By Lyndon Kelley
Over the wide variety of planting and soil-applied herbicide situations, most irrigated producers will gain from an early-season irrigation application. The limiting factor is whether the irrigation system is ready to go.
Aggressive cover crop growth and sometimes lack of timely spring rain may leave fields lacking moisture for optimum planting or seedling germination. Achieving the maximum uniform germination and emergence can often be improved through proper early season water management. Irrigating fields prior to or just after planting can keep the planter moving and still meet the “plant into moisture” requirement if rainfall is lacking in your area.
Late spring tillage and the delays in killing cover crops are two reasons we see drier than normal planting conditions in some fields. Late spring planting of some seed and vegetable crops may result in a greater need for early season irrigation for developing crops as we enter into the typical drier weather of summer.
Irrigation water applied at 0.5 to 0.75 inch will moisten dry soil down to 6 inches to replace water lost to tillage or a cover crop. An inch of irrigation will often be needed in a field that has not received rainfall since the cover crop was destroyed. Heavier soil will take a slightly larger application to wet soil down to the 6-inch level. Monitoring newly emerged crops that were “irrigated up” is essential. It is important to water enough to keep roots growing down into the moisture. In most years, rainfall is plentiful enough to replenish water lost to tillage or a cover crop, but a dry layer 6 to 8 inches deep can greatly hinder crop development and needs to be replenished by rain or irrigation.
Some producers fear irrigation water may contribute to imbibitional chilling injury, a condition that may happen when seeds are exposed to cold soil and water temperatures during the initial 24 hours for soybeans to 36 hours for corn as seedlings begin the germination process when the seeds imbibe water and potentially rupture during the swelling process.
Most agronomists believe that soil temperatures at seed depth below 50 degrees may result in imbibitional chilling injury. Since most irrigation water is within a few degrees of 50 degrees, consider soil temperature and weather forecasts before making irrigation applications at germination. Soil temperature in the mid-50s or higher should temper a 0.5-inch irrigation application resulting in minimal risk. For more information about cold injury and seedling germination, see “Early-Planted Corn & Cold Weather” by Purdue University.
Early season irrigation can be the cause and solution to soil crusting and emergence problems. Depending on soil type, crop residue and irrigation application equipment, early season irrigation can create some soil crusting, which can be accelerated by rapid surface drying. Small applications of water 0.2 to 0.3 inch may help to allow emergence of seed through the crust.
Planting after harvested forage like wheat hay or cereal rye silage has the double crop advantage, but rainfall or irrigation is required to replace the depleted soil moisture. Newly emerged corn and soybeans use less than 0.5 inch water per week, but many annuals like wheat and rye will dry the soil to a depth of 2–3 feet, leaving the crop dependent on timely rain or irrigation. Unless the forecast promises a significant chance of rain, 1 to 1.5 inches of irrigation is needed to create the moist soil crops need to successfully develop.
Many herbicide options can be assisted by a timely rain or irrigation. Applications of 0.3 to 0.5 inch of water will move activated soil-applied herbicides if rainfall does not occur within two days after herbicide application. Many soil-applied herbicide labels contain information on improving effectiveness by timely rains or irrigation.
Irrigating to activate herbicides can also create the problem of different levels of weed control between the dry corners and the irrigated portion of the field. Timely and directed scouting for weeds in dry corners will be needed later in the season.
Early season irrigation can be more accurately scheduled for monitoring soil moisture in the root zone rather than using a checkbook irrigation scheduling system for newly emerged crops. Later in the season, checkbook irrigation scheduling will show its advantages over scheduling by soil moisture in the root zone alone. To learn more about checkbook irrigation scheduling, see the Soil Water Balance Sheet
or Irrigation Scheduling Tools
by Purdue University Extension and Michigan State University Extension.
Delayed planting and slow root growth may increase the need for monitoring soil moisture and early season irrigation. Soil probing below the developing root is a good indication of the need for early season irrigation. Soil below the roots should still be able to form and hold a ball when squeezed if adequate moisture is present. The USDA offers an easy-to-use guide on hand-feel method of soil moisture monitoring