Check if nullable int has value and compare value to another integer
There is really no need to check HasValue in this situation. It is sufficient to compare with highestWeightOfParcel because if collo.WeightGrammes is null the comparison is false for any value of highestWeightOfParcel:

int highestWeightOfParcel  = 0;

if (collo.WeightGrammes &gt; highestWeightOfParcel)
{
    highestWeightOfParcel = collo.WeightGrammes.Value;
}