Validation of the Forage Brassica Calculator: A fertiliser forecasting system for forage brassica crops grown in New Zealand. Pasja and bulb turnips

Abstract

Forage brassica calculators can be used to assist fertiliser consultants and farmers plan nitrogen (N) and phosphorus (P) applications for site-specific brassica production. The cost of each nutrient and the predicted productivity are used to recommend both economic and environmentally sound application rates. In this study, simulated yield was that modelled by PARJIB (a model that analyses and forecasts yield responses to nutrients) from the soil available nutrient levels, the potential yield and the fertiliser rates. The actual (observed) yields are those measured from the field. Simulations for ÂePasjaÂf (R2=0.96; root mean square of the deviation (RMSD) .3% (0.25 t/ha)) followed observed data closely with individual harvests at 60 and 90 days after sowing, consistent with observed data (R2>0.93; RMSD.4% (0.20 t/ha)). The ÂePasjaÂf model is sufficiently robust to predict site specific N and P fertiliser requirements. However, the model may need to be extended to more than two grazing events. Simulations for bulb turnips (R2=0.10; RMSD.28.1% (2.70 t/ha)) was poor. Simulations predicted the same yield for different treatments within sites, while observed yields differed widely (4.12 t/ha). This was because experiments included establishment and seeding method treatments which are not currently accounted for by the calculators. Thus, practical use of the brassica calculators for assisting with fertiliser rates currently depends on other agronomic factors being managed optimally. Key words: agro-ecological, leachate, supplementary feed, water-holding capacity