Prediction of time to harvest and its variability of broccoli (Brassica oleracea var. italica) part II. Growth model description, parameterisation and field evaluation

• A dynamic model to predict head size and variation was developed.
• The model was evaluated against independent field data.
• Head size and variation could be predicted based on weather data.
• Variation in time to heading is one main cause of head size variation.
• Simulated harvest percentages of 80% can be archived with less than two harvests.

A lack of uniformity of broccoli heads in practice leads to the need for multiple hand harvests which increase production costs significantly. Decisions about harvest operations have to take economic aspects and existing price fluctuations into account. For planning of cutting and optimization of harvesting a forecast of the exact amount of harvestable heads per date is necessary. A model for growth, development and variability is presented here. Heading and variation in time to heading were considered in a piecewise linear regression model in response to temperature combined with one stochastic subroutine to describes the occurring variability. This model was implemented into a growth model to validate the hypothesis that variation in heading can explain the variation in head size. For this task a dry matter production and portioning model for cauliflower was used and reparameterised for broccoli growth and improved through implementation of dynamic LUE and SLA parameters. The model was positively evaluated using independent field data. The model explains on average 79% (22–94%) of the measured variability in head size at final harvest by variation in heading. For prediction of harvest dates a harvest criterion of 500 g fresh weight with a stem part of 0–10 cm is defined. To maximise reaping percentages of individual harvests and for optimisation of harvest operations the model calculates percentages of harvestable heads per date. Simulations showed that harvest percentages not less than 80% can be archived by 1.8 selective hand harvests on average.