The 28-chromosome normal hybrid was backcrossed to wheat to produce a heptaploid in the first backcross generation (BC1) and subsequently monosomic (21 11+11) and double monosomic (21 11+11+11) additions of barley chromosomes to wheat were isolated in the BC2 generation. The monosomic additions could be divided into five different phenotypic groups and disomic additions were isolated from among their progeny with a very low frequency (063 per cent). However, some monotelo-disomic additions (21 11+ it II) obtained from the progeny of selfed monosomics yielded both disomic and ditelosomic additions in their progeny with a much higher frequency. A sixth addition line was obtained independently from three unusual F1 hybrids exhibiting 221, 211 + 111 and 251 + 1111 at meiosis.Altogether six of the seven possible disomic additions and seven of the 14 ditelosomic additions of barley chromosomes to wheat have been produced. The chromosome 5 addition could not be obtained in a disomic form because the plants carrying this chromosome are self-sterile. The addition lines were initially characterized by their morphological differences from the wheat parent, and subsequently from isozyme studies and heterochromatic banding (N-banding) of chromosomes. Most of these lines showed more asynapsis at meiosis than the wheat parent and all, except addition line 4, were less fertile than the wheat parent.

Seed dispersal, interacting with environmental disturbance and management across heterogeneous landscapes, results in irregular weed spatial distributions. Describing, predicting, and managing weed populations requires an understanding of how weeds are distributed spatially and the consequences of this distribution for population processes. Semivariograms and kriged maps of weed populations in several fields have helped describe spatial structure, but few generalizations can be drawn except that populations are aggregated at one or more scales. Limited information is available on the effect of weed arrangement, pattern, or field location on weed population processes. Because weeds are neither regular nor uniform in distribution, mean density alone is of limited value in estimating yield loss or describing population dynamics over a whole field. Sampling strategies that account for spatial distribution can increase sampling efficiency. Further research should focus on understanding processes that cause changes in spatial distributions over time to help predict rates of invasion and potential extent of colonization.

The nonuniform spatial distribution of weeds complicates sampling, modeling, and management of weed populations. Principles of a rational approach to analysis of weed spatial distribution, combining classical and spatial statistics, are presented using data for cumulative emergence of common lambsquarters in no-tillage soybean fields in 1990 and 1993. Classical statistics, dispersion indices, mean/variance relationships, and frequency histograms confirmed that raw and loge-transformed data were not normally distributed, that populations were aggregated, and that large-scale trends in population means violated assumptions of spatial statistics. Detrending was accomplished by median polishing loge-transformed data and confirmed by evaluation of standardized residuals and frequency histograms. Detrended residuals were used to construct omni-directional and uni-directional semivariograms to describe the spatial structure of the populations. A spherical model fit to the data was verified by cross validation. Semivariograms showed that common lambsquarters density was spatially autocorrelated at distances to 16 m, with more than 30% of the variance in density due to distance between field locations. Comparisons of kriged estimates and their standard deviations with and without detrending indicated that estimates using detrended data were more appropriate and more precise. Kriged estimates of common lambsquarters density were used to draw contour maps of the populations.

Accurate prediction of potential weed seedling density would allow growers to implement control measures more effectively and could help avoid inappropriate and over application of preemergence herbicides. We compared three methods for handling soil samples to predict potential weed seedling emergence in plow-disk and no-tillage corn: seedling emergence from greenhouse trays, emergence from intact cores, and seed extraction following sieving. Seedbank numbers were highest for common lambsquarters followed by annual grasses and redroot pigweed, and seed numbers were higher in no-tillage than plow-disk plots. Coefficients of variation typically exceeded 60%. Density of seedling emergence from cores was more similar to field density than was emergence from trays. The percent of seeds in the seedbank that emerged was commonly more than 90% for annual grasses and usually less than 20% for common lambsquarters. All methods gave equivalent and relatively poor predictions of field population density. Spearman rank correlation between predicted and actual populations ranged from low negative values for common lambsquarters in no-tillage to 0.60 for annual grass emergence from trays in 1991. No method consistently gave highest correlations in both years and both tillage systems. Seedling emergence from intact cores, evaluated for 4 yr in plow-disk and no-tillage soybean fields, was significantly correlated (rs= 0.15 to 0.68) with emergence in the field. Pooling data from three to five neighboring sample sites increased the correlation between core and field emergence densities.

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