  We present the contributions of the non-linear chiral scalar field S to cross
sections of $pp\to d\pi ^{+}, pp\to pn\pi ^{+}$ and $pp\to pp\pi ^0$ reactions
at threshold. We compare our results with the meson $\sigma$ contribution from
the $\sigma$-linear model. We show that the chiral scalar field S is almost 5
times bigger than the other contributions, which indicates that the two-pion
exchange dynamics embebbed in the S field is the most important term for the
pion production at threshold.
