1058 to 2239 kg ha-1 (11 to 24 percent) as compared to subplots without smother plants. Weed infestations were not heavy enough at any of the experimental sites to reduce corn yield, and therefore smother plant presence in the weedy subplots did not increase corn yield compared to weedy subplots without smother plants.
Dry weight of annual grass and broadleaf weeds harvested eleven weeks after smother plant emergence generally decreased with increasing smother plant seeding rate, but the data were too variable to obtain a reliable estimate of weed suppression.
Smother plant evaluation under conditions of more intense weed competition than observed in this study will be required to fully assess the technology. In the absence of the smother plant, corn in weedy plots developed as rapidly and yielded as well as corn grown under weed-free conditions. In this study, therefore, even an ideal smother plant would not have had a beneficial effect. This suggests that a threshold level of weed competition may be necessary for smother plant use to be advantageous. Smother plant research employing several levels of weed competition could be used to investigate this possibility.
The dwarf Brassica smother plant evaluated in this research was competitive with the main crop in two of the three testing environments. Smother plants that reduce main crop yield by 10 percent or more in some years are not likely to be adopted by producers. Researchers involved in smother plant development may be able to make the technology more economically feasible by employing a combination of the following approaches.
First, researchers need to develop a better understanding of the factors involved in determining whether the crop and smother plant will compete for the same resources. Corn development and yield results indicate that the dwarf Brassica smother plant may have a minimal impact on corn under some (favorable) environmental conditions, but that it could have a negative effect if the availability of nitrogen or other nutrients is limited. Additional research investigating the effects of the availability of nitrogen, water, and other nutrients on the competitive interactions between corn, smother plants, and weeds could facilitate the development of smother plant lines with greater selectivity than observed with the dwarf Brassica.
It is possible that manipulation of the smother plant genotype alone will not be successful. Main crop cultivars may also need to be selected for performance in a smother plant system before smother plant technology becomes feasible. Current crop cultivars have been selected for performance in high nutrient status monoculture cropping systems, and may not be adapted for use with smother plants. Interdisciplinary research involving plant breeders, weed scientists, soil scientists, and perhaps others is needed to investigate this question.
Some method of controlling smother plant height, life cycle length, or both would be a useful way of regulating its interference with crop growth. It is possible that a gene or genes could be altered or inserted into the smother plant so that the