Given a limit to crop yield improvement, Waggoner (1994) observed that a logistic model was more suitable Soybean [Glycine max (L.) Merr.] yields in the USA have risen for crop yield vs. time data 22.6 kg ha Ϫ1 yr Ϫ1 from 1924 to 1997, but in the last quarter century (1972-1997) have risen 40% faster, 31.4 kg ha Ϫ1 yr Ϫ1 . This upwardtrend in on-farm yield is fueled by rapid producer adoption of technologies emerging from agricultural research. Published estimates of the The logistic response curve is sigmoid because the expoannual gain in yield attributable to genetic improvement averaged nential change in yield (Y) over time (T) is modulated about 15 kg ha Ϫ1 yr Ϫ1 prior to the 1980s, but is now averaging about by the degree to which present yield differs from its 30 kg ha Ϫ1 yr Ϫ1 in both the public and proprietary sectors. Periodic limit (K). The inflection point (Tm) separates the posiadvances in agronomic technology, and a relentless rise in atmospheric tive and negative exponential phases of the sigmoid.CO 2 (currently 1.5 LL Ϫ1 yr Ϫ1 ), also contribute to the upward trend Wagoner (1994) used average U.S. corn (Zea mays L.) in on-farm yield. In Nebraska, irrigated yield averages 800 kg ha Ϫ1 yields from 1940 to 1992, and set K to 21 Mg ha Ϫ1 (the more than rainfed yield, and is improving at a 40% faster annual rate highest grain yield recorded in U.S. corn production), (35.1 vs. 24.9 kg ha Ϫ1 ). About 36% of the annual variation in the to show that U.S. corn yield was rising at a logistic rate irrigated-rainfed yield difference is attributable to annual variation in absolute rainfed yield. Inadequate water obviously limits absolute of 3.6% yr Ϫ1 . crop yield, but also seems to be an obstacle in terms of the rate of Attaining the theoretical yield maximum on each U.S. yield improvement. Several physiological traits changed during six farm each year for each crop would unquestionably redecades of cultivar releases in Ontario that led to a genetic gain in yield quire optimization of every yield-affecting biotic or abiof about 0.5% yr Ϫ1 . Changes in some traits were obvious (improved otic factor in every production environment. The degree lodging), but more subtle in others (greater N 2 -fixation, greater stressto which such multiple-factor optimization is possible, tolerance). In terms of photosynthate supplied to sinks across a wide or economically feasible, will determine what fraction range of environments, recent cultivars seem to be superior to obsolete of the crop's yield potential that can actually be realized ones. To sustain and enhance soybean yield improvement in the future, on the average U.S. farm on a year-to-year basis. In any technological innovation must be continually injected into the agriculevent, mitigation of the yield-limiting factors will require tural enterprise.

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Soybean [Glycine max (L.) Merr.] production in short‐season areas has increased greatly because of improvements in cultivars and production practices. Previous studies of genetic improvement in soybean have reported yield increases of 0.5 to 1% per year. To evaluate the genetic improvement of short‐season germplasm, 41 cultivars ranging from maturity group 000 to 0, released from 1934 to 1992, were grown for 8 station‐years in Ontario and Quebec. Yield, maturity, plant height, lodging score, 100‐seed weight, seed protein and oil levels, and yield stability were regressed on year of release to determine if improvements have been made. Yield has been improved about 0.5% per year during the period under study; however, since 1976 yield has been improved about 0.7% per year. There is evidence that the rate of genetic improvement of seed yield is accelerating. Significant lodging reduction was also observed. Seed protein levels have been reduced 4 g kg‐1 yr‐1 and seed oil levels have increased 4 g kg‐1 yr‐1. Although yield has been increased, yield stability has remained constant.

South Africa (SA) is undergoing a rapid epidemiological transition and has the highest prevalence of obesity in subSaharan Africa (SSA), with black women being the most affected (obesity prevalence 31.8%). Although genetic factors are important, socio-cultural, environmental and behavioural factors, as well as the influence of socio-economic status, more likely explain the high prevalence of obesity in black SA women. This review examines these determinants in black SA women, and compares them to their white counterparts, black SA men, and where appropriate, to women from SSA. Specifically this review focuses on environmental factors influencing obesity, the influence of urbanisation, as well as the interaction with socio-cultural and socio-economic factors. In addition, the role of maternal and early life factors and cultural aspects relating to body image are discussed. This information can be used to guide public health interventions aimed at reducing obesity in black SA women.

Genetic improvement of short‐season soybean [Glycine max (L.) Merr.] cultivars has resulted in a 0.5% annual gain in yield. Although yield is the product of dry matter (DM) accumulation and partitioning, the relative contributions of these two components of yield to genetic improvement has not been documented. Furthermore, the mechanism by which higher DM accumulation or harvest index (HI) is accomplished in the newer cultivars is unclear. The objective of the current study was to characterize DM accumulation and partitioning in cultivars which differ in yield potential, and determine the role of these traits in yield improvement. Two older (low yield potential) and two newer (higher yield potential) soybean cultivars of similar maturity were grown in side‐by‐side trials in 1996 and 1997. Plant samples were taken during each growing season and separated into leaves, stems + petioles, roots, and seeds. Dry matter accumulation and leaf area indices were measured. Seed yield of the new cultivars was 30% greater than their older counterparts. Increased DM accumulation contributed 78% and increased HI contributed 22% towards the genetic gain in yield. Total plant dry weight increased to a maximum around R4/R5 and subsequently declined during the seed‐filling period (SFP) as pod development increased and leaf senescence began. This decline in dry weight during the SFP was greater for the old than for the new cultivars. The newer cultivars maintained leaf area further into the SFP than the old cultivars enabling continued dry matter accumulation. The results of this experiment indicate that genetic yield improvement in the short‐season soybean cultivars examined was mainly associated with longer leaf area duration and the subsequently greater DM accumulation.