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   541篇
  免费   36篇
林业   28篇
农学   16篇
基础科学   8篇
  101篇
综合类   54篇
农作物   26篇
水产渔业   25篇
畜牧兽医   269篇
园艺   17篇
植物保护   33篇
  2021年   11篇
  2020年   7篇
  2019年   10篇
  2018年   4篇
  2017年   7篇
  2016年   9篇
  2015年   8篇
  2014年   11篇
  2013年   26篇
  2012年   26篇
  2011年   34篇
  2010年   24篇
  2009年   21篇
  2008年   38篇
  2007年   33篇
  2006年   44篇
  2005年   28篇
  2004年   30篇
  2003年   30篇
  2002年   36篇
  2001年   9篇
  1999年   6篇
  1998年   4篇
  1997年   4篇
  1996年   9篇
  1995年   3篇
  1994年   7篇
  1993年   6篇
  1991年   4篇
  1989年   3篇
  1987年   3篇
  1986年   6篇
  1985年   3篇
  1984年   3篇
  1983年   4篇
  1980年   3篇
  1979年   3篇
  1977年   3篇
  1976年   6篇
  1975年   3篇
  1974年   5篇
  1973年   4篇
  1972年   5篇
  1971年   3篇
  1969年   3篇
  1968年   2篇
  1966年   2篇
  1965年   2篇
  1928年   2篇
  1898年   2篇
排序方式: 共有577条查询结果,搜索用时 15 毫秒
81.
Advanced Irrigation Engineering: Precision and Precise   总被引:1,自引:0,他引:1  
  相似文献   
82.
Radiographic osteoarthritis scores were determined in 60 dogs up to 3 years following tibial plateau leveling osteotomy. Radiographs taken immediately following surgery and at long-term follow-up 1 to 3 years later were evaluated by 2 certified radiologists using a modified 32-point osteoarthritis scale. Changes in osteoarthritis scores were evaluated by paired t-tests and regression analysis. Sub-groups of dogs were formed to evaluate if osteoarthritis scores changed differently for follow-up periods of < 24 months compared with those > 24 months. There was a significant increase in osteoarthritis score from post-operative to follow-up evaluations for all dogs studied. Regression analysis of the relationship of months after surgery to change in osteoarthritis score was not significant. These results indicate that osteoarthritis did progress following tibial plateau leveling osteotomy, but not in a linear fashion over time.  相似文献   
83.
In the Padma floodplain of Bangladesh, the traditional system of agriculture has become unsustainable due to high population growth. Mango-based agroforestry which has been practiced by the farmers since the 1990s, is a promising alternative and is considered as one of the few options to lift farmers out of poverty and improve livelihood security. This paper examines the potential of mango-based agroforestry to improve livelihoods, using data collected by rapid rural appraisal, farmer participatory research, stakeholder analysis and a farm household survey in six representative villages in the floodplain. Farmers with the least land were found to allocate a higher percentage of their land to agroforestry, and the increased income from agroforestry compared to other agricultural systems helps reduce relative poverty. This income maintains basic household needs, providing food security and fuelwood, and contributes to healthcare, housing and sanitation conditions, and meeting educational expenses.  相似文献   
84.
Between February 15 and May 17, 2011, a total of 88 broodmares (10 maiden, 10 barren, and 68 foaling) maintained on pasture in southeast Texas were examined three times weekly (Tuesday, Thursday, Saturday) by transrectal palpation and ultrasonography. On Tuesday or Thursday, mares in estrus with uterine edema, a relaxed cervix, and a dominant follicle ≥34 mm in diameter were alternately assigned to treatment with the following: group (1) 2,500-unit human chorionic gonadotropin (hCG), intravenous; group (2) 1.0-mg BioRelease Histrelin (Biorelease Technologies, Lexington, KY), intramuscular; or group (3) 0.5-mg BioRelease Histrelin, intramuscular. Ovulation was confirmed by ultrasonographic examination. The percentage of mares ovulating within 2 days appeared to be similar between maiden, barren, and foaling mares, so responses for all mares were totaled for analysis. A nonsignificant trend for higher ovulation rates within 2 days was noted for both dose rates of histrelin compared with hCG treatment (31/37, 84%; 34/37, 92%; and 33/36, 92% for groups 1-3, respectively) (P = .45). Ovulatory responses appeared to improve for both products as the season progressed, yet no differences were detected between response rates to histrelin or hCG for any month (P ≥ .50). The use of 1.0- or 0.5-mg BioRelease Histrelin was found to be at least equally effective as hCG treatment for inducing ovulation within 2 days of treatment throughout the breeding season.  相似文献   
85.
The objective of this study was to examine the effects of dietary ω-3 fatty acid supplementation on insulin sensitivity (SI) in horses. Twenty-one mares were blocked by age, body weight (BW), and body condition score (BCS) and randomly assigned to one of three dietary treatments. Treatments consisted of (1) 38 g of n-3 fatty acids via fish and algae supplement and diet (MARINE), (2) 38 g of n-3 fatty acids via a flaxseed meal from the supplement and diet (FLAX), and (3) control (CON) no supplemental fatty acid. Treatments were supplemented for 90 days. Frequent sampling intravenous glucose tolerance tests were performed on days 0, 30, 60, and 90. Blood samples were analyzed for glucose and insulin. The minimal model was applied for the glucose and insulin curves using MinMod Millennium. SI increased 39% (P < .007) across all treatment groups. Acute insulin response to glucose decreased 22% (P < .006) between days 30 and 60 and increased (P = .040) again at day 90. Disposition index (combined SI and β pancreatic response) increased (P = .03) by 53% in the MARINE- and 48% in the FLAX-supplemented horses and did not change with time in the CON group. In insulin-resistant mares, MARINE- and FLAX-treated horses had an increase in SI (P = .09). It would be interesting to test this supplement in a larger group of insulin-resistant horses. If proven effective, supplementation with ω-3 fatty acids would help to reduce problems associated with insulin resistance in horses.  相似文献   
86.
Abstract

Chelonid herpesvirus 5 (ChHV5), the likely etiologic agent of sea turtle fibropapillomatosis (FP), is predicted to be unevenly distributed within an infected turtle, in which productive virus replication and virion shedding occurs in cutaneous tumor keratinocytes. In this study, we measured and compared ChHV5 DNA quantities in tumors, skin, urine, major organs, and nervous tissue samples from green turtles Chelonia mydas. These samples were taken from the carcasses of 10 juvenile green turtles with and without clinical signs of FP that stranded in Florida during 2014. Quantitative PCR for ChHV5 UL30 was used to identify ChHV5 DNA in tumors, skin, heart, kidney, nerves, and urine sampled from five out of five FP-positive and three out of five FP-free turtles. The most frequently co-occurring sites were cutaneous tumor and kidney (n = 4). Novel data presented here include the identification of ChHV5 DNA in kidney, heart, and nerve samples from three FP-free turtles. These data support candidate nontumored anatomic sites of ChHV5 DNA localization and mobilization during two different disease states that may be involved in the ChHV5 infection cycle.

Received September 8, 2016; accepted April 17, 2017 Published online July 26, 2017  相似文献   
87.
Most turtle species require high adult survivorship to maintain stable populations. Translocations are often implemented to conserve turtle populations but may cause demographic disturbance as a result of increased mortality or dispersal of released animals. The gopher tortoise (Gopherus polyphemus) is one of the most frequently translocated turtle species. Short-term monitoring indicates that dispersal by released tortoises is common, but few long-term data are available to determine if losses of translocated animals continue for multiple years. We used 12 years of mark-recapture data to investigate long-term apparent survival of two groups of gopher tortoises translocated during separate periods to St. Catherines Island, Georgia, USA. We analyzed capture histories in program MARK to compare apparent survival of newly released tortoises and previously established translocated tortoises and also to determine whether apparent survival varied with sex or maturity. Apparent annual survival did not vary between adult males and females (0.98 ± 0.01), but was lower in sexually immature tortoises (0.84 ± 0.05). We documented a temporary reduction in apparent survival of newly released adult (0.75 ± 0.06) and immature tortoises (0.45 ± 0.26) during the first year after release that may be attributed in part to permanent dispersal. However, for both maturity classes, apparent survival of newly released tortoises was consistently high and matched that of previously established animals during the remainder of the study. Additional long-term studies of both translocated and naturally-occurring populations are needed to improve management of remaining tortoise populations.  相似文献   
88.
Polyphenol oxidase (PPO) activity in leaf extracts of wild type (WT) red clover and a mutant line expressing greatly reduced levels of PPO (LP red clover) has been characterized. Both latent and active forms of PPO were present, with the latent being the predominant form. PPO enzyme and substrate (phaselic acid) levels fluctuated over a growing season and were not correlated. Protease activation of latent PPO was demonstrated; however, the rate was too low to have an immediate effect following extraction. A novel, more rapid PPO activation mechanism by the enzyme's own substrate was identified. Rates of protein breakdown and amino acid release were significantly higher in LP red clover extracts compared with WT extracts, with 20 versus 6% breakdown of total protein and 1.9 versus 0.4 mg/g FW of free amino acids released over 24 h, respectively. Inclusion of ascorbic acid increased the extent of protein breakdown. Free phenol content decreased during a 24 h incubation of WT red clover extracts, whereas protein-bound phenol increased and high molecular weight protein species were formed. Inhibition of proteolysis occurred during wilting and ensilage of WT compared with LP forage (1.9 vs 5 and 17 vs 21 g/kg of DM free amino acids for 24 h wilted forage and 90 day silage, respectively). This study shows that whereas constitutive red clover PPO occurs predominantly in the latent form, this fraction can contribute to reducing protein breakdown in crude extracts and during ensilage.  相似文献   
89.
Methods devised for oil extraction from avocado (Persea americana Mill.) mesocarp (e.g., Soxhlet) are usually lengthy and require operation at high temperature. Moreover, methods for extracting sugars from avocado tissue (e.g., 80% ethanol, v/v) do not allow for lipids to be easily measured from the same sample. This study describes a new simple method that enabled sequential extraction and subsequent quantification of both fatty acids and sugars from the same avocado mesocarp tissue sample. Freeze-dried mesocarp samples of avocado cv. Hass fruit of different ripening stages were extracted by homogenization with hexane and the oil extracts quantified for fatty acid composition by GC. The resulting filter residues were readily usable for sugar extraction with methanol (62.5%, v/v). For comparison, oil was also extracted using the standard Soxhlet technique and the resulting thimble residue extracted for sugars as before. An additional experiment was carried out whereby filter residues were also extracted using ethanol. Average oil yield using the Soxhlet technique was significantly (P < 0.05) higher than that obtained by homogenization with hexane, although the difference remained very slight, and fatty acid profiles of the oil extracts following both methods were very similar. Oil recovery improved with increasing ripeness of the fruit with minor differences observed in the fatty acid composition during postharvest ripening. After lipid removal, methanolic extraction was superior in recovering sucrose and perseitol as compared to 80% ethanol (v/v), whereas mannoheptulose recovery was not affected by solvent used. The method presented has the benefits of shorter extraction time, lower extraction temperature, and reduced amount of solvent and can be used for sequential extraction of fatty acids and sugars from the same sample.  相似文献   
90.
OBJECTIVE: To determine prevalences of various hemoplasma species among cats in the United States with possible hemoplasmosis and identify risk factors for and clinicopathologic abnormalities associated with infection with each species. DESIGN: Cross-sectional study. Animals-310 cats with cytologic evidence of hemoplasmosis (n = 9) or acute or regenerative anemia (309). PROCEDURES: Blood samples were tested by means of a broad-spectrum conventional PCR assay for hemoplasma DNA and by means of 3 separate species-specific real-time PCR assays for DNA from "Candidatus Mycoplasma haemominutum" (Mhm), Mycoplasma haemofelis (Mhf), and "Candidatus Mycoplasma turicensis" (Mtc). RESULTS: Overall prevalences of Mhm, Mhf, and Mtc infection were 23.2% (72/310), 4.8% (15/310), and 6.5% (20/310), respectively. Mixed infections were detected in 20 (6.5%) cats. Cats infected with hemoplasmas were more likely to be male than were uninfected cats. Infection with FeLV or FIV was significantly associated with infection with Mhf. Compared with uninfected cats, cats infected with Mhf had higher reticulocyte counts, nucleated RBC counts, and mean corpuscular volume; cats infected with Mhm had higher mean corpuscular volume; and cats infected with Mtc had higher monocyte counts. CONCLUSIONS AND CLINICAL RELEVANCE: Results supported the suggestion that these 3 hemoplasma species commonly occur among cats in the United States and that pathogenicity of the 3 species varies.  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