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.

Seedlings of the cotton cultivar Sicot 1 were treated with either 1%, 2% or 5% colchicine in lanolin which was topically applied to the apical meristem four days after emergence . Of 293 1%-treated seedlings 64 survived to produce fertile C, shoots which gave rise to 84 C2 single boll families . C2 and C3 generations were examined in field experiments for morphological variants and for a number of quantitative characters (yield components and fibre quality) . No morphological mutants were seen but considerable heritable variation for metrical characters of economic importance was detected . Of 18 extreme C2 families consisting of 54 C3 progenies, 19 were shown in a replicated spaced-plant trial to be significantly different to the Sicot 1 parental material for at least one (usually more) characters . The 222 C3 progenies of the 66 non-extreme C2 families were tested in an unreplicated plot experiment and at least 14 were suspected of exhibiting variation in metrical characters . Although no light has been shed on the mechanism responsible for the production of variants by this method, it is argued that the colchicine technique results in higher levels of genetic variation than found in commercial cultivars ; variation which can be readily exploited by cotton breeders .

Hayman analysis for lint percentage, boll weight, fibre quality and bacterial blight resistance of a ten-parent half-diallel set of crosses was conducted in Upland cotton . The ten parents represented a fixed sample of the best germplasm available to Australian breeders for the characters of commercial importance . Heterosis and inbreeding depression were detected, particularly for boll weight and blight resistance . An analysis of genetic components and parameters indicated that since additive effects were substantial and heritability high, early-generation selection of spaced plants and pure-line breeding should be successful . Some genotypes were identified as being potentially good donors for hybridization since they possessed dominant genes for improved character expression . Acala C-1 was identified as the first-choice parent for increased boll size, Coker 315 for increased span length, while Namcala was the best parent for breeding improved fibre strength . McNair 220 possessed the most dominant genes for high lint percentage . Reba P279 and Siokra were the only parents with resistance to the prevelant race of bacterial blight .