Alfalfa and Ryegrass Responses to Irrigation Frequency in Shallow Soils.

Plant biomass production can be constrained in shallow soils because water availability becomes limited when rainfall is scarce. This study assessed irrigation effects on dry matter (DM) productivity of alfalfa and perennial ryegrass in shallow soils. An experiment was planted on March 2011 in complete randomized block design with four replicates in Lincoln, New Zealand in a soil less than 30 cm deep over gravels and coarse sand. Treatments were a rainfed control (non-irrigated) and three irrigation frequencies (every-3.5-days, every-7-days, and every-14-days) to fully replenish atmospheric water demand (Penman calculation from an on-farm meteorological station). We measured soil water by time domain reflectometry, and canopy cover by multispectral radiometry [normalized difference vegetation index (NDVI)]. For this establishment year, we examined two selected harvest intervals (i.e., 20-Dec-2011 to 25-Jan-2012, and 25-Jan to 29-Feb). Accumulating both harvests, DM productivities of alfalfa and ryegrass resulted respectively in 5767 and 6734 kg ha^-1 71 days^-1 for the every-3.5-days irrigation, while rainfed controls resulted in only 9 and 15% of these DM productivities, respectively (Ps<0.001). Furthermore, on the 25-Jan harvest, DM for both species were about one-fifth lower under every-14-days irrigation than with every-3.5-days (Ps<0.03). Soil water contents were much lower under rainfed than irrigation (0.12 vs. 0.23 m³ m^-3; Ps<0.05) and temporal variations increased with decreasing irrigation frequency. Compiling data for both species, NDVI measurements on 8-Feb were approximately double under irrigation vs. rainfed (0.80 vs. 0.33, Ps<0.001), and NDVI was well correlated to the subsequent DM harvest (r=0.98, Ps<0.001), implying restricted capacity of rainfed plants to intercept sunlight. Reduced DM under rainfed likely occurred through a water stress effect on canopy expansion. Results suggest an irrigation frequency of every-7-days can fully overcome water limitations in shallow soils. Understanding these ecophysiological controls on biomass production can inform improved management practices for enhancing these systems.