First- and second-year seedbank emergence of 23 summer annual weed species
common to U.S. corn production systems was studied. Field experiments were
conducted between 1996 and 1999 at the Iowa State University Johnson Farm in
Story County, Iowa. In the fall of 1996 and again in 1997, 1,000 seeds for
most species were planted in plastic crates. Seedling emergence was counted
weekly for a 2-yr period following seed burial (starting in early spring).
Soil temperature at 2 cm depth was estimated using soil temperature and
moisture model software (STM2). The Weibull function was fit to
cumulative emergence (%) on cumulative thermal time (TT), hydrothermal time
(HTT), and day of year (DOY). To identify optimum base temperature
(Tbase) and base matric potential (ψbase) for
calculating TT or HTT, Tbase and ψbase values ranging
from 2 to 17 C and −33 to −1,500 kPa, respectively, were evaluated for each
species. The search for the optimal model for each species was based on the
Akaike's Information Criterion (AIC), whereas an extra penalty cost was
added to HTT models. In general, fewer seedlings emerged during the first
year of the first experimental run (approximately 18% across all species)
than during the second experimental run (approximately 30%). However,
second-year seedbank emergence was similar for both experimental runs
(approximately 6%). Environmental effects may be the cause of differences in
total seedling emergence among years. Based on the AIC criterion, for 17
species, the best fit of the model occurred using Tbase ranging
from 2 to 15 C with four species also responding to ψbase = −750
kPa. For six species, a simple model using DOY resulted in the best fit.
Adding penalty costs to AIC calculation allowed us to compare TT and HTT
when both models behaved similarly. Using a constant Tbase,
species were plotted and classified as early-, middle-, and late-emerging
species, resulting in a practical tool for forecasting time of emergence.
The results of this research provide robust information on the prediction of
the time of summer annual weed emergence, which can be used to schedule weed
and crop management.