September 4, 2013

We first evaluated simulated corn yields across Nebraska in an Aug 16, 2013 CropWatch article. Since then, most parts of Nebraska have experienced more than a week of high temperatures between 90°F to 100°F without significant rainfall. This article provides an updated evaluation of simulated yields, taking into account the recent heat wave across the state.

We used Hybrid-Maize, a corn simulation model developed by UNL scientists, to evaluate possible corn yield outcomes as well as the possible consequences of frost before maturity. The evaluation is based on this year’s average and late planting dates (Table 1) as reported by UNL extension specialists and educators for different areas of the state, up-to-date weather data at selected locations (Table 1), and long-term historical weather data (26 years on average) at those locations. All weather data are provided by the High Plains Regional Climate Center.

Yield evaluations are of yield potential as predicted by Hybrid-Maize for both fully irrigated and rainfed crops when growth and yields are not limited by nutrient deficiencies, weeds, insect pests, or diseases. Because of the variations in actual crop management, including planting date, hybrid maturity, seeding rate, fertilization, etc., simulated yield expectations are presented as percentage of simulated long-term average yield (Figures 1 and 2).
 

Figure 1. Expected corn yield potential in fully irrigated fields based on this year’s average and late planting dates (evaluated on Sept 2, 2013).

Expected Yields for Fully Irrigated Fields

Figure 2. Expected loss of GDD (i.e., growing degree day, 50°F based) due to possible frost before maturity (evaluated on Sept 2, 2013). For Concord and Mead, the possible losses of GDD account for only about 3-4% of total GDD for the hybrids used in the simulations.

The recent heat wave has resulted in a small decrease in simulated yields compared to estimates from August 16. Even with this small reduction, fields with an average planting date are estimated to be 2-8% higher than long-term averages with the same planting date (Figure 1). For fields planted around June 1, however, expected yields are 0-13% lower than simulated long-term averages with the same late planting date. There was little change in this estimate from August 16.

In general, warmer temperatures have two opposite effects to corn yield formation: higher daily rate of grain filling, but fewer days for grain filling, with the latter typically having the greatest impact. On the other hand, the recent warmer temperatures have accelerated crop development and therefore reduced the possibility of yield loss from an early frost (Figure 2). On average across the seven locations, GDD accumulation was 80 GDD higher than expected on Aug 16, 2013 (Table 1).

Expected Yields for Rainfed Fields

The amount of crop available water in the soil profile at the beginning of the growing season is an important factor in determining crop yield in rainfed systems. We don’t have representative initial soil moisture data for all rainfed fields. Instead, we assumed, for all locations in the simulation, 100% soil available water in the top foot, but ¾ of soil available water in the subsoil due to severe water depletion last year, in silt clay loam soils. The maximum rooting depth was assumed to be 5 feet at all locations. Other settings for these simulations were the same as for fully irrigated fields.

Figure 3. Expected rainfed corn yields based on 2013 average and late planting dates (evaluated on Sept 2, 2013).

For fields planted on the average planting date at each site, expected yields remain considerably below long-term averages for most areas except for parts of northeast Nebraska, such as in Elgin and Concord where some areas received adequate rainfall (Figure 3). For fields planted June 1, simulated yields are also lower than an average year for most locations except where precipitation was above normal. Compared to the evaluation released on Aug 16, 2013, expected yields are lower for most areas. Compared with irrigated fields, simulations of rainfed fields show much greater variation due to variation in growing season precipitation across locations. Likewise, due to large variability in rainfall amounts over relatively short distances, there is greater uncertainty in making inferences about potential yields in areas nearby each of the rainfed locations simulated in this evaluation. In addition, differences in actual soil texture, soil moisture status in the beginning of the season, tillage system, and topographic location of a field, will contribute to yield variation of rainfed fields.

Summary

Yields for irrigated corn planted before or around mid-May across Nebraska could be 2-8% higher than an average year based on a September 2 evaluation. For irrigated corn planted as late as June 1, yields remain 0-13% lower than an average year with the same hybrid maturity and late planting dates. For late planted corn, frost damage before maturity is a concern, but the concern is less than the last evaluation due to recent high temperatures. (See CropWatch article.)

For rainfed corn, expected yield potential could vary widely across the state. Many locations could see considerably lower rainfed yields than in an average year for both average and late planting dates because of insufficient water recharge in subsoil at planting and spotty rainfall later. We plan to update the corn yield evaluation regularly until the end of the growing season.

Haishun Yang, Associate Professor, Agronomy and Horticulture
Charles Shapiro, Extension Soils Specialist
Jenny Rees, Extension Educator
Greg Kruger, Extension Crops Specialist
Patricio Grassini, Research Assistant Professor, Agronomy and Horticulture
Ken Cassman, Professor, Agronomy and Horticulture