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鄂北大牛地气田固井取得重大突破
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作者 周仕明 《石油钻探技术》 CAS 2002年第6期47-47,共1页
关键词 鄂北大牛地气田 固井 水泥浆体系 油层套管 固井质量
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大牛地石盒子组盒3段储层测井响应特征
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作者 李文庆 《中国石油和化工标准与质量》 2012年第S1期5-5,共1页
本文试图在前人研究成果的基础之上,利用分析测试资料,实现储层的测井定量解释,从而达到预测储层的目的。
关键词 牛地 岩心特征 测井响应 特征
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无黏土相钻井液及生物完井液在DF2井的应用 被引量:4
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作者 郑锋辉 薛玉志 +3 位作者 刘宝锋 张敬辉 李海滨 李公让 《油田化学》 CAS CSCD 北大核心 2008年第4期302-304,324,共4页
鄂尔多斯大牛地地区山1储气层低孔低渗,物性差,钻井过程中极易受到严重而难以恢复的污染,以往该层所钻井不经改造都无油气产出。该储气层DF2井水平段长达900 m,夹有两段煤层和一段泥岩层,在钻井中使用了含甲酸钾、防水锁剂,不含滑润剂... 鄂尔多斯大牛地地区山1储气层低孔低渗,物性差,钻井过程中极易受到严重而难以恢复的污染,以往该层所钻井不经改造都无油气产出。该储气层DF2井水平段长达900 m,夹有两段煤层和一段泥岩层,在钻井中使用了含甲酸钾、防水锁剂,不含滑润剂的低密度无黏土相钻井液,钻进中钻井液性能稳定,井壁稳定,携砂正常,起下钻畅通,完钻后电测一次成功。介绍了该钻井液的配制、维护及全水平段钻井液的性能。水平段完钻后用一种生物完井液浸泡裸眼井段24小时,随后试气,获得了无阻流量8×104m3/d的天然气自然产能。图1表1参4。 展开更多
关键词 水基钻井液 甲酸钾钻井液 元黏土相 低密度 生物完井液 水平井段 油气层保护 鄂尔多斯大牛地地区
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鄂北工区水平井提速提效技术探讨 被引量:4
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作者 闫永辉 《探矿工程(岩土钻掘工程)》 2013年第6期43-47,共5页
随着鄂北工区大牛地气田的持续建产,水平井规模应用已成为气田开发的重点技术。但由于地层岩性变化、可钻性差异,泥岩稳定性,低破裂压力层段漏失,钻井液润滑和携岩能力,岩屑床的清除,以及钻遇泥岩煤层发生坍塌掉块等问题和不可控因素,... 随着鄂北工区大牛地气田的持续建产,水平井规模应用已成为气田开发的重点技术。但由于地层岩性变化、可钻性差异,泥岩稳定性,低破裂压力层段漏失,钻井液润滑和携岩能力,岩屑床的清除,以及钻遇泥岩煤层发生坍塌掉块等问题和不可控因素,对水平井钻井提速提效产生了不利的影响。系统分析了影响水平井提速提效的"瓶颈"问题,提出较为完善的技术方案,以实现水平井施工安全、快速、高效钻进的目的。 展开更多
关键词 水平井 提速提效 岩屑床 轨迹控制 钻井液 鄂北工区大牛地气田
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Phylogenetic Relationships and Status Quo of Colonies for Gayal Based on Analysis of Cytochrome b Gene Partial Sequences 被引量:13
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作者 马国龙 常洪 +5 位作者 李世平 陈宏宇 冀德君 耿荣庆 常春芳 李永红 《Journal of Genetics and Genomics》 SCIE CAS CSCD 北大核心 2007年第5期413-419,共7页
Thirty-three mutations and four different haplotypes were found when cytochrome b(Cytb) gene partial sequences of 12 gayals were analyzed. Together with sequences of Bos indicus, Bos taurus, Bos grunniens, and Bos g... Thirty-three mutations and four different haplotypes were found when cytochrome b(Cytb) gene partial sequences of 12 gayals were analyzed. Together with sequences of Bos indicus, Bos taurus, Bos grunniens, and Bos gaurus with Bubalus bubalis as the out group, the partial sequences of Cytb gene of gayals were aligned and base composition and nucleotide variation of Cytb gene were analyzed. The phylogenetic trees were constructed by the NJ method and the MP method respectively, both supporting almost the same topology. Gayal is an independent species of Bos from Bos indicus, Bos taurus, and Bos gaurus. The results also indicate that a great proportion of gayal bloodline was invaded by other species, and the protection of gayal is facing a formidable situation. 展开更多
关键词 GAYAL cytochrome b gene phylogenetic relationship status quo of colony
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陇西药市部分品种价格表
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作者 陈亮 《全国药材商情》 2003年第35期14-14,共1页
关键词 陇西药市 药材价格 党参 木香 土龙骨 桔梗 牛地 天麻
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Supplementary description of a female and two newly-recorded species in the subfamily Oedemerinae(Coleoptera:Oedemeridae) from China
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作者 田颖 任国栋 李强 《Entomotaxonomia》 CSCD 2015年第2期123-128,共6页
The female ofDiplectrus bistigmaeus Zhang, Ren et Ba, 2012 from Xizang was newly reported and supplementarily described. Two species of Oedemerinae were reported for the first time from China: Nacerdes (Xanthochroa... The female ofDiplectrus bistigmaeus Zhang, Ren et Ba, 2012 from Xizang was newly reported and supplementarily described. Two species of Oedemerinae were reported for the first time from China: Nacerdes (Xanthochroa) brendelli Svihla, 1987 and N. (Asiochroa) mimoncomeroides Svihla, 1998. The potential geographical distribution of these two species based on the known distribution was predicted by DIVA-GIS software. 展开更多
关键词 false blister beetles new record geographical distribution
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Spatial Trend of Foot and Mouth Disease Virus (FMDV) Serotypes in Cattle and Buffaloes, Pakistan 被引量:2
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作者 Muhammad Abubakar Muhammad Javed Arshed +1 位作者 Qurban Ali Manzoor Hussain 《Virologica Sinica》 SCIE CAS CSCD 2012年第5期320-323,共4页
The present study describes the frequency of Foot and Mouth Disease (FMD) virus serotypes (O, A and Asia-l) in major regions (all provinces) of Pakistan using Indirect Sandwich ELISA. Also, spatial distribution ... The present study describes the frequency of Foot and Mouth Disease (FMD) virus serotypes (O, A and Asia-l) in major regions (all provinces) of Pakistan using Indirect Sandwich ELISA. Also, spatial distribution of various FMD serotypes and their comparison is discussed. A total of 590 samples (Epithelial tissue) have been analyzed during a period of five years (2005-2009). Out of 590 samples, 180 were found positive, giving an overall confirmation of FMDV about 33.2 %. Of the prevalent serotypes, FMDV 'O' serotype caused most outbreaks (20.7 %), followed by serotype A (6.6 %) and serotype Asia-1 (4.6 %) while there was no positive case of type 'C'. The study clearly showed that the disease was more frequent in the agro-climatic zones than in hilly areas. Based on the data of 590 samples (〉50 outbreaks), the overall prevalence of FMDV in cattle and buffaloes in Pakistan was 33.2 %, while in cattle alone, it was 37.1%, higher than in buffalo (28.7 %). There were eight cases of mixed serotypes infection, indicating the presence of endemic state of disease. Another significant feature was the change over time. In phase-I (2005-2007), there was an overall prevalence of 29.4 %, while the occurrence of the serotype O, A and Asia-1 was 20.4 %, 2.9 % and 4.7 %, respectively. During phase-II (2008-2009), the overall prevalence was 59.21%, while those of serotype O, A and Asia-1 were 22.4 %, 31.6 % and 4.0 %, respectively. This clearly indicated a shift from serotype O to A, which may help to explain the occurrence of more severe outbreaks, despite vaccination. 展开更多
关键词 Spatial and Temporal Distribution Serotype shift Foot and Mouth disease (FMD) Virus Indirect Sandwich ELISA SEROTYPES
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Notes on the genera Anaesthetobrium Pic and Microestola Gressitt(Coleoptera:Cerambycidae:Lamiinae)
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作者 Meiying LIN Siqin GE 《Entomotaxonomia》 CSCD 2020年第4期298-310,共13页
This paper deals with the genera Anaesthetobrium Pic,1923 and Microestola Gressitt,1940 belonging to the tribe Desmiphorini.Microestola Gressitt,1940 is reinstated from synonyms of Cylindilla Bates,1884.Two new synony... This paper deals with the genera Anaesthetobrium Pic,1923 and Microestola Gressitt,1940 belonging to the tribe Desmiphorini.Microestola Gressitt,1940 is reinstated from synonyms of Cylindilla Bates,1884.Two new synonyms are proposed:Eunidiopsis Breuning,1939 syn.nov.=Anaesthetobrium Pic,1923 and Mimopothyne Breuning,1956 syn.nov.=Microestola Gressitt,1940.Two new combinations are made:Anaesthetobrium bicolor(Breuning,1939)comb.nov.previously in genus Eunidiopsis Breuning,1939,and Microestola flavolineata(Breuning,1956)comb.nov.previously in genus Mimopothyne Breuning,1956. 展开更多
关键词 longhorn beetle REINSTATEMENT new synonym new combination TAXONOMY
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Genetic Parameters for Yields and Reproductive Traits of Chinese Holstein Cows in Heilongjiang Province
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作者 Kewei DING Takeo KAYABA 《Agricultural Science & Technology》 CAS 2013年第3期520-522,526,共4页
[Objective] This study aimed to estimate the genetic parameters for milk yield, fat percentage, fat yield and age at first freshening of Chinese Holsteins first lactation cows in Heilongjiang under frigid-temperature ... [Objective] This study aimed to estimate the genetic parameters for milk yield, fat percentage, fat yield and age at first freshening of Chinese Holsteins first lactation cows in Heilongjiang under frigid-temperature and temperature zone. [Method] Records of yield and reproduction from 8 868 Chinese Holstein first lacta- tion cows from 21 herds were utilized to evaluate genetic parameters for Hei- Iongjiangs' Chinese Holsteins. Restricted maximum likelihood (REML) method was used with an animal model. [Result] The heritabilities were 0.16 to 0.34 for yields and fat percentage, 0.38 for reproduction (age at the first freshening), which were similar to estimates for temperate areas from similar methods, except for constituent percentage. Also, the estimates of genetic correlation were high between milk yield and fat yield (0.97), but were low between milk yield and fat percentage (-0.24), age at the first freshening (-0.29) and generally antagonistic. [Conclusion] Thus, es- timates of genetic parameters in this frigid and frigid-temperate environment did not differ appreciably from those that occur in temperate dairy areas. 展开更多
关键词 Heilongjiang Chinese Holsteins PRODUCTION REPRODUCTION Genetic pa rameters
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Growth Response of Artemisia annua by Effect of Types and Composition of Organic Fertilizer in Lowland
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作者 Pambudi AjiNugroho Yuli Widyastuti +1 位作者 Samanhudi Ahmad Yunus 《Journal of Agricultural Science and Technology(B)》 2017年第5期339-345,共7页
Artemisia annua is a plant used to cure malaria diseases. Artemisia plant contains artemisinin as secondary metabolite that used to eliminate parasite that caused malaria, such as Plasmodium falciparum. Artemisia grow... Artemisia annua is a plant used to cure malaria diseases. Artemisia plant contains artemisinin as secondary metabolite that used to eliminate parasite that caused malaria, such as Plasmodium falciparum. Artemisia growth affects production of artemisinin content in plant. Therefore, necessary environment conditions and appropriate organic manure application are needed to support the growth of Artemisia. This research aimed to determine the effect of fertilizer type and proportion in the medium on the Artemisia growth. This research was conducted at greenhouse of Faculty of Agriculture, Universitas Sebelas Maret, Surakarta, in October 2015 to January 2016. This research used a completely randomized design (CRD), consisting of two factors of treatment with three replications. The first factor was type of fertilizer that consists of three types: horse manure fertilizer, compost filter press mud and cow manure fertilizer. The second factor was proportion of fertilizer with media consisted of five levels: fertilizer as media, proportion of fertilizer with media 4:1, 3:2, 2:3 and 1:4. Data were analyzed using analysis of variance and Duncan's multiple range test with level of 5%. It can be concluded that treatment with compost filter press mud provided the highest of plant height, root length, days to flowering, root volume, fresh weight and dry weight of crop. 展开更多
关键词 Artemisia annua PRODUCTIVITY COMPOST manure.
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Comparison the Genetic Potential of Some Growth & Milk Trait, s of Hybrid (Crossed) Saanen with Native Goat (Lori Black Goat) versus Native Goats in Qom Province
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作者 S. M. Hoseini Ghazikhani M. Kalantar 《Journal of Agricultural Science and Technology(A)》 2011年第8期1297-1299,共3页
The performance of some growth and milk traits of selected Qomi native (Lori black) goats and hybrids of saanen with native goats during 2008-2010 was compared. Growth traits include birth weight (BW), one-month w... The performance of some growth and milk traits of selected Qomi native (Lori black) goats and hybrids of saanen with native goats during 2008-2010 was compared. Growth traits include birth weight (BW), one-month weight (1 MW), weaning weight (WW) and 6-month weight (6 MW). Also the milk production traits was monthly milk production on 2, 3, 4, 5th months (quantitative traits) and some milk composition included dry matter (%DM), fat (%F), and protein (%P) percentage on during of production (qualitative traits). There were no significant differences in sex, birth type and genetic group for BW. Also the effect of genetic groups on 1 MW, WW, and 6 MW was significant, while effects of sex and birth type on 1 MW, WW and 6 MW was not significant. Also MW (Mothe's weight) had no significant effect on BW, 1 MW, WW and 6 MW. The average of daily milk records was 0.48 kg for nativel and 1.1 kg for crossed goats. The maximum monthly milk records in two groups were in second month (native: 0.64 -4- 0.05, crossed: 1.44 ~: 0.12) and the minimum were in fifth month (native: 0.36 + 0.03, crossed: 0.77 -4- 0.05). %DM, %F and %P in 5th month were higher than other months in two groups. Effects of genetic groups on milk production and also on average of%F, %DM and %P were significant. This study showed that the crossbreeding has a significant effect on most milk production and growth traits in Iranian native goats. 展开更多
关键词 Saanen goat qomi native goat growth traits milk traits
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Estimating the Consumers' Benefits of the Demand for Organic Fresh Milk in Taiwan
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作者 Huang Chin-Huang 《Journal of Modern Accounting and Auditing》 2011年第11期1284-1291,共8页
Certified agricultural standards (CAS) of the organic food label serve to increase food safety and restore the confidence and consumption level in Taiwan. The organic CAS represents food quality without the use of s... Certified agricultural standards (CAS) of the organic food label serve to increase food safety and restore the confidence and consumption level in Taiwan. The organic CAS represents food quality without the use of synthetic chemicals to protect consumers' health. As a consequence, production costs and consumer prices have increased. The aim of this study is to analyze consumers' perception of the organic CAS label and how purchasing behavior for the product is influenced by converting the Fresh Milk Logo to the organic CAS certified label, which is based on contingent behavior analysis. This study adopts factor analysis to extract the perceptions and preferences of the respondents. In addition, the on-site Poisson model to revise the truncated and endogenous stratification and assess consumer welfare precision. The incremental consumer surplus for consumer consumed organic fresh milk is approximately NT$8,979. 展开更多
关键词 food quality organic food contingent behavior consumer surplus
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Evaluation of Productive and Reproductive Performance of Cattle in Major Dairy Production Area of Chitwan and Nawalparasi Districts
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作者 Uddhav Paneru Mohan Sharma +1 位作者 Mana Raj Kolachhapati Bhola Shanker Shrestha 《Journal of Agricultural Science and Technology(A)》 2016年第1期69-75,共7页
A study was conducted to determine the productive and reproductive performance of dairy cattle reared in major milk production area of Chitwan and Nawalparasi districts of Nepal, as well as determining the existing br... A study was conducted to determine the productive and reproductive performance of dairy cattle reared in major milk production area of Chitwan and Nawalparasi districts of Nepal, as well as determining the existing breeding scenario of cattle farming followed by the farmers. Five years (2008-2012) of data of 728 animals send to data management section of Animal Breeding Division under Dairy Cattle Improvement Program (DCIP) were reviewed and analyzed. Similarly, monthly data recording of 380 animals taken from May 2012 to April 2013 were used to find out information on reproductive performance. Average milk yield, fat percentage and protein percentage of cattle were observed 2,841 ± 84.9 kg, 4.43% ± 0.66% and 3.33% ± 0.17%, respectively, and the effect of genotype on these traits were found significant (P 〈 0.001, P 〈 0.001 and P 〈 0.01, respectively). Similarly, calving interval, post-partum estrus, post-partum conception, age at maturity and age at first calving were observed 422.8 ± 7.71, 108.9 ± 6.13, 149.93 ± 27.04, 498.5 ±16.32 and 816.0 ±17.41 d, respectively. Age at maturity and age at first calving was significantly affected by genotype (P 〈 0.01 and P 〈 0.05, respectively). In addition, average number of insemination per conception was observed 1.38 in survey household. Breeding system of cattle followed by the farmers were found satisfactory in the survey household, but needs gradual improvement. It was observed that offspring of cross between Jersey and Holstein performed better in terms of productive and reproductive performance in farmer managed condition of Nepal. Therefore, rotational crossing of Jersey and Holstein for maintaining blood level between 50% and 75% of either breed is the best choice for farmers. 展开更多
关键词 Calving interval post-partum estrus rotational crossing blood level.
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STRUCTURAL CHARACTERISTICS OF THE NIUSHOU MOUNTAIN AND ITS GEOLOGICAL SIGNIFICANCE IN CENTRAL NINGXIA
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作者 ZHANGJin MAZongjin +1 位作者 RENWenjun LEIYongliang 《Geotectonica et Metallogenia》 2003年第1期165-178,共14页
There are many thrust-related structures occurring in the Paleozoic strata of the Niushou Mountain in the central part of Ningxia Hui Autonomous Region. The fault-related folds are the typical structures in this area.... There are many thrust-related structures occurring in the Paleozoic strata of the Niushou Mountain in the central part of Ningxia Hui Autonomous Region. The fault-related folds are the typical structures in this area. Based on the analysis about these structures and their relationships, the processes by which these structures of the Miboshan Formation were formed are reconstructed, and the strata underwent about three stages of deformation: (1) horizontal shortening, (2) folding, and (3) thrusting. And the fact that the Niushou Mountain is the leading edge of an old thrust sheet was proved, the Niushou Mountain,the Daluo Mountain and the Xiaoluo Mountain together constitute the front part of this old thrust zone,so the Niushou Mountain and the Ordovician strata in the central and southern parts of Ningxia now are likely allochthons. In the period from middle Ordovician to Devonian, the areas of the central and southern Ningxia belonged to the back-arc foreland basin of North Qilianshan orogen, which was adjacent to the continent in the north. In the later part of the early Paleozoic period, the Niushou Mountain was formed after the closure of the back-arc foreland basin. 展开更多
关键词 the Niushou Mountain fault-related fold CLEAVAGE
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Application of Interval Newton Method to Solve Nonlinear Equations and Global Optimization
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作者 LI Shuang XU Caijun WANG Xinzhou 《Geo-Spatial Information Science》 2003年第1期24-27,33,共5页
The basic principle of interval arithmetic and the basic algorithm of the interval Newton methods are introduced.The prototype algorithm can not find any zero in an interval that has zero sometimes,that is,it is insta... The basic principle of interval arithmetic and the basic algorithm of the interval Newton methods are introduced.The prototype algorithm can not find any zero in an interval that has zero sometimes,that is,it is instable.So the prototype relaxation procedure is improved in this paper.Additionally,an immediate test of the existence of a solution following branch_and_bound is proposed,which avoids unwanted computations in those intervals that have no solution.The numerical results demonstrat that the improved interval Newton method is superior to prototype algorithm in terms of solution quality,stability and convergent speed. 展开更多
关键词 interval algorithm interval Newton method global optimization
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Current Status and Possibilities for the Development of the Cattle Breeding in the Primorsko-Goranska County, Croatia
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作者 P. Mijic M. Domacinovic +3 位作者 I. Klaric T. Bobic M. Brajan D. Sostaric 《Journal of Agricultural Science and Technology(A)》 2011年第4X期536-543,共8页
At the beginning of 2007 the milk producers from the Primorsko-Goranska County, Croatia were surveyed through the questionnaire in order to determine the current status and to highlight the obstacles and the possibili... At the beginning of 2007 the milk producers from the Primorsko-Goranska County, Croatia were surveyed through the questionnaire in order to determine the current status and to highlight the obstacles and the possibilities of development and improvement of the modern cattle breeding in the observed county. The questionnaire spanned 2001-2006 period and all together 260 families were surveyed. The survey aimed to get as much as possible useful information's that would indicate the problems concerning the cattle breeding in this area, and based on these results to give certain guidelines for the improvement of current status. Older age of milk producers, significant number of family farms with small number of cattle and difficult implementation of selective measures due to the small number of cows that were included in milk recording were determined as the main obstacles for cattle breeding development in the surveyed county. It is emphasized that development of cattle breeding in this area should be based upon the increased number of family farms with larger herds. To achieve this it is necessary to retain existing cattle fund, and then gradually increase the number of cattle in the mentioned county through the import of breeding heifers of dairy and combined breeds from neighboring European countries with highly developed cattle breeding. Besides that, it is also necessary to create conditions for the production of organic (ecological) products, which represents the future of agriculture, livestock production, bearing in mind that demanding European market has recognized the value of organic agricultural products that were produced through environment friendly production, which Primorsko-Goranska County, due to its significant natural resources, could easily assure. 展开更多
关键词 CATTLE Primorsko-Goranska County STATUS PERSPECTIVE Croatia.
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Production of Afyon Kaymak with Traditional and Technological Methods
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作者 Goekhan Akarca Oktay Tomar Abdullah Caglar 《Journal of Food Science and Engineering》 2014年第3期115-119,共5页
Afyon kaymak (milk cream) is traditional dairy product of Afyon city which is a junction of Middle Anatolia and West regions and famous with its thermal tourism and food. However, challenging production process led ... Afyon kaymak (milk cream) is traditional dairy product of Afyon city which is a junction of Middle Anatolia and West regions and famous with its thermal tourism and food. However, challenging production process led Afyon kaymak to lose its importance. Although buffalo milk is the raw material of traditional Afyon kaymak, cow milk can be used in the process. In traditional process, milk is filtered with double cheesecloth and gotten into the aluminum or tinned copper milk cream pans where the capacity is around 2.5-3.0 L. Half-filled pan is warm up to 90-95 ℃. Pans are carried to cool area and left until cooling. Milk cream on the cooled pans is lined by a pin. New milked milk is added to the lined cream in pans and heated for 1 h. It is covered and held on for 6-8 h. While holding period is until afternoon on summer time, it is until mid-afternoon in winter. Covers of the pans are opened and hold in a cool place until morning and, thus, cream is chilled. Cream layer on pans are lined and gotten out of pans. Cream as circle is divided into four pieces and left to the cream package after turning down. In this review, we aimed to give some information about Afyon kaymak and its production methods. 展开更多
关键词 Milk cream kaymak BUFFALO Afyon COW quality.
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Lowland pasture in Himalayan highland: edaphic properties and species composition
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作者 Kesang WANGCHUK 《Journal of Mountain Science》 SCIE CSCD 2016年第3期455-464,共10页
This study was carried out to investigate if on-farm cut vs. grazed pastures differed in soil fertility and species density in the temperate Himalayan highlands. Soil fertility and species relative density were measur... This study was carried out to investigate if on-farm cut vs. grazed pastures differed in soil fertility and species density in the temperate Himalayan highlands. Soil fertility and species relative density were measured from 30 cut and 30 grazed dairy pastures. In both types of pasture, soil pH, available phosphorus(P) and exchangeable potassium(K) were negatively correlated with pasture age and slope. In cut pasture, cocksfoot(Dactylis glomerata) and pasture age were positively correlated,whereas in grazed pasture, they were negatively correlated. In grazed pasture, unsown species and pasture age were positively correlated. Soil available P was significantly greater in cut pastures whereas soil exchangeable K was significantly greater in grazed pastures. In terms of species density, cut pasture had greater densities of cocksfoot and Italian ryegrass(Lolium multiflorum), whereas grazed pasture showed greater densities of white clover, sedge and local grass. Our study suggests that, if there are no improvements in the current method of pasture management, the cut pastures in the future are likely to have a simple pasture mixture constituting only cocksfoot and white clover(Trifolium repens).Whereas in grazed pasture, the pasture mixture is likely to be comprised of white clover and unsown species such as sedge, local grass and broadleaf weeds. 展开更多
关键词 Cutting Grazing Pasture age Soil fertility Species relative density
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Does landscape structure contribute to the long-term maintenance of generalized parasitism in cowbirds?
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作者 Scott ROBINSON Wendy SCHELSKY Jeffrey HOOVER 《Chinese Birds》 CSCD 2013年第1期31-38,共8页
Unlike most brood parasites, several species of cowbird (Molothrus) are generalists that parasitize multiple host species across their range and within the same communities; likewise, there is little evidence that ind... Unlike most brood parasites, several species of cowbird (Molothrus) are generalists that parasitize multiple host species across their range and within the same communities; likewise, there is little evidence that individuals within a population specialize on host species. This situation has variously been attributed to the recency of cowbird evolution (the 'evolutionary lag' hypothesis) or to hidden costs of rejection by hosts (the 'equilibrium' hypothesis). Both hypotheses have some support as cowbirds are indeed a relatively young clade compared with more specialized cuckoos and cowbirds are capable of sophisticated behaviors such as retaliation against rejection ('mafia' behaviors) that would select for acceptance of cowbird eggs. Nevertheless, many species in the Americas have evolved specialized defenses against cowbird parasitism, almost all of which live in more open habitats (e. g., grasslands, shrublands, riparian strips), which indicates that coevolutionary processes can operate in ways that select for host defenses in spite of cowbird counterdefenses. We propose that the structure of landscapes in North America may explain why forest-nesting birds lack defenses against parasitism and reinforce the long-term maintenance of generalized brood parasitism in cowbirds. Because cowbirds require open habitats in which to feed, they are rare or absent in large forest tracts, which dominate much of the forest cover of the Americas. These tracts act as 'source habitats' that produce surplus young that recolonize populations in smaller, fragmented forest patches in which rates of both cowbird parasitism and nest predation are very high ('sink' habitats). Evolution of antiparasite adaptations would be very slow in this situation because most hosts are produced in areas where there is little or no cowbird parasitism. In addition, the interplay of host breeding dispersal, source-sink metapopulation dynamics, and fragmented forest habitat could further deter the evolution of host defenses against parasitism. Therefore, as long as large forest tracts remain widespread in North America, most forest birds will likely continue to lack defenses against cowbird parasitism, guaranteeing a steady supply of na ve hosts in forest habitats, even in fragmented landscapes. This situation will, in turn, favor host generalist cowbirds that actively avoid more open habitats in favor of parasitizing forest bird communities. These forest communities may also act as source populations for cowbirds, which might pump surplus generalist cowbirds into more open habitats further slowing the coevolutionary process. As long as large forest tracts are a common part of the landscape, generalist parasitism may persist indefinitely. 展开更多
关键词 brood parasitism COWBIRD forest fragmentation geographic mosaic theory of coevolution nest predation source-sink metapopulation dynamics
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