The mass of water in corn follows a seasonal cycle. During the vegetative stages, corn is around 80% water. Corn water mass quickly accumulates during the vegetative stages. Maximum water mass occurs around 1830 Fahrenheit GDD after planting for many Iowa corn cultivars, aligning with the end of its R2 (blister) stage. At this stage around 70% of the plant’s mass is water. From then on, water content decreases until harvest, when it makes up only around 15% of the plant’s mass. Corn gravimetric water content, defined as water mass divided by biomass (dry mass) of vegetation, is used as a measure of plant moisture. A recent history of measured corn gravimetric water content vs thermal time is shown in Figure 1, which projects gravimetric water content to peak early in the growing season (Togliatti et al. 2019). The length of the seasonal cycle in gravimetric water content varies with maturity rating; we expect cultivars that silk early will peak in gravimetric water content before those silking later and take longer to reach maturity. The goal of our research study is to compare the seasonal cycle in gravimetric water content of a typical Iowa cultivar to one which is typical of the upper Midwest that utilizes a shorter growing season.
This study consists of two adjacent corn plots both planted on the same day, one of a cultivar common in central Iowa, and the other of a cultivar common in central Minnesota. This trial is being conducted from 2019 - 2021 south of Ames, IA. The Iowa hybrid has a maturity rating of 113 days to silking while the Minnesota hybrid has a rating of 79 days to silking. We expect the Iowa hybrid to peak in gravimetric water content around 750 GDD, while the Minnesota hybrid should peak at 535 GDD (Togliatti et al. 2019). Therefore, to capture these peaks in gravimetric water content, crop sampling was conducted throughout the vegetative stages based on growing degree day accumulation: at 390, 570, 750, 930, and 1110 accumulated GDD.
Fig. 1. Corn whole plant gravimetric water content for hybrids typical for Mead, NE and Ames, IA.
At each sampling day, plants of both hybrids were cut and weighed for total plant biomass weight. Plants were then dried for one week and weighed again for dry biomass weight. This year P1366Q was planted as the Iowa hybrid and P7907AM is the short-day hybrid. Based on the estimations of timing of peak gravimetric water content, our hypotheses are that for the 390 and 570 GDD sampling days that gravimetric content will be higher for the 79-day hybrid and for the final three sampling days gravimetric water content will be higher for the 113-day hybrid.
2021 Observations
We have complete fresh and dry biomass observations for the first two sampling days of 2021. This year, our plots were planted on April 27, and the first two sampling days were on May 26 and June 3. Sampling was conducted between 2 pm and 3 pm and aligned with thermal times of 410 and 535 GDD. Both days were dry and sunny; May 26 was comfortable with a high in the mid 70’s F and June 3 was warm with a high in the mid 80’s F. Both hybrids were V3 on the first sampling day (May 26). On the second sampling day (June 3) the Iowa hybrid was V4/V5 and the Minnesota hybrid was V5. Mean measurements and confidence intervals from these two sampling days are displayed in Figure 2.
Fig. 2. Vegetation measurement summaries of the first two sampling days of all years of the study. Top: dry biomass per plant. Bottom: Gravimetric water content. Horizontal axis is growing degree day accumulation since planting. Error bars are to two standard error, capturing a 95% confidence interval. Iowa points are plotted on GDD +1 for the purpose of better data visibility.
Visibly, the short-day hybrid plants appeared to be larger than those of the Iowa hybrid. The measurements support that the Minnesota corn hybrid was more massive, in terms of water and dry mass, than the Iowa hybrid. Although the mean value of gravimetric water content was larger for Minnesota plants than Iowa plants on the first two sampling days, it is not a significant difference at 95% confidence.
Annual Variability
So far in 2021, the measurements do not support hypotheses for the first two sampling days; there is no significant difference in gravimetric water content between the two hybrids at 95% confidence. In 2020 we did find that the gravimetric water content of the Minnesota hybrid was higher than the Iowa hybrid on the 390 GDD sampling day, but results were inconclusive for the 570 GDD sampling day. One concerning observation in Figure 2 is decreasing gravimetric water content from the first sampling day to the second this year, especially for the Iowa hybrid. This pattern was not observed in 2019 nor 2020 and contradicts what is expected early in the growing season. This summer central Iowa is in a drought, which decreases the storage of water in vegetation during the afternoon when the rate of transpiration exceeds the rate of water uptake through roots (Ahuja et al. 2008). This is especially true if it has been more than one week since rain. Sunny skies, hot high temperatures, and low dew points have been common this growing season, contributing to increased plant transpiration and soil evaporation thus far. Needed rain fell over much of central Iowa this week, and additional chances for rain and cooler temperatures within the next week look promising. Some areas will not receive significant rain this week, and grain fill could be impacted. The drought of 2021 thus far is more severe than in 2020, but not as bad as 2012 in terms of crop conditions (in the early phases of the 2020 growing season drought was only slight). Some drought related yield loss is expected throughout the region, with severity varying locally.
References
Ahuja, L., Reedy, V., Sasendran, S., and Yu, Q., 2008: Response of Crops to Limited Water, Vol. 1. 1st ed., American Society of Agronomy, 667 South Segoe Road, Madison, WI, 53711.
Togliatti, K., Hartman, T., Walker, V. A., Arkebauer, T. J., Suyker, A. E., VanLoocke, A., and Hornbuckle, B. K., 2019: Satellite l–band vegetation optical depth is directly proportional to crop water in the us corn belt. Remote Sensing of Environment, 233, 5.