max(0, x - threshold): the broken-stick transform that turns an environmental
gradient into a heat-load covariate, zero in the comfort zone and linear above it.
The threshold is part of the model choice, 68 to 72 THI points are the usual pig and
cattle comfort limits, but the honest way to pick one is to compare fits.
Arguments
- x
environmental gradient (typically the
thi())- threshold
comfort limit; heat load is zero at or below it