Iowa Corn: Grain Filling Progress

Developing corn ear. Photo: Ohio State University

According to the latest NASS report, the 2019 corn crop is about 2-weeks behind normal in terms of reaching dent stage. What are the implications of this delay in grain filling period and end of season yield? We address this question by examining two key weather variables; solar radiation and temperature.

Solar radiation interception is expected to be 15% lower during the dent phase in 2019 compared to past years (Fig. 1). This is because daylength, and therefore amount of solar radiation, decreases as we move from August to September. An unknown factor is the clouds during 2019 dent phase, which may compensate for the decreased radiation energy.

Solar radiation is the main driver for crop photosynthesis, which for the present week is estimated at 210 lbs dry matter/acre/day for Iowa. Crop photosynthesis is strongly related to corn grain accumulation, which for the present week is estimated at 4.5 bushels/acre/day for Iowa (Fig. 2). For reference, the maximum grain accumulation rate can be up to 7 bushels/acre/day.

Fig 1. Average solar radiation (left panel), average daily temperature (middle panel) and minimum temperature (right panel) for central Iowa. Data points are averages over 10-day period (e.g. 5 June is the average from 1 June to 10 June). Horizontal green lines in the middle and right panel indicates the base temperature of 50oF and killing frost temperature of 32oF, respectively. Blue and red horizontal lines indicate the normal grain fill duration (blue line) and the expected 2019 grain fill duration (red line).  Click Image to Enlarge

Fig 2. Simulated current grain accumulating rates (in bushels per acre per day) across three I states. Data are from the FACTS project simulations. Click Image to Enlarge

Temperature during the dent phase is also expected to be around 5oF lower in 2019 than past years (Fig. 1). A 2-week delay in the start of dent stage will probably delay corn maturity by 3 weeks because temperatures begin decreasing from late August to September. When crops can be expected to reach maturity this year depends on the planting date and hybrid maturity combinations.

In general, crops planted before June 1 should not have an issue reaching maturity in Iowa. Historically, the average temperature will be above 50oF until October 10 for central Iowa, while the first frost likely would occur after October 20 in central Iowa, thus crops will likely make it to maturity.

The key issue this year will be the dry down period for grain moisture to reach 15-20%. While it is expected that with normal fall conditions, even late planted corn will reach maturity, grain dry down will be slower resulting in wetter corn at harvest

For the state of Iowa, the 2019 corn yields can be expected to be lower than the past 3 years because temperature and radiation during the grain filling period is less than would typically be expected due to later than normal planting dates. However, there is a lot of variability across the state for planting date, corn maturity, and weather conditions that will be addressed in a follow up article.

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