The simplest crop development model assumes a linear relationship between temperature (x) and rate of development (y) and can be written as y =β1+β2x +€. In crop development models non-linear functions of the parametersβ1andβ2have certain physical meanings. For example TO=−β1β2is the‘apparent’threshold temperature at which development begins to take place; while k=1/β2is the value of the summation constant, the number of degree days, or the sum of the daily temperature remainder indices required for the crop to pass through the phonological phase in question. In this article statistical inferences (hypothesis testing and the construction of confidence intervals) on TOand k are made in the case of five wheat cultivars, namely, Wilge, Betta, Karee, Scheepers 69 and SST 102. It is concluded that there were no real differences between threshold temperatures and summation constants for the cultivars considered.