UGA IRRIGATION UPDATE

Irrigation: Keep Efficiency And Crop Progress In Mind

This year can hardly be described as “normal” by any sense of the word. Therefore, it is no surprise that weather has been variable as well. Early season temperatures were at or below normal, with low evapotranspiration rates and adequate rainfall across most parts of the state, says David Hall, University of Georgia Extension water educator.

“This is good from an irrigation requirement standpoint, but cooler temperatures can mean slower growth for the cotton plant.”

However, at some point in the season, cool, wet weather will likely give way to higher temperatures and dry spells, leading to the need for irrigation. Crop progress and irrigation efficiency are important factors to keep in mind.

Plan Based On Crop Progression

With cool, wet conditions in some parts of the state but not in others, the Georgia cotton crop is anywhere from a few weeks past emergence to flowering.

Hall says in the first month the cotton root profile is fairly shallow in the soil.

“If a cotton plant has just fully emerged and your root profile is in the range of 8 inches to 10 inches, in this scenario you do not need to irrigate.”

However, crop water requirements increase from squaring and flowering.

“From 30 to 50 days after planting, water consumption almost doubles,” Hall says. “Water use typically increases in late June through July when we usually see hot, dry weather. Keep this in mind and stay on top of irrigation applications. Conversely, don’t over-irrigate the crop as there are yield penalties for doing this, too.”

Estimating water needs and planning irrigation is best based on the crop’s physiological progression rather than days after planting. BY AMANDA HUBER SOUTHEAST EDITOR

Cotton Irrigation Schedule

Growth Stage DAP Weeks after Planting

First Flower to First Open Boll

57-63 64-70 71-77 78-84 85-91 92-98 99-105 106-112 9 10 11 12 13 14 15 16

113-119 17

120-126

18 127-133 19 First Open Boll to 134-140 20 >60% Open Bolls 141-147 21 148-154 22 155-161 23 Harvest 162-168 169-175 24 25 Source: UGA Cotton Team Newsletter, June 4, 2020

Inches/Week Inches/Day

1.28 1.47 1.52 1.48 1.42 1.30 1.16 0.88 0.69 0.51 0.35 0.22 0.12 0.05 0.02 0.00 0.00 0.18 0.21 0.22 0.21 0.20 0.19 0.17 0.13 0.10 0.07 0.05 0.03 0.02 0.01 0.00 0.00 0.00

“Now is a good time to review the cotton irrigation schedule, determine where you currently are and decide what your water requirements are.”

Take Efficiency Into Account

University of Georgia Extension precision ag and irrigation specialist Wes Porter says producers also need to account for efficiency reductions based on environmental conditions and irrigation capacity.

“Hotter days with lower humidity mean irrigation efficiency is going to be lower than normal. With a water requirement of around 0.25 inch per day, an irrigation event of 0.75 of an inch would not last three full days when efficiency is accounted for.

“The typical efficiency of an overhead center pivot with drops is around 80% to 85%, meaning you are losing about one-fifth of the irrigation you are applying during each event. Thus, it is critical that weather, environmental conditions, irrigation timing and amounts all be accounted for when making scheduling decisions.”

Porter says later in the season, it is important not to over-irrigate the crop, “especially when we are receiving rainfall and have high humidity.” The additional irrigation in the canopy can cause boll rot.

“Monitor your field closely between the seventh to ninth weeks of bloom for signs of open bolls. Bloom is a critical time to ensure the crop is receiving the required amount of water. However, use caution, note how much rainfall each field receives and adjust irrigation applications accordingly.”