing cold-tolerant genotypes is a primary requisite to maximize the productivity under such challenging environmental conditions. However, lack of methods for early evaluation of cold tolerance in the newly developed clones and the extensive time required for assessing their tolerance in the field are major constraints for clonal selection. The present study was initiated with an objective to identify and characterize cold stress responsive miRNAs from H. brasiliensis that show stronger association with cold tolerance. Next generation sequencing using Illumina HiSeq method revealed the expression of 21 and 29 conserved miRNA (from clone RRIM 600) families in cold-stressed and control samples, respectively. Forty-two novel miRNAs were identified from this study. Upon differential expression analysis, eight conserved miRNAs were found commonly expressed in both the samples. When expression analyses were performed subsequently with six selected miRNAs in two Hevea clones (viz. RRII 105 and RRIM 600), miR169 showed a strong association with cold tolerance. miRNAs such as miR482 and miR159 also exhibited association with cold tolerance. This study suggests the possibility of employing these miRNAs as markers for cold tolerance after validation in more number of genotypes with varying levels of cold tolerance.  相似文献   
77.
This study evaluated the performance and carcass characteristics of lambs fed diets with increasing levels of Mimosa tenuiflora (Willd.) hay replacing Buffel grass (Cenchrus ciliaris). Twenty-eight Santa Inês male lambs with an average body weight (BW) of 20.3 ± 1.49 kg(mean ± SD) were allocated in individual stalls and distributed in a completely random design with four treatments (0, 20, 40, and 60 g/100 g total DM M. tenuiflora hay replacing Buffel grass hay in diet) with seven replications. M. tenuiflora hay at the level of 20% dry matter (DM) total replacing Buffel grass hay increased final weight (P = 0.006), total weight gain (P < 0.001), average daily weight gain (ADWG; P < 0.001), DM intake (P < 0.001), and feed efficiency (P < 0.001). Intake of crude protein, NDFap, ADFap, ash, ether extract, total and non-fibrous carbohydrates, and total digestible nutrients presented a positive quadratic effect with M. tenuiflora hay replacing Buffel grass hay and 40 g/100 g total DM level presented greater intake. There were positive quadratic effects by M. tenuiflora hay inclusion at 20 g/100 g total DM level on slaughtering weight (P = 0.005), hot carcass weight (P = 0.002), cold carcass weight (P = 0.002), empty body weight (P = 0.001), hot carcass yield (P = 0.002), cold carcass yield (P = 0.003), and increase linear on biological yield (P = 0.003). There was no influence on cooling weight loss (P = 0.284). M. tenuiflora hay may be included in lamb diets at amounts up to 20 g/100 g total DM substitution of Buffel grass hay because increase in the nutrients intake, growth performance, and carcass characteristics.  相似文献   
78.
A nitrogen (N) balance digestion trial was conducted to determine the protein requirement of collared peccaries (Pecari tajacu). In a 4 × 4 Latin square design, four captive adult male peccaries were fed four isoenergy diets containing four different levels of N (11.7, 16.3, 22.8, and 26.7 g N/kg of dry matter—DM). After 15 days of adaptation, a total collection of feces and urine was carried out for five consecutive days. Regression analyses between N intake and N in feces and urine allowed to calculate the metabolic fecal nitrogen (MFN = 2.3 g N/kg of dry matter intake—DMI) and daily endogenous urinary N (EUN = 185 mg N/kg0.75). Likewise, by regression analyses between consumption of nitrogen and the nitrogen balance (NB = N ingested ? N excreted, mg N/kg0.75), a daily requirement of 514 mg N/kg0.75 was calculated. Therefore, if food intake is unrestricted, collared peccaries require a minimum in their diet of about 5.4% crude protein on DM basis. These values are almost as low as those found for browsing and frugivorous wild ruminants, which reinforce the proposition that peccaries’ digestive physiology is nearer to that of domestic and wild ruminants than domestic pigs. This relatively low protein requirement of collared peccary and its great ability to digest protein reveal the relevance of the forestomach for the species on nitrogen/protein metabolism and allow the use of diets with lower crude protein levels than the commercial ones used for the domestic pig, which reduces feed costs.  相似文献   
79.
The objective of this study was to analyse the effects of different levels of rumen-degradable protein (RDP) on intake, digestibility and microbial protein synthesis by supplementing mustard oil cake (MOC) on rice straw-based diet of cattle (Bos indicus) in Bangladesh. A 4 × 4 Latin square design was applied. Four diets having constant energy (7.0 MJ/kg of dry matter (DM)) with varying levels of RDP (M 0 = 4.1 g/MJ (control), M 1 = 6.3 g/MJ, M 2 = 8.3 g/MJ and M 3 = 12.4 g/MJ of metabolizable energy (ME)) were received by each animal for a period of 28 days. A metabolism trial was conducted for 7 days. Results indicate that with increasing levels of RDP, crude protein (CP) and RDP intake increased significantly (P < 0.01). The significant (P < 0.01) increase in digestibility values are obtained for DM, organic matter, CP and digestible organic matter in the rumen. The digestibility of neutral detergent fibre and acid detergent fibre was also increased significantly (P < 0.05). The total nitrogen (N), ammonia-N and total volatile fatty acids increase significantly (P < 0.01) while the rumen pH increased from M 0 to M 2 and decreased thereafter. The efficiency microbial N intake increased significantly (P < 0.01) but showed a curvilinear response with higher RDP level (12.40 g/RDP/MJ ME). This study concludes that supplementation of RDP from MOC enhances the intake, digestibility and microbial protein synthesis which ultimately increases utilization of low-quality feed resources that can be used for developing cost-effective feeding systems on a straw-based diet in tropical regions.  相似文献   
80.
Indigenous chicken (IC) and their production systems were characterized to understand how the whole system operates for purposes of identifying threats and opportunities for holistic improvement. A survey involving 594 households was conducted in six counties with the highest population of IC in Kenya using structured questionnaires. Data on IC farmers’ management practices were collected and analysed and inbreeding levels calculated based on the effective population size. Indigenous chicken were ranked highest as a source of livestock income by households in medium- to high-potential agricultural areas, but trailed goats in arid and semi-arid areas. The production system practised was mainly low-input and small-scale free range, with mean flock size of 22.40 chickens per household. The mean effective population size was 16.02, translating to high levels of inbreeding (3.12%). Provision for food and cash income were the main reasons for raising IC, whilst high mortality due to diseases, poor nutrition, housing and marketing channels were the major constraints faced by farmers. Management strategies targeting improved healthcare, nutrition and housing require urgent mitigation measures, whilst rural access road network needs to be developed for ease of market accessibility. Sustainable genetic improvement programmes that account for farmers’ multiple objectives, market requirements and the production circumstances should be developed for a full realization of IC productivity.  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