Ecohydrology of irrigated silage maize and alfalfa production systems in the upper midwest US

Increased reliance on maize silage as a primary dairy forage in place of perennial crops like alfalfa may result in tradeoffs with on-farm water balances and water quality. Our objective was to evaluate how modern dairy forage crop, manure, and irrigation management influence field-scale ecohydrology of silage maize and alfalfa production systems. This study utilized year-round measurements of precipitation, irrigation, evapotranspiration (ET), soil water storage, subsurface tile drainage, and drainage concentrations and loads of nitrate (NO3-N), dissolved reactive P (DRP), and total suspended solids (TSS) to calculate annual water balances, water-use efficiency, and water quality impacts of these crops. Mean annual ET (µ = 499 mm yr−1) and tile drainage (µ = 90 mm yr−1) were similar between crops, due in part to high variability, though cumulative drainage over three paired years was 326 mm for silage maize and 214 mm for alfalfa. Annual soil water storage was similar for alfalfa (25 ± 44 mm yr−1) and silage maize (27 ± 50 mm yr−1), and confidence bounds indicated water budgets were balanced. Inherent water use efficiency (IWUE*) was 70% greater for maize than alfalfa (30.8 versus 18.1 g C hPa kg H2O−1 d−1). Paired-watersheds analysis indicated alfalfa reduced NO3-N but increased DRP concentrations in drainage compared to silage maize. However, weekly loads of NO3-N and DRP were reduced by 72% and 33%, respectively, and annual TSS loads reduced by 37% with alfalfa compared to silage maize. Results highlight the value of alfalfa for managing on-farm water flows and water quality during critical periods of the year and suggest that increased usage of maize silage has facilitated gains in dairy productivity at the expense of water quality in the Upper Midwest US.