A method of determining the weight of animals and the development of plants in a controlled environment. The method includes measuring the increase in CO.sub.2 production over a test period. CO.sub.2 production is a measure of the metabolic rate and this is related to weight of animals and development of crops which can often be determined by weight. There is illustrated a poultry rearing house having vents, lights and an air heater. CO.sub.2 sensors are connected to a CO.sub.2 controller and in turn to a central controller. To test the increase in CO.sub.2 production over time, the birds are settled into a passive state with no feed and the lights turned off. The level of CO.sub.2 produced is measured. The vents are closed. After a time, the rate of Co.sub.2 produced is measured and the increase calculated.
