Roger D. Cousens

An empirical model relating crop yield to weed and crop density and a statistical comparison with other models

SUMMARYA hyperbolic model relating crop yield to weed density is extended to include crop density as a further variable. Other models were obtained from published sources, eight being originally applied to yield of above-ground biomass and six to marketable yield. Data were obtained from a field experiment in which spring wheat and spring barley were planted either in monoculture or together and at a range of densities. Further data were obtained from a published experiment on Sinapis alba and barley grown in containers. The models were fitted to data using maximum likelihood estimation. Comparisons of residual sums of squares showed that for the wheat and barley field experiment biomass yield and marketable yield were sufficiently described by a three-parameter model. The Baeumer & de Wit (1968) equation proposed for replacement series experimental designs is considered reasonable for the analysis of field additive designs provided the parameters are interpreted in agronomic terms. For the Sinapis alba and barley experiment more complex models could be justified.

Genetic improvement and agronomy for enhanced wheat competitiveness with weeds

The rapid development of herbicide resistance in weeds, and environmental imperatives, have forced the consideration of non-chemical tactics such as crop competition for weed management. This review of wheat–weed competition examines the plant traits associated with wheat competitiveness, and the opportunities for plant breeding or manipulating crop agronomy to differentially favour the growth of the crop. Many studies have proven that enhancing crop competitive ability can reduce weed seed production and crop yield loss, although a number of difficulties in conducting this research are identified and suggestions are made for improvement. It remains to be seen whether crop competitiveness will be considered as a priority by farmers and plant breeders. Farmers require precise information on the reliability of agronomic factors such as increased crop seeding rate or choice of variety for enhancing crop competitive ability in different environments. Plant breeders need to know which plant traits to incorporate in varieties to increase competitive ability. A thorough analysis of the benefits and costs of enhancing wheat competitiveness is needed. Competitive wheat crops should be available as part of reliable and economical integrated weed management packages for farmers.

Invasiveness and comparative life-history traits of exotic and indigenous Senecio species in Australia

Abstract. A comparative ecological study of closely related invasive and non-invasive species, Senecio madagascariensis and S. lautus (Asteraceae), investigated what traits might confer invasive ability in very similar species. Life-history attributes of the weed S. madagascariensis were compared to five habitat-specific subspecies of S. lautus: S. l. alpinus, S. l. dissectifolius, S. l. lanceolatus and two forms of S. l. maritimus. Field populations of each taxon were monitored to compare their population ecology. Relative rates of phenological development were compared at a single location. Seed germination was studied in a laboratory experiment. Transplant experiments were conducted in a range of S. madagascariensis and S. lautus habitats to compare performance in different environments. In monitored field populations S. madagascariensis produced seedlings and reproductive cohorts more frequently, flowered for longer periods, produced more seeds and had fewer germinable achenes in the soil compared to S. lautus taxa. S. madagascariensis achenes had higher rates of germination than S. lautus in both light and dark conditions. S. madagascariensis was found to have higher rates of survival than S. lautus taxa in a range of habitats and to be faster to flower in both transplant and standard glasshouse environments. Overall S. madagascariensis performed better than S. lautus ecotypes in terms of seedling, growth and fecundity measurements and second best for achenes. Despite relatively good performance in terms of life-history traits there is no evidence that S. madagascariensis is invading S. lautus habitats. We speculate that physiological and morphological adaptations to specialised environments are a better explanation for success of Senecio taxa/ecotypes than generalised life-history trait performance. We suggest that invasiveness is essentially unpredictable, due to habitat/plant specific interactions between invader and area of introduction. In the absence of predictive theory, quarantine authorities should use a combination of methods to assess invasive potential including a database of known weeds, performance comparisons between congeneric natives and exotics in a range of habitats at proposed point of introduction and monitoring of introduced species to determine if they spread.

A model of the economics of controlling Alopecurus myosuroides Huds. in winter wheat

Abstract A simplified mathematical model of black-grass (Alopecurus myosuroides Huds.) reproduction, growth and competition with winter wheat has been constructed. Using the model, an attempt has been made to study the long-term economic implications of controlling black-grass infestations in winter wheat through various cultivation practices and the use of herbicides. The sensitivity of the results has been tested in respect of assumptions about the susceptibility of black-grass to herbicides, the degree of seed kill achieved by straw burning and the value of the wheat crop. The indications from the model are that, in general, the highest long-term benefits are to be attained if herbicide is applied every time the black-grass population attains 7 · 5 plants m 2 . However, a strategy of applying herbicide every year involves little economic loss compared with the optimum and reduces the risks. Equally, as long as herbicide is being used, whether or not the straw is burnt is likely to be of little economic significance, except in the extreme case of continuous direct drilling.

Reliability of higher seeding rates of wheat for increased competitiveness with weeds in low rainfall environments

SUMMARY Increasing crop competitiveness using higher seeding rates is a possible technique for weed manage- ment in low input and organic farming systems or when herbicide resistance develops in weeds. A range of wheat seeding rates were sown and resulted in crop densities between 50-400 plants/m 2 (current recommendations are 100-150 plants/m 2 ) in the presence and absence of annual ryegrass (Lolium rigidum Gaud.) in three wheat cultivars at nine experiments in southern Australia. Wheat densities of at least 200 plants/m 2

Screening bioenergy feedstock crops to mitigate invasion risk

The cultivation of plants for the production of hydrocarbon fuels is gaining global momentum. These biofuels offer potential benefits as alternatives to fossil fuels, but mitigating any environmental risks posed by large-scale cultivation of bioenergy feedstock (biofeedstock) species poses new challenges: some proposed biofuel crops have become naturalized, and even invasive; the relative risks and benefits of growing monocultures versus polycultures of biofeedstock species require more thorough examination; and prompt development of a robust ecological risk assessment framework and careful screening are needed before these biofuel species are widely cultivated. Greater collaboration between agronomists developing crops for biofeedstock production and invasion biologists could substantially lower the risk of new plant invasions.

A mathematical analysis of factors affecting the rate of spread of patches of annual weeds in an arable field

Abstract A model is presented of the spread of a weed from a point source, with dispersal being both unaided and aided by a combine harvester. Four “type” weeds were modeled, chosen to represent species of differing population and dispersal ecologies and based broadly on Avena spp. (wild oats), Raphanus raphanistrum (wild radish), Bromus spp. (bromegrass), and Fumaria spp. (fumitory). Wind-adapted species were excluded. The greatest rate of spread was predicted to be for R. raphanistrum and the least for Fumaria, regardless of whether dispersal by combine harvester was included. Rate of spread was more sensitive to dispersal parameters than to demographic parameters and increased up to 16-fold as soon as any seeds were dispersed by the harvester. Given that uptake by the harvester is a function of phenology of seed production and dispersal, better data on such processes is required for weeds. Because rate of spread is more dependent on dispersal than on demographic factors, greater attention needs to be given to describing dispersal frequency distributions and especially to analysis of their shapes. Nomenclature: Bromus spp., bromegrass; Raphanus raphanistrum L. RAPRA, wild radish; Avena spp., wild oats; Fumaria spp., fumitory.

Model-based analysis of the likelihood of gene introgression from genetically modified crops into wild relatives

The proliferation of genetically modified crops has created a need for methods to predict the likelihood of gene introgression into related species in situ. We present a model of a modified crop and an associated unmodified plant population removed spatially from the modified crop but not completely isolated from it, reflecting standard practices for isolation of field trials. We develop models for two kinds of life histories, broadly based on Brassica and Gossypium, taxa that are targets for genetic modification. We find that current prescriptions for field trials are likely to lead to escape of transgenes into wild populations when outcrossing rates are moderate and hybrids are fertile. The results are sensitive to pollen rain within plausible bounds for model parameters, suggesting buffer widths are an important aspect of the design of field trials. When gene introgression requires the spontaneous development of a polyploid, the likelihood of gene introgression is lower but still appreciable in realistic circumstances. Events that are unlikely over periods of a few years become almost certain within scales of a few decades, emphasising the need for gene risk assessments to be set in specified time frames. The models serve to identify the parts of the system that are poorly known and that are important in determining outcomes, providing a focus for future research. There is a need for research on the consequences of changes in fitness due to the transgenes, competitive interactions between related species, and the broader ecological consequences of changes in agricultural practice resulting from the use of genetically modified crops.

Sampling strategy is important for producing weed maps: a case study using kriging

Abstract Weed maps are typically produced from data sampled at discrete intervals on a regular grid. Errors are expected to occur as data are sampled at increasingly coarse scales. To demonstrate the potential effect of sampling strategy on the quality of weed maps, we analyzed a data set comprising the counts of capeweed in 225,000 quadrats completely covering a 0.9-ha area. The data were subsampled at different grid spacings, quadrat sizes, and starting points and were then used to produce maps by kriging. Spacings of 10 m were found to overestimate the geostatistical range by 100% and missed details apparently resulting from the spraying equipment. Some evidence was found supporting the rule of thumb that surveys should be conducted at a spacing of about half the scale of interest. Quadrat size had less effect than spacing on the map quality. At wider spacings the starting position of the sample grid had a considerable effect on the qualities of the maps but not on the estimated geostatistical range. Continued use of arbitrary survey designs is likely to miss the information of interest to biologists and may possibly produce maps inappropriate to spray application technology. Nomenclature: Capeweed, Arctotheca calendula (L.) Levyns.

Effects of soil type on crop yield-weed density relationships between winter wheat and Bromus sterilis

Department of Science, Anglia College of Higher Education, Cambridge CB] IPT; *Department of Agricultural Sciences, University of Bristol, Long Ashton Research Station, Long Ashton, Bristol BS18 9AF; t Department of Environmental and Evolutionary Biology, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, and I Crop Science Department, Harper Adams Agricultural College, Newport, Shropshire TFEO 8NB