Tell me, Bob...
Hey Bob...
In looking at that controller, and re-re-reading your question....
Does that controller simply turn the fan array on and off based on temperature setting? Zooming in close to the controller board doesn't reveal any hints of substantial componentry.
With the stirring-fan motors I've seen, they're an arrangement that will SOMETIMES respond okay to a thyristor-chopper type control, like the 'dimmer' type that Rob mentions, but if you're running an array of stirrers in your pen, and they're on 240v line, you can't just use a 120v dimmer.
One COULD, however, employ a gadget that uses a solid-state relay and switches power on and off... a few cycles on, and then a few off... so that the motors are never really running 'at speed'... the downside is that some motors will get really hot under those circumstances.
If I were faced with this situation, I'd probably consider something half-luddite... take the motor out, and install a shaft on each frame so the fan blades could be belt-driven from a lineshaft, and then put a 1hp 3-phase motor at some convenient location, and VFD that bugger, and wire the VFD to a thermistor sensor, so that fan speed increases with ambient temperature, and put the thermistor in such a location that airflow provides direct feedback to the thermistor's sense location, which means the temperature will attempt to be made self-regulating and constant based on your setpoint.