The cold, rainy weather this past weekend has presented challenging conditions for corn fields that have already been planted. Now that the weather is improving it’s good to start thinking about what the consequences might be and what to look for. Heat unit accumulation has been negligible since April 26. Rainfall had been near the climatological average prior to the recent rains. The effect of this weather pattern has resulted in April planted corn taking more days to emerge. And that will be no different with corn planted the last week of April. All it matters is the day of emergence, not of planting.
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The impact of low temperatures and 2 inches of rain last weekend on yield potential is minimal according to a simulation model (FACTS) that accounts for biophysical factors on seedling emergence and crop growth. This simulation model does not account for cold stress associated with stand loss, disease infection, or injury from herbicides and insecticides. If the degree of seedling damage is minor then the yield loss will be minor. Furthermore, the model showed that for a 110-day corn maturity the optimum planting window for maximizing 2017 yield potential is between 5/5 and 5/12 in central Iowa, which is very encouraging given that more than 80% of Iowa’s corn crop is yet to be planted.
While much of April had 4-inch soil temperatures above 50oF for a good portion of the state there is concern about how much fluctuation there was at the 2-inch seed placement depth and what effect the fluctuation may have had. The first most noticeable effect is a longer number of days for emergence to occur. There have been reports of corn planted April 10 taking 14 days to emerge. This is simply a matter of heat unit accumulation. Fluctuations in 2-inch soil temperatures compounded by soil and planting depth variance is likely the cause of uneven emergence dates from one plant to another. Cooler soil temperatures lead to vigor loss (lower crop growth rates) which effect stand establishment; make the seedling more susceptible to herbicide (ALS) and insecticide (organophosphates) injury; more vulnerable for disease pathogen infection; and, ultimately, reduction in yield potential.
The best way to assess the full impact of the weather on April planted corn is to get into the field and conduct a thorough stand establishment assessment. Determining the plant population is the first step. While determining the plant population make note of emergence delays and any ‘skips’ where plants should be. Plant ‘skips’ should be evaluated to determine: (1) if there was no seed due to planter error, (2) if the seed did not germinate, or (3) if the seedling died before emergence. Lost yield potential from stand reductions and uneven emergence are dependent upon how severe they are. Additionally, evaluate emerged plants for injury due to ALS herbicides, organophosphates insecticides, and disease pathogen infections. Symptoms to look for are brown or discolored and pinched mesocotyls and coleoptile. It is likely that plants with stunted growth or delayed emergence will have symptomology and lower yield potential.