Greater than 40 percent of soybean acreage in Arkansas, Louisiana, and Mississippi is irrigated. Most of those irrigated acres are in the Delta portions of the states.

The impact of that irrigated soybean acreage is significant because yields from properly irrigated fields are about 20 bushels per acre greater than yields from nonirrigated acres.

Often there is confusion about when to initiate irrigation of soybeans to realize maximum yield potential.

Six planting dates for MG 4 soybeans are shown in the accompanying table. For each planting dates, estimated dates for R1 (beginning bloom), R3 (beginning podset), and R6 (full seed) are shown, along with average rain and pan evaporation (potential evapotranspiration) between the indicated stages at Stoneville, Miss.

Estimated water use was derived from pan evaporation multiplied by a crop coefficient for the period. Weather and water use at other locations in the lower Mississippi River Valley are similar to those for Stoneville.

Between planting and R1 for every planting date, water use does not exceed rainfall in an average year. Thus, irrigation in an average year will not be needed before R1 for MG 4 varieties.

Water deficit (water use minus rainfall) between R1 and R3 for all plantings is less than the 2-inch deficit normally used to trigger irrigation. Thus, irrigation will not be needed prior to R3 in an average year because the soil can easily supply the deficit amount.

The R3 to R6 period of all planting dates in an average year experiences water deficits that range from 6.5 to 7.7 inches. When added to the R1 to R3 deficits, the R1 to R6 water deficits range from 7.3 to 8.2 inches.

Thus, in an average year, plantings of MG 4 varieties will need 7.3 to 8.2 inches of irrigation water to realize maximum yield potential. Irrigation should be initiated no later than R3 when planting date is between March 20 and May 10.

Of course, no year has weather exactly like that for an average year. Some years will have conditions that result in larger deficits than those shown here for the various periods. This means that irrigation should be initiated before R3 in some years, and/or more water will need to be applied during the season to realize maximum yield potential.

The information presented here can be used as a benchmark for irrigation planning in a given year at a given location. Actual rainfall amount received on a given field can be compared to that shown in the table to determine when irrigation should begin and how much irrigation water should be applied to supplement rainfall during the growing season.