An innovation systems approach to understanding the impacts of grass grub damage in irrigated Canterbury dairy pastures

Abstract

An innovation systems approach involving agribusiness representatives, researchers and farmers identified that damage caused by grass grub (Costelytra giveni) was a key factor contributing to areas of reduced yield within high-producing pastures. Using a recognised yield mapping technique, areas of ‘Low’ and ‘High’ pasture height were identified in different paddocks over 3 years; ‘Low’ areas had significantly higher numbers of grass grub larvae than ‘High’ areas. Pasture production was measured for the ‘Low’ and ‘High’ pasture height areas, and the difference was calculated to be 6800 kg DM/ha for 2018/19. This difference persisted after grass grub larvae were no longer active. Farm systems modelling analysis estimated this ‘yield gap’ led to a $650/ha/year difference in profit. A survey of farmer perceptions of grass grub damage on 23 central Canterbury dairy farms estimated 19% of pastures were affected by grass grub, with 11% of the area in those paddocks being damaged. Econometric modelling showed differences in farmer perceptions of grass grub damage. Data from the survey and the pasture measurements were combined with the farm systems modelling results to estimate a $1,870,000/year regional-scale impact of the ‘yield gap’. An integrated pest management approach is suggested to control grass grub damage.