The balance agri-nutrients brassica calculator - improvement of the turnip model.

Forage brassica calculators were developed to assist in the planning of nitrogen (N) and phosphorous (P) management for site-specific forage brassica production. These calculators are based on the PARJIB model which analyses and forecasts yield responses to nutrients. Previous validation of the calculators showed that simulation of the summer („Barkant‟) turnips model was poor (RMSD≤28% (2.7 t/ha) (R 2 =0.10). This was attributed to yield responses to P being masked by low yield potential due to factors such as poor crop establishment and other agronomic factors. To recalibrate the model, three turnip P response experiments were conducted in the 2012-13 season at sites (Southland, Manawatu and Central North Island) with soil Olsen P values between 5 and 17 mg/kg soil. Fertiliser P was either broadcast or banded at five rates (0-200 kg P/ha). There was a response to P fertiliser only at the site with the lowest soil P (Southland). At this site, the total dry matter (DM) yield at the final harvest increased with P supply, from 0.4 t/ha for the 0 kg P/ha crops to 10 t/ha for the 100 kg P/ha crops. This was associated with a 3fold increase in plant population with P supply. However, the method of P application had no effect on either final DM yield or plant population, across the three sites. These and previous data were used to refit the PARJIB model to take into account soil P retention and reserve K. This approach explained a larger proportion of yield variation with an RMSD of <10% (0.23-1.1 t/ha) (R 2 =0.65) compared to the initial validation. The turnip P response curve is now comparable to other brassica species and can be used with greater confidence for strategic N and P fertiliser recommendations. Additional keywords: Brassica campestris subsp. rapifera, agronomic factors, „Barkant‟ turnip, calibrated, nutrient requirement, PARJIB model, predicted yield, validated.