Fine-tune schedule for corn irrigation

Water-deficit stress during the reproductive period in corn can increase the interval from silking to pollen shed and reduce kernel number and/or weight. The most critical period for irrigating corn is from approximately one week before to two weeks after silking.

Even within this period, the exact effect of water deficits depends on timing and duration. For example, water deficits at seed set may result in a low number of seeds, and water deficits after seed set may result in a high percentage of small seeds.

Yield in grain crops is largely determined by seed number. This may be due to the more limited exposure interval for seed set as compared to grain fill. (See table for corn stages and corresponding information for irrigation management).

Corn yield is extremely sensitive to normal water deficits during and after but not before beginning silk. The sensitivity of crop growth stages to water-deficit stress declines in the order of silking and pollination to grain filling (blister to dent) to vegetative development (pre-silking).

However, corn grown on sandy soils is more prone to yield-reducing effects of pre-silking water deficits than is corn grown on finer-textured soils, and should be monitored for stress that may limit vegetative growth.

Plants grown on droughty soils can be stressed during the vegetative period to the point that they will be too small and too damaged to produce a credible yield even if adequate water is available during the reproductive period. Recording plant growth stage and soil water status (insure that root zone soil has Ž 50 percent plant-available water during the vegetative period) before silking is the best approach to managing soil water prior to reproductive development on sandy, droughty soils.

Proper irrigation management requires accurate irrigation scheduling; i.e., deciding when to irrigate. The most difficult decisions are when should irrigation begin and end.

If water for irrigation is limited, the first period of the corn growing season to consider not irrigating is the vegetative or pre-silking phase. If water is limited for required irrigation during the reproductive period, it should be used during the silking-to-blister period when seeds are being formed.

If water for reproductive irrigation is not limited, grain yields will be maximized by applying irrigation during this 45- to 50-day period whenever the soil water deficit is between 1 and 2 inches.

Water use during the irrigation period will average about 0.3 inch per day or about 14 inches from beginning silk to full dent (one-half milk line) stages.

To determine the last irrigation for corn, use the “milk layer” or “milk line” as the criterion for estimating days to maturity. The milk layer is defined as the borderline between the bright, clear yellow of the seed coat outside the hard starch layer (top of kernel) and the milky, dull yellow of the seed coat outside the dough layer (base of kernel) (See photo by Erick Larson picture at http://msucares.com/crops/corn/images/milklinehalf.jpg [1]).

It takes about 20 to 25 days for the milk-line to progress from the top of the kernel (beginning dent stage) to the bottom or base of the kernel (maturity or black layer) where it is attached to the cob. Thus, a final irrigation when the milk-line is halfway down the kernel coincides with 10 to 12 days to maturity.

Basically, the coarser the soil texture, the later the last irrigation is needed. The last soil-recharging irrigation of corn grown on silt loam and clay loam soils should occur at about dent stage (20 days to maturity), whereas the last irrigation of corn grown on a sandy soil should occur at full dent stage (about 10 to 12 days to maturity).

These guidelines should be considered in conjunction with the time of year that these stages occur, since a greater water use rate between the last irrigation and maturity than that used here (0.1 to 0.2 inch per day) results in more rapid depletion of soil water between the planned last irrigation and maturity.


Larry G. Heatherly is a retired USDA-ARS Research Agronomist and current crop consultant (email [email protected] [2]).

Stages of corn development (based on 125-d hybrid) that are important for irrigation management
Stage* Description DAE/DTM# Pertinent information for irrigation management
V12 Twelfth leaf fully emerged 42/83 Ovule number (potential kernels) and size of ear are being determined. Water deficits at this time may reduce potential number of kernels and size of ears.
V15 Fifteenth leaf fully emerged 53/72 Ten to 12 d away from silking. Beginning most crucial period in seed yield determination. Number of ovules that develop silks is being determined. Large seed yield reduction results from water-deficit stress that occurs between this stage and 2 wk after silking.
V18 Eighteenth leaf fully emerged 60/65 Water-deficit stress during this time delays ear and ovule development, which can delay silking until after pollen shed is partially or mostly done. Ovules that silk after pollen shed are not fertilized and do not produce a kernel.
R1 Silks emerging 66/59 Number of ovules to be fertilized is being determined. Water-deficit stress at this time results in poor pollination and seed set, and thus reduced number of kernels.
R2 Blister 78/47 Beginning of rapid increase in seedfill or kernel weight. Irrigate if needed to assure adequate water for translocation of nutrients and rapid seedfill.
R3 Milk 86/39 Rapid increase in grain weight. Water-deficit stress at this time reduces both number of kernels and final size or weight of kernels.
R4 Dough 92/33 Rapid increase in grain weight. Water-deficit stress at this time results in unfilled and/or undersized kernels. About one-half of individual kernel weight has accumulated.
R4.7 Beginning dent 101/24 See below.
R5 Dent 105/20 Kernel drydown has started, and the hard starch layer (milk-line) advances from the top to the base of the kernel. This line is used in deciding when to terminate irrigation. Water-deficit stress at this time reduces yield by reducing kernel weight.
R5.4 Full dent 113/12 Equivalent to about one-half milk-line. Irrigation of corn growing on most soils can be terminated at this time.
R6 Physiolocal maturity/black layer 125/0 No further irrigation needed.
*V indicates vegetative; R represents reproductive. #DAE = days after emergence; DTM = days to maturity.