A.H. AbdelGadir, M. Dougherty, J.P. Fulton, L.M. Curtis, T. W. Tyson, H.D. Harkins and B.E. Norris
Fluctuations in cotton (Gossypium hirsutum, L.) yield in the Tennessee Valley of Alabama are common and usually related to drought or irregular rainfall. A sprinkler irrigation study was established from 1999 to 2004 to evaluate the minimum design flow rate to produce optimum cotton yields and economic gain. A replicated randomized block design consisting of four irrigation treatments ranging from one inch every 12.5 days (equivalent to 1.5 gpm acre-1 design flow rate or system capability) to one inch every 3.1 days (6.0 gpm acre-1) and a control, rainfed treatment. Daily plant water requirement was determined using soil moisture sensors and a spreadsheet-based scheduling program (MOISCOT) developed by Alabama Cooperative Extension engineers. Significant yield differences between irrigated and rainfed cotton were noted during the study period, with rainfall variability and treatment effects accounting for most of the yield response. The minimum design flow rate (1.5 gpm acre-1) increased mean seed cotton yield by more than 500 lb acre-1 over rainfed yields. The most economically efficient design flow rate (4.5 gpm acre-1) increased mean seed cotton yield by more than 996 lb acre-1. A positive relationship was observed between cotton yield and total seasonal irrigation depth during dry years. Across all six years of the study, irrigated treatments produced significantly higher yields than rainfed cotton. The highest six-year cotton lint yield and net economic returns were obtained with the 4.5 gpm acre-1 irrigation treatment. This result provides a rule of thumb for estimating the extent of irrigated area based on available water supply rate.
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