Simplified methods for on-farm prediction of yield potential of grazed lucerne crops in New Zealand

Abstract

Analyses of New Zealand farm systems require estimates of herbage yield and quality. These are available for perennial ryegrass and white clover pastures but not for other common pasture species. We used five lucerne datasets to derive functional relationships between growth rate and temperature. This required three functions to account for partitioning in lucerne between spring and autumn and differences between seedling and regrowth crops. The maximum mean daily growth rate of 206 ± 5.95 kg DM/ha/d occurred with full canopy cover at mean air temperatures between 15°C and 22°C. This represents the upper end of the temperature range available. The relationship was summarised into a matrix that could be incorporated into existing forage forecasters for farm system analyses. Alternatively, a simplified model assessed growth rate in relation to thermal time. This showed after 140°Cd accumulated from 1 July, lucerne initiated spring growth at 11.9 kg DM/ha/°Cd until mid-January and then decreased to 6.7 kg DM/ha/°Cd from mid-January to 30 June for fully irrigated crops. An exponential heightchron relationship was also calculated to estimate crop height based on the thermal time required per mm as photoperiod changed. This relationship allows lucerne quality to be predicted from the leaf:stem ratio.