From Chris Main, Brian Leib, David Verbree, and Larry Steckel.
Rainfall continues to be spotty across Tennessee. Some locations have continued to have adequate moisture while other areas continue to miss the passing thunderstorms. UT Agronomists have received many questions about irrigation of crops. Almost no local data is available on the benefit of early irrigation in our crops. At this point wheat is too mature to improve yield with water. The best you could hope for is to help maintain soil moisture for double crop beans. However, irrigating wheat that is trying to dry down may promote other problems with grain quality. Early season corn, cotton and soybean all have low water use requirements. Dr. McClure wrote an excellent blog on irrigating corn and soybeans and can be found here. Her comments on helping emergence, activating herbicides and incorporating fertilizer hold true for cotton as well.
Early season water use in cotton is very low until blooming begins. Generally, cotton uses 0.5 inches of water per week prior to bloom. The exception would be in conditions like we experienced the first week of May. High temperatures, low humidity, south wind and conventionally tilled soil depleted soil moisture. Also, cotton grown in sandy soil with a very low water holding capacity is more likely to benefit from early irrigation. However be careful when watering cotton early. In UT research watering heavy prior to bloom in good water holding soils prevented cotton from reaching maximum yield potential. In fact watering too early was more detrimental to yield than watering too late.
Additionally, we need to remember that pumping water increases costs. Estimates from UT Irrigation Specialist Dr. Brian Leib of $3.60-$15.00 per acre-inch can eat up profit potential quickly with higher irrigation cost created by higher lift of water and the use of a diesel power plant. In general, electricity is less expensive to operate and more expensive to install than diesel. However, the operating cost for electricity can be more expensive than diesel for early irrigation when less than an 1” of water is applied in a month due to the electrical demand charge. The demand charge for a 100 kW motor can be $750 just for turning the pump on one time in a month. Center pivots can be operated with an “Off-Season Rate Plan” that does not incur a demand charge but does increases the rate by $0.01 per kW-hr.. Irrigation water cost for your center pivot can be estimated from tables located below. These tables also included the projected payback period for switching from diesel to electric.
We are aware that with the installation of new pivots across Tennessee we are tempted to run them. Regardless of crop, water is much more important during reproductive stages and we should focus our water resource during those periods for maximum economic return.
Herbicide Activation:
Herbicides that provide residual weed control must be “activated” to work. The definition for an “activated herbicide” is that it is worked into the top soil solution by precipitation and has become available to be taken up by germinating weeds. Fortunately, herbicides can be readily activated by overhead irrigation.
Recent research from Arkansas suggests that herbicides like Valor or Reflex need about 0.25″ of overhead irrigation to become active. Others like Dual, Cotoran or Caparol need at least 0.5″ of water to become active. The general rule of thumb is to apply 0.5” of water to incorporate most cotton herbicides. Herbicides vary in the amount of time they can lay on top of dry soil and still provide residual weed control once watered in. A good rule of thumb is 10 days. Once most herbicides lay on top of dry soil for 10 days or more they degrade to the point that they cannot provide much residual weed control once they are worked into the soil with water. Therefore the sooner the better when planning to irrigate to activate a herbicide.
Comparison of pumping systems costs from Brian Leib.
Diesel-Electric Pump Payback Table simplified
