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   441篇
  免费   40篇
  国内免费   1篇
林业   10篇
农学   12篇
基础科学   4篇
  75篇
综合类   33篇
农作物   13篇
水产渔业   35篇
畜牧兽医   265篇
园艺   7篇
植物保护   28篇
  2023年   1篇
  2022年   2篇
  2021年   12篇
  2020年   7篇
  2019年   10篇
  2018年   15篇
  2017年   15篇
  2016年   13篇
  2015年   10篇
  2014年   27篇
  2013年   37篇
  2012年   25篇
  2011年   40篇
  2010年   17篇
  2009年   14篇
  2008年   22篇
  2007年   18篇
  2006年   23篇
  2005年   21篇
  2004年   19篇
  2003年   17篇
  2002年   14篇
  2001年   7篇
  2000年   12篇
  1999年   15篇
  1998年   5篇
  1997年   4篇
  1996年   1篇
  1995年   3篇
  1994年   1篇
  1993年   2篇
  1992年   4篇
  1991年   5篇
  1990年   3篇
  1989年   6篇
  1988年   6篇
  1987年   2篇
  1986年   4篇
  1985年   1篇
  1984年   3篇
  1983年   5篇
  1980年   2篇
  1978年   3篇
  1976年   2篇
  1975年   1篇
  1974年   1篇
  1972年   1篇
  1970年   1篇
  1969年   1篇
  1968年   1篇
排序方式: 共有482条查询结果,搜索用时 31 毫秒
431.
A 4‐year‐old spayed female mixed breed dog was referred to the Michigan State University, Veterinary Teaching Hospital (MSU‐VTH) with vomiting, lethargy and anorexia of 2 weeks duration. Abdominal radiographs and ultrasonography showed hepatosplenomegaly. Cytological evaluation of ultrasound‐guided fine needle aspirates of the liver and spleen revealed fungal organisms and pyogranulomatous inflammation; fungal culture documented Paecilomyces variotii infection. The dog received antifungal therapy and supportive care. Multiple firm plaque‐like skin lesions, predominantly involving the inguinal region, developed 18 days after initial presentation and were diagnosed histopathologically as calcinosis cutis. While generalized calcinosis cutis has been reported in three dogs with blastomycosis and one dog with leptospirosis, the association with disseminated Paecilomyces spp. infection is novel.  相似文献   
432.
433.
The purpose of this work was to determine the tissue accumulation of lead (Pb) and its effects on osmoregulatory processes of the freshwater fish Prochilodus lineatus. Juvenile fish were exposed to Pb (from 1.7 to 0.7 mg of dissolved Pb L?1) for 6, 24 and 96 h and Pb accumulation was analyzed in the gills, liver, kidneys, blood cells and muscle. The following parameters were also analyzed: hematologic (hemoglobin content, hematocrit and number of red blood cells), metabolic (blood glucose), endocrine (blood cortisol), osmo ionic (plasma osmolality and Na+, K+, Cl? and Ca+2 concentrations), gill enzymes (Na+/K+-ATPase and carbonic anhydrase), chloride cell (CC) density and CC location in the gills. Pb accumulated in all the analyzed tissues, with the kidneys showing the highest concentration, followed by the gills and liver. The lowest Pb concentrations were found in blood cells and muscle. Pb promoted an increase in blood glucose after 6 and 24 h exposure. Gill Na+/K+-ATPase was inhibited after 24 h of exposure, but its activity was restored at 96 h, probably due to the increase in CC in gill lamellae. Plasma Na+ was reduced after 6 and 96 h, while K+ concentrations increased at all the experimental times. Fish exposed to Pb showed reduced plasma Ca+2 at all experimental periods. Hematologic parameters remained unchanged. Overall, this study demonstrated that Pb interferes in osmoregulatory processes of P. lineatus and the proliferation of CC in the gills is a response in order to reestablish adequate ion concentrations.  相似文献   
434.
In hydroponics, nutrient management is the limiting factor to obtaining optimal production, and nitrogen (N) is the key component to consider when optimizing nutrient management in these types of systems. The objective of this study is to evaluate different combinations of N fertilizer concentrations in order to optimize the yield of basic seed potato minitubers in a three-phase hydroponic system. Treatments consisted of five combinations of N concentrations, applied before and after 21 days after plant transplant as follows, respectively: 1) 13 and 13, 2) 13 and 0, 3) 13 and 7.8, 4) 13 and 16, and 5) 13 and 26 mmol L?1. Propagation was performed by transplanting 3–4 cm potato plantlets cv. Agata from sprouts. There were significant effects of N treatments on all measured variables (root, leaf, stem, and plant dry weight and minituber number and weight). To obtain the maximum minituber number yield, 9.51 minitubers/plant, corresponding to 67 minitubers/m2, post 21-day adjusted N concentration was 18.4 mmol/L. Treatment 4 promoted higher basic seed potato minituber yield in a three-phase hydroponic system.  相似文献   
435.
The purpose of this work was to evaluate the suitability of a method based on the presence of two restriction sites (for Hae III and Hindf I) in the mitochondrial NADH dehydrogenase subunit 5 (mt ND5) gene to identify Scomber species. The evaluation was performed on 144 reference and market samples by sequencing of the entire 505-bp fragment of the mt ND5 gene and of a 464-bp fragment of the Kocher fragment of the cytochrome b gene (mt Cytb). Sequence analysis of any of the two fragments allows the identification of each of the four Scomber species, but S. japonicus and S. colias had the same restriction sites at the ND5 amplicon and would not have been differentiated by this analysis. Similarly, loss of the Hae III site in some S. scombrus individuals would have misidentified them as not being Scomber. All the market products were correctly labeled except one acquired in Spain labeled as originating in the Atlantic and containing S. japonicus.  相似文献   
436.
Abstract

This paper presents a study of operating costs in shrimp hatcheries. Size effects are presented for fixed, variable, unitary, and marginal costs. Cost structures by functional production department and by cost‐category are discussed for a hypothetical 100‐million postlarvae/month hatchery in an industrialized country, and compared to those of a hatchery in a developing country. Health management issues, mainly measures commonly used to manage bacterial diseases, and their economic impact are discussed, focusing on the trade off between survival rates and sanitation down‐time.

Salaries and benefits represent the largest cost‐component (37%) for hatcheries in industrialized countries, while feeds and operating supplies constitute the most important costs in developing countries (60%). The need to import hatchery feeds and other operating supplies into developing countries works against savings from lower labor costs typical of developing countries.

Total fixed costs depend on the size of a hatchery and are constant for all possible levels of output. Average fixed cost per unit (thousand postlarvae) can be reduced by producing high postlarval outputs. Hatcheries in areas with seasonal shrimp crops face the challenge of covering fixed costs with revenue from a short production season. Hatcheries with large fixed costs can operate in a cost‐efficient manner only by producing high levels of output. Average total cost and average variable cost curves follow a U‐shaped curve, in which costs first decline as output increases, then reach a minimum, and finally increase as output increases above a point of diminishing returns.

In hatcheries total costs are independent of larval survival, but lower output resulting from low survival increases unit costs. This effect is more pronounced at lower levels of output. Bacterial diseases decrease survival rates and increase unit costs.  相似文献   
437.
Breeding efforts in cereal crops directed toward developing or improving end-use products of grain require assessment of existing phenotypic variance and an understanding of the genetic control of grain quality traits. To this end, a grain sorghum [Sorghum bicolor (L.) Moench] mapping population consisting of 113 F2:7 recombinant inbred lines (RILs) derived from a cross between Sureño and RTx430 was evaluated in multiple environments for grain composition (fat, fiber, protein, starch) using near-infrared reflectance spectroscopy (NIRS), and size estimates of grain parts (embryo, vitreous endosperm, floury endosperm, kernel area) using an image-based phenotyping software system. Estimates of broad-sense heritability of grain compositional traits ranged from 0.11 to 0.90, whereas those of grain size ranged from 0.16 to 0.72. Composite interval mapping (CIM) was applied to a single nucleotide polymorphism (SNP)-based linkage map to identify marker-trait associations, and through these efforts, a total of 37 quantitative trait loci (QTL) for grain quality were identified across environments. Each QTL explained between 7 and 23% of the phenotypic variation for a given grain trait. Three of the five QTL that colocalized were for traits with significant negative correlation, which included grain protein content that was negatively correlated with grain starch content. In addition, several traits that were positively correlated (e.g. fat and fiber content) also revealed colocalized QTL. Finally, we compared the present study with previous studies identifying grain composition trait loci in an effort to identify genomic regions controlling grain traits across a diversity of environments and sorghum genotypes.  相似文献   
438.
Apple (Malus?×?domestica Borkh.) trees, either abandoned or cared for, are common on the North American landscape. These trees can live for decades, and therefore represent a record of large- and small-scale agricultural practices through time. Here, we assessed the genetic diversity and identity of 330 unknown apple trees in northern Minnesota with 9 simple sequence repeat (SSR) markers. The unknown (not identified by cultivar name) trees were compared to >?1000 named cultivars in the U.S. Department of Agriculture Plant Genetic Resources Unit Malus collection and also to each other to identify repeated genotypes. Overall, the 330 unknown trees had high diversity (average He?=?0.75), and consisted of 264 unique genotypes. A total of 76 of the unknown trees were matched to 20 different named cultivars, and these cultivars were mainly derived from either the local breeding program at the University of Minnesota, or were Russian cultivars imported for horticulture in the northern US. This study demonstrates the importance of local breeding programs, and also the challenges associated with identifying clones in a genetically diverse crop like apple.  相似文献   
439.
The effect of goat nutritional condition on the response to biostimulation with sexually active males during different stages of anestrus was determined. Fifty‐eight Criollo × Nubian females on high and low body mass index (BMI) diets were used. Each BMI group was divided into two for biostimulation with sexually active males during May (mid‐anestrus) or July (transition period). Ovulatory responses to biostimulation were characterized from serum progesterone, as well as the delay for response (first and second ovulations followed by a normal length luteal phase, O‐WNLP). The percentage of goats showing one O‐WNLP was greater in the high BMI group than in the low BMI group and greater during the transition period than in the mid‐anestrus. However, the interaction between factors revealed that the difference between BMI groups was only significant in the transition period and the difference between stages was only significant in goats with high BMI. Occurrence of a second O‐WNLP tended to be greater in the high BMI group than in the low BMI group. Response delay was shorter in the transition period than in mid‐anestrus. In conclusion, female nutritional status interacting with the stage of anestrus determined the ovulatory response to male biostimulation in crossbred Criollo goats.  相似文献   
440.
The objective of this study was to compare active drug concentrations in the plasma vs. different effector compartments including interstitial fluid (ISF) and pulmonary epithelial lining fluid (PELF) of healthy preruminating (3‐week‐old) and ruminating (6‐month‐old) calves. Eight calves in each age group were given a single subcutaneous (s.c.) dose (8 mg/kg) of danofloxacin. Plasma, ISF, and bronchoalveolar lavage (BAL) fluid were collected over 96 h and analyzed by high‐pressure liquid chromatography. PELF concentrations were calculated by a urea dilution assay of the BAL fluids. Plasma protein binding was measured using a microcentrifugation system. For most preruminant and ruminant calves, the concentration–time profile of the central compartment was best described by a two‐compartment open body model. For some calves, a third compartment was also observed. The time to maximum concentration in the plasma was longer in preruminating calves (3.1 h) vs. ruminating calves (1.4 h). Clearance (CL/F) was 385.15 and 535.11 mL/h/kg in preruminant and ruminant calves, respectively. Ruminant calves maintained higher ISF/plasma concentration ratios throughout the study period compared to that observed in preruminant calves. Potential reasons for age‐related differences in plasma concentration–time profiles and partitioning of the drug to lungs and ISF as a function of age are explored.  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