Understanding yield and water use of dryland forage crops in New Zealand

Abstract

Lucerne (Medicago sativa L.) was found to be a more productive dryland forage option than chicory (Cichorium intybus L.) or red clover (Trifolium pratense L.). This was concluded from superior annual dryland yields of 20 t DM/ha from lucerne compared with 14-16 t DM/ha for chicory and red clover. This yield advantage was achieved by higher growth rates during both cool spring/autumn periods and dry summer periods. Lucerne was also the most persistent species maintaining a botanical composition of 94% six seasons after establishment, compared with 65% for chicory and 0% for red clover. All three species had similar herbage quality (25% crude protein, 11.5 MJ ME/kg DM) and grazing stock consumed 30% more protein and energy from lucerne than chicory or red clover crops.

The superior lucerne production during dry periods was due to increased water extraction up to 2.8 m depth, compared with ~1.9 m for chicory and red clover. All three crops displayed a top-down perennial water extraction pattern with an extraction front velocity of ~15 mm/day. Depletion of available water capacity in each layer of the soil profile was exponential following the arrival of the extraction front.

A detailed examination of lucerne physiology was conducted to understand seasonal variation, and the effects of water shortages on forage yield. Total DM production under non-water and non-temperature limiting conditions was related to total intercepted radiation. The total radiation use efficiency (RUE) was found to be 1.6 g/MJ. However, there was a seasonal change in DM partitioning between shoot and perennial organs (roots and crowns) and its influence on forage yield was quantified by converting total RUE to shoot RUE. The shoot RUE was 1.3 g/MJ in September, gradually decreased to a constant 1.0 g/MJ from mid-December late-January and then abruptly decreased to 0.6 g/MJ in March/April. Temperature also influenced shoot production and this was quantified by multiplying RUE by a linear factor that declined from unity at a mean regrowth cycle air temperature of 18°C to zero at 0°C.

Seasonal changes in radiation interception were quantified by studying the influence of temperature and photoperiod (Pp) on the components of leaf area index (LAI) expansion. Specifically, main-stem node appearance was linear in response to Tt and the phyllochron was 37±7°Cd for from August-January. However, phyllochron increased to 60 °Cd when the Pp on the day 150 °Cd before the first node decreased to 16 h (24 January). Continued decrease in Pp gave a 5.6 °Cd/h Pp reduction in phyllochron returning, it to 37±7 °Cd at a Pp of 13.5 h (15 March). There was a poor relationship between main-stem node appearance and LAI expansion, suggesting branching and leaf expansion have different seasonal responses to environment.

Water shortages were quantified by crop transpiration (ET) relative to the crops ET demand. Crop ET was calculated from water balance by removing evaporation losses from the soil and outer canopy. Crop ET demand (EPT) was calculated from Penman evapotranspiration potential (EP) multiplied by crop cover and a calibration coefficient (0.86), determined by regressing the ET of irrigated crops against EP. The RUE and LAI of dryland crops was expressed as a fraction of irrigated crops (fD/I) to quantify the effects of water stress. The LAI expansion of lucerne was the most sensitive process with fD/I of 1.0 at an ETIEPT of 0.97 decreasing to 0.1 at an ET/EPT of 0.22. There was a 1: 1 relationship between the fD/I of RUE and ET/EPT.

It is concluded that the improved understanding of lucerne environmental responses presented in this thesis must be considered when examining yield variability of lucerne.