How important is pH fall rate and final pH?

The faster the pH value falls the sooner the wasteful activities of the live plant material and undesirable microorganisms will stop.  This will reduce losses and result in a more palatable silage.  It will also conserve more sugars for conversion to lactic acid so that less sugars will be required to achieve a successful fermentation. If the pH is slow to fall and/or sugars run out before a stable pH is reached, undesirable microorganisms such as clostridia can take over as they can convert lactic acid to butyric acid, a much weaker acid, so the pH increases (clostridial secondary fermentation).

The lower the crop DM, the lower the final pH will need to be to stabilise the fermentation

Effect of pH on grass silage fermentation quality