oalib

Publish in OALib Journal

ISSN: 2333-9721

APC: Only $99

Submit

Any time

2020 ( 6 )

2019 ( 409 )

2018 ( 635 )

2017 ( 637 )

Custom range...

Search Results: 1 - 10 of 448482 matches for " L. J. Lourens "
All listed articles are free for downloading (OA Articles)
Page 1 /448482
Display every page Item
The effect of unrestricted milk feeding on the growth and health of Jersey calves
J. L. Uys,D. C. Lourens,P. N. Thompson
Journal of the South African Veterinary Association , 2012, DOI: 10.4102/jsava.v82i1.33
Abstract: This study was conducted to determine the effect of feeding high milk volumes on the growth rate, health and cross-sucking behaviour in group-fed Jersey calves. Three-day-old heifers (n = 120) in a seasonal calving dairy herd were randomly assigned to one of 6 treatment groups. Three groups received high milk volumes (HMV), consisting of ad libitum milk or milk replacer feeding twice a day, while 3 groups received restricted milk volumes (RMV), consisting of 2 twice daily, during the pre-weaning period. After a pre-weaning period during which feeding was reduced to once daily, all calves were weaned at 42 days and monitored until 60 days of age.Adjusting for birth mass, birth date,damparity and sire, average daily mass gain (ADG), both pre-weaning (days 0–42) and overall (days 0–60), was higher inHMVthan inRMVcalves (P<0.001).After weaning, growth rates showed no differences and at 60 days of age the HMV calves maintained a 6.74 kg advantage in mean body mass (P < 0.001). The mean intake of dry starter feed was higher in RMV than in HMVcalves. Overall feed conversion rate ofHMVcalves was 9.6%better thanRMVcalves. However, the variable cost per kg mass gain was 12 % higher for HMV calves. In the RMV groups 75 % of calves showed cross-sucking behaviour pre-weaning and 18 % post-weaning, whereas in HMV calves the proportions were 2 % and 7 %, respectively. There was no significant effect of milk volume on the incidence of diarrhoea.We conclude that the feeding of high volumes of milk to Jersey calves has a positive effect on growth rate, without compromising health or reducing solid feed intake after weaning. However, the higher cost of such a feeding system may limit its implementation.
Simulating sub-Milankovitch climate variations associated with vegetation dynamics
E. Tuenter, S. L. Weber, F. J. Hilgen,L. J. Lourens
Climate of the Past (CP) & Discussions (CPD) , 2007,
Abstract: Climate variability at sub-Milankovitch periods (between 2 and 15 kyr) is studied in a set of transient simulations with a coupled atmosphere/ocean/vegetation model of intermediate complexity (CLIMBER-2). Focus is on the region influenced by the African and Asian summer monsoon. Pronounced variations at periods of about 10 kyr (Asia and Africa) and about 5 kyr (Asia) are found in the monsoonal runoff in response to the precessional forcing. In the model this is due to the following mechanism. For low summer insolation (precession maximum) precipitation is low and desert expands at the expense of grass, while for high insolation (precession minimum) precipitation is high and the tree fraction increases also reducing the grass fraction. This induces sub-Milankovitch variations in the grass fraction and associated variations in the water holding capacity of the soil. The runoff does not exhibit sub-Milankovitch variability when vegetation is kept fixed. High-latitude vegetation also exhibits sub-Milankovitch variability under both obliquity and precessional forcing. We thus hypothesize that sub-Milankovitch variability can occur due to the dynamic response of the vegetation. However, this mechanism should be further tested with more sophisticated climate/vegetation models.
Simulating sub-Milankovitch climate variations associated with vegetation dynamics
E. Tuenter,S. L. Weber,F. J. Hilgen,L. J. Lourens
Climate of the Past (CP) & Discussions (CPD) , 2007,
Abstract: Climate variability at sub-Milankovitch periods (between 2 and 15 kyr) is studied in a set of transient simulations with a coupled atmosphere/ocean/vegetation model of intermediate complexity (CLIMBER-2). Focus is on the region influenced by the African and Asian summer monsoon. Pronounced variations at periods of about 10 kyr (Asia and Africa) and about 5 kyr (Asia) are found in the monsoonal runoff in response to the precessional forcing. In the model this is due to the following mechanism. For low summer insolation (precession maximum) precipitation is low and desert expands at the expense of grass, while for high insolation (precession minimum) precipitation is high and the tree fraction increases also reducing the grass fraction. This induces sub-Milankovitch variations in the grass fraction and associated variations in the water holding capacity of the soil. The runoff does not exhibit sub-Milankovitch variability when vegetation is kept fixed. High-latitude vegetation also exhibits sub-Milankovitch variability under both obliquity and precessional forcing. We thus hypothesize that sub-Milankovitch variability can occur due to the dynamic response of the vegetation. However, this mechanism should be further tested with more sophisticated climate/vegetation models.
High Arabian Sea productivity conditions during MIS 13 – odd monsoon event or intensified overturning circulation at the end of the Mid-Pleistocene transition?
M. Ziegler, L. J. Lourens, E. Tuenter,G.-J. Reichart
Climate of the Past (CP) & Discussions (CPD) , 2010,
Abstract: Marine isotope stage (MIS) 13 (~500 000 years ago) has been recognized as atypical in many paleoclimate records and, in particular, it has been connected to an exceptionally strong summer monsoon throughout the Northern Hemisphere. Here, we present a multi-proxy study of a sediment core taken from the Murray Ridge at an intermediate water depth in the northern Arabian Sea that covers the last 750 000 years. Our results indicate that primary productivity conditions were anomalously high during MIS 13 in the Arabian Sea and led to extreme carbonate dissolution and glauconitization in the deep-sea sediments. These observations could be explained by increased wind driven upwelling of nutrient-rich deep waters and, hence, by the occurrence of an exceptionally strong summer monsoon event during MIS 13, as it was suggested in earlier studies. However, ice core records from Antarctica demonstrate that atmospheric methane concentrations, which are linked to the extent of tropical wetlands, were relatively low during this period. This constitutes a strong argument against an extremely enhanced global monsoon circulation during MIS 13 which, moreover, is in contrast with results of transient climate modelling experiments. As an alternative solution for the aberrant conditions in the Arabian Sea record, we propose that the high primary productivity was probably related to the onset of an intensive meridional overturning circulation in the Atlantic Ocean at the end of the Mid-Pleistocene transition. This may have led to an increased supply of nutrient-rich deep waters into the Indian Ocean euphotic zone, thereby triggering the observed productivity maximum.
Monsoonal response to mid-holocene orbital forcing in a high resolution GCM
J. H. C. Bosmans,S. S. Drijfhout,E. Tuenter,L. J. Lourens
Climate of the Past Discussions , 2011, DOI: 10.5194/cpd-7-3609-2011
Abstract: In this study we use a sophisticated high-resolution atmosphere-ocean coupled climate model, EC-Earth, to investigate the effect of Mid-Holocene orbital forcing on summer monsoons on both hemispheres. During the Mid-Holocene (6 ka), there was more summer insolation on the Northern Hemisphere than today, which intensified the meridional temperature and pressure gradients. Over North Africa, monsoonal precipitation is intensified through increased landward monsoon winds and moisture advection as well as decreased moisture convergence over the oceans and more convergence over land compared to the pre-industrial simulation. Precipitation also extends further north as the ITCZ shifts northward in response to the stronger poleward gradient of insolation. This increase and poleward extent is stronger than in most previous ocean-atmosphere GCM simulations. In north-westernmost Africa, precipitation extends up to 35° N. Over tropical Africa, internal feedbacks completely overcome the direct warming effect of increased insolation. We also find a weakened African Easterly Jet. Over Asia, monsoonal precipitation during the Mid-Holocene is increased as well, but the response is different than over North-Africa. There is more convection over land at the expense of convection over the ocean but precipitation does not extend further northward, monsoon winds over the ocean are weaker and the surrounding ocean does not provide more moisture. On the Southern Hemisphere, summer insolation and the poleward insolation gradient were weaker during the Mid-Holocene, resulting in a reduced South American monsoon through decreased monsoon winds and less convection, as well as an equatorward shift in the ITCZ. This study corroborates the findings of paleodata research as well as previous model studies, while giving a more detailed account of Mid-Holocene monsoons.
Anomalously high Arabian Sea productivity conditions during MIS 13
M. Ziegler,L. J. Lourens,E. Tuenter,G.-J. Reichart
Climate of the Past Discussions , 2009,
Abstract: Marine isotope stage (MIS) 13 (~500 000 years ago) has been recognized as atypical in many paleoclimate records and, in particular, it has been connected to an exceptionally strong summer monsoon in East Asia. Here we present a multi-proxy study of a sediment core taken from the Murray Ridge at intermediate water depth in the northern Arabian Sea that covers the last 750 000 years. Our results indicate that upwelling driven primary productivity conditions were anomalously high during MIS 13 and led to extreme carbonate dissolution and glauconitization. We argue that an extreme summer monsoon circulation was probably not responsible for these aberrant conditions, because such an event does not show up in the Antarctic methane record and transient modeling results. As an alternative, we propose that high productivity was related to the onset of an intensive meridional overturning circulation in the Atlantic Ocean at the end of the Mid-Pleistocene transition. This led to an increased supply of nutrient-rich deep waters into the Indian Ocean euphotic zone, thereby triggering the observed productivity maximum.
Preven??o de doen?as sexualmente transmissíveis em mulheres: associa??o com variáveis sócio-econ?micas e demográficas
Jiménez, Ana Luisa;Gotlieb, Sabina Léa Davidson;Hardy, Ellen;Zaneveld, Lourens J. D.;
Cadernos de Saúde Pública , 2001, DOI: 10.1590/S0102-311X2001000100005
Abstract: sexually transmitted diseases (stds) have been a subject of discussion both among scientists and in the mass media, especially because of their association with the human immunodeficiency virus (hiv). we studied the adoption of specific protective behaviors for the prevention of stds among women, as well as the associations between these behaviors and socioeconomic and demographic variables. this was a descriptive study based on secondary data from a previous study carried out in campinas, s?o paulo state, brazil. a total of 635 women were selected using the social network ("snowball") technique. subjects were classified into four groups: adolescents and adults of upper middle and lower socioeconomic status, respectively. condoms were the std prevention method most frequently mentioned by interviewees. a negative association was observed between having a steady partner and condom use in all the groups. the main reason mentioned for not using condoms was "having a single partner and trusting him". among adolescents, a positive association was observed between schooling above the 8th grade and condom use, and a negative association was observed between age and condom use. among adults, only condom use in general was also positively associated with socioeconomic status.
Preven o de doen as sexualmente transmissíveis em mulheres: associa o com variáveis sócio-econ micas e demográficas
Jiménez Ana Luisa,Gotlieb Sabina Léa Davidson,Hardy Ellen,Zaneveld Lourens J. D.
Cadernos de Saúde Pública , 2001,
Abstract: As doen as sexualmente transmissíveis (DST) têm sido debatidas no ambiente científico e nos meios de comunica o de massa, em especial, por sua associa o a maior risco de infec o pelo Vírus da Imunodeficiência Humana (HIV). Estudou-se a ado o de comportamentos por mulheres para prote o das DST, tal como a associa o destes a variáveis sócio-econ micas e demográficas. Trata-se de estudo descritivo, com dados secundários de pesquisa feita em Campinas, S o Paulo, na qual foram entrevistadas 635 mulheres selecionadas mediante a técnica de amostragem "bola de neve". Foram classificadas em: adolescentes e adultas de status sócio-econ mico médio-alto ou baixo. Grande propor o delas n o se prevenia das DST, em particular, as de status baixo. Em todos os grupos, o condom masculino foi o método de preven o mais referido. Houve associa o negativa entre parceiro fixo e uso de condom, e a principal raz o para n o usá-lo foi "só ter um parceiro e confiar nele". Em meio às adolescentes, ocorreu associa o positiva entre escolaridade acima da oitava série e uso de condom, bem como negativa entre idade e uso desse método. Entre adultas o uso exclusivo de condom esteve, em geral, positivamente associado a status sócio-econ mico.
Reconstruction of a continuous high-resolution CO2 record over the past 20 million years
R. S. W. van de Wal,B. de Boer,L. J. Lourens,P. K?hler
Climate of the Past (CP) & Discussions (CPD) , 2011, DOI: 10.5194/cp-7-1459-2011
Abstract: The gradual cooling of the climate during the Cenozoic has generally been attributed to a decrease in CO2 concentration in the atmosphere. The lack of transient climate models and, in particular, the lack of high-resolution proxy records of CO2, beyond the ice-core record prohibit, however, a full understanding of, for example, the inception of the Northern Hemisphere glaciation and mid-Pleistocene transition. Here we elaborate on an inverse modelling technique to reconstruct a continuous CO2 series over the past 20 million year (Myr), by decomposing the global deep-sea benthic δ18O record into a mutually consistent temperature and sea level record, using a set of 1-D models of the major Northern and Southern Hemisphere ice sheets. We subsequently compared the modelled temperature record with ice core and proxy-derived CO2 data to create a continuous CO2 reconstruction over the past 20 Myr. Results show a gradual decline from 450 ppmv around 15 Myr ago to 225 ppmv for mean conditions of the glacial-interglacial cycles of the last 1 Myr, coinciding with a gradual cooling of the global surface temperature of 10 K. Between 13 to 3 Myr ago there is no long-term sea level variation caused by ice-volume changes. We find no evidence of change in the long-term relation between temperature change and CO2, other than the effect following the saturation of the absorption bands for CO2. The reconstructed CO2 record shows that the Northern Hemisphere glaciation starts once the long-term average CO2 concentration drops below 265 ppmv after a period of strong decrease in CO2. Finally, only a small long-term decline of 23 ppmv is found during the mid-Pleistocene transition, constraining theories on this major transition in the climate system. The approach is not accurate enough to revise current ideas about climate sensitivity.
Rapid shifts in South American montane climates driven by pCO2 and ice volume changes over the last two glacial cycles
M. H. M. Groot,R. G. Bogotá,L. J. Lourens,H. Hooghiemstra
Climate of the Past Discussions , 2010, DOI: 10.5194/cpd-6-2117-2010
Abstract: Tropical montane biome migration patterns in the northern Andes are found to be coupled to glacial-induced mean annual temperature (MAT) changes; however, the accuracy and resolution of current records are insufficient to fully explore their magnitude and rates of change. Here we present a ~60-year resolution pollen record over the past 284 000 years from Lake Fúquene (5° N) in Colombia. This record shows rapid and extreme MAT changes at 2540 m elevation of up to 10 ± 2 °C within a few hundred of years that concur with the ~100 and 41-kyr (obliquity) paced glacial cycles and North Atlantic abrupt climatic events as documented in ice cores and marine sediments. Using transient climate modelling experiments we demonstrate that insolation-controlled ice volume and greenhouse gasses are the major forcing agents causing the orbital MAT changes, but that the model simulations significantly underestimate changes in lapse rates and local hydrology and vegetation feedbacks within the studied region due to its low spatial resolution.
Page 1 /448482
Display every page Item


Home
Copyright © 2008-2017 Open Access Library. All rights reserved.