Emergence Prediction of Common Cocklebur and Redroot Pigweed in Maize Using Thermal Time Models

The knowledge of the relationship between seedling emergence time and the prevailing environmental condition such as temperature is useful to timely application of herbicides. Two years of study was conducted to investigate seedling emergence of Xanthium strumarium and Amaranthus retroflexus in maize in two contrasting environments of Karaj and Tonkabon. Thermal time was used for predicting cumulative weed emergence. The Gompertz model was found more likely to predict weed emergence patterns for different sites and years. X. stramurium started its emergence with receiving 200 GDD. It required 500 GDD to reach its maximum emergence. The emergence of X. stramorium continued through the season up to 900 GDD. For A. retroflexus, emergence started at 400 GDD and with receiving a GDD of 600 reached its maximum level. A. retroflexus showed a relatively whole season emergence and continued its emergence up to 1200 GDD. Therefore, choosing the right time for herbicide spraying is accompanied with complexities. Predicting the start and the duration of seedling emergence in fields could optimize weed control timing.