Academic research and HIV/AIDS in South Africa

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Relative risk of HIV infection among young men and women in a South African township

The prevalence of HIV infection in Africa is substantially higher among young women than it is among young men. Biological explanations of this difference have been presented but there has been little exploration of social factors. In this paper we use data from Carletonville, South Africa to explore various social explanations for greater female infection rates. This paper reports on data from a random sample of 507 people between 13 and 24 years old. Subjects were tested for HIV, as well as other sexually transmitted infections (STIs), and answered a behavioural questionnaire. The age-prevalence of HIV infection differs between men and women with considerably higher rates of increase with age among young women. The age of sexual debut did not differ significantly between men and women (15.9 and 16.3 years, respectively) and below the age of 20 years there was no difference in the number or distribution of the number of sexual partners reported by men or women. The risk of infection per partnership was substantially higher among women than among men. Women have sexual partners who are, on average, about five years older than they are with some variation with age. Scaling the age-prevalence curve for men by the age of their sexual partners gives a curve whose shape is indistinguishable from that for women but is about 30% lower for men than for women. In terms of social explanations for HIV rates among women, the data indicates that this difference can be explained by the relative age of sexual partners, but not by other factors explored. In addressing the epidemic among young women it will be essential to deal with the social factors that lead young women to select their partners from older-age cohorts and that shape their sexual networking patterns

Yield and protein of wheat and durum in Brown Soil Zone as affected by long-term tillage system and crop rotation

Non-Peer ReviewedThree tillage-rotation experiments in the Brown soil zone showed that spring wheat and durum grain yield and protein were affected by tillage system, length of time in tillage system, crop sequence. During the initial 15 years of no-tillage (NT) monoculture wheat, grain yield and protein were equal or lower than minimum-till (MT) or conventional tillage (CT). These differences were attributed to reduced N availability with NT, likely from greater N immobilization (sequestration) in soil organic matter under NT. However, during the last several years, grain yield and protein with NT continuous wheat has been equal to higher than with MT practices. This may be due to improved management, specifically better control of foxtail barley and side banding of N at time of seeding, and/or simply a longer time in NT. When following non-cereal crop, spring wheat and durum had equal or higher yield and protein than wheat following wheat. In diversified rotations, the wheat or durum had highest yield under NT practices. Wheat on MT and NT fallow, with an extra 23 kg/ha fertilizer N applied, had higher grain protein than wheat on CT fallow but not higher grain yield. Durum grown on fallow after a pulse crop had higher yield and protein than that grown after fallow after durum

A simple model for quantifying change in soil organic C as influenced by tillage and crop rotations on the Canadian prairies

Non-Peer ReviewedSimulation models are required for quantifying the impact of crop rotations and tillage on soil organic C dynamics, and for aggregating C sequestration over a relatively large area. However, most current models of soil organic C have been built based on kinetically defined discrete pools with different turnover times. Those pools of soil organic C only exist conceptually. They have not been determined experimentally, thus validation of kinetic models describing soil organic C turnover is usually difficult or not independent from actual measurements. Thus, there is a need to develop a simulation model that can be easily validated and used for estimating future projection of C sequestration under specified management practices. A simple model has been developed to quantify the impact of crop rotations and tillage on soil organic C and validated using long-term field experiments conducted on the Canadian prairies. This simple model required a few input parameters and accurately predicted the change of soil organic C with a relative error of 5% or better. Crop rotation in cereal-dominant cropping systems, affected the amount of soil organic C due to differences in the amount of crop residue inputs. Clay content of soil played a vital role in determining the soil organic C sequestered under conservation tillage compared to tilled systems. This study also showed that the rate constant of soil organic C turnover was about the same for all systems in the drier region of the Canadian prairies, regardless of soil texture and the cropping system

Economic and risk considerations of nitrogen fertilizer use in the Brown Soil Zone

Non-Peer ReviewedOptimum use of nitrogen fertilizer requires consideration of factors that influence plant response and those that govern the decisions of producers. The response of spring wheat to soil moisture and N fertilizer was assessed in a 9-yr zero tillage study conducted on a medium texture soil at Swift Current, Saskatchewan. These data were used to assess the economic merit and risk considerations of alternate N fertilizer management systems when combined with snow-trapping to enhance soil moisture reserves. The N fertilizer system included rates from 0 to 100 kg ha-1, spring versus fall applications and deep-banding versus surface broadcasting. The results showed that the optimum rates of fertilizer N (FN) varied directly with spring soil moisture reserves (SM) and the probability distribution for 1 May to 31 July precipitation, and inversely with soil N (SN), the ratio of FN cost to wheat price, and the level of risk aversion held by producers. The optimum FN rates were highest for spring- and fall-banding; they were 3 to 14 kg ha-1 lower for spring broadcasting and 7 to 22 kg ha-1 lower with fall broadcasting. The optimum rates increased 3.7 to 5.7 kg N ha-1 for each 10 mm increase in SM, with the higher rates associated with high SN. The FN rates declined 5 kg ha-1 for each additional year that the land was cropped continuously. For producers seeking to maximize expected profit or those with low risk aversion, the optimum FN rates were considerably higher than those recommended by the Saskatchewan Soil Testing Laboratory (SSTL) . In contrast, the FN rates for producers with high risk aversion were generally lower than those of the SSTL. The SSTL recommended rates were most appropriate for producers with medium risk aversion. The study found no single combination of timing and method of N fertilizer placement to be superior in all cases. Spring- and fall-banding provided higher net margins than broadcasting N fertilizer when SM or wheat prices were high, or if banding fertilizer can be combined with a tillage operation for weed control. The economic benefit from snow-trapping averaged $9 to$32 ha-1 depending on FN rate and wheat price; however, little benefit or a small loss was incurred in some years when infiltration of melt water was low or winter snowfall was minimal

Water use efficiency and precipitation use efficiency of crops in the semiarid prairie

Non-Peer ReviewedThe importance of water use efficiency (WUE) in crop production, in the semiarid prairie, is based on the fact that the available water is the most limiting factor influencing crop production. This poster compares water use efficiency of four crop rotations from the Swift Current, SK, long-term rotation experiment: fallow-wheat-wheat (F-W-W), F-flax-W (F-Flx-W), continuous wheat (Cont W) and wheat-lentil (W-Lent). We found that, the WUE of flax and lentil averaged 50% and 64%, respectively, of wheat following wheat. The precipitation required per unit of produce from the complete cropping system (PUE) increased with cropping intensity on a yield basis (kg ha-1 mm-1): Cont W (4.8) > W-Lent (4.2) > F-W-W (4.1) > F-Flx-W (2.9) (opposite response to WUE) and when PUE was calculated on a dollars produced per rotation basis ($ ha-1 mm-1): W-Lent (1.0) was higher than the other two rotations (0.6 to 0.7)

Effects of crop rotation and fertilization on soil carbon sequestration in the semiarid region

Non-Peer Reviewe

Relationship between soil texture and soil organic carbon at small field scale

Non-Peer ReviewedThe capacity of a soil to store organic carbon is related to its particle size distribution, or soil texture, mainly because the capacity of clay particles to stabilize organic materials. A study of the relationship of soil organic carbon (SOC) and particle size distribution at field level in two soils of Saskatchewan indicated that making broad assumptions about the relationship between soil texture and SOC in soils within a filed might lead to erroneous conclusions. At field scale dominant dynamic processes affecting the spatial distribution of soil texture, or other factors may create local conditions that override the fundamental texture SOC relationship of soils

Long-term tillage and crop rotation effect on soil aggregation

Non-Peer ReviewedTillage and cropping sequences play a key role in controlling soil aggregation. We measured water-stable aggregate (WSA), wind erodible fraction (WEF), and geometric mean diameter (GMD) for six mid to longterm (8 to 25 years) experiments comparing tillage and cropping sequences in the Brown, Dark Brown, and Black Chernozemic soils of Saskatchewan. In the coarse-textured soil, no-tillage (NT) had a higher value of WSA by 49% more than in the wheat-phase of fallow-wheat (F-W), and had a lower value of WEF by 27% less than in the fallow-phase of F-W compared with minimum tillage (MT). In the medium-textured soils, NT had a higher WAS, ranged from 17 to 38%, and a lower WEF, ranged from 37 to 64% compared with conventional tillage (CT), depending on crop rotation systems. The reduced WEF under NT in the medium-textured soils was due mainly to increased GMD. In the fine-textured soils, NT had a higher WSA, ranged from 10 to 19% compared with MT or CT, and a lower WEF by 47% compared with MT only in the heavy clay soil. Change in GMD was not detectable in the light- and fine-textured soils. Continuous cropping compared with rotations containing fallow improved soil physical properties by increasing WSA, reducing WEF in the medium and fine-textured soils, and increasing GMD only in the medium-textured soils. Of the three soil physical properties determined in this study, WSA was the most sensitive to changes in tillage and crop rotations, then WEF and the least GMD

Nitrate leaching after 17 years in no-tilled crop rotations in southwestern Saskatchewan

Non-Peer Reviewe