Ellen M. Bauske, Paul L. Mask, Karl Harker, C. Dale Monks, and Joseph Kemble

Many corn producers believe that the best way to maximize com yield is to plant as early as possible, but this may not always be true. AAES computer simulations indicate delayed planting in South Alabama can ensure that corn reaches its most critical growth stage during periods of peak rainfall.

Alabama growers generally plant corn from early March until mid-May. The most critical stage in corn development is during tasseling and silking. Drought, even for a short period of time during this stage, can cause serious yield reduction. Using a simulation model, AAES researchers correlated long-term weather patterns with plant developmental stages to best utilize natural rainfall in dryland crop production. Results of the study indicated that planting too early on farms in South Alabama can result in lower soil profile moisture at mid-silk.

Researchers used 30 years of weather data to drive the corn growth simulation model used to explore the relationship between planting date and soil profile moisture. Resulting analyses allowed researchers to estimate the amount of moisture available to corn at mid-silk in an average year at different planting dates and locations.

An initial step in this project involved determining the number of “heat units” common corn varieties require to reach mid-silk in Alabama. The measurement of heat units reflects the fact that temperatures must be high enough over a period of time to make corn grow. To calculate heat units, subtract 50 from the average temperature each day of the growing season; add the remainder to the ongoing accumulation of heat units. It is possible to accumulate 0-36 heat units per day. Heat units required to reach mid-silk were determined for 30 corn varieties grown in AAES variety trials from 1989 to 1994. The average accumulation was 1,420 heat units, 95% of the varieties were at mid-silk between 1,338 and 1,502 heat units.

To address the issue of moisture availability in the corn growth model, researchers used a water budgeting balance sheet. In water budgeting, a method used to balance available soil moisture, precipitation is a credit entry and evapotranspiration (water used by the plant and water lost to the atmosphere estimated by pan evaporation) is a debit entry. A water budgeting balance sheet for a fine sand soil type was used in the planting date model. This soil type has a very low available-water holding capacity (one inch per foot), and represents a worst-case scenario. Using 30-year rainfall data, researchers graphed precipitation probabilities for Belle Mina, Sand Mountain, Headland, Milstead, Fairhope, and Brewton (Figure 1). These graphs were used to identify time periods with the greatest probability of precipitation at each location. Pan evaporation data and information on water use rates at each stage of corn development also were used in the model.

Figure 1. Probability that more than one-half inch of precipitation will occur in Brewton

Rainfall and temperature data were programmed into the simulation model for each of the six locations. The model was run for six planting dates at two-week intervals beginning on March 1 and ending May 15. Soil profile moisture at mid-silk was graphed for each planting date at each location to determine when mid-silk occurred at favorable soil moisture levels (Figure 2).

Figure 2. Effect of planting date on soil moisture at mid-silk at Brewton.

Delayed planting in Milstead, Belle Mina, and Sand Mountain (Central and North Alabama) did not increase the likelihood of adequate soil moisture at mid-silk (see table). However, this was not the case in South Alabama. Delayed planting at Brewton, Fairhope, and Headland placed mid-silk in more favorable soil moisture conditions.

Planting dates suggested by this study are still within current Alabama Cooperative Extension Service recommendations, but are later than many growers currently plant. However, field tests must be done to confirm these results, and to identify other potential drawbacks to delayed planting, such as increased insect pressure.

Bauske is an Extension Associate and Kemble is an Assistant Professor in Horticulture. Mask is an Associate Professor and Monks is an Assistant Professor of Agronomy and Soils. Harker is an Agricultural Meteorologist with the Southeast Agricultural Weather Service Center in Auburn.