Louisiana receives relatively high annual rainfall amounts but unfortunately it is not evenly distributed during the crop season. For this reason, water is generally the most limiting factor in corn production in our area for maximizing yields. This makes irrigation timing a critical component of successful corn production.
Rainfall has been more than plentiful so far this growing season meeting water demands of the corn crop, but important yield determining factors begin before tasseling. The number of kernels per row and kernel rows per ear begins to be determined between V6 and V8 as the ear shoots are formed. Kernels per row will not be complete until about one week from silking making these vegetative stages important in final yield. Water demand from plants will dramatically increase at silking with continued relatively high water demands through the milk and dent stages. Weekly water demands during silk will peak at 1.75” to 2.0” or higher and gradually decrease to 1.5” – 1.0” at dent stage. These weekly demands will vary depending on temperature, humidity and wind. Uptake of water and nutrients continue at a lower rate through black layer. This is the point that a black abscission layer is formed and dry weight accumulation ends. Sufficient soil moisture should be available through black layer.
Several methods of determining when to irrigate are available, including soil moisture measuring devices such as tensiometers or watermarks, but in most cases these are not practical on large acres and can be expensive.
A much more practical and less time consuming method is a water balance approach. This involves a combination of daily crop use, or evapotranspiration (ET), rainfall received and allowable soil water deficit in the rooting zone. Corn, as with other crops, will begin to stress when soil water depletion reaches 40-60 percent of a soils’ water holding capacity in the rooting zone with an average of 50 percent. This normally occurs before visible plant stress occurs such as twisting of corn. If visible plant stress occurs, some degree of yield loss occurs that cannot be recovered which will be greater if plant stress occurs during critical stages of silking through dent. The goal should be to replenish soil moisture before visible stress occurs.
Fifty percent soil water deficit should be the trigger for irrigations. Ideally, irrigations should be complete by the time 50 percent is reached. Soil water deficits are measured in inches of water. Below are suggested soil water corn deficits for soil types.
Sandy soils & Ridge silt loam soils/w hardpan - 1.50”
Alluvial silt loams/fine sandy loams - 2.00”
Clay/clay loams – 1.75”
Sandy soils & Ridge silt loams/w hardpan: 1.00 – 1.25”
Alluvial silt loams/fine sandy loams – 1.50”
Clay/clay loams – 1.25”
Daily ET rates in corn during critical stages will peak at 0.23-0.3 plus inches per day depending on daily high temperatures. During normal temperatures (max 94 degrees F) daily ET rates will peak at approximately 0.25” but will increase with temperatures. These rates will be fairly constant for three weeks beginning with silking before slowly declining. It is simple math to calculate the number days at 0.25” per day to reach allowable deficits. An example would be if using a 2.00” deficit, you would reach this in eight days (0.25 x 8 = 2.0). If it takes two days to irrigate a particular field, you would begin irrigations two days prior to reaching the 2.0” allowable deficit.
The third part of the water balance approach is rainfall. Rainfall should be measured on a field to field basis to be most accurate. Using the example above, if 0.50” rain was received, you would simply subtract from the running total which would delay irrigations two days if ET rates are 0.25” per day. If adequate water is supplied from either rain and/or irrigation that equals or exceeds the total water deficit, the deficit would be reduced to 0” where you would start again. Keeping track of rainfall will help you account for all water supplied to the crop and may save an irrigation or two in some years which is good for your bottom line with high fuel prices and other costs associated with each irrigation.
Furrow irrigations will normally saturate soils and will bring water deficits back to 0” providing there is adequate lateral water movement when watering every other middle and irrigation runs long enough to saturate the lower end of fields. Crusting occurs on some of our low organic matter soils of the Macon Ridge and/or sandier soils that may require watering every row to ensure saturation.
Center pivots will only saturate soils several inches so it is important to start earlier with these systems to ensure adequate moisture during extended rain free periods. Once soil moisture is depleted below the depth a pivot can saturate, only a rainfall will replenish soil moisture to prevent crop stress and yield loss. This is the reason for the recommended lower 50 percent allowable deficits which will trigger first irrigations sooner and subsequent ones more often than furrow irrigation.
Another important issue with center pivots is the amount of water applied each time. Most fields where pivots are used are undulating with steep slopes which favor high runoff rates. In these situations, a maximum of 0.7” – 0.8” per circle should be applied to reduce runoff and increase efficiency. Applying larger amounts of 1.0” – 1.25” in one circle usually result in increased runoff and less water infiltrating the soil profile and root zone. This becomes a critical issue during extended periods of no rainfall between silking and dent making it important irrigate with pivots more frequently with less volume to ensure the crop does not stress during this period. There may be instances with extreme temperatures that it is difficult to bring the allowable deficit back to 0,” but if you are maintaining the moisture below allowable deficits you can prevent crop stress and yield loss, or “deficit” irrigation. This has been done successfully on Macon Ridge soils under center pivots with yields of 180-190 bushels during extremely dry years.
The water balance approach to irrigation is a simple way to ensure crop water demands are met to maximize yields, but at the same time can save you irrigations by accounting for all the water supplied to the crop.