Comparative analyses of variable and fixed rate irrigation and nitrogen management for maize in different soil types: Part II. Growth, grain yield, evapotranspiration, production functions and water productivity

Comparisons of variable rate irrigation (VRI) and variable rate fertigation (VRF) with fixed rate irrigation (FRI), no irrigation (NI) and conventional fertilizer management for maize (Zea mays L.) under three soil types (S1, S2 and S3) were made for three years (2015, 2016 and 2017). The research quantified and compared maize growth and development [leaf area index (LAI) and plant height], grain yield, crop evapotranspiration (ETc), irrigation-yield production functions (IYPF), evapotranspiration-yield production functions (ETYPF) and crop water productivity (CWP) under VRI, FRI and NI at fixed rate fertigation (FRF), VRF and pre-plant nitrogen (PP) management in the same environment and under the same agroomic management practices. The VRF treatment used 20% less fertilizer as compred with PP and FRF treatment without significantly (P > 0.05) reducing the grain yield. In the higher elevation soil S1, the grain yield was not significantly different (P > 0.05) between FRI and VRI treatments. However, in S2 and S3 which have lower elevation, yield in FRI was 43% and 55% greater than the yield in VRI, respectively. On average, under VRI management total irrigation amount 24% lower than FRI in S1, with only 4% reduction in yield as compared with FRI. Soil type impacted the response of maize grain yield to ETc and the responses also varied between FRI and VRI. For all soil types and years, higher ETc was observed in FRI treatment, except in 2015 for S1 and S2 where highest ETc was observed in VRI. FRI had greater productivity per unit of ETc than VRI. Observing the linear relationship of the pooled data for each soil type, a 25.4 mm of ETc (beyond the intercept) resulted in 0.48 and 0.35 ton ha−1 maize yield for FRI and VRI treatment, respectively, in S1; 0.75 and 0.45 ton ha−1 for FRI and VRI, respectively, in S2; and 0.71 and 0.43 ton ha−1 for FRI and VRI, respectively, in S3. VRI strategy increased the variability in grain yield, ETc and CWP as compared with FRI management. S1 had the lowest grain yield variability as compared with S2 and S3. The coefficient of variation (CV) of grain yield ranged from 8% in S1-FRI to as high as 35.3% in S3 VRI. FRI treatment in all soil types and years had less variation in yield as compared with VRI. Soil type had impact on CWP. On a three-year average basis, CWP was 2% higher in VRI in S1 whereas 11% lower in S2 and S3 than FRI CWP. Stronger grain yield response to irrigation and ETc (IYPF and ETYPF) were observed for FRI than VRI and NI at all nitrogen levels. The results of this research indicated that, in most cases, FRI had superior performance in terms of maintaining optimum crop yield and reducing yield variations than VRI. VRI management based on soil water status has potential of maintaining maize grain yield and improving CWP as compared with FRI in certain conditions or soil types such as in S1. However, further research is needed to validate/justify its adoption for the fields with significant spatial soil heterogeneity (both in horizontal and vertical domains) and to understand the economics of VRI-VRF systems. These results would be beneficial for maize growers and their advisors in terms of understanding the productivity responses to water under VRI and FRI management strategies with VRF, FRF and PP nitrogen application.