Research emphasizing slower plant growth and delayed maturity in continuous maize ( Zea mays L.), no-till (MM-NT) systems has often led to the conclusion that lower grain yields in this environment are associated with reduced plant heights. Yet prior research has shown that early-season and mature plants are not always shorter in MM-NT systems, suggesting that overall plant height may not be an accurate morphometric indicator of decreased yield in MM-NT environments. Given that plant-to-plant morpho-physiological uniformity is strongly associated with higher yield in maize, we hypothesized that greater plant height variability would provide a better agronomic explanation for yield loss in MM-NT environments than overall plant height reductions. This 14-year study primarily examined the effects of crop rotation {maize-soybean [ Glycine max (L.) Merr.] and continuous maize} and tillage system (no-till and moldboard plow) on the yield, 4-week plant population, and 4- and 8-week plant height and plant height variability of a single maize cultivar. Due to sizeable year-to-year variation, actual crop response means for the MM-NT; maize-soybean, no-till (MB-NT); and continuous maize, moldboard plow (MM-PL) treatment combinations were expressed relative to the accompanying means for the maize-soybean, moldboard plow (MB-PL) treatment. In numerous years, the MM-NT system exhibited reduced actual and relative yields and lower 4- and 8-week plant heights compared to the other treatment combinations. Both actual and relative 4- and 8-week plant height variability were rarely greatest for the MM-NT treatment, and in only a few years were actual and/or relative plant density lowest for this system. However, single-factor regression analyses between relative yield and the aforementioned relative agronomic measures revealed that a decline in relative MM-NT yield was most strongly associated with an increase in relative 4-week plant height variability. Multi-factor regression analyses between relative yield, relative 4-week plant height variability, and various weather parameters suggested that this strong inverse relationship was potentially a manifestation of (i) non-uniform germination, emergence, and early seedling growth and (ii) later-season intra-specific competition. Regression analyses between relative 4-week plant height variability and various weather parameters suggested that phenomenon (i) was potentially promoted by cool and moist or warm and dry pre-plant weather conditions while phenomenon (ii) was possibly encouraged by low precipitation and/or high temperatures during rapid stem elongation. While MM-NT systems should be managed to limit plant density reductions and minimize growth and developmental delays, increased focus should be placed on minimizing the occurrence of plant-to-plant variability in these environments.
