MAIZE CROP COEFFICIENTS UNDER VARIABLE AND FIXED (UNIFORM) RATE IRRIGATION AND CONVENTIONAL AND VARIABLE RATE FERTILIZER MANAGEMENT IN THREE SOIL TYPES

Maize (Zea mays L.) evapotranspiration crop coefficients (Kc) that are needed to estimate crop evapotranspiration (ETc) using the two-step approach for variable rate irrigation and nitrogen management under different soil types have not been investigated or quantified. In this research, alfalfa- and grass-reference crop coefficients (Kcr and Kco) curves were developed for fixed rate or uniform rate fertigation (FRF), variable rate fertigation (VRF) and pre-plant nitrogen (PP) management under fixed rate or uniform rate irrigation (FRI) and variable rate irrigation (VRI) for three soil types [Crete silt loam (S1), Hastings silty clay loam (2) and Hastings silt loam (S3)] in 2015, 2016 and 2017 growing seasons. Irrigation and nitrogen management strategies, as well as soil type, all influenced the Kcr and Kco values, which exhibited inter-annual variation. On average, greater variation in Kc curves between FRF, VRF and PP nitrogen treatment were observed under VRI treatments as compared with FRI. Results showed that Kc values are more dependent on the amount rather than the timing of the nitrogen application. In all three seasons, higher Kc values were observed in the FRI treatment than VRI with Kcr, ranging from 0.07 to 1.30 in FRI and 0.07 to 1.20 in VRI. Kc curves also differed between nitrogen treatments and the difference was more prominent in the VRI treatments than in the FRI in all years. In general, maximum Kc was observed in PP nitrogen treatment, followed by FRF and VRF. On a monthly average basis, maximum Kc values were observed in July and August in all soil types and minimum Kc values were observed in June. When soil types are considered, overall, the maximum Kcr value was observed in FRI-PP treatment in S1 (1.02), FRI-VRF treatment in S2 (1.06) and FRI-VRF treatment in S3 (1.02). The Kcr and Kco equations as a function of growing degree days were developed and monthly average Kcr and Kco values were tabulated for practical applications. To the best of the authors’ knowledge, this research is the first that investigated and quantified the impact of VRI and VRF strategies under FRF, VRF and PP fertilizer management strategies on maize Kc values. The Kcr and Kco values quantified in this research can aid irrigators, state agencies and other water management and agricultural professionals for more accurate crop water use determinations under different irrigation and nitrogen management strategies and different soil types.