  We study the effects of the interaction terms between the inflaton fields on
the inflationary dynamics in multi-field models. With power law type potential
and interactions, the total number of e-folds may get considerably reduced and
can lead to unacceptably short period of inflation. Also we point out that this
can place a bound on the characteristic scale of the underlying theory such as
string theory. Using a simple multi-field chaotic inflation model from string
theory, the string scale is constrained to be larger than the scale of grand
unified theory.
