A system for determining performance data of plants growing in a field. The system comprises a mass flow meter for generating mass data related to the mass of plant product passed through the mass flow meter, and that is communicatively connected to a computer based data processing system; a temperature sensor for determining the temperature of the air adjacent the mass flow meter or within the mass flow meter, and that communicatively connected to the computer based data processing system, and a moisture sensor for determining the moisture of air adjacent the mass flow meter or within the mass flow meter, and that is communicatively connected to the computer based data processing system. The computer based data processing system is structured and operable to utilize the mass data, the air temperature and the air moisture to determine a yield of the plants from which the plant product was harvested.