October 12, 2012

Nebraska, like many parts of the western Corn Belt, is experiencing one of the most severe droughts in decades. Soil moisture storage throughout the rooting depth was largely depleted by the end of the growing season, particularly in dryland fields. Soil water recharge during the coming winter and spring will be critical to setting plant population for next year's corn in dryland systems. This is because yield response to plant population in dryland fields could be muted if soil water storage is low at the start of the growing season and rainfall during the growing season cannot meet crop water demand.

Using the Hybrid-Maize model, we simulated dryland corn yield responses to different plant populations at two locations: North Platte (representing west central Nebraska) and Mead (representing southeast Nebraska). Each simulation scenario was a combination of two factors:

• plant population of 20,000, 25,000 or 30,000 per acre, and
• soil moisture, of silt loam soil, at planting of 100% (about 3.1 inches water/foot), 75% (about 2.6 inches water/foot), or 50% (about 2.2 inches water/foot) of field capacity (F.C.) throughout a 5-foot rooting depth.

For both locations, simulations were run over 31 years (1982 to 2012) of weather data, using May 1 as the sowing date. A hybrid maturity of 2500 GDD (CRM of about 103 days) was used for North Platte while a maturity of 2700 GDD (CRM of about 113 days) was used for Mead. Simulation results presented below are means of 31 years of simulations at each site.

In North Platte (Figure 1, upper graph), dryland corn yield does not respond to increasing population from 20k to 30k per acre if soil moisture in the 5-foot rooting zone is poor (i.e., 50% F.C., or 2.2 inches/foot) at time of planting and the growing season has average rainfall (i.e., 10.4 inches). This is because low water availability throughout the growing season limits yield response to greater plant numbers. If soil water is 75% F.C. (i.e., 2.6 inches/foot) or higher by the time of planting, there is a moderate response in dryland yields to increased plant population for a season with average rainfall.

At Mead in southeast Nebraska, response of dryland corn yield to plant population (Figure 1, lower graph) is strongly positive even when soil moisture at planting is as low as 50% F.C. (i.e., 2.2 inches/foot) for a year with average rainfall during the growing season (i.e., 14.8 inches). And the response is even steeper if soil moisture at planting further improves. In this area, typical growing season rainfall can significantly compensate for low soil water storage at start of the growing season, and as a result, the impact of a drought year to the next season is less than in lower-rainfall areas, such as west central Nebraska.

Summary

Dryland corn growers in areas with conditions similar to west central Nebraska should consider taking into account the amount of stored soil moisture at planting when determining seeding rates. This may well be the situation in 2013 if the current low soil water storage exists at planting time. The amount of precipitation during the coming winter and spring will be important for recharging soil moisture and thus in making decisions about 2013 seeding rates in dryland fields of west central Nebraska.

For Mead, our simulations suggest that dryland corn yields are not very sensitive to soil moisture status at planting. Note that these conclusions are based on crop model simulations and thus must be used to guide management decisions, along with past experience and any differences in soil properties compared to those specified in this study. Real-time soil moisture at various locations across the state can be checked at http://www.hprcc.unl.edu/awdn/soilm/.

(Special thanks to Suat Irmak, Department of Biological System Engineering, for comments and suggestions.)

Haishun Yang, Associate Professor / Crop Simulation Modeler, Department of Agronomy and Horticulture
Patricio Grassini, Research Assistant Professor, Department of Agronomy and Horticulture
Jenny Rees, UNL Extension Educator
Keith Glewen, UNL Extension Educator
Kenneth G Cassman, Professor, Department of Agronomy and Horticulture